Petrology of 

Paricutin Volcano 

Mexico 

By RAY E. WILCOX 

GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA, MEXICO 

-GEOLOGICAL SURVEY BULLETIN 965-C 

Prepared in cooperation with the Secretarta de Ia Economfa Nacional de 

Mexico, Direccion de Minas y Petroleo, 

and the Universidad Nacional Autonoma de Mexico, lnstituto de Geolog{a, 

under the auspices of the Foreign Operations Administration, Department of 

State 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1954 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas M·cKay, Secretary 

GEOLOGICAL SURVEY 

W. E. Wrather, Director 

For sale by the Superintendent of Doc:uments, U. S. Government Printing Office 

Washington 25, D. C. · Price 30 cents (paper cover) 
CONTENTS 

Page 

Abstract---------------------------------------------------------- 281 Introduction _____________________ ·- _________________________ -______ 282 

History of the eruption_____________________________________________ 284 

Petrography _____________________ ·-________________________________ 289 

Representative lava specimens ___________________ --------------- 292 

Individual constituents_________________________________________ 300 

Plagioclase _____________ --·----_____________________________ 300 

Olivine___________________________________________________ 302 

Clinopyroxene_____________________________________________ 304 

Orthopyroxene____________________________________________ 304 

Opaque and accessory minerals______________________________ 305 

Glass-------~-------------------------------------------- 306 

JCenoliths_____________________________________________________ 306 

Transition zones between lavas and xenoliths_____________________ 312 

Petrochemistry____________________________________________________ 313 

Origin of the petrographic and chemical trends________________________ 319 

Paragenesis of minerals ____________________________ ------_______ 320 

~egaphenocrysts------------------------------------------ 321 

~icrophenocrysts------------------------------------------ 323 ~icrolites and glass _____________________________ ----------- 325 

Chemical relationships _____________________________ ----- ____ --- 326 

Thermal requirements _________________________________ ----_____ 340 

Literature cited ___________________________________________ ---_---- 341 

Index------------------------------------------------------------ 351 

ILLUSTRATIONS 

PLATE 10. Photomicrographs of lavas and pumice of Paricutin volcano__ 296 

11. Photomicrographs of Paricutin xenoliths and granodiorite____ 297 

FIGURE 95. ~ap of central ~exico showing location of Paricutin and other 

volcanoes active since 1519 _____________________ -- ____ -- 282 

96. Area covered by lava of Paricutin volcano to end of 1944____ 286 

97. Area covered by lava of Paricutin volcano to end of 1952____ 287 

98. Volume percentages of megaphenocrysts in some Paricutin 

ejecta_______________________________________________ 301 

99. Relative abundances of groundmass materials in successive 

Pari cutin ejecta______________________________________ 301 

100. Compositions of lavas of Paricutin volcano, plotted according 

to date of eruption____________________________________ 317 

101. Silica-variation diagram of lavas of Parlcutin and nearby 

volcanoes-------------------------------------------- 318 

III 
IV CONTENTS 

Page 

FIGURE 102. Occurrence of megaphenocrysts and microphenocrysts in successive ejecta of Parfcutin volcano of 1943-50_ _ _ _ _ _ _ _ _ _ _ _ 320 

103. Silica-variation diagram of lavas, phenocrysts, an.d xenoliths of 

Paricutin volcano_____________________________________ 329 

104. Silica-variation diagram showing graphical operations in addition and subtraction of materials from bulk compositions____ 331 

105. Silica-variation diagram comparing actual· bulk compositions 

with trends developed by graphical addition or subtraction 

of olivine, plagioclase, and xenolith taken two at a time_____ 333 

106. Silica-variation diagram comparing actual bulk compositions 

with trend developed by graphical addition or subtraction 

of olivine, plagioclase, and xenolith______________________ 334 

107. Schematic cross section of Parfcutin volcano and its supposed 

magma cupola________________________________________ 345 

TABLES 

TABLE-1. Volume percentages of megaphenocrysts and groundmass 

(vesicle-free) and of vesicles in Pari cutin lavas_______________ 292 

2. Analyses of lavas from Parfcutin volcano _____________________ -294 

3. Analyses of xenoliths from Parfcutin volcano_________________ 296 

4. Summary of anorthite contents of central portions of plagioclase microphenocrysts ______ 7' ________________ - _----- _--- 302 

5. Refractive indices and inferred compositions of olivine megaphenocrysts in lavas of Pari cutin volcano __ -:________________ 303 

6. Optic angles of orthopyroxene, with compositions inferred from 

curves of Winchell (1951, fig. 283)------------------------ 305 

7. Refractive indices of glassy mesostases of some Paricutin lava 

specimens_____________________________________________ 306 

8. Specific gravities of Paricutin lavas ____ -- ____ -- ________ ----_- 319 

9. Average divergences of test trends from actual trends of the 

Pari cutin silica-variation diagram_________________________ 337 
PETROLOGY OF THE PARICUTIN VOLCANO 

By RAY E. WILCOX 

ABSTRACT 

Paricutin volcano, Mexieo, remained in active eruption from its birth, February 

20, 1943, until March 4, 1g52. During this period of 9 years, the eruption of lava 

and vapors was remarkably continuous; eruption of pyroclastic material was 

somewhat spasmodic, with some periods of a month or more during which practically no pyroclastic material was thrown out. With rare exceptions the sites of 

lava eruption were at either the northeastern or southwestern base of the main 

cone, and lava emission al.ternated between these sites from time to time. The 

total area of old land covered by lava during the 9-year eruption is 24.8 square 

kilometers, and the total rock material erupted is estimated to have occupied 

about 1.4 cubic kilometers in the magma chamber. 

A progressive change in composition is noted in successive ejecta of Paricutin 

volcano. Ejecta of 1943 are of olivine-bearing basaltic andesite containing 55 

percent silica. Succeeding ejecta are progressively more salic, until in 1952 the 

ejecta are of orthopyroxene andesite containing over 60 percent silica. With the 

exception of a sharp decrease in magnesia and increase in silica in 194 7, this change 

in composition of successive ejecta was fairly regular. The ejecta of 1943 and of 

the first part of 1944 are characterized by small numbers of olivine and plagioclase 

megaphenocrysts in groundmasses of plagioclase, olivine, clinopyroxene, orthopyroxene, opaque oxide, n.nd glass. Plagioclase megaphenocrysts are virtually 

absent in ejecta after 1944. In ejecta of 1945-1952, the olivine megaphenocrysts 

carry coronas of fine-grained hypersthene; and in ejecta of late 1947, olivine megaphenocrysts are scarce and remain so in successive ejecta through the close of 

eruption in 1952. Clinopyroxene megaphenocrysts occur only rarely and clinopyroxene microphenocrysts only in the ejecta of 1943-44 and in some ejecta of 

1946. Orthopyroxene, which becomes more abundant and larger in size in the 

groundll'.l&lses of successive pre-1947 ejecta, occurs also as megaphenocrysts in 

ejecta of 1947 and later. In the individual mineral series there appears to be only 

a slight tendency towards more salic compositions of the minerals themselves in 

the later erupted materials, although the bulk rock compositions become definitely 

more salic. Composition::; of plagioclase megaphenocrysts average near An1o, 

those of olivine near Fo80, and those of orthopyroxene near En15, as inferred from 

optical properties. Groundmass orthopyroxene is consistently poorer in magnesia 

than the megaphenocrysts of the same specimen. The glassy mesost.ases of later 

erupted specimens have lower refractive indices than those of early erupted specimens, indicating more salic compositions. 

Xenoliths of granite, quartz monzonite, dacite tuff and other rock types are 

found in the ejecta of Paricutin volcano and are presumed to represent some, at 

least, of the types of coun,Gry rock at depth. All stages of melting, inflation, and 

mutual solution of the mineral components of the xenoliths are seen, but there 

seem to be only a few easily recognized examples of intimate strewing of xenolithic 

281 
282 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

material through the normal ejecta, and the visible transition zones between lava 

and xenolith do not usually exceed a few millimeters in width. Xenocrysts of 

quartz and feldspar occur sporadically in a few lava specimens that appear to be 

otherwise normal. · ' 

On the basis of graphical tests using the chemical and. petrographic data, it is 

concluded that fractional crystallization alone could not have caused the observed 

differences between successive ejecta. A combination of fractional crystallization 

(involving olivine and plagioclase) and assimilation of salic country rock is shown, 

by use of the silica-variation diagram, to satisfy closely the chemical relationships 

between the lavas; and rough calculations indicate that the heat required for this 

process appears to be available by convection without superheat if a r~a&onable 

configuration of the magma cupola is assumed. 

INTRODUCTION 

Paricutin volcano is located in the State of Michoamin, Mexico, 

about 320 kilometers due west of Mexico City, at lat. 19°29'33" N., 

long. 102°14'59" W. (fig. 95). The eruption began suddenly February 20, 1943, and continued without major interruption for 9 years. 

The eruption of lava ceased suddenly on February 25, 1~52; and 

after a few explosive surges, the eruption of pyroclastics cease'd''c'ompletely on March 4, 1952. The purpose of this paper will be to examine the remarkable continuity of eruptive activity and an equally 

remarkable progressive change in composition of successively erupted 

material from basaltic andesite of 55 percent silica in 1943 to andesite 

of slightly over 60 percent silica at the end of the eruption in 1952. 

104' 

EXPLANATION 

.. Active volconoes 

100 0 200 Miles 

~------~---------~------~ 

100 0 300 Kilomete;e 

~--~--~--~--~ 

100' 

Mexico0 City A 

POPOCATEPETL 

104 100' 96" 

FIGURE 95.-Map of central Mexico showing location of Parfcutin and other volcanoes active since 1519. 
PETROLOGY OF-PARICUT~ VOLCANO 283 

Many scientific pilg;rimages have been made to Paricutin volcano, 

which, with its easy ac~cessibility, has offered opportunity to study the 

life cycle of a volcano J[rom its very beginning, as well as to study the 

effects of a prolonged eruption on the ecology and economy of a rural 

area. Bibliographies of published results of some of these studies 

have been included in earlier chapters of this bulletin (Segerstrom, 

1950, p. 4-6, and Williams, 1950,. p. 272-274), and it is necessary here 

only to mention a few other references that have appeared more 

recently. Reports of the eruptive activity for 6-month intervals 

have been made by F1ries and Gutierrez (1950a, b; 1951a, b, 1952a, 

b; and 1954. A detailed description of the activity of the volcano 

during the first 2 years is being furnished by Foshag and Gonzalez. 

The pre~ent study began with a period of essentially continuous 

observation of the activity of Paricutin vo]cano by the writer from 

September 1946 through May 1948, and thereafter observations were 

confined to short monthly visits until December 1948. Specimens of 

the erupted material collected then, together with specimens collected 

by other investigatom before and after the period of the writer's 

field work, form the basis of the petrographic and chemical portions 

of this study. 

The field investigation during 1946-48 was carried out with the 

support of the U. S. State Department's program of Scientific and 

Cultural Cooperation with the American Republics. For aid and 

advice during the investigation, the writer is especially indebted to 

Carl Fries, Jr., Richard E. Fuller, Howe! Williams, Eduard-O Schmitter, 

J. A. Hernandez Velasco, S. Shoup Oropeza, Celedonio Gutierrez, 

Antonio Saldana, and Jesus Saldana. The house at Cuzeno Station, 

5 kilometers north o:f the active cone, was made available as an 

observatory by the Geological Society of America, which also supported the work in many ways through the U. S. Committee for the 

Study of Paricutin _volcano. Calculations of tidal force at Paricutin 

from 1946 through 1B48 were kindly furnished by the U. S. Coast 

and Geodetic Survey. 

For specimens and thin sections of Paricutin lavas, bombs, and 

xenoliths to augment his own collection, the writer thanks Eduardo 

Schmitter and J. A. Ilernandez Velasco, of the Instituto de Geologia 

de Mexico; S. Shoup Oropeza, of Recursos Hidralicos de Mexico; 

Konrad B. Krauskopf, of Stanford University; F. H. Pough, of the 

American Museum of J~ atural History; and Carl Fries, Jr. and Donald 

E.' White, of the U. S. Geological Survey. The writer is grateful to 

Konrad B. Krauskopf for permission to use six unpublished chemical 

analyses. Field notes of early activity were kindly made available by 

Celedonio Gutierrez, :s:owel Williams, Konrad B. Krauskopf, Donald 

E. White, and the h~te Ezequiel Ord6nez. The manuscript of the 
284 GEOLOGIC INVESTIGATIONS IN THE P ARICUTIN AREA 

present report was read and criticized by Richard E. Fuller and 

several of the writer's colleagues in the Geological Survey, to whom 

the writer owes many thanks, while retaining responsibility for any 

misstatements or faulty reasoning. 

HISTORY OF THE ERUPTION 1 

The eruption of Paricutin volcano began February 20, 1943, afterseveral weeks of local earthquakes of increasing frequency and 

intensity. After the onset of the eruption, no intense local earthquakes occurred. No previous vent appeared to have been situated 

at the particular point of outbreak (Gonzalez and Foshag, 1947) 

although there are many extinct cones within a radius of a few kilometers (Williams, 1950, pl. 8). The site of outbreak is 6~ kilometers. 

SSW. of the village of Angahuan, near the northeast base of Cerros. 

de Tancitaro, at an elevation estimated to have been· 2,398 meters. 

above sea level, (Fries and Gutierrez, 1951b, table 4). 

The growth of the resulting cinder cone was rapid. On the morning 

of February 21, it was estimated to be 10 meters high and by noon tobe some 30 to 50 meters high. On the night of February 22, Ord6fiez 

(1947, p. 2-6) estimated the height to be 60 meters and on February 

26, some 150 meters. On March 2 it was measured instrumentally as. 

165 meters in height and 560 meters across the base in an east-west 

direction. Huge masses of plastic lava were thrown out bodily ontothe flanks, in addition to the great quantities of moderate sized bombs. 

and lapilli, to contribute significantly to the rapid growth of the cone. 

The first lava had already started to flo"', by the night of February 

22 (Ord6fiez, 1947, p. 26 and 29), and to move eastward from the now 

sizable cone. Although the record of lava emission until October of 

the first year is not complete, it appears that the emission was all 

from the north and northeast sides of the cone and that, although 

especially strong surges or fountaining occurred from time to timer 

there were few periods during which lava was not flowing. Eruption 

of ash and bombs from the main vent was generally very copious: 

during the first 8 months. 

On October 18, 1943, a series of new lava vents broke out along a 

line about N. 60° E. from the NE. base of the cone and within a few 

days lava emission became concentrated at one vent, named Sapichu,z 

about 800 meters northeast of the main vent of the cinder cone. 

Appreciable amounts of pyroclastics were erupted from this vent, 

while steam, but no significant amounts of pyroclastics, were erupted 

1 A detailed account of the first 2 years of eruption of Paricutin is being prepared by W. H. Foshag and 

J. Gonzaiez and a compilation of subsequent activity is being prepared by the writer for publication by 

the Geological Survey. For purposes of the present paper, only a -brlet resum6 will be given. 

2 In the spellings of Tarascan geographic names the writer follows tbose·auggesf;eti by Fries· (hi ·segerstrom 

1950, p, 153-161), which depart to a minor extent from the spellings given in previous papers. 
PETROLOGY OF PARICUT~ VOLCANO 285 

from the main cone during the several months life of the Sapichu 

vent. The flow of lava from Sapichu kept the northeastern side of 

its own cinder cone Qpen, so that the resulting pile of pyroclastics 

became crescentic in shape. 

On January 8, 1944, immediately after the cessation of activity of 

Sapichu, multiple lava vents were opened at and near the southwest • 

base of the main cone, and strong eruption of pyroclastics was resumed from the vent of the main cone. The area of lava vents 

became known as the Mesa de los Hornitos because of the profusion 

of hornitos. With the possible exception of part of April, the effusion 

of lava was copious and continuous, giving rise to the great San Juan 

flow, which overwhelmed the town of San Juan Parangaricutiro in 

July and continued for a total distance of 9 kilometers before finally 

ceasing in August or September 1944 (fig. 96). 

In September 1944 a new vent, called Taqui, and in early November 

another, called Ahmin, opened on the southwestern base of the cone. 

Effusion of lava from the Taqui vent was not well recorded and may 

or may not have been continuous. During November and December 

the Ahmin vent furnished copious effusions of lava, which spread 

eastward and northeastward from the cone. The general outline of 

the Paricutin lava field had been delineated by the end of 1944 

(fig. 96), and subsequent flows to the end of the eruption in 1952 were 

to pile up on previous flows for the most part and to extend the 

ultimate border only slightly (fig. 97). 

During 1945 the eruption of lava continued, apparently alternating 

between the Ahuan and Mesa de los Hornitos vents near the south· 

west foot of the cone (Krauskopf and Williams, 1946). Observations 

during April to October were sporadic and not resumed systematically 

until October from which time until the end of the year lava emission · 

from the sou,thwestern vent complex was continuous (Krauskopf, . 

1948a, p. 721-726). 

The lava activity continued from the Mesa de los Hornitos vents 

during the first part of 1946 until March 17 when a resurgence of 

strong lava emission was marked by the reopening of the Ahuan vent 

(Kennedy, 1946), becoming less copious in June and more copious in 

July and August (J. A. Hernandez V., field notes). About September 

11, 1946, a new Mesa de los Hornitos vent took over from the Ahuan 

vent, building up to a climax of fountaining in mid-October and 

continuing with decreasing volume through the remainder of 1946 

(Wilcox, 194 7 a). 

On January 14, 1947, with the further decline of the Mesa de los 

Hornitos vent, ''Puertecito," a new vent, opened at the immediate 

southwest base of the cone and furnished moderate amounts of lava 

285997-54-2 
286 GEOLOGIC INVESTIGATIONS IN · THE PARICUTIN AREA 

-.... ~~~ 

- -· 

EXPLANATION 

0 

Vent 

CORUCJUATA 

/f1\'..l --"' 

~~--~--~--0~----------~-· 

Grid ticlls correspond 1o grid of figure 97 

--.... ·~\\I/', TZINT~~GO ·~ /.,.... :::,: I 

'''~''')' 

--, 1,11&' 

r::: T~IRAFII\N 

..... ,,~ ... ,~ _, 

"--

+ 

+ 

FIGURE 96.-Area covered by lavas of Paricutin volcano .to end of 1944. 

until it finally ceased on March 2. Meanwhile, on January 19 lava 

began to issue from the northeast base (the new Sapichu or Nuevo 

Juatito vent) for the first time since January 1944. The flow was 

sluggish at first; but with the closing of the Puertecito vent on the 

opposite side of the cone March 2, the new Sapichu vent effusion 

became appreciable. On August 11 fountaining occurred from the 

new Sapichu vent, and on August 14 the Puertecito vent reopened 
PETROLOGY OF PARICUT~ VOLCANO 287 

and remained strongly active while the Sapichu vent became weak, 

finally ceasing on October 1. On September 1 the old Ahu.an vent 

was rejuvenated and took over the major portion. of J.a,va emission, 

which continued in moderate to large amounts until the end of 194 7 

~WilCox, 1947b, 1948a, b; Wilcox and Shoup. 1948). It was noted 

thatthe·lsva from the new Ahuan vent tended to pile up more around. 

the vent and spread from the vent in broad lobes of much greater 

it 

NATION 

c Limit of lava an Dec.31 01951 

•rea covemt 

~ b, l;.,a flaw .. .Jaftuary- March 1952 

. . 

Vent 

0 

g 0 

0 

N . ' 

0 

-·-.... ~EL OBISPO I '" ~ '\ ,, '..- TZINTZUNGO ~ ' ' ~URETO ~ 'I \''/JI\' , __ 

80005 

FIGURE 97.-Area covered by lavas of Parlcutin volcano to end of 1952 (atter Fries and Gutimez. 1954). 
288 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

thickness than in previous flows (Wilcox and Shoup 1948, p. 79). 

This may have been due in part to the smaller gradients in the dir~_ction of this flow and in part to greater viscosity of the lava,- which; 

it is found (fig. 100), had by this time reached a siiica content of over 

58 percent. 

On February 7, 1948, the lava vent at the northeast base of the 

cone opened again, and by February 16 the Ahuan vent had closed. 

Thereafter, until final sudden cessation of lava activity of the volcano 

February 25, 1952, the effusion of lava was continuous from the 

northeast vent, (Fries and Gutierrez, 1950a; 1950b, 1951a, 1951b, 

1952a, 1952b, 1954). The rate of effusion varied from moderate to • 

strong from February 1948 through January 1949 and was generally' 

weak from the end of February 1949 to July 1949, increasing to 

moderate from July through December 1949. From January to June 

1950 the effusion of lava was generally strong, with several brief 

periods of weak eruption of a few da))S duration. From June 20 

to August 25 the eruption of lava was moderate, and from A~gust 

25 to October 15 it vacillated between strong, moderate, and weak 

at intervals of several days. Thereafter to the end of 1950, the 

eruption of lava was moderate except for the last 10 days when it 

varied between moderate and weak. 

The variable behavior of lava effusion continued through January 

1951, becoming predominantly large during the last half of February. · 

The effusion of lava was very small from March 8 to 19, and thereafter 

to June 20 was moderate for the most part. The effusion of lava was 

large from June 21 to November 9, thereafter to the end of the year 

varied from small to large. During January and February the 

effusion of lava was moderate to large, ceasing altogether on February 

25, 1952, just five days after the ninth birthday of the volcano. 

A general chronology of the pyroclastic eruptions would be difficult, 

other than to say that the amount of pyroclastics erupted during the 

first couple of years was greater than subsequently and that the 

intensity varied greatly from both the short-term and long-term -viewpoints. The almost complete cessation of pyroclastic eruption (but 

not vapor eruption) from the main cone during the active period 

of the Sapichu lava vent, October 18, 1943, to January 6, 1944, 

is notable. There were weeks-long periods of little pyroclastic 

activity during the latter part of 1945 and early 1946, mid-February 

to mid-March 1947, July 26-August 7, 1947, and mid-March to 

mid-June 1948. Subsequently, until the end of June 1951, the 

eruption of pyroclastics was weak for the most part, punctuated by 

brief periods of voluminous eruptions of ash and bombs and by 

spasmodic surges of pyroclastics and strong explosions of increasing 

frequency and intensity. From July through October 1951 the pyro.-
PETROLOGY OF PARICUTLN VOLCANO 289 

clastic eruption was uniformly strong, becoming moderate for most 

of November and strong to very strong for December 1951 and 

January and early February 1952. After lava eruption ceased on 

February 25, only weak explosions occurred; and eruption of pyroclastics ceased on March 4, 1952. 

The total areal extent of the new lava field at the end of the eruption, 

as shown on figure 97, is about 24.8 square kilometers (Fries and 

Gutierrez, 1954). The total weight of rock material erupted, including 

both lava and pyroclastics, is computed by Fries (1953, p. 611) 

as about 3596X 106 metric tons .. Assuming an average specific gravity 

of 2;6 in the molten state, it would have occupied about 1.4 cubic 

kilometers in the Paricutin magma chamber. 

In order to determine whether variations in atmospheric pressure 

might have some control on the variations of pyroclastic activity, a 

record of barometric pressure was kept at Cuzefi.o station, 5 kilometers 

north of Par!cutin volcano starting in September 1946. No obvious 

correlation between atmospheric pressure and intensity of volcanic 

activity was noted. Likewise, there was no apparent ~orrelation 

betwee11 the pyroclastic or lava activity and the variations in tide 

producing force, calculated by the U. S. Coast and Geodetic Survey 

for the vicinity of Paricutin volcano. (See eruptive diagrams in 

Wilcox, 1947a and b, 1948a and b; Wilcox and Shoup, 1948; Wilcox 

and Gutierrez, 1948.) 

PETROGRAPHY 

During the protracted eruption of Par!cutin volcano, the petrographic character of successive ejecta changed from au olivine-bearing 

basaltic andesite to an orthopyroxene-bearing andesite, and this 

change was accompanied by a gradual change of chemical compositions. In the petrographic descriptions which follow, the megaphenocrysts will be discussed separately from the groundmasses9 

which include nticrophenocrysts, microlites, and glassy mesostasis. 

The material erupted during 1943 and early 1944 is characterized 

by small numbers of olivine and plagioclase megaphenocrysts in a 

groundmass composed of plagioclase, olivine, clinopyroxene microphenocrysts (with or without orthopyroxene microphenocrysts) in a 

clear glass or microli te-charged glass mesostasis. The rna terial 

erupted after the first part of 1944 is notably lacking in plagioclase 

megaphenocrysts, and until 1947 the only megaphenocrysts are 

olivine, lying in a groundmass of plagioclase, decreasing amounts of 

olivine and clinopyroxene microphenocrysts, and increasing amounts 

of orthopyroxene microphenocrysts in the glassy mesostasis. The 

material erupted after 1947 and until the end of 1950 is characterized 

by scattered orthopyroxene Inegaphenocrysts and, rarely, corroded 
290 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

olivine megaphenocrysts in a groundmass composed of plagioclase and 

orthopyroxene microphenocrysts and glassy mesostasis. 

Petrographic descriptions of some of the lavas and bombs erupted 

from Paricutin volcano during 1943~5 have been given by Schmitter 

(1945, p. 127), Milton (1945), and Williams (1950, p. 256-265). 

·Schmitter classed the rocks of 1943 as latitic olivine basalt ("basalto 

latitico de olivino"), while Milton regarded them as andesite of 

"basaltic habit." Williams (1950, p. 256) chose to call them basaltic 

andesites on the basis of low but positive qz values, shown ·by chemical 

analyses, and the presence of modal olivine. Because of the limited 

number of analyses at his disposal, Williams was led to characterize 

the ejecta of Paricutin as monotonously similar, with no evidence of 

serial trends. No detailed petrographic descriptions of material 

erupted since 1945 have been published, and the definite compositional 

trends have only been revealed as analyses on later erupted material 

have become available. 

In the present study more than 125 specimens of lavas, bombs, and 

xenoliths of Paricutin volcano have been examined in hand specimen 

and in thin section. The dates of eruption of these specimens cover 

the period from 1943 through 1952-with unfortunate gaps during the 

first portion of 1946, for which specimens are not available, and during 

the first portion of 1948, specimens of which were lost in shipment. 

Optic angles and extinction angles were measured directly in thin 

section on the universal stage. Refractive indices, nX', of plagioclase 

cleavage fragments were determined in immersion liquids with sodium 

light as were values of nZ of some orthopyroxene cleavage fragments. 

Measurements of nX, nY, and nZ of other orthopyroxene and of 

'olivines were made using the method of Rosenfeld (1950). Compositions of individual minerals of the Paricutin lavas have been inferred 

from their optical properties, using 2V and nZ of orthopyroxene 

(Winchell, 1951, fig. 283), refractive indices of olivine (Winchell, 1951, 

fig. 395), and extinction angles X' /\010maz and X' /\010.L-a .for 

plagioclase {Winchell, 1951, figs. 148 and 176 respectively). 

· Attention should be drawn here, however, to the different compositions that.can be inferred by applying the same optical data to curves 

·compiled by different authors. The orthopyroxene of specimen 

W-48-5 (see description below) was found to have (negative) 2V=80° 

and nZ=l.695. Applied to Winchell's (1951) figure 283, both 2V 

and nZ lead to an inferred composition of En75• Applied to Kennedy's 

· (1947) figure 3 (derived largely from Hess and Phillips, 1940), 2V 

leads to an inferred composition of Ens4 while nZ leads to an inferred 

composition of En77• . This. is in spite of the apparent excellence of 

the data of Hess and Phillips for the .particular orthopyroxenes .that 

they investigated. The curves of Winchell are used in the present 
PETROLOGY OF P ARICUTIN VOLCANO 291 

investigation simply because they seem to give consistent values of 

composition for both 2V and nZ. If the values are in error it is 

probable that the error of m.ost are in the same direction and of 

similar magnitude, so that at least the relative differences in composition between the orthopyroxenes of the Parfcutin lavas can be 

depended upon. 

Compositions of plagioclases of volcanic rocks, inferred from conventional extinction angle curves, may be in error by several percent 

if one gives credence to· the existence of a separate set of optical properties for high-temperatur~ plagioclases, as suggested by the work 

of Kt)hler (1949) and others. The original complete curves of 

Rittmann (1929) for extinction angles in different orientations were 

computed from the optical data on plutonic plagioclases, that is, 

plagioclases that had cooled very slowly and had low-temperature 

optical properties. Using the complete Rittmann procedure, the 

writer in fact has found that eurves of extinction angles of particular 

Paricutin plagioclases do not fit in the family of complete curves 

published by Rittmann. In view of the disagreement still existing 

concerning the optical properties of volcanic and plutonic plagioclases, 

the conventional "low-temperature" curves of Winchell (1951, fig. 148) 

have been used here, recognizing that the anorthite-contents thus 

inferred are on the order of 10 percent greater than those to be inferred 

from the "high-temperature7J' curves recently compiled by Troger 

(1952, p. 113). 

The volume percentages of megaphenocrysts, measured by the 

Chayes (1949) point-counting method, are given in table 1 for all 

but five of the analyzed specimens and for a few additional speci.:. 

mens. The results are graphed according to dates of eruption in 

figure 98. Because in some specimens there is no sharp demarcation 

in size-frequency distribution between large and small phenocrysts, 

megaphenocrysts have been arbitrarily defined as crystals larger than 

0.3 millimeter in longest dimension, while microphenocrysts are defined as those crystals ranging in size from 0.3 millimeter down to 

0.03 millimeter, the lower limit depending somewhat on morphology. 

The relative abundance of microphenocrysts and microlites of plagioclase, olivine, orthopyroxene, and clinopyroxene, estimated by eye in 

all the thin sections available, are graphed according to dates of 

eruption in figure 99. Where two or more specimens are so close in 

date of eruption that they could not be shown separately, the average 

of the results is graphed for the group of specimens of that date. 

In the following paragraphs the petrography will first be described 

for several individual specimens, then for the individual constituents 

through the series. This will be followed by a description of repre-
292 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

sentative xenoliths and a discussion of the mutual effects between 

xenolith and enclosing lava. 

TABLE 1.-Volume percentages of megaphenocrysts, and ground mass (vesicle-free) 

and of vesicles in Paricutin lavas 

Analysis 

number 

Specimen 

number Date Erupted 

Mega phenocrysts 

OrtboOlivine pyroxene Clinopyroxene 

G~;:sd- Vesicles 

Plagioclase 

----·-1-----1-·-------1---1----------------

L ---------- -ii.:i&:c---- Feb. 22, 1943 ________ 2.2 0 0 1.1 96.7 17.7 4 ___________ Apr.(?) 1944 _________ 3.3 0 0 0 96.7 2.0 6 ___________ 11-29-1 Nov. 1945 ___ -------- 2. 5 0 0 0 97.5 19.5 

7_ ---------- 11-26-2 Nov------------- ____ 3.6 0 0 0 96.4 17.3 

8----------- 12-7-1 Dec.---------------- 3.2 0 0 0 96.8 9.5 

9_ ---------- 12-3-1 Dec _____ ------------ 3.3 0 0 0 96.7 26.6 10 __________ 12-19-1 Dec_---------------- 3.0 0 0 0 97.0 20.5 

2-7-1 Feb. 1946__ __________ 4.0 0 0 0 96.0 14.5 11 __________ W-4&-27 Sept ______ ---------- 2.6 tr 0 0 97.4 7.3 

W-47-6 Feb. 1947__ __________ 2. 5 0 0 0 97.5 11.8 12 __________ W-47-9 May __ ------------ __ 4. 5 0 0 0 95.5 24.7 13 __________ W-47-14 June __ -------------- 3.5 .I 0 0 96.4 15.1 14 __________ W-47-19 Sept. ___ ------------ 2.3 .7 0 .3 96.7 5. 7 15 __________ W-47-30 NOV----------------- 1.8 tr 0 0 98.2 30.8 

W-47-31 Dec.-------------- __ 1.7 0 0 0 98.3 15.0 16 __________ W-48-5 Aug. 1948 ___ -------- .9 .7 0 0 98.4 14.0 

17---------- FP-5-49 May 1949 ___________ .2 1.4 0 0 98.4 23.5 18 __________ FP-20-49 Dec- ___ ------------- .6 1.3 0 0 98.1 12.0 19 __________ FP-20-50 Sept.I950 ___________ 0 1.2 0 .1 98.7 29.1 

FP-55-50 Dec----------------- .3 2.4 0 0 97.3 14.0 

22---------- FP-16-52 Feb. 1952 ____________ .3 .7 0 .1 98.9 18.2 

REPRESENTATIVE LAVA SPECIMENS 

The thin section of the specimen of lava of February 22, 1943 

(analysis 1, table 2), collected by G. A. Cooper and chemically 

analyzed by Charles Milton (1945), shows 2.2 percent by volume of 

euhedral olivine megaphenocrysts up to 0.5 millimeter in diameter 

and 1.1 percent by volume of plagioclase megaphenocrysts up to 0.4 

millimeter in length. The hand specimen of this thin section was not 

available to the writer. An olivine megaphenocryst from lava erupted 

in March 1943 (specimen 51-W-18), however, showed the following 

refractive indices: 

nX=1.667 ±0.002 

nY=1.691 to 1.693±0.002 

nZ=1.713±0.003 

Inferred composition is Fos2-79· 

Extinction angles X' 1\0lO.La of four plagioclase megaphenocrysts 

range from 39° to 36° with slight progressive normal zoning. The 

inferred avera.ge range of composition is An7s--72. Groundmass is 

composed of abundant microphenocrysts of plagioclase, showing slight 

progressive normal zoning (inferred average range An74_ 68), many 

euhedral microphenocrysts, and grains of olivine (inferred composition 

near Fo70 by 2V), and abundant microlites of orthopyroxene (?), 

chiefly as stubby prisms up to 0.005 millimeter length in a mesostasia 
PETROLOGY O:E' PARICUTIN VOLCANO 293 

of brown glass composing about 5 percent of the volume of the rock. 

A lava erupted in March 1943 (Instituto de Geologia de Mexico 

thin section labeled "l-ava de Marzo 1943") has many euhedral 

megaphenocrysts of olivine up to 0.8 millimeter in diameter and many 

lath-shaped megaphenocrysts of plagioclase from 0.3 to 0.6 millimeter 

in length. (See pl. 10, fig. L) Judging from the descriptions by 

Schmitter (1945), the occurrence of mega phenocrysts of plagioclase is 

not uncommon in the earliest la.vas. The groundmass of this specimen 

is composed of abundant plagioclase; many stubby prisms of clinopyroxene (augitic?), some as long as 0.15 millimeter; microlites of 

orthopyroxene (?) ; dendrites and skeletons of opaque oxide; and innumerable tiny bubbles in a glassy mesostasis, which is estimated to 

compose about 25 percent of the volume of the rock. 

A specimen, estimated to have been erupted about April 1944 and 

collected by K. B. Krauskopf from the middle reaches of the great 

San Juan flow (specimen 11-16-1, analysis 4, table 2), is dense black 

with scattered olivine phenocrysts. In thin section the texture is 

porphyritic intersertal. Olivine megapheT}ocrysts compose 3.3 percent by volume of the rock and range up to 1.3 millimeter in length. 

They are generally euhedral but show minor embayment and attachments of (or reaction to form) small orthopy.toxene crystals. Plagioclase mega phenocrysts are rare; one em bayed and internally corroded 

fragn1ent of plagioclase (xenocryst?) was found in the section. In 

the groundmass, plagioclase 1nicrophenocrysts predominate. Most 

are less than 0.2 millimeter in length and not well terminated. Extinction angles X/\010 .La of the uentral portions of 10 crystals 

ranged from 37° to 34°, from which an average composition of An68 is 

inferred. Zoning in most crystals is normal-progressive, being much 

more marked in the thin rims which commonly range in composition 

down to An50 and less. Orthopyroxene microphenocrysts are common 

as stubby prisms up to 0.1 millimeter in length, showing slightly higher 

interference color than in the orthopyroxene of most of the Paricutin 

lavas. Optic angles were measured in two crystals as (negative) 

2V=70° and 68°, from which compositions of En67 and En65 are inferred. A lesser number of elongate clinopyroxene crystals up to 0.6 

millimeter in length are present; none lend themselves to determination of optic angles. Many polyhedrons (cubes?) of opaque oxide 

up to 0.05 millimeter are present and are presumed to be magnetite. 

Also present in the groundn1ass are scattered irregularly shaped 

grains of olivine, usually with adhering orthopyroxene crystals. 

Microlites of orthopyroxene (?), clinopyroxene (?), and dendrites of 

opaque oxide are distributed through a brown translucent glassy 

mesostasis which composes about 10 percent of the volume of the rock. 

285997-54-3 
294 GEOLOGIC INVESTIGATION8 IN THE PARICUTIN AREA 

TABLE 2.-Analyaes of lavas from Paricutin volcano, Mexico 

2 3 5 6 7 8 10 11 

Bulk Analyses 

SiOt------ 55.04 54.88 55.51 55.21 55.59 56.41 56.15 56.48 56.41 56.61 56.13 

AbOa----- 18.82 18.38 18.19 17.94 17.72 17.67 17.57 17.57 17.60 17.61 17.34 Fe,o, _____ 1. 92 1. 31 1.63 1.60 1.33 1.39 1.80 1.63 1.67 1. 45 1. 74 FeO ______ 5.69 5.97 5.38 5.96 5.99 5.40 5.08 5.18 5.18 5.35 5.42 MgO _____ 5.68 5. 57 5.31 5.37 5.60 5.66 5.68 5.56 5.62 5.64 5.58 cao ______ 7.17 7.40 7.19 6.98 6.99 6.89 6.95 6.95 6.90 6.89 6.99 

NatO ..••• 3.88 3.88 3.92 3.87 4.00 3.87 3.85 3. 70 3.81 3.84 3. 79 K,O ______ .85 .86 1.10 1.26 1.13 1.19 1.18 1.34 1.20 1.21 1.30 

HtO+---- } .16 { .13 .08 .05 .03 .04 .04 .08 .05 .05 .20 HtO- ____ .05 .01 .01 .04 .03 .04 .00 .00 .03 .06 COt--~--- n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. .06 TiOt~----- .94 .95 .97 1.08 1.05 .93 .95 .94 .93 .93 1.02 p,o. ______ Mno .21 .29 .31 .41 .36 .30 .30 .37 . 31 .30 .36 _____ .07 .13 .12 .13 .13 .12 .12 .11 .12 .11 .12 

--------------------------------- TotaL. 1100.51 99.80 99.72 99.87 99.96 99.90 99.71 

Normative minerals 

Q _________ Or 3.47 2.40 3.54 3.30 3.46 4.62 4.86 ________ 4.99 5.56 6.67 7. 77 6.62 7.22 7.23 Ab ________ An 32.40 33.01 33.52 32.44 33.82 32.45 32.49 ________ Wo 31.32 30.02 28.63 27.76 26.78 27.48 27.24 _______ 1.04 2.20 2.32 1.86 2.18 2.09 2.32 En ________ 14.16 14.00 13.28 13.38 13.91 14.19 14.20 Fs ________ 

Mt. 7.37 8.58 7.26 8.04 8.26 7.52 6.60 ______ 2. 77 1.86 2.32 2.32 1.85 2.09 2.55 fl _________ 1.82 1.67 1.82 2.13 2.12 1. 67 1.82 

AP-------- .67 .67 .67 1.01 1.00 .67 .67 

Calculated groundmass compositions 

SiOa ______ 55.53 ........................ -------- 56.12 57.20 57.30 

AbOs.---- 19.22 ...................... -------- 18.80 18.34 18.53 

Total Fe 88 FeO. 7.15 -------- -------- 6.92 6.24 6.15 MgO _____ 4.66 -------- -------- a. 76 4.41 3.94 cao ______ 7.29 ....................... -------- 7.31 7.15 7.33 Na,o _____ 4.00 -------- -------- 4.05 4.02 4.06 KJO ______ .88 -------- -------- 1. 32 1.23 1.25 ·No, ______ .98 -------- -------- 1.13 .96 1.01 p,o. ______ .22 -------- -------- .43 .31 .31 Mno _____ .07 -------- -------- .14 .12 .12 

TotaL. 100.00 -------- -------- 99.98 -------- 99.98 100.00 

t Includes 0.04 S and 0.06 BaO. 

Analysis Specimen 

Data on specimens analysed 

Date erupted Collector 

1 ------------------ Feb. 22, 1943 ________ C. A. Cooper _________ _ 2 USNM 108058 Mar. 1943 ___________ W. H. Fosbag ________ _ 

3 USNM 108073 Nov.1943 ________________ do ________________ _ 

4 11-16-1 

5 USNM 108100 

Apr.(?) 1944 ________ K. Krauskopf ________ _ Nov. 1944 ___________ W. H. Foshag ________ _ 

6 11-29-1 Nov. 1945 ___________ K. Krauskopf ________ _ 

7 11-26-2 

8 12-7-1 

Nov. 21, 1945. ____________ do ________________ _ Dec. 1945 _________________ do ________________ _ 

9 12--3-1 _____ do ____________________ do ________________ _ 

10 12-19-1 _____ do ____________________ do ________________ _ 

11 W-46--27 Sept. 18, 1946 ________ 1. A. Herliandez V __ _ 

99.91 99.80 100.02 100.11 

5.63 5. 59 4.86 4.87 

7. 77 6.69 7.23 7.80 

31.39 32.04 32.46 32.04 

27.48 27.87 27.21 26.48 

1.86 1. 98 2.20 2. 79 

13.88 14.13 14.10 13.93 

6.86 7.02 7.52 6.88 

2.32 2.33 2.09 2.56 

1.82 1.68 1.67 1.98 

1. 01 .67 .67 .67 

57.42 57.45 57.42 56.94 

18.39 18.46 18.36 18.02 

6.15 6.16 6.18 6.60 

4.01 4.02 4.18 4.34 

7.27 7.24 7.18 7.26 

3.87 4.00 4.01 3.93 

1.40 1.26 1.26 1.36 

.98 .97 .97 1.06 

.39 .32 .31 .37 

.11 .12 .11 .12 

99. 99 100. oo I oo. 98 too. oo 

Anal yet 

Charles Milton (1945). 

E. Chadbourn (William11, 

1950). 

Do. 

lames Kerr. 

E. Chadbourn (William!!, 

1950). 

lames Kerr. 

Do. 

Do. 

Do. 

Do. 

H. Hyman. 
PETROLOGY OF PARICUTIN VOLCANO 295 

TABLE 2.-Analyses of la11as from Paricutin volcano, Mexico--Continued 

12 13 14 15 16 17 18 19 20 21 22 

Bulk Analyses-Continued 

57.05 57.63 58.13 58.39 59.09 59.41 59.771 59.93 60.24 60.38 60.07 17.27 17.50 17.59 17.78 17.55 17.30 17.291 17.31 17.30 17.27 17.28 1. 42 1. 38 1. 27 1. 87 2.04 1. 57 1. 21 1. 23 1.19 1.10 1.37 5. 21 5.12 5.20 4. 51 4.27 4. 78 4.95 4.95 4. 59 4.66 4.39 5.64 5.16 4. 55 4.03 4.03 3. 81 3. 72 3.55 3.55 3.59 3. 73 6.94 6. 77 6. 72 6. 75 6.46 6.36 6.28 6. 21 6.14 6.16 6.16 3. 71 3. 71 3. 79 3.86 3. 92 3. 71 3. 74 3. 73 4.01 3.89 4.00 1. 23 1. 38 1. 41 1. 30 1.50 1. 67 1. 67 1. 72 1.66 1.69 1. 67 .17 .04 .10 .11 .08 .12 .12 .10 .04 .05 .03 .02 .02 .03 .01 .03 .00 .00 .00 .04 .05 .05 .02 .01 .01 .01 .00 .02 .00 . 01 .00 .00 .01 .89 .86 .84 .86 . 78 .84 .83 .83 .80 .80 . 81 .29 .29 .30 .30 . 30 . 31 . 31 .30 .29 .28 .28 .12 .12 .11 .12 .11 .11 .11 .11 .10 .10 .10 ------------------------------ {19.98 

6.30 

7.23 

31.42 

26.95 

2.44 

14.10. 7.13 

2.09 

1. 67 

.67 

58.37 

18.40 

5. 74 3.42 

7.40 

3. 95 

1. 31 

. 95 

. 31 

.13 

99.98 

Analysis 

00.99 

7.01 

8.32 

31.37 

26.91 

2.09 

12.88 

6.99 

2.09 

1. 67 

.67 

58.65 

18.40 

5. 76 3.39 

7.11 

3.90 

1.45 

.90 

.30 

.12 

00.98 

Specimen 

12 W-47-9 13 · W-47-14 14 W-47-19 15 W-47-30 16 W-48-5 17 FP-5-49 18 FP-20-49 

19 FP-20-50 20 FP-5-51 21 FP-13-51 22 FP-16-52 

100.05 99.90 100.16 100.01 100.00 99.98 99.95 100.01 00.9.'1 

Normative minerals-Continued 

7.59 9. 71 10.13 10.77 10.80 11.28 10.68 11.58 10.51 8.32 7. 77 8.89 10.04 10.01 10.01 10.02 10.01 10.02 32.46 32.44 32.98 31.52 31.96 31.96 34.07 32.49 34.10 26.62 27.47 25.82 25.64 25.30 25.30 24.20 25.02 24.22 2. 21 1. 86 15:g~ i 1. 86 1. 74 1. 63 1. 97 1.63 1. 97 11.38 10.09 9. 52 9.30 8.90 8. 91 {1.00 9.31 7.37 5.54 5.02 !).36 6.86 6: 86 6.34 6.47 5.82 1. 86 2. 78 3.02 2.10 1. 86 1. 86 1. 62 1.62 1.86 1. 52 1. 67 1. 52 I. 52 1. 52 1. 52 1. 52 1. 52 1. 52 . 67 .67 .67 .67 .67 .67 .67 .67 .67 

Calculate1il groundmass compositions-Continued 

58.91 59.10 59.44 59.71 60.16 60.16 --------- __ .,. ______ --------- 18.29 18.28 17.95 17.71 17.76 17.60 --------- --------- --------- 5. 89 5.92 5.87 5.99 5. 77 5.93 .......................... __ ., ______ --------- 3.19 3.14 3.38 3.29 3.01 3.19 --------- --------- --------- 6. 98 6.94 6. 61 n. 51 6.46 6.32 --------- --------- --------- 3. 95 3. 97 4.01 3.80 3.84 3. 79 ----· ---- --------- --------- 1. 47 1. 34 1.53 1. 70 1.71 1. 75 --------- --------- --------- .88 .88 .80 .86 .85 .85 --------- --------- --------- . 31 . 31 • 31 .32 .32 . 31 --------- --------- --------- .11 .12 .11 .11 .11 .11 --------- --------- ---------

99. 98 100. 00 100. 01 100. 00 99.99 100.01 --------- --------- -----·----. 

Data on specimens analysed-Continued 

Date erupted Collector 

Apr. 9, 194i'--------- J. A. Hernandez V. June 10, 1947 ________ C. Guti~rrez __ ------- Sept. 5, 1947__ _______ S. Shoup 0------------ Nov. 1947 .. ·--------- R. E. Wilcox _________ _ Aug. 1948 .... _____ _________ do ___ _____________ _ 

May 19, 19<l9 ________ C. Fries, Jr ___ _______ _ Dec. 13, 19•19 __ __ _________ do ________________ _ Sept. 1, 1950 ______________ do ________________ _ May 1951 ... _____________ _ do _________ _______ _ 

Nov. 195L ·--·------- C. Gutil!rrez _ .. __ ------ Feb. 25, 19ii2 .. _ __ _ ___ C. Fries, Jr __________ _ 

Analyst 

H. HYman 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. Lucile Tan-ant. Do. 

Do. 
296 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

TABLE 3.-Analyses of xenoliths from Paricutin volcano, Mexico 

Bulk Analyses Normative minerals 

X-1 X-2 X-3 X-4 X-1 X-2 X-3 X4 

--------- SiOt--------- 70.88 71.99 71.10 75.95 Q ___________ 26.79 32.27 29.97 33.02 

AhOa------- 14.27 15.95 14.31 13.51 Or __________ 21.75 14.43 22.22 27.82 

Fe20s---~--- 1.52 .68 1.43 .25 Ab __________ 35.75 34.51 27.22 33.03 

FeO --------- 1. 53 1.22 1.63 .27 An __________ MgO 7.53 11.38 13.33 5.28 ________ 1.17 .71 1.14 .05 c ___________ .71 2.44 cao ________ --------- ------:io 1. 65 2.49 2.88 1.05 En __________ 2.91 1.80 2.90 Na2o _______ 4.18 4.03 3.20 3.90 Fs __________ 2.52 1. 45 1.19 .13 

K20--------- 3.64 2.43 3. 76 4. 74 Mt __________ .93 .93 2.09 .46 H20+----~-- .11 .13 .07 .13 !!_ __________ • 76 .46 • 76 .15 

RtO-------- .05 .02 .01 0 AP---------- .34 .34 .34 co, _________ .02 . 01 0 0 

'ri02-------- .36 .21 .37 .04 

:P20s-------- .08 .09 .07 .02 Mno _______ .05 .05 .05 .03 

TotaL_- _ _ 1 99. 64 100. 01 100. 02 99. 94 

1 Includes 0.268. 

Data on specimens analyzed 

Analysis Specimen Date erupted Collector Analyst 

:X-L------------- 51-W-L ________ May 1943 _______ F. H. Pough __________________ E. Engleman. 

:x:-2-------------- 51-W-5 __________ 1943 _____________ Inst. de Geol. de M6xico______ Do. :x:-a______________ 51-W -9__________ 1944-45 _______________ do _____ -------------------- Do. 

:X-4-------------- 51-W -8---------- 1944-45 _______________ do_________________________ Do. 

PLATE 10 

FIGURE 1. Lava erupted March 1943 (Instituto de Geologia de Mexcio), thin 

section showing megaphenocrysts of plagioclase and olivine. 

Crossed nicols. 

2. Pumice fragment erupted April1944 (Instituto de Geologia de Mexico), 

thin section showing microphenocrysts of olivine and plagioclase 

and microlites of olivine and clinopyroxene (rods) in clear glass. 

Plane polarized light. 

3. Lava erupted April 9, 1947 (analysis 12, table 2), showing olivine 

mega phenocryst with thin reaction rim of fine-grained orthopyroxene. 

Crossed nicols. 

4. Lava erupted September 1950 (analysis 19, table 2), showing orthopyroxene megaphenocryst cluster. Plane polarized light. 

5. Lava erupted February 1952 (analysis 22, table 2), showing orthopyroxene and plagioclase microphenocrysts in clear glass. Plane 

polarized light. 

6. Lava erupted June 1947 (analysis 13, table 2), showing clinopyroxene 

flanking plates and extensions. Plane polarized light. 
GEOLOGICAL SURVEY BULLETIN 965 PLATE 10 

2 

3 4 

PHOTOMICROGRAPHS OF LAVAS AND PUMICE OF PARICUTIN VOLCANO 
GEOLOGICAL SURVEY 

BULLETIN 965 PLATE 11 

PHOTOMICROGRAPHS OF PARlcUTTN XENOLITHS AND GRANODIORITE 
PETROLOGY OF PARICUT~ VOLCANO 297 

Lava that issued from the northeast lava vent in May 1947 (Specimen W-47-9, analysis 12, table 2) is quite vesicular and contains 4.5 

percent by volume of olivine megaphenocrysts ranging up to 0.8 

millimeter in diameter, showing corroded outlines and carrying 

coronas of small orthopyroxene crystals (pl. 10, fig. 3). Refractive 

indices determined on cleavage fragments of olivine megaphenocrysts 

showed 

nX=l.675 

nY=l.695 

nZ=approximately 1.712, 

corresponding to a composition of about Fo80• No plagioclase megaphenocrysts are present. The groundmass consists of abundant 

plagioclase laths up to 0.25 millimeter in length, abundant orthopyroxene prisms up to 0.20 millimeter in length, a moderate number 

of c1inopyroxene needles, scattered grains of olivine (rimmed with 

orthopyroxene), and dust-size, opaque oxide in a cloudy brown glass 

mesostasis which composes about 25 percent of the volume of the rock. 

Centers of groundmass plagioclase show X' /\010 maximum, ranging 

from 35° to 38° (implying An80 to An611) and rims range from 25° to 

35° (A144 to An60). Three groundmass crystals of orthopyroxene 

have optic angles (negative) 2V=72° to 75° (implying En 49 to En 71). 

PLATE 11 

FIGURE 1. Quartz monzonite xenolith erupted May 1943 (analysis X-1, table 

3) showing orthoclase (or), plagioclase (pl), and quartz {q) with 

black glass in fractures and along grain boundanes. Crossed nicols. 

2. Granodiorite (specimen F P-20-52) from outcrop area 30 kilometers 

southeast of Parlcutin volcano. · 

3. Frothy xenolith erupted 1943 (specimen collected by Instituto de 

Geologia de Mexico, Wilcox 51-W-4), showing quartz (q) and feldspar (f) and glass. Plane polarized light. 

4. Vesicular xenolith (pyroclastic?) erupted in 1943 (specimen collected 

by Instituto de Geologia de Mexico, Wilcox 51-W-6), showing 

quartz (q) and feldspar (f) phenocrysts in inflated groundmass of 

quartz, feldspar, and glass. Plane polarized light. 

5. Transition zone between xenolith and lava erupted October 1949 

(specimen FP-11-49), showing quartz (q) and fritted orthoclase 

(or) xenocrysts in transitional glass groundmass containing plagioclase and orthopyroxene microphenocrysts and microlites. Plane 

polarized light. 

6. Glassy xenolith erupted July 1945 (collected by F. H. Pough, Wilcox 

specimen 51-W-2), showing phenocrysts and fragments of orthopyroxene (about En77) and plagioclase (An80- 86) in salic glass. Plane 

polarized light. 
298 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

A specimen of lava, issued from the northeast lava vent on June 

10, 1917 {specimen W-47-14, analysis 13, table 2), has a porphyritic 

intersertal texture with about 15 percent by volume of vesicles and 

1Q-15 percent by volume of clear brown glass mesostasis. There is 3.5 

percent olivine megaphenocrysts up to 1.2 millimeter diameter, somewhat corroded and carrying fine-grained orthopyroxene rims, and 0.1 

percent by volume of orthopyroxene megaphenocrysts. Abundant 

microphenocrysts of plagioclase and orthopyroxene stand out clearly 

in the glass base. There are a few independent microlitic needles of 

clinopyroxene and many outgrowths and overgrowths of microlitic 

clinopyroxene on orthopyroxene microphenocrysts (pl. 10, fig. 6). 

Scattered cubes of opaque oxide up to 0.02 millimeter occur in the 

olivine crystals but are rare in the groundmass. 

Lava of November 1947 from the active front of the lava flow 

issuing from the southwestern vent (specimen W-47-30, analysis 15, 

table 2) is quite vesicular, and olivine megaphenocrysts compose 1.8 

·percent by volume of the rock (vesicle-free). The olivine occurs as 

fractured and corroded crystals up to 1.5 millimeter diameter which 

carry coronas of fine-grained orthop:yroxene. No mega phenocrysts 

of plagioclase or orthopyroxene were observed. The groundmass 

consists of abundant plagioclase laths and tablets up to 0.15 millimeter 

in length, abundant prisms of orthopyroxene up to 0.1 millimeter in 

length, and occasional ragged needles of clinopyroxene (also found as 

flanking plates on some orthopyroxene prisms). Opaque oxide 

crystals adhere to the clinopyroxene and are scattered through a 

slightly cloudy, brown glass basis, composing about 35 percent of the 

volume of the rock. Centers of groundmass plagioclase show 

X' /\010maz=35° to 40° {An80 to An70). Determinations of optic 

angles of groundmass orthopyroxene showed a range of (negative) 

2V=70° to 79° (En ss to En 74). 

A specimen erupted in September 1948 (specimen W-48-5, analysis 

16, table 2) is moderately vesicular and contains 0.9 percent of olivine 

megaphenocrysts and 0.7 percent by volume (of vesicle-free rock) of 

orthopyroxene megaphenocrysts. The olivine megaphenocrysts, 

ranging up to 0.8 millimeter diameter, are corroded and carry coronas 

of fine-grained orthopyroxene crystals. The few orthopyroxene 

mega phenocrysts occur as euhedral prisms up to 0. 7 millimeter in 

length. The refractive index, nZ= 1.695 on cleavage fragments, and 

optic angle determination, (negative) 2V=80°, of one orthopyroxene 

crystal imply a composition of En75• The groundmass is composed 

of abundant orthopyroxene prisms up to 0.15 millimeter in length, 

· with clinopyroxene as flanking plates on a few of them, and abundant 

tiny crystals and chains of opaque oxide (both adhering to and included in the orthopyroxene) and dustlike particles in the dark-brown, 
PETROLOGY OF PARICU~ VOLCANO 299 

nearly opaque glass basis which composes about 25 percent of the 

volume of the rock. The centers of groundmass plagioclase crystals 

show extinction angles X' 1\ 010 maz ranging from 34 ° to 40° 

(An58 to An70), with rims ranging down to compositions as low as 

An4s· Optic angles of six groundmass orthopyroxene crystals range 

from (negative) 2V=65° to 68° (En81 to En65). These exceptionally 

low values of En content are confirmed by the higher birefringence 

and stronger pleochroism observed in thin section. 

A specimen of lava erupted in September 1949 (specimen FP-2o-49, 

analysis 18, table 2) is moderately vesicular and porphyritic hyaloophitic in texture. It contains 0.6 percent by volume (vesicle-free) 

of olivine megaphenocrysts and 1.3 percent by volume of orthopyroxene megaphenocrysts. The olivine crystals show less embayment than those of previously described specimens but carry distinct 

rims of small orthopyroxene crystals. The orthopyroxene megaphenocrysts are euhedral prisms ranging up to 0.8 millimeter in 

length and show optic angles (negative) 2V ranging from 76° to 86° 

(implying En72 to En80). The groundmass consists of abundant 

plagioclase laths up to 0.15 millimeter in length (with rare tablets up 

to 0.4 millimeter in length), abundant prisms of orthopyroxene up to 

0.1 millimeter in length, as well as some orthopyroxene microlites in 

an abundant clear brown glass basis which composes about 25 percent 

of the volume of the rock. No clinopyroxene or opaque oxide was 

observed. Optic angles of two groundmass orthopyroxene crystals 

show (negative) 2V=75° (En71 ). 

A specimen of lava erupted in September 1950 (specimen FP-20-50, 

analysis 19, table 2) is very vesicular and has a hyaloophitic 

texture. It contains no olivine megaphenocrysts but 1.2 percent by 

volume of orthopyroxene megaphenocrysts, which occur as euhedral 

prisms up to 0.8 millimeter in length, some in clusters of several 

crystals (pl. 10, fig. 4). Refractive index on cleavage fr~gments of 

megaphenocrysts was found to be nZ=1.704-1.706 implying compositions of En68 to En69• Optic angles of five megaphenocrysts 

were found to range from (negative) 2V =75° to 79° (implying Eh71 

to En74). The groundmass consists of abundant laths and tablets of 

plagioclase up to 0.2 millimeter in length and abundant prisms of 

orthopyroxene up to 0.1 millimeter in length (a few with flanking 

plates of clinopyroxene), in an abundant clear brown glass basis composing about 25 percent of the volume of the rock. Refractive 

indices of fragments of the groundmass plagioclase showed nX' = 1.557 

approximately, implying compositions near An55. Optic angle of one 

orthopyroxene crystal of the groundmass showed (negative) 2V=69° 

(En05). Refractive index of the glass showed a range of n= 1.538 to 

1.543. 
300 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

Specimen FP-16-52 (analysis 22, table 2, pl. 10, fig. 5), from the 

last lava erupted, was collected at the vent a few days after lava 

activity had ceased on Feb. 25, 1952. It is quite vesicular and in 

thin section shows 0.3 percent by volume of olivine megaphenocrysts, which range up to 0.6 millimeter diameter and carry thin 

reaction rims of orthopyroxene. Also present is 0. 7 percent of 

euhedral orthopyroxene mega phenocrysts up to 0.5 millimeter in length 

and 0.1 percent of plagioclase mega phenocrysts up to 0.35 millimeter in 

length. One orthopyroxene megaphenocryst in the thin section had 

(negative) 2V=79° and grains in immersion liquids showed nZ= 1.692, 

implying a composition about En75. 

Groundmass consists of abundant plagioclase microphenocrysts, 

generally better formed than in most earlier lavas; many orthopyroxene 

microphenocrysts; abundant microlites of orthopyroxene and clinopyroxene; dust-size magnetite in some areas; and a brown-glass· 

mesostasis (10-20 percent by volume), which in some areas is made 

nearly opaque by dusty inclusions and in other areas is translucent. 

The central portions of the plagioclase microphenocrysts show 

X' l\010max=33°-38° (An56- 66) and X' A010.ia=32°-36° (An6o-7o). 

Refractive index on cleavage, nX' = 1.562, implying compositions 

about An65. Zoning is normal progressive and only slightly marked, 

except at the extreme edges of the crystals where X' I\010.La ranges 

down to as low as 30 ° (An56) in some crystals. Orthopyroxene 

microphenocrysts have (negative) 2V=75°-78°, implying compositions of En71 _7a. Quite a few are intergrown or coupled with plagioclase microphenocrysts, and still others show narrow flanking plates 

of clinopyroxene. 

INDIVIDUAL CONSTITUENTS 

The major constituents of the successive lava ejecta of Paricutin 

volcano include plagioclase, olivine, orthopyroxene, clinopyroxene, 

opaque oxide, and glass. In this section the occurrence of each mineral 

as megaphenocrysts, microphenocrysts, and microlites will be described, with the discussion of paragenetic relations being left to a 

later section. Figure 98 shows the volume percentages of megaphenocrysts in those specimens on which modal analyses have been made 

(table 1). Figure 99 shows relative abundance of microphenocrysts 

and microlites, estimated from all thin sections available, with no 

distinction being made between lavas and bombs. 

PLAGIOCLASE 

Megaphenocrysts are rare in all Parlcutin ejecta except in those of 

1943 and early 1944. In one thin section of March 1943 ejecta 

(collected by the Instituto de Geologia de Mexico), plagioclase 

megaphenocrysts up to 1 millimeter in length are common, as they 
PETROLOGY OF PARICUT~ VOLCANO 301 

:

1 1 1 

ORTHl~o···· r·:ffi:NtCRYSTs

1 1 , ,1 , 1 

~ :I I I CLINOPt~··· lEGAPH~OtRYSTS I I· Iw 

tl. 

z OLIVINE! MEGAPHENOCRYSTS 

~ :1..---1 -.--11--.----11 --rr-j I ----.---r---+1111---r--,,j ----.-1 I ----.-. ~1-. J 

:

11 1 1 

~G·rcL··· .G.PHE~OrRYSTS 1 1 . 

1 

1943 1944 1945 1946 1947 1948 1949 1950 

DATE OF ERUPTION 

FIOUI'tE 98.-Volume percentages of megaphenocrysts in some Parfcntin ejecta, plotted according to date · · of eruption. 

11 il,,~~ i,,,, ~ ,1,, GR~iir .. i,llilfli~ilr· 11111 1 ~ 111111 

~ 1 ~ ,, 1111. tl: ~ 1111 GR~J~··, ri·;~:;or· ~: 1 ~ 1 1 J.: ~ 1 l 

z 

::> i ~r---~~~~~~----~--.----!J~,~;""oM~.~lol:~;·~J ------r-1-.-1-1 

1 u11111111! 111 ! 1111 rmrDMAii 11Ii~1rnr 11111 11111111 

1943 1944 1945 1946 1947 1948 1949 1950 

DATE OF ERUPTION 

FH1UR:Ji: 99.-Relative abundances of groundmass minerals in Parlcutin ejecta, plotted according to date of 

emption. Microphenocrysts are shown by solid bars, reacting olivine by circle bars, and microlites by dotted bars. 

285997-54-4 
302 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

appear to have been in several of the 1943 specimens examined by 

Schmitter (1945, table 2). Zoning is generally slight, and extinction 

angles in thin sections examined by the writer imply compositions of 

An6s-74· Thin rims of some crystals are progressively zoned to as 

low as Ansa· 

Plagioclase is present as a groundmass constituent in all the ejecUL 

of Par!cutin volcano, much in contrast to its limited occurrence as 

megaphenocrysts. Groundmass plagioclase occurs as lath- and 

tablet-shaped microphenocrysts, generally not more than 0.25 

millimeter long, and microlites (lengths less than 0.03 millimeter) are 

not generally developed. The plagioclase microphenocrysts typically 

show weak, progressive normal zoning in their central portions and 

strong, progressive normal zoning in their narrow rims. The Rittmann zone method was used to measure the extinction angles of 10 

crystals in each of 7 specimens listed in table 4. The results in most 

specimens indicate a range of about 10 percent in apparent anorthite 

content from crystal to crystal. In order of dates of eruption, the 

average compositions for each specimen are 7 4, 68, 62, 63, 65, 65, 65, 

and 63 percent anorthite. It is thus apparent that the change in 

average anorthite content of plagioclase microphenocrysts from 

1944-52 is not great and that the trends have not been entirely 

consistent. It is, of course, possible that more extensive determinations might reveal a more consistent trend. The anorthite contents 

of the thin rims of the plagioclase microphenocrysts range down to as 

low as An48 (specimen W-47-9). 

TABLE 4.-Summary of anorthite contenta of central portion1 of plagioclase 

microphenocrysts 

[Ten crystals determined in each specimen on universal stage) 

Extinction angle 

Analysis Specimen Date of eruption 

X't\010mu X'A010.la 

1 -------------- Feb.1943 ____________ -------------- 4 11-16-2 ______ Apr.(?) 1944 .• ______ -------------- ---------- W-47-24----- Oct. 1944____________ 34°-38° 

12 W-47-9 ______ May 1947----------- 35°-38° 

15 W-47-30 _____ Sept. 1941----------- 35°-40° 

16 W-48-5______ Sept 1948.----------- 34°-40° 

22 FP-16-52.... Feb. 1952------------ 33°-38° 

------34;;-----

320-360 

Molecular percent anor- thite (after Winchell 

1951) I 

Range 

70-78 

65-73 

68-66 

60-66 

60-70 

58-70 

56-70 

Average 

74 

68 

62 

63 

65 

65 

63 

to!v~rhtlt~~~t~~~:S~kU::~: curves of Tr(iger (1952, p. 113) imply ~northite contents from 6 to 11 percent 

OLIVINE 

Megaphenocrysts of olivine are consistently present in all thin sections of specimens erupted before 1948. In specimens on which modal 

analyses have been made, olivine megaphenocrysts are present in 

. amounts up to 4.5 percent by volume (see fig. 98), and in a few other 
PETROLOGY OF PARICUTIN VOLCANO 303 

specimens are estimated to amount to as much as 6 or 7 percent by 

volume. In the successive ejecta of the second half of 1947, a sudden 

and remarkably uniform decrease in amounts of olivine megaphenocrysts took place, and this finds its expression also in the sharp decrease 

in MgO-content of the analyzed specimens (fig. 100). Subsequent 

ejecta are characterized by a paucity of olivine megaphenocrysts. 

Little reaction or corrosion is shown by the olivine megaphenocrysts 

of 1943 and 1944 ejecta; but in later ejecta, reaction rims of fine-grained 

orthopyroxene become more evident. 

TABLE 5.-Refractive indices and inferred compositions of olivine megaphenocrysta 

in lavas of Paricutin volcano 

[.-.indicates approximately} 

Specimen number Date of Eruption nZ nY nX Composition 

51-W-18 Mar. 1943 ______ 1.713 1. 692 1.667 FoBO-M 

51-W-20 May 1943 ______ 1.711 1. 693 1.676 Fo1~1 

W-47-9 May 1947 _ ----- ,_1. 712 1. 695 1.675 Fo1e-ao 

FP-2Q-49 Dec. 1949 ______ ,_1. 720 ----- 1. 676 Fo1r-s1 

Refractive indices and inferred chemical compositions of olivine 

megaphenocrysts of several lavas of Paricutin volcano are given in 

table 5. The~range of composition is from Fo78 to Foa. with the major 

bulk of megaphenocryst material judged to be of compositions between Fo80 and Fo78• A decrease of 2Vover x of 2° to 3° from center to 

rim of some crystals implies 4 to 6 percent less Fo in the rims than 

in the center. The olivine megaphenocrysts characteristically carry 

tiny euhedral crystals of opaque oxide up to 0.01 millimeter diameter, 

as scattered individuals or in clusters or strings. Those examined by 

the writer have been found to be opaque even in the most intense 

light and are presumed to be magnetite or titanomagnetite; it is noted, 

however, that Schmitter (1945, table 2) identified spinel inclusions in 

olivine phenocrysts of several specimens erupted in 1943. Because of 

the presence of these inclusions, the bulk compositions of the olivine 

megaphenocrysts no doubt is slightly higher in iron than indicated by 

optical properties. 

The olivine of the groundmass occurs as microphenocrysts and 

grains in widely varying amounts in the ejecta of 1943-46; in a few 

specimens the texture is seriate, and the distinction between megaphenocrysts and microphenocrysts (and grains) of olivine is made by 

using the arbitrary 0.3 millimeter limit. The optic angle of one 

groundmass crystal of olivine in a lava of 1944 (Krauskopf specimen 

11-23-2) is (negative) 2V=88°, corresponding to a theoretical composition of Fou,. Excellently formed microphenocrysts of olivine are 
304 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

found in some of the pumice and lapilli ejected from the main vent 

during 1943 (see pl. 10, fig. 2), and these show little or no reaction. 

Reaction rims of orthopyroxene around grains of groundmass olivine 

appear in some ejecta of 1943-45 and become increasingly wider in 

ejecta of 1946 and 194 7. No groundmass olivine was found in post194 7 ejecta. 

CLINOPYROXENE 

Clinopyroxene does not occur as megaphenocrysts in Paricutin 

specimens except in rare circumstances. In the groundmasses of 

several of the earlier lavas and bombs of Padcutin volcano, microphenocrysts of clinopyroxene, rarely as much as 0.1 millimeter long, 

are moderately well developed. Extinction angles Z to c ranging from 

45° to 51° would indicate that these crystals are augitic rather than 

pigeonitic. Microphenocrysts of clinopyroxene do not occur in ejecta 

of 1945 and 1946 but appear again in some of the ejecta of 1947. 

The most common occurrence of clinopyroxene is as microlitic 

prisms and needles in the glassy mesostasis of the ejecta. Clinopyroxene microlites are present in the majority of lavas erupted during 

1943-47 and are spasmodically present in lavas erupted after 1947. 

Microlitic clinopyroxene commonly occurs also as platelike overgrowths on the flanks of orthopyroxene microphenocrysts, sometimes 

extending as long swallowtails and bundles beyond the ends of the 

orthopyroxene crystals into the glass, (see pl. 10, fig. 6). These 

flanking plates of clinopyroxene apparently lie against the (100) faces 

of the orthopyroxene, with coincidence of the b crystallographic axes 

(compare Kuno, 1950, p. 978). Opaque oxide adheres so thickly to 

the microlitic prisms of clinopyroxene of some specimens as to obscure 

their optical character, while the orthopyroxene of the same specimens 

have ·few if any adherent opaque oxide. Extinction angles Z to c 

ranging from 45° to 50° for the clinopyroxene microlites and flanking 

plates imply that they are augitic rather than pigeonitic. 

ORTHOPYROXENE 

Megaphenocrysts of orthopyroxene are characteristically present in 

the Paricutin material erupted after 1946, but they are rarely present 

in earlier ejecta. In figure 98 it is seen that olivine gives way to 

orthopyroxene as the predominant megaphenocrysts in the later lavas 

and that the transition occurred mainly during 194 7. Orthopyroxene 

megaphenocrysts occur generally as thick prisms up to 1.0 millimeter 

in length, rarely up to 1.4 millimeter, and as cruciform growths and 

clusters of several crystals. (See pl. 10, fig. 4.) They show weak but 

perceptible pleochroism, and optic angle measurements imply compositions from En70 to En80• (See table 6.) Refractive index of orthopyroxene megaphenocrysts of specimen W-48-5 (analysis 16, table 2) 

showed nZ= 1.695, from which a composition of En75 is inferred. 
PETROLOGY OF PARICUTIN VOLCANO 305 

TABLE 6.-0ptic angles of orthopyroxene, with compositions inferred from curves of 

Winchell (1951, fig. 283) 

Mega phenocrysts 

Specimen Date of Num- Molecular percent En NumMicro phenocrysts 

Molecular percent En 

Eruption ber de- Range ber de- Range ter- (-) 2V ter- (-) 2V I--.,---.--• 

mined Mini- Maxi- Aver- mined Mini· Maxi- Aver- mum mum age mummum age 

-·---1·----1---1---1---·1--·1-- -----------

11-16-L _____ Apr.(?)1944 ------------------------------------- ~ 

W:-47-9 ______ May 1947_ ------------------------------------- 3 

W-47-30 _____ Nov. 1947_ ------- ------------------------------ 7 

W-48-5 ______ Sept. 1948_ 1 80° -------------- 75 6 

W-48-9 ______ Oct. 1948_ ------------------------------------- 1 FP-20-49 ____ Dec. 1949_ 5 76°-86° 72 80 75 2 

FP-20-50_ --- Sept. 1950_ 5 75°-79° 71 74 73 5 

FP-55-50 ____ Dec. 1950_ 1 74° -------------- ·70 2 

FP-16-52_ _ __ Feb. 1952_ 1 79° ------- ------- 74 4 

Orthopyroxene occurs in the groundmasses of the great majority of 

the Paricutin lavas and bombs. In only a few of the rocks is it lacking 

or doubtfully present, notably those erupted during 1943. In most of 

the 1943 and 1944lavas, it is found as microlites and microphenocrysts. 

of varying length and slimness from specimen to specimen, mostly too 

small for measurement of optic angles. In the later lavas it is found 

as larger prisms, and through successively erupted specimens the maximum lengths of the orthopyroxene prisms increases gradually until in 

1947 the maximum exceeds 0.3 millimeter, the arbitrary boundary 

size between microphenocryst and megaphenocryst set here. Optic 

angles of orthopyroxene microphenocrysts, given in table 6, show a 

range of (negative) 2V from 65° to 79°, from which a compositional 

r~ng~ of En61 to En74 is inferred. There is only a slight, if any, trend 

towards lower En content in later ejecta, but it is apparent that the 

compositions of the microphenocrysts are consistently lower in En 

content than the megaphenocrysts in the corresponding specimens. 

In many specimens clinopyroxene overgrowths are common (pl. 10, 

fig. 6) as thin plates on the (100) faces of the orthopyroxene, as 

described above in connection with clinopyroxene. 

OPAQUE AND ACCESSORY MINERALS 

No attempt was made to distinguish between the various possible 

opaque minerals encountered in the examination of the thin sections 

except to establish that yellow metallic sulfides were not present in 

observable amounts or grain sizes. On the basis of the octahedrallike form of some of the better formed crystals and on the bulk chemical analyses, it is concluded that most of these opaque crystals are 

magnetite or titaniferous magnetite. The opaque material occurs in 

several situations: as isolated, tiny, well-formed crystals up to 0.02 millimeter in diameter and groups included in olivine and orthopyroxene 

phenocrysts, as tiny crystals, cluster and skeletal groups in the ground-
306 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

mass, and as minute dustlike particles distributed through the glassy 

mesostasis of many of the rocks. In many specimens the opaque 

cryntals are seen to adhere individually or in clusters to the microlites 

and larger crystals of ferromagnesian minerals, particularly to the 

clinopyroxene microlites. Accessory minerals were not searched for 

systematically other than to determine that the great preponderance 

of the tiny, low-birefringent microphenocrysts were orthopyroxene 

rather than apatite, which, from the bulk chemical compositions, can 

be expected to be present in small amount. 

GLASS 

Glass is consistently present in the suite of rocks from Parfcutin. It 

varies in amount from a diffuse mesostasis to an appreciable portion 

of the groundmass. All gradations in transparency are found, from 

clea.r light-brown glass carrying scattered well-developed microphenocryBts of ferromagnesian minerals and plagioclase to nearly opaque 

material which is seen under high magnification to be croweded with 

ferromagnesian microlites, opaque dustlike particles and minute vesicle~:~. There seems to be a tendency for more common occurrence of 

clea,r glass in the very early ejecta (1943-44) and in the late ejecta 

{194:9-52). Representative refractive indices of the clearer glasses are 

shown in table 7, and indicate a trend towards more s~lic compositions 

of the glass in the later ejecta. 

TABLE 7.-Refractive indices of glassy mesostases of some Parieutin lava specimenB 

S!X'cimen Date erupted Description Refractive index of 

glass 

51-W-23 1943 Bomb 1.565-1.568 

·w-48-9 Oct. 1948 Lava 1.542-1.547 

FP-20-50 Sept. 1950 Lava 1.538-1.543 

XENOLITHS 

Some 20 xenoliths, mostly from Parfcutin bombs, have been examined in thin section. In hand specimen all of these are light colored, 

contrasting with the black and dark brown of the normal Paricutin 

material. The majority consist of highly vesicular glass through which 

are distributed grains of quartz, feldspar, and ferromagnesian minerals 

in -various stages of destruction. Both plutonic and extrusive rock 

types appear to be represented in the specimens studied and include 

granite, granodiorite, quartz monzonite, and as near as could be determ.ined, relics of dacitic and rhyolitic porphyries. 

One xenolith (specimen 51-W-1, analysis X-1, table 3) erupted as 

the nucleus of a bomb in 1943 is only slightly altered and shows a 
PETROLOGY OF PARICUT~ VOLCANO 307 

coarse granitic texture. It contains abundant anhedral grains of 

fractured and cloudy orthoclase up to 7 millimeters in diameter, many 

roughly euhedral crystals up to 3 millimeters in diameter of fractured 

and cloudly plagioclase of composition about An6 (nX' on cleavage 

ranges from 1.527 to 1.529), about 20 percent by volume of anhedral 

quartz grains up to 0.4 millimeter in diameter, and about 15 percent 

by volume of masses of opaque material, some of which, by their outline and internal structure, must be relics of ferrmnagnesian minerals. 

Titanite, zircon, and apatite are present as accessories. The start of 

thermal breakdown of the rock is revealed by the thin veinlets of 

vesicular glass in the feldspars and along crystal contacts (see pl. 11, 

fig. 1), by the opaque pseudomorphs of the ferromagnesian minerals, 

and by incipient fracturing and development of streaks of tiny bubbles 

along fractures in the quartz. Otherwise the rock is a normal quartz 

monzonite, similar to the quartz monzonite-granodiorite suite outcropping along the edge of the plateau not far to the south of the Paricutin 

region. 

A similar xenolith, (specimen· 51-W-9, analysis X-3, table 3) 

erupted during 1944 or 1945, contains about equal amounts of anhedral 

orthoclase grains up to 2 millimeters in diameter and subhedral 

crystals of plagioclase up to 4 millimeters in length. About 20 

percent by volume of anhedral quart~ grains and 15 percent of irregular 

to rectangular masses of opaque oxide, no doubt relict ferromagnesian 

minerals, make up the remainder of the rock. Much of the plagioclase 

is both polysynthetically twinned and oscillatorily zoned. Predominant compositions lie between An2s and An2s (nX' on cleavage= 

1.541-1.543) while some range down to An,G (nX'= 1.535). Both 

orthoclase and plagioclase are much fractured and carry many minute 

inclusions (both bubbles and solid material?). The feldspars are 

veined by vesicular glass as in specimen 51-W-1, but in !addition 

narrow border zones of some feldspar grains are "fritted." The term 

"fritted" is used here to describe the minutely wrinkled or reticulate 

texture frequently encountered in feldspars that have been heated to 

high temperatures (Larsen and Switzer, 1939, p. 564 and pl. 2; Wilcox, 

1944, p. 1058, and pl. 5). The intense thermal expansion apparently 

·causes separation along the two cleavages of the feldspar and subsequent internal corrosion in these fractures to give, upon quenching, 

a regularly spaced reticulite of glass in which the now isolated blocks of 

feldspar retain their original orientation fairly closely. The process 

of solution under dry conditions would consist chiefly of simple 

melting; in the presence of mineralizers such as water, the breakdown 

to liquid would be facilitated and could take place at lower temperatures. The mechanism in orthoclase above 1170° C should also 

include incongruent melting to form leucite as well as liquid (Morey 
308 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

and Bowen, 1922), in which case, however, the leucite could not ... be 

distinguished easily from the glass under the microscope. 

Quartz grains in this specimen show marked undulatory extinction 

and incipient fracturing, with development of streaks of minute 

bubbles. The masses of opaque oxide, some rectangular in outline, 

show streaks and thin lenses of translucent birefrigent material 

paraJlel to their long dimension that, under intense illumination, are 

reminiscent of cleavage in amphiboles. The translucent material is 

brown in reflected light. This rock is classed as a quartz monzonite 

near granodiorite. 

Another xenolith (specimen 5t-W-4, erupted in 1943) illustrates an 

advn.nce stage of disintegration of this type of granodiorite or quartz 

mon.zonitic rock. Frothy glass is predominant in this xenolith (see 

pl. 11, fig. 3), and its refractive index ranges from 1.484 to 1.490. 

Scattered through the vesicular glass are irregularly embayed quartz 

grains up to 0. 7 millimeter in diameter showing strong undulatory 

and patchy extinction with development of streaks of tiny bubbles 

along some of the fractures. Scattered plagioclase relics of irregular 

shape show wide borders of fritting. The unfritted central portion of 

one grain showed an extinction angle of x·, I\010max -:20°, which 

impllies a composition of about An37 • Most of the scattered relics of 

orthoclase and microcline up to 1.3 millimeter in diameter are fritted 

throughout. The optic angles of four fritted grains of orthoclase, 

measured on the universal stage, are (negative) 2V=59°, 62° 62°, and 

66°. Ferromagnesian relics are virtually absent in this specimen 

although they may be represented by the patches of light-brown 

glasB that occur throughout. 

Specimen 51-W-8 (analysis X-4, table 3), erupted during 1944 or 

194S, is composed predominantly of pumiceous glass through which 

are scattered fragments and relics of quartz n,nd feldspar with some 

macroscopically visible streaks of darker mnte:rial. Original fractures 

in the rock are emphasized in the relic by their dark borders and high 

degree of vesiculation. The vesicular glass is clear and colorless with 

refrnctive index ranging from 1.489 to 1.493. The many irregularand cusp-shaped fragments of feldspar range in size up to 0.9 millimeter 

in diameter and show marginal fritting and numerous veinlets of vesicular glass, very few show lamellar twinning. Scattered cusp-shaped 

grains of quartz range up to 0. 7 millimeter in diameter and commonly 

show undulatory extinction and incipient fracturing. Opaque oxide 

and relics of ferromagnesian minerals are rare, and the rock may 

originally have been granite or rhyolite tuff. 

A xenolith representative of extrusive or hypabyssal rock, perhaps 

originally a dacite or quartz latite porphyry, is illustrated by specimen 

51-1W-6, erupted in 1943 (see pl. 11, fig. 4). In hand specimen it 
PETROLOGY OF PARicUTIN VOLCANO 309 

is finely vesicular, carrying abundant phenocrysts, buff to white feldspar, and white quartz. In thin section the phenocrysts are seen to 

consist of many stubby plagioclase crystals up to 4 millimeters in 

diameter, some stubby odhoelase crystals up to about 4 millimeters, 

some subhedral quartz crystals up to 0.3 millimeter, and a few masses 

of opaque oxide up to 0.8~ millimeter irregular to rectangular in outline (relics of ferromagnesian minerals?). The groundmass is predominantly of vesicular gliass, refractive index ranging from 1.485 to 

1.492, through which are seattered quartz grains· up to 0.05 millimeter in diameter with inegularly sealloped outline, clots of finegrained material of low index and relief (feldspar relics?), and elots 

and streaks of dark material that eontain a few mierolitie prisms up 

to 0.03 millimeter in length of moderate birefringenee and relief and 

small globular masses of high birefringence and relief. The plagioclase phenoerysts are not partieularly fritted but are elouded by many 

tiny bubbles and (?) glass inclusions. Although inclusions in the 

plagioclase are too abund3lnt for eonfident determination of refractive 

indiees, the well-developed polysynthetic twinning permits determination of extinction angles on some crystals. Two crystals with 

albite-carlsbad twins gave extinction angles that imply compositions 

of An 40 and An30 • These and others show some zoning. The orthoclase· 

phenocrysts are similar to the plagioclase in content of tiny vesicles 

and inclusions. The quartz phenocrysts have rounded corners, few 

inclusions, and show moderate cracking and incipient shattering. 

One direction of parting in the quartz appears well developed. The 

rectangular masses of opaque oxide show streaks of translucent material parallel to the long dimensions similar to those in the quartz 

monzonite of specimen 51-, W -9. Some of these masses are surrounded 

by zones of brown glass with index greater than balsam. 

Specimen 51-W-7, a xenolith erupted in 1944 or 1945, is similar 

in most respects to speeinien 51-W :-6 and is regarded as having been 

originally a dacite or quartz latite porphyry. Refractive index of the 

vesieular glass ranges froll!l 1.490 to 1.494. The feldspar phenocrysts 

are more rounded and mpre elouded by tiny vesicles and inclusions 

than those of specimen 51r--W-6. Precise refractive index determinations were not possible, but the extinction angle for one crystal of 

plagioclase imply a composition of about An45 • Measured optic angle 

of one orthoclase crystal gave (negative) 2V=80°. This specimen 

contains a few clear fragment:s that are presumed to be olivine; one· 

crystal has a measured optie angle (negative) 2V=83°, implying: 

a composition of Fo70 • These fragments were found in only one· 

portion of the slide, not clearlly assoeiated with any other dark materials yet definitely inclosed by the glassy groundmass so that they, 

285997-54-5 
810 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

cannot be regarded as having been introduced during grinding of the 

thin sec·~ion. It is possible that the thin section here cuts across a 

portion of the transition zone between xenolith and lava and that the 

olivine is an advance crystalline representative of the surrounding 

lava (compare description of "Inclusi6n de Marzo" on page 312). 

A further variation of the xenoliths supposed to be dacite or quartz 

latite porphyry is shown in specimen 51-W-5, (analysis X-2, table 3) 

erupted during 1943. The phenocrysts consist of much damaged 

feldspar and ferromagnesian relics with some euhedral quartz crystals 

in a groundmass of highly vesicular glass that carries abundant small 

-quartz grains of scalloped outline and relics of feldspar. The unique 

feature of this specimen is the profuse development of vesicles in the 

feldspar phenocryst relics. These vesicles, mostly less than 0.4 

millimeter in diameter, are distributed uniformly through the crystals, 

which in this inflated condition range up to 5 millimeters in diameter. 

The central portions of a few of the crystals are less vesiculated and 

retain some of the optical characters of feldspars. The vesiculated 

portions are composed of glass in which occur innumerable tiny 

elongate particles up to 0.005 millimeter in length having moderate 

birefringence and parallel extinction. These impart brown to the 

crystal when viewed at low magnifications. It seems reasonable to 

suppose that the rock was already greatly altered, either hydrothermally or by weathering, before it was taken into the Paricutin 

magma and that the uncommon inflation of the feldspar phenocrysts 

took place when the heat of the magma caused the evolution of the 

water from hydrous alteration products of the original feldspar. 

A unique xenolith of gray obsidian (specimen 51-W-2), which was 

erupted as a bomb and was collected by F. H. Pough in July 1945, does 

not fit in either of the above-described classes of xenoliths. Two types 

of material compose this xenolith. Type 1, which predominates, is 

fairly dense, gray obsidian in hand specimen; and as seen in thin 

section, it consists of slightly -to moderately vesicular glass crowded 

with microlites and also containing some fragments of plagioclase and 

a few fragments of orthopyroxene. Type 2 occurs as dark streaks and 

whorls· in the hand specimen; and as seen in thin section, it consists of 

abundant plagioclase crystals and fragments and minor amounts of 

orthopyroxene crystals in a mesostasis of clear glass (see pl. 11, fig. 6). 

·The glass of tyPe 1 contains so many microlites that its index could 

-only be determined to lie between 1.491 and 1.497. Hs microlites 

:show very low inte.tference colors, have indices much higher than the 

-enclosing glass and range in size up to 0.002 millimeter. The index 

,of the glass mesostasis of type 2 was found to lie in the same range of 

1.491 to 1.497. The plagioclase occurs as sharply angular fragments 

up to 1 millimeter in diameter, while many of the smaller· plagioclase 
PETROLOG~ OF PARICUTLN VOLCANO 311 

crystals appear to be complete crystals sttictly euhedral in outline. 

They are clear, moderately'zonnd and twinned, and show no fritting. 

Refractive indices, nX' on qleavage, range from 1.558 to 1.562, implying compositions of An00 to An64• The central portion of one albitecarlsbad twin showed extin~tion angles of 22° versus 32° implying a 

composition of An64 • The outer portion of the same crystal showed 

extinction angles of 20° vetsus 27°, implying An56• Another crystal 

showed X' l\010max=34° implying a composition of An63 • Orthopyroxene occurs as scattered crystals and fragments, some showing 

corrosion and some carrying adherent opaque oxide particles. Their 

pleochroism is slightly mor~ marked than that of the orthopyroxene 

of the normal Paricutin lava, and their refractive indices (n~ on 

cleavage about 1.702) imply a composition of about En70 • Rare relics 

of other ferromagnesian minerals (xenocrysts?), now mostly opaque 

oxide, are seen. The moderately calcic nature of the plagioclase 

(An60_ 64) and high magnesian content of the orthopyroxene (En77) 

seems incongruous when compared to the apparently salic nature of 

the abundant enclosing glass (n=1.491-1.497). Such relations are 

found, however, in some tr$,nsition zones between xenoliths and lava 

(see description of transition zone of specimen FP-11-49, below), 

and it may be that this spe¢imen is part of a broad transition zone or 

of a once completely melted salic xenolith. 

It is interesting to note t:Q:at in the pre-Paricutin lavas of the region, 

the xenolithic material which is occasionally encountered is much the 

same as has just been described for the ejecta of Paricutin volcano. 

Thus, specimen W-48-7; a ;xenolith collected at Cuezefio from a lava 

flow of nearby Tzintzungo v~lcano (see Williams, 1951, pl. 8), resembles 

very closely the Paricutin 'xenolith specimen 51-W-9. A block of 

light-colored pumiceous material (specimen W-48-4) was found by the 

writer among blocks of normallnva in a wall in Angahuan village; and 

from what could be learned from the residents, all the blocks had been 

gathered in the immediate. vicinity years before when the wall was 

built. No doubt the pumic~ous block was a xenolith of the prehistoric 

eruption of Tzintzungo voldano, lava of which covers the area. This 

specimen represents a mor~ advanced sta.ge of disintegration than 

W-48-7 and is comparable in most respects to the Paricutin xenolith, 

.51-W-4, described above. 

A granodiorite (specimen FP-20-52, pl. 11, fig. 2), from what is 

probably the nearest outcrop area of salic plutonic rock 30 kilometers 

SE. of Paricutin volcano, is of interest for comparison with the 

Paricutin xenoliths. Its texture is xenomorphic-granular with 

plagioclase grains up to 1.5 miUimeter in diameter composing about 

35 percent by volume. Prpgressive normal zoning is common, and 

some crystals range from as, high as An7o at their centers to as low as 
312 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

An23 at their edges. The major portion appears to be between An30 

and An40 • Orthoclase grains up to 1 millimeter in dia,meter compose 

about 25 percent by volume and quartz grains up to 0.5 millimeter 

about 20 percent, both being generally interstitial to the plagioclase. 

Clinopyroxene grains up to 0.7 millimeter in diameter are the most 

common of the ferromagnesian constituents and compose about 10 

percent by volume. Orthopyroxene, sometimes i.ntergrown with 

clinopyroxene and always much altered, occurs sporadically. Biotite, 

amphibole (?) and opaque oxide crystals, and clots compose the 

remainder of the rock with very minor amounts of apatite and zircon 

as accessories. Except for its somewhat more mafic character, the rock 

is similar to many of the coarse-grained Paricutin xenoliths. 

TRANSITION ZONES BETWEEN LAVAS AND XENOLITHS 

Of the specimens in which both lava and xenolith are present, the 

contacts between dark lava and light xenolith. are sharply defined 

to the unaided eye, and obvious effects of mixing and strewing of one 

material in the other are absent. In thin section the gradation in 

color from the dark-brown glass mesostasis of the lava to the colorless 

glass of the xenolith occurs across a zone usually less than a millimeter 

wide at most contacts. The gradation in mineral content, however, 

may extend several more millimeters into the xenolith and is revealed 

by the growth of isolated microphenocrysts of plagioclase and ferromagnesian minerals in the otherwise normal appearing, colorless 

glass of the xenolith. In the thin section of a few of the specimens, 

stringers of brown glass carrying plagioclase and ferromagnesian 

crystals are seen to penetrate for distances of a centimeter or more 

into the xenolith. 

A transition zone is well developed in a thin section of an "Inclusi6n 

de Marzo 1943" loaned by the Instituto de Geologia de Mexico. 

This xenolith is apparently of the quartz monzonite type, described 

above, with irregularly shaped grains of quartz and fritted orthoclase 

and plagioclase distributed through a matrix of colorless vesicular 

glass of index much lower than that of balsam. The surrounding 

lava contains scattered euhedral phenocrysts of olivine up to 1.5 

millimeter in diameter and a few lath-shaped phenocrysts of plagioclase 

up to 0.6 millimeter in length in a groundmass that consists of abundant 

plagioclase laths, many microphenocrysts and grains of olivine, 

abundant prisms of orthopyroxene, and a glassy mesostasis which 

contains variable amounts of microlites and dusty particles. The 

width of gradation from brown to colorless glass in the transition 

zone between lava and xenolith is variable but rarely exceeds 0.8 

millimeter. The glass of this zone not only carries fragments of 

xenolith crystals (quartz and feldspar) but also slender euhedral 
PETROLOG:Y OF PARICUTIN VOLCANO 313 

laths of plagioclase up to 0.03 n1illimeter in length and stubby euhedral 

crystals of olivine up to 0.01 rnillimeter in diameter. No corrosion or 

reaction coronas are seen: on the olivine crystals, even where they 

occur sparsely out in the colorless glass. 

An inclusion in Pari:cutin lava collected from the vent cascade 

October 17, 1949, (specimen FP-11-49) shows an exceptionally 

broad transition zone sev¢ral millimeters wide. The xenolith proper 

is composed of many irregular grains of quartz and orthoclase up to 

1.5 millimeters in diameter immersed in clear, colorless glass of 

refractive index lying between 1.486 and 1.491. Large plagioclase 

grains are sparsely present, one crystal showing extinction angles 

that indicate a composition of about An30• Ferromagnesian minerals 

appear to be absent. The enclosing lava is similar to the normal 

lava that was erupted at that time (see description of specimen 

FP-20-49, above) with a' glass mesostasis of refractive index about 

1.524-1.528. The transition zone is variable in the proportion of 

crystalline material to glass, and the glass becomes generally lighter 

in color towards the xenolith. Streaks of lava penetrate the xenolith 

for several millimeters, and conversely the xenolithic material penetrates the lava proper. Grains of quartz and orthoclase, the latt~r 

more fritted than in the ~enolith (see pl. 11, fig., 5), as well as phenocrysts of orthopyroxene and olivine are scattered through the transition zone. From the lava proper to the xenolith proper the groundmass crystals ("micropheiJlocyrsts") are consistently euhedral calcic 

plagioclase laths and orthopyroxene prisms, and they become less 

abundant towards the xerlolith. 

It is noted for the norl!llal Paricutin lavas in general that isolated 

quartz grains are encountered in some hand specimens and thin 

sections, always of irregular shape and showing evidence of reaction. 

Individual alkali feldspalr grains are rarely found, being almost 

unrecognizable in hand specimen and recognized in thin section by 

their fritted texture and rernnants of feldspar optical characters. 

The greater persistence' of quartz xenocrysts may be · explained 

chiefly by their physical structure, that is, lack of cleavage or parting 

that would cause rapid dl.sintegration under thermal stress. 

PETROCHEMISTRY 

Twenty-two chemical. analyses of lavas of Paricutin volcano, 

selected as satisfactory 'fron1 the standpoints of both quality of 

analysis and of definiteness of date of eruption (within 2 months), 

are listed in table 2 along with four analyses of xenoliths in table 3. 

Participating analysts were Charles Milton, Erma Chadbourn, James 

Kerr, Harry H. Hyman, • Edythe Engleman, and Lucile Tarrant, all 

except Milton working under the supervision of Lee C. Peck. Norma-
314 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

tive mineral compositions are given in table 2, and Niggli values of 

analyses 1, 2, 3, and 5 have been published by Williams (1950, table 3). 

Calculated groundmass compositions of several specimens are given 

in table 2. Spectrographic analyses of these lavas and xenoliths, as 

weU as of lava specimens of Williams (1950) for the Paricutin region, 

have been made by K. J. Murata and will form the basis of a separate 

paper by him. 

Chemical compositions of lava specimens have been plotted against 

dates of eruption in figure 100. For purposes of this and subsequent 

diagrams, the ferric iron of each analysis has been recalculated as 

ferrous oxide and combined with the ferrous oxide actually found in 

order to eliminate confusion owing to the erratic but generally reciproc~l relations between ferrous and ferric iron. The new value of 

ferrous oxide and the values of the nine other major oxides (Si02 , 

AI2<Da, MgO, CaO, Na20, K20, Ti02, P20 5 and MnO) were adjusted 

to total 100 percent and were plotted as qpen circles on the diagram 

according to dates of eruption of the specimens. 

'.Uhe resulting graph of figure 100 shows in a striking manner that 

the !oxide constituent of successively erupted lavas have varied serially 

during the course of the eruption. Silica increased from about 55 

percent in 1943 to slightly over 60 percent in 1952; potash increased 

con~istently if only a small amount; soda remained nearly constant, 

while alumina, iron oxide, lime, and magnesia decreased. During 

194 7, an extra rapid increase in silica took place while alumina also 

incr~ased slightly and magnesia decreased rapidly. The specimens of 

analyses 6-10 of table 2 were collected by K. B. Krauskopf from lava 

erupted during November and December 1945 in order to determine 

the variation of lava composition over a short period of eruption and, 

in the case of analyses 8 and 9, the variation between toe and vent 

of the same flow. It is seen from the analyses and from figure 100 

that the variations of analyses 6-10 are remarkably small, all being 

within one-half of 1 percent of silica and proportionately less for the 

oth~r oxide constituents. Reversals of trend are represented, however, 

by analyses 4 and 11, which are noticeably lower in silica and higher 

in iron than their immediate predecessors. 

The chemical relations of the lavas of Paricutin volcano are further 

illustrated in the silica-variation diagram of figure 101, where the 

recast analyses are plotted as large circles. For comparison the 

analyses of lavas of other volcanoes in the vicinity (recast in the same 

manner from Williams, 1950, table 1) are plotted as small circles. 

The plots of Ti02, P20 5, and MnO are not shown in this diagram, 

beca;use their variations with respect to silica are very small. 

The close similarity in chemical trends of the lavas of Paricutin 

volcano to those of the volcanoes of the surrounding region is well 
PETROLOdY O:F PARICUTIN VOLCANO 315 

illustrated by the silica-vl;\riat:ion diagram, even on the large scale of 

figure 101. This similarity would be even more noticeable by contrast if there were room on the same diagram to plot the analyses of 

the lavas of the Neo-Voldanic Zone of Mexico as a whole (Williams, 

1950, fig. 93). It may :be remarked that the most aberrant of 

Williams' specimens, No. 6, a hornblende andesite, was collected from 

the Mesa de Zirim6ndirp at the extreme southwestern corner 'Of 

William's mapped area. ]ts abnormally high alumina content and the 

presence in it of amphibole to the exclusion of other ferromagnisian 

minerals set it apart from the normal lavas of the Paricutin region. 

While the silica range of the Paricutin lavas is only about half that of 

the Paricutin region and :the Paricutin lavas are quite consistently 

slightly lower in alumina a;nd slightly higher in iron and magnesia than 

the lavas of comparable silica content from the surrounding volcanoes, 

the correspondence is othe!rwise remarkably close; and this correspondence may imply that similar factors have controlled the chemical 

trends of Paricutin and neighboring lavas. 

The alkali-lime index {the silica percentage at which the sum of 

the alkalis equals the lime, as defined by Peacock, 1931) for the lavas 

of Paricutin volcano is about 62 (by extrapolation) and that of the 

lavas of the surrounding J!egion is 61.5 when determined according to 

the plot of figure 101. The alkali-lime index of the N eo-Volcanic 

Zone of Mexieo (exclusiv~ of the Paricutin region?) is given as 60 by 

Williams (1950, p. 265). Thus, all three groups of these Mexican 

roeks lie elose to the border between Peacock's ealcic and calc-alkalic 

igneous series. 

Calculated compositions of groundmasses, given in table 2, are 

plotted as crosses on figure 100. In calculating the groundmass of 

each specimen, its megaphenocrysts were subtracted from the bulk 

analyses (total Fe as FeO, and excluding H20, S, C02 and BaO) and 

the residue readjusted to total 100 percent. In these calculations , 

the compositions of mod$)1 megaphenocrysts were taken as Fo80 for 

olivine, En75 for orthopyiroxene, and An70 for plagioclase. Specific 

gravities were taken as 3i.5 for olivine, 3.4 for hypersthene, and 2.7 

for plagioclase. Lacking values of the specific gravity for the individual analysed specimens, that of the bulk rocks is assumed to 

be 2. 7. Although the actual specific gravities apparently range from 

about 2.8 for the earlier lavas to slightly below .2.7 f.or the later lavas 

(as seen in table 8), the assum.ption of a value of 2.7 for each specimen in_troduees no errors that approach the effects of the possible 

errors in determining the amount of phenocrystic material by modal 

analysis. It should be npted in table 8 that the percent vesicles of 

specimen W -46-27 detenmined by modal analysis in thin section 

would lead one to expect a much higher value of powder specific 
316! GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 
6 

MgO 

x xx---"-. 

x* ' 

5 

4 

'-x-llo... -JC---JC- -x · "'~x---- --.__x--- 3 

--!.4 

.... 

i'5 ~ KzO ~ ~---==0=--.::::::::-if 

~ 

~~--o-- ~-:s- 0 0--- -r I Ti02 

-j I 

1& P•O• ~---_...._. 0 0 0 0 0 c-.o t I 

0 

FIGURE 100.-Compositions of lavas of Paricutin volcano, plotted according to date of eruption. Bulk analyses (table 2), recast with all iron as FeO and 

excluding volatiles, are plotted as circles. Calculated groundmass compositions (table 2) are plotted as crosses. 

'"d 

t:r:l 

~ 

:;:o 

0 

t"' 

0 

0 

~ 

0 

>:rj 

'"d 

> 

~ 

c:;~ 

d 

8 

H 

z 

~ 

0 

t"' 

0 

~ 0 

~ 

-l -
318 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

tz 

1.1.1 

(.) 

0: 

1.1.1 

a. 

50. 55 60 

23 4 5 7 6 89 If 1012 1314 15 161817 

"'--Williams (1950) analyses numbers_..,. 

20 

19 

18 

17 

:c 

_.o...----";::;i7~~6 ~es numbers of Paricutin lavas (table I) 

1 2 4 53 11 7 9e1012 13 14 15 16 17 1e 192 ~ 

0~--,----r--~~--~--~-L~-ll~~~-.~~~~~~~--~--~~ 

50 55 60 

PERCENT Si02 

FxnuRE 101.-Silica-variation diagram of lavas of Pari cutin and nearby volcanoes. Large circles show lavas: 

of Parfcutin connected in order of eruption. Small circles show lavas of nearby volcanoes connected in 

order of analysis number (Williams, 1950, table 1). Both sets of analyses have been recast with all iron as: FeO and excluding volatiles. 

gravity than was actually found. It is concluded that the vesicu--

lal-ity ·of the thin section is too high, owing to loss of material during· 

grinding; and it is probable that such error is greater, the greater 

the initial vesicularity of the rock. 
Specimen 

51-W-19 

51-W-21 

11-16-1 

W-46-27 

W-47-19 

FP-53-'"50 

PETROLOGY OF PARICUTIN VOLCANO 

TABLE 8.---..,Specific gravities of Paricutin lavas 

Date eriupted Pen ent vesicles 

:~arch 1943 _ ,- _________ _ N egligib e ___________ _ ~ ·une 1943 __ __, __________ - Negligible ___________ _ April1944 __ .; __________ _ 2.0 _________________ _ 

Heptember 1946 ________ _ Beptember H}47 ________ _ November 1950 ________ _ 

7.3 _________________ _ 5.7 _________________ _ 

N egligibie ___________ _ 

319 

Bulk specific gravity 

2. 76 

2. 73 

2. 68 1 2. 68 

2. 62 

2. 67 

1 Specific gravity o powder determip.ed as 2.78 by Chemical Analysis Laboratory, U.S. Geological Survey. 

ORIGIN (}F PETRQGRAPHIC AND CHEMICAL TRENDS 

The series of rock ~pecimens from the protracted eruption of 

Pari cutin vo] cano furnishes petrographic and chemical data that 

are perhaps as completj3 and definite in respect to time and place 

as have bec()me available from any volcanic suite. In this suite, 

for instance, ~ihere can tie no doubt that all the specimens came from 

the same ma:~a cham~er, that only a limited number of types of 

intratelluric (rystals exiSted in the chamber and that there were no 

significant in~erruption~ of the process or processes that brought 

about the observed dive11sification of rock type. Among other things, 

these circumstances prol-ide a welcome opportunity to test quantitatively whether fractional crystallization alone produced the resulting 

rock series-fl. casual po~tulate made so often for composite suites of 

other geographic areas ~here, because of the many possible variables 

of time, spaco, and par~cipating minerals, the postulate can neither 

be supported or refuted.l 

LimitationE of the data at hand must be recognized at the outset 

and be kept in mind throughout. The successive lava flows from 

which the annlyzed spectimens were collected represent no more than 

a continuous sampling of the Paricutin magma body during the 

years 1943-5~. It istementary, but·nevertheless intpQr.tant, to 

appreciate thHt .the rna a body was not necessarily homogeneous in 

1943 and thaL it did no necessarily change throughout from basaltic 

to andesitic (Ompositio during only those years in which material 

was issuing from it. 1 

The chemi ~al and u.· trographic differences between successively 

erupted materials are s rikingly apparent-the more so because of 

their serial nttture. T e problem is to deduce the mechanism by 

which they were produbed. In the discussion which follows only 

two mechanisms, ass~ilati0n and fractional crystallization, are 

considered in respect td their individual and combined abilities to 

account for the observed features of the rock series. Considering 

only those tVI·o, the writer has been led to the conclusion that the 
320 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

observed trends could not have been produced by fractional crystallization alone but that they could have been produced by assimilation of granitic country rock combined with fractional crystallization 

of olivine and plagioclase. It is hoped that the inclusion of the basic 

data in some detail in the previous sections will enable other workers 

to evaluate the possible involvernent of other mechanisms or perhaps 

to modify or drastically revise the relative importance of the roles 

permitted here to assimilation and to fractional crystallization. 

PARAGENESIS OF MINERALS 

The association of the fourmajorcrystalline constituents in the Parfcutin ejecta of 1943-50 are generalized in figure 102 from the detailed data 

of figures 98 and 99. As noted previously, the size limit between 

megaphenocrysts and .microphenocrysts has been chosen rather 

arbitrarily as 0.3 millimeter and that between microphenocrysts and 

microlites (the latter not shown on figure 102) as 0.03 millimeter. 

The occurrence of olivine relics-crystals that are marginally converted to pyroxene-is indicated on the diagram by hatching. The 

overlapping of occurrence of relict and fresh olivine in the diagram is 

brought about not by the occurrence of both types in the same specimens but by alternation of the two types from specimen to specimen. 

OAfE OF ERUPTION 

FinURE 102.-0ccurrence of mega phenocrysts (upper) and micro phenocrysts (lower) in successive Paricutin 

ejecta of 1943 .to 1950 (generalized from data of figures 98 and 99). 
PETROLOGY OF PARICUTIN VOLCANO 321 

In most of jhe specim¢ns three stages of cooling are implied by the 

presence of megaphenocrysts, microphenocrysts, and glass with 

microlites. VThereas the conventional assignment of these three to 

intratelluric, hypabyssal (conduit), and surficial cooling stages might 

be justified here, consideration will be given also the the possible cooling effect of admixture :af foreign material in producing the microphenocryst gnneration while still at depth. A sharp demarcation 

between meg:tphenocry~ts and microphenocrysts does not exist in 

some specimens, and her~ we must suppose that the change in crystallizing conditions must have been gradual. Likewise, the glassy 

mesostases mhy not havfj been developed in all specimens as a result of 

any sudden increase in the rate of cooling but simply by the inability 

of crystallization to keep pace with a steady cooling that had been 

going on for some time previously. 

The degree of confide:p.ce with which one may deduce equilibrium 

relationships between the component minerals is, of course, not the 

same for the different g~nerations. The megaphenocrysts may well 

represent minerals that were in equilibrium with the liquid and with 

each other. The microphenocrysts, however, are producti of a more 

rapidly changing envirohment in which true equilibrium was never 

established, shown, for instance, by the progressive zoning in the 

plagioclase. But, although their crystallization was no doubt forced 

and somewhs t removed from equilibrium, the degree of departure 

from equilibrium may have been roughly the same from specimen to 

specimen throughout the years of the continuous eruption, and one 

may be justif[ed in makting cautious comparisons through the series. 

The microlites, develop~d during the rapid congelation of the interstitial liquid, cannot be used to infer equilibrium relationships; and 

the condition; of this final congelation may have been so fortuitous 

from specimeo. to specimen that only the most general comparisons. 

can be made between the microlites of the different members of the, 

rock series. : MEGAPHENOCRYSTS 

The earlies j generatioh of crystals represented are the megapheno-. 

crysts: olivin3 and plagioclase in 1943-44 specimens, olivine alone in 

the 1945-47 specimens, and orthopyroxene and olivine in the 1947-52 

specimens. r~he olivine and plagioclase megaphenocrysts of the 

1943-44 specimens can no doubt be regarded as developed at depth;~ 

and lacking corrosion or reaction rims, they must have been essentially 

in equilibriunt with each other and with the liquid until eruption at, 

the surface. Schmitter 1 (1945, photograph Q) notes occasional gross. 

skeletal form!: of olivine mega phenocrysts; but these, because of their 

symmetry, probably shoruld be regarded as growing forms rather than 

dissolving or reacting forms. 
322 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

It should be noted here that the general absence of plagioclase 

megaphenocrysts sets the later Paricutin ejecta apart from the large 

proportion of andesites and basaltic andesites of other localities. 

While volcanic rocks bearing ferromagnesian phenocrysts but no 

plagioclase phenocrysts are perhaps not rare, they certainly have not 

received much attention in literature. For lavas of the circumpacific 

zone, (calc-alkaline suite) Johannsen (1937, p. 174-175) cites "boninite" as an abnormal andesite that contains olivi.J;le and pyroxene 

phenocrysts to the exclusion of plagioclase phenocrysts; and possibly 

some varieties of "sanukite" mentioned by Johannsen may be included 

here. Larsson (1941, p. 319-321) describes four lavas of the Brazo 

del Viento area, northern Patagonia, in which olivine is the only 

phenocrystic constituent and one lava in which olivine and diopside 

are the only phenocrystic constituents. It is interesting that among 

the lavas from some of the volcanoes near Paricu tin there are several 

examples free of plagioclase phenocrysts, notably the augite-hornblende andesite of the Mesa de Huamiracua, the olivine basaltic 

andesite of Cerro de Cutzato, and the olivine basalt of Cerros de 

Jabali (Williams, 1950, p. 241, 248, and 253, respectively). · In 'addition, plagioclase phenocrysts are remarkably scarce in the porphyritic 

lavas on the northwest flanks of Cerros de Tancftaro (Williams, 1950, 

p. 255). Olivine phenocrysts, but no plagioclase phenocrysts, are 

noted in a specimen of basalt from Jorullo volcano by Schmitter (1945, 

table 2, specimen 10). 

The reasons for the absence of plagioclase megaphenocrysts from all 

but the earliest ejecta of Paricutin volcano is not at once clear. Perhaps only olivine was forming at depth, or perhaps olivine was crystallizing at a few centers to give large crystals while plagioclase was 

crystallizing at many centers to give small crystals. It hardly seems 

likely that much previously formed plagioclase could have been removed by gravity, for olivine, which would have had a much greater 

gravitational advantage, was not completely removed. 

The loss of stability of the olivine c;ystals in the magma of the 

specimens erupted after 1944 is indicated by the appearance of reaction 

rims of fine-grained orthopyroxene on the olivine megaphenocrysts. 

Despite this loss of stability, olivine megaphenocrysts are found as 

reacting relics in appreciable quantities in lavas erupted as late as 1947 

and are found in small numbers in most of the specimens erupted after 

1947. There is even a suggestion in the graph of figure 98 and estimates of oljvine in other thin sections that the volume percentage of 

olivine megaphenocrysts increases slightly from specimen to specimen 

of the ejecta from 1943--47. Taken together with the sudden decrease 

during the latter part of 1947, this might be interpreted as indicating 

that there had been a progressive concentration of intratelluric olivine 
PETROLOPY OF PARICUT~ VOLCANO 323 

in the portion:; of the m&gma erupted successively between 1943 and 

1947 and an iinpoverishnn.ent in those portions of the magma erupted 

late in 194 7. This latter lin fact is supported by the chemical relationships, to be dif:cussed mote fully below, and there seems to be no basis 

for proposing ·;hat the olivine disappeared siinply by reaction to form 

orthopyroxene, even thoUjgh that process was operating. It is possible 

that the sharp break in 11947 in the olivine content may be related to 

some mechaninm permitt~d by a flank rather than apical outlet of the 

magma chamber, as will be suggested at a later pojnt in the discussion. 

The first ap)earance oi fine-grained orthopyroxene reaction riins on 

olivine megap:1enocrysts !was not accompanied by the appearance of 

orthopyroxene megaphenocrysts. Orthopyroxene microphenocrysts 

are found in ('ccasional ~peciinens as early as 1943, but megaphenocrysts do not appear until the latter part of 1947 and then only in small 

numbers. In subsequently erupted speciinens successively greater 

numbers of or Ghopyroxe:Qe mega phenocrysts are found, and these are 

generally of larger size tn the later specimens. Indeed, there is a 

remarkably consistent te:pdency towards increase in maxiinum size of 

orthopyroxene crystals, ~tarting with the microlites and microphenocrysts of the ~~arly eject&;and in many specimens there appears to be 

no sharp line of demarcat.on in size-frequency distribution between the 

orthopyroxene megaphenocrysts and microphenocrysts. The tiny 

crystals of opaque oxide ~ouild as inclusions in most of the olivjne and 

orthopyroxene megaphenpcrysts in spite of their small size should be 

mentioned here, because ,their origin is intratelluric also. 

Summarizin?: the relationships of the megaphenocrysts, it is inferred 

that in those I ortions of the magma erupted during 1943 and the first 

part of 1944 ('livine, plagioclase, ard magnetite were in equilibrium 

with the liqui~~ at depth~ In those portions of the magma erupted 

during the re~ t of 1944 a,nd durmg parts of 1945, only olivine and 

magnetite wer3 in equilibrium. In those portions erupted dul'ing the 

other parts of 1945 and during 1946 to 1950, olivine no longer was in 

equilibrium; and that already formed was reacting to form orthopyroxene, not as bomoge:peous single-crystal shells around the olivine 

but as ma.tted mantles of cry.:;tals of microphenocryst size. 

~ICROPHENOCRYSTS 

Whereas plagjoclase is, of very lllnited occurrence as megaphenocrysts, it is abundant a$ microphenocrysts in all speciinens of the 

Paricutin eje( ta, as sh<)wn in figure 99. Olivine, clinopyroxene, 

orthopyroxene, and, in most specimens, opaque oxide occur together 

with the plagioclase as microphenocrysts in the groundmasses of most 

specimens of 1943 lava; qut clinopyroxene microphenocrysts are present in only a few speciinens erupted after 1943. Olivine "micro-
324 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

phenocrysts" (irregular grains) in most of the ejecta of 1944-46 are 

obviously reacting to form orthopyroxene, and olivine microphenocrysts are not found in the material erupted after 1946. Orthopyroxene microphenocrysts, on the other hand, are generally present 

in the ejecta of 1944 and subsequently although in some of the early 

ejecta they are so small that it becomes a matter of judgment whether 

to class them as microphenocrysts or microlites. It is clear that the 

development of orthopyroxene is partly at the expense of the olivine in 

the microphenocrysts and that broadly at least it is a result of the 

olivine-orthopyroxene reaction relation. 

The clinopyroxene microphenocrysts of the early ejecta have been 

presumed, on the basis of their high extinction angles, to be a.ugitic; 

and their association with orthopyroxene would fit with the suggestion 

of Poldervaart and Hess (1951, fig. 6) that augitic pyroxenes should 

form a reaction series separate from that of olivine-orthopyroxene. 

It is of interest, however, to inquire why clinopyroxene largely 

ceased to form as microphenocrysts in the magma erupted after 1945, 

while orthopyroxene continued to be formed in all subsequently 

erupted. portions. With progressive crystallization in a homogeneous 

liquid, it would seem that "augitic" clinopyroxene should continue to 

crystallize along with orthopyroxene (or pigeonite) at least until the 

edge of the two--pyroxene field was reached. The fact that the "augit.ic" phase drops out without significant progress across the twopyroxene field would therefore lead one to suspect that external factors 

were involved; ljhat is, the temperature had risen, or the magma was 

not truly homogeneous, and crystallization was proceeding in successively unlike portions of the magma.· The possibility of increase 

of temperature seems remote because of the lack of appropriate effects 

on the megaphenocrysts. The few reliable temperature measurements 

that have been made on the lavas of Paricutin indicate that there has 

probably been a small but general decrease in temperature of successively erupted n1aterials. In December 1944 the temperature of flowing lava 5 kilmneters from the vent was found by Zies (1946, p. 179) 

to be 1,110° 0 by use of a thermocouple. During November 1945 to 

February 1946, a series o.f thermocouple measurements by Krauskopf 

(1948b, p. 1272) gave a maximum of 1,070° C at the vent. During 

October fl,nd November 1946, measurements by the writer with an 

optical pyromet.er in subdued daylight gave a maximum of 1,040° C 

for flowing lava half a kilometer from the vent. The more favorable 

possibility seems to be that change of composition by progressive 

contamination of the liquid was the cause of the anomalous crystallization sequence. 

From the relationships of the microphenocrysts, we may concludethat, during extrusion and perhaps prior to extrusion, the magma of 
PETROLOGY OF PARICU~ VOLCANO 325 

n10st of the specimens erupted in 1943 and the first part of 1944 was 

precipitating plagioclase, olivine and opaque oxide and in addition, 

orthopyroxene in some portions. In some specimens of pumice such 

a,s that. erupted in April1944 (see pl. 10, fig. 2), the microphenocrysts 

a,re fresh olivine and plagioclase, which shows that no significant 

a,mount of pyroxenes had crystallized prior to leaving the conduit. 

In much of the magma e11upted during the latter part of 1944 and in 

1945, clinopyroxene had been crystallizing only in small quantities 

while plagioclase and orthopyroxene had been forming consistently 

a~nd olivine had become unstable in certain portions of the magma. 

In the magma being erupted during and after 1946, the only a<'tively 

forming crystals were plagioclase and orthopyroxene, with the exception of some erupted in 1946 in which clinopyroxene was formed 

sporadically. 

MICROLITES AND GLASS 

In some cases, especially in lava in the central portions of the thicker 

flows, crystallization no doubt continued at about the same rate as it 

had in the conduit and resulted in no marked change in texture to 

rnark the new generation. Most of the specimens of the Parfcutin 

ejecta, however, have been collected from the margins of flows or 

f'rom bombs, and in such situations the chilling is rapid enough to 

produce a recognizably finer grained or glassy mesostasis. Glass composes as much as 40 percent of the volume of some specimens examined, 

while in only a very few was it found to be less than 5 percent of the 

volume. 

Since the formation of microlites is complexly involved in the 

initial stages of congelation of the glass, the mineral type, habit, and 

n,bundance of the microlites must be expected to vary greatly according to the rate of chilling and the many other highly variable factors 

affecting nucleation. Both orthopyroxene and clinopyroxene are 

found as microlites in many of the specimens, and the thin flanking 

plates and swallow-tail extensions of clinopyroxene on orthopyroxene 

1nicrophenocrysts (pl. 10, fig. 6) are regarded as belonging to this 

final generation of microlitic development, as are the opaque oxide 

crystallites scattered through the glass and adhering to the pyroxene 

of some specimens. Whereas microphenocrysts of clinopyroxene do 

not generally occur in rocks erupted after 1945, microlites of clinopyroxene are commonly ~ound in most rocks erupted until the end of 

1950, although in decreasing amounts. In these conditions of formation the crystalline products may be stable or metastable forms, and 

the mineral association means little in regard to equilibrim relationships. Calcic plagioclase is not well represented as microlites, and it 

1may be that the flanking plates and micn;>lites of clinopyroxene are 
326 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

calcic pyroxene, the crystallization of which in the rapidly changing 

conditions was easier than that of calcic plagioclase. 

CHEMICAL RELATIONSHIPS 

It can be granted at once that the Paricutin lavas had their source 

in a single magma chamber, and this offers an advantage not ordinarily 

present in an inquiry into the mode of origin of a volcanic suite. In 

the usual study, specimens come from various vents that had been 

active at widely separated intervals, thus quite probably from different 

chambers or from different parts of a master chamber at quite different 

stages of evolution. In contrast, the blood relationship between the 

members of the Parfcutin suite must be very close, with proportionately less vari1tbility in the factors of time, space and environmental 

conditions. 

A rigorous treatment would demand that all conceivable modes of 

origin be examined for their possible contributions to the observed 

features of the chemistry of the Paricutin igneous suite. The writer 

is not prepared to give such a rigorous treatment here but rather will 

consider only two possible mode3 of origin that. have already been 

suggested: that of fractional crystallization and that of "bodily" 

assimilation of country rock involving no significant differential 

movement of chemical constitutents. 

Schmitter (1945, p. 130) has concluded that the presence of orthopyroxene in the groundmasses of the 1943 lavas may be a result of 

assimilation of monzonitic country rock such as represented by the 

xenoliths. Although the percentage of Al20a in the Paricutin xenoliths is less thnn in the lavas (see tables 2 and 3), the ratio of AhOa: 

CaO is much higher. Thus the addition of xenolithic material to the 

magma may have forced the crystallization of orthopyroxene rather 

than clinopyroxene for much the same reasons as suggested by Bowen 

(1928~ p. 208-210) and others in the case of ~;tssimilation of aluminous 

sediments. Similarly, the presence of orthopyroxene in certain lavas 

of Hakone volcano is attributed by Kuno (1950, p. 998) to assimilation 

of granitic wall rock, the low content of normative Wo and high 

content of water in which is regarded as favoring crystallization of 

orthopyroxene rather than clinopyroxene in the contaminated magma. 

Williams (1950, p. 270) in discussing the whole of the igneous suite 

of the Paricutin region, including only the lavas erupted from Paricutin volcano in 1943 and 1944, found no reason to doubt that the 

major control was fractional crystallization and no proof that the 

course of differentiation had been influenced by contamination of 

the magma by country rock. Based on tests of the much more complete chemical and petrographic data now available, however, it is 

concluded here that neither assimilation nor fractional crystallization 
PETROLOGY OF PARICUT~ VOLCANO 

acting alone could have; produced the suite of lavas of Paricutin 

volcano but that assimilation and fractional crystallization acting 

together could have produced it. In the following discussion the 

ability of these two mechanisms to produce the chemical and petrogqtphic trends will be Qonsidered first and the. thermal and other 

requirements of the proposed mechanism second. 

To test the adequacy of fractional crystallization. and assimilation 

as factors in the production of the observed chemistry of the Paricutin 

lava, the variation diagram will be the main tool. Starting with any 

one of the bulk analyses, which for this purpose may be presumed to 

represent the composition of the magma (including suspended crystals) 

before eruption, one may determine graphically whether removal or 

addition of appropriate materials could have produced the compositions of the other lavas. It should be noted that for the tests made 

here we are not attempting to delineate a liquid line of descent of the 

magma (Bowen 1928, p. 92) but rather a "bulk line of descent." If, 

for instance, the graphically determined line of descent simulating 

fractional crystallization matches that of the plotted analyses, the 

conclusion would be that fractional crystallization could have been 

the causal mechanism-if a close match is not obtained it could not 

have been the sole meQhanism. Similar reasoning would apply to 

the tests of assimilation 1alone and of assimilation together with fractional crystallization. 

Of the several types ·of variation diagrams in common use, the 

most practical for the purpose at hand appears to be the conventional 

silica-variation diagram of the form proposed and applied by Harker 

(1900, p. 390; 1909, p. 118-146 and 333-350). This diagram has the 

advantages of ease of oonstruction, simplicity of chemical relationships, and straightforwatdness in the graphical addition or subtraction 

of components from a given material. The objection that errors in 

analytical determinations of silica may give a false impression of 

departures from an otherwise smooth trend (Larsen, 1938, p. 506) 

does not hold in the case of the Paricutin suite, in which all but one 

of the analyses were made by analysts under the supervision of the 

same chemist; and therefore, if silica percentages are in error at all, 

they are probably in error by similar magnitudes in the same direction. 

vYhile it would be possible to use other types of variation diagrams 

here, all are less convenient than the silica-variation diagram. Besides requiring preliminary calculations the diagram of Niggli (Niggli 

and Beger, 1923, p. 197, 297), in which the molecular values al, jm, 

c, and alk are plotted as ordinates against si as abscissa, obscures 

the relations of iron and magnesia to silica and to each other by 

-combining both into th~ index fm. This diagram likewise obscures 

the interrelations of soda and potash by combining into the index 
328 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

alk. Further, the geometry of the diagram, and hence the graphical 

operation, is complicated by the manner in which the abscissa, si, 

is calculated. Thus, the si value of a mixture of equal parts of materials A and B does not lie half way between the separate si values 

of A and of B. 

The variation diagram proposed by Larsen (1938, p. 506), willie 

fulfilling most of the geometric requirements, involves several arbitrary 

preliminary calculations and has so complex an abscissa that interpretation becomes difficult. In that diagram the weight per.cent of 

each oxide constituent is plotted as ordinate against the index 

~ Si02+K20-FeO-MgO-CaO, a rough indication of the "acidity,. 

or "basicity" of the rock. When plotted in this manner, almost all 

igneous rock suites conform more closely to smooth curves than when 

plotted agains1i silica alone; and this is put forward, somewhat circuitously, as an advantage of the method. Actually, the artificial 

expedient of loading the abscissa with the same constituent that is 

being plotted ns ordinate cannot help but give greater conformance 

of plotted points to smooth lines, the extreme case, of course, being 

the straight line obtained by plotting one constituent against itself. 

The writer doubts that such deemphasis is justified in the usual case 

and certainly not in the carefully analyzed Paricutin lava series in 

which divergences from smooth chemical trends are significant and 

should be explained rather than deemphasized. 

The general chemical setting of the lavas of Paricutin can be best 

illustrated on a small scale, silica-variation diagram, such as figure 103" 

which shows the relationships between the lavas, their phenocrysts 

(olivine, orthopyroxene, and plagioclase), and their xenolithic inclusions. The mega phenocryst compositions used here are olivine, Fo80 ; 

orthopyroxene, En75 ; and plagioclase, An70, the plagioclase being taken 

as somewhat n1ore calcic than the optically determined micropheno-. 

crysts. The effects of possible departures from these assumed compositions will be considered later. The four analyses of xenoliths 

have been combined to give a plotted position of the "average xenolith," to be used in the following discussion as an approximate representation of the xenolithic n1aterial. On the variation diagram at. 

this scale, the lavas form a close-knit series in the middle silica range,. 

whereas their olivine and plagioclase phenocrysts lie at lower silica 

percentages, their orthopyroxene phenocrysts within the silica range 

of the lavas, and their xenoliths in a much higher silica range. 

As an example on figure 103, the simple subtraction of olivine, Foso, 

from the bulk analysis of no. 12 gives successive compositions lying 

along straight line extensions away from the corresponding oxide constituents of the olivine. In the residuum silica, alun1ina, lime, soda~ 

aod potash are seen to increase while iron and magnesia decrease. At 
.... 

z 

w 

0 

0:: 

w 

A. 

35 

3 

2 

I 

FeO 0 

2 

I 

~~g 0 

2 

I Alzos 0 No2o 2 

I 

40 

MgOj41.9 

8 

iii 

If) 

oi 

e 

0 

.., 

{l. 

.. 

c 

:~ 

0 

~L 

PETROLOGY OF PARICUTIN VOLCANO 

45 

.. 

Cit .. 

Cl c 

~ :: ·o ~ Cl A. 

PHENO~RYSTS ~ 0 

60 65 70 

EXPLANATION 

Compositions of lavas and xenoliths 

Calculotecl·compositions • 

Expected - trend 

Ali>J31.8 Mg0~278 

FeO •16.6 

I 

75 

LAVAS 

XENOLITHS 

CaO •14.3 

Mgo.o.o 

Na2(111! 3.4 

···; • ., FeO 

·~·· .. 

. · •. ·~29~ .... ... ~..;;0\. ·· .... 

~ r~ .•FeO 

·.L 

.. ·: :r~:~ . 

" ·~o 

329· 

80 

Kz00b---~~~----~------~~---z.-----~.-----~-----,~--~~r-----~ 

Si02 

FIGURE 103.-Silica·variation diagrams of lavas, phenocrysts, and xenoliths of Parlcutin volcano. Dots 

represent bulk compositions of lavas and xenoliths (analyses 6-10 averaged). Values for bulk composi- tions are counted from 0 point for each oxide at bottom of scale. Arrows from analysis 13 show trends to be expected by simple subtraction of olivine, Foao, from mixture of composition of no. 12. Heads of arrows are at silica percent of analysis 15. 

a given silica percentage, say that of analysis 15 (58.6 percent silica), 

the content of the other oxide constituents in the residuum can be 

read off; and it is found that they do not even approximate the values 

for corresponding constituents of no. 15. The conclusion is that 

simple removal of olivine, Fo80, alone from the magma represented by 

analysis 12 could not have produced the magma represented by no. 15. 

By similar operations it can be shown that no pairs of Paricutin lavas 

can be related by simple addition or subtraction of olivine, Fo80, alone 

or of plagioclase, An70, alone. 
330 GEOLOGIC INVESTIGATIONS IN THE PARlCUTIN AREA 

To test the effects of simultaneous addition or withdrawal of two 

mineral constituents, such as olivine and plagioclase, one may proceed 

as illustrated a.t the left in figure 104, an enlargement of the central 

portion of figure 103. Whereas only one oxide control, silica, was 

needed for the subtraction of a single mineral constituent, here two 

will be required; and for these, silica and alumina have been chosen 

arbitrarily. The pair of specimens to be compared are taken as no. 1 

and no. 11, and just enough olivine and plagioclase have been removed 

from no. 1 to attain the silica and alumina percentages of no. 11. Th& 

graphical procedure is as follows: A radius is drawn from the alumina 

point of olivine (outside of the diagram) through the alumina point of 

analysis 1, and another radius is drawn from the alumina point of 

plagioclase through the alumina point of analysis 11. The intersection 

of these radii at 55.5 percent silica determines the relative proportions 

of olivine and plagioclase subtracted. A radius is drawn from the FeO 

point of olivine through the FeO point of no. 1 ; the radius then intersects the 55.5 percent silica-vertical at 7.0 percent FeO. Through 

this point a radius is drawn from the FeO point of plagioclase and,. 

extended, it intersects the Si02 vertical of no. 11 at 8.2 percent FeO,. 

about 1.3 percent higher than the plotted value of FeO in no. 11. In 

a similar manner it is found that the test value of CaO is too low by 

0.8 percent and· that of MgO is too low by 0.3 percent. From the 

large discrepancies in both FeO and CaO it may be concluded that 

simultaneous subtraction of Fo80 and An70 does not account wholly 

for the differences in composition between no. 1 aad no. 11. 

It should be noted that it does not matter which constituent, 

olivine or plagioclase, is subtracted first. The test is equally valid 

if performed in reverse, that is, by addition of olivine and plagioclaSe 

to no. 11, for it leads to the same relative discrepancies in MgO, CaO 

and FeO upon duplicating the Si02 and Al20 3 values of no. 1. In 

fact, one may start with any member of the related series of rocks 

in this narrow range of silica content and test the correspondence 

of any or all of the others of the series to the graphically determined 

compositions. Because there is no compelling reason to insist that the 

least silicic of the lavas (no. 1) represent~ the direct parent of the 

others, one may choose an intermediate member as a convenient point 

of departure from which to test the entire series, keeping in mind that 

the procedure is justified whether the lavas are direct derivatives 

of the previously erupted lavas or all derived from a common parent.· 

The right hand portion of figure 104 illustrates the graphical operations in adding or subtracting three materials, here olivine, Fo80 ; 

plagioclase, An70 ; and "average xenolith." Although the constructions are more tedious than those for only two variables, the solution 

again is . unique and essentially unaffected by the sequence in which 
·-PETROLOGY OF' PARICU'I'IN VOLCANO 331 

the operations are performed. It is seen that, with three materials 

available for addition or subtraction, three oxide controls must be 

used, and for these silica, alumina, and magnesia have been chosen. 

11 10 19 

19 

Al203 

CONTROL 

AI203 

- Plus U:Otith CONTROL Minus 

oliVine 

close 

8 

FeO 7 

... 6 FeO 6 

z 

.... 

(.) 

0: 

.... 

a. 5 5 

PERCENT SILICA (CONTROL) 

FIOUBJ: lM.-Silica-varlation diagrams, showing graphical operations in addition and subtraction of 

materials from bulk compositions (represented by open circles) of lavas of Parfcutin volcano. Dots oppo- site FeO, CaO, and MgO of no. 11 show ~rcentages in final product subtracting olivine, Foao, and plagio- clase, An7o, from no. 1, using SiOs and AbOs of no. 1 and no. 11 as controls. Dots opposite FeO and CaO 

of no. 19 show pet:Centages in final product obtained by subtracting olivine, adding "average xenolith, .. and subtracting plagioclaSe from no. 10, using SiOs, AbOa, and MgO of nos. 10 and 19 as controls. 
332 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

Using analysis 10 as the starting material and no. 19 as the end 

. material, radii are drawn from the alumina point of olivine throttgh · 

the alumina point of no. 10 and from the magnesia point of olivine 

through the mu.gnesia point of analysis 10. Similarly, radii are drawn 

from the alun1ina and magnesia points of plagioclase respectively 

through the alumina and magnesia points of analysis 19. Then a 

series of closely spaced radii are drawn from the alumina and magnesia 

points of the "average xenolith," cutting the olivine and plagioclase 

radii. One mtty then choose the radii, one from xenolithic alumina 

and the other from xenolithic magnesia, which intersect the olivine 

and plagioclase radii at corresponding silica percentages (here 57.6 

percent and 5~}.6 percent silica). A radius from the FeO point of 

olivine through the ·FeO point of analysis 10 cuts the 57.6 percent 

silica-vertical at 6.0 percent FeO, and a straight line from this point 

towards the FeO point of the "average xenolith" cuts the 59.6 percent 

silica-vertical ttt 5.5 percent FeO. A straight line from this point 

through the FeO point of plagioclase cuts the silica-vertical of analysis 

19 at 5.8 percent FeO, about 0.3 percent lower than the actual plotted 

value for the FeO of analysis 19. By a similar procedure the CaO 

content of the graphically derived product is found to be 0.1 percent 

lower than that of analysis 19. , · 

Here again the magnitudes of the discrepancies between graphically 

determined end points and their actual counterparts may be taken 

as a measure of the ability, from the compositional standpoint only, 

of the process under test to account for the derivation of the end 

mnterial from the starting material or of both from a common parent. 

In this example the total discrepancies for the four oxides, Al20a, FeO, 

CaO, and MgO, is 0.40 percent; and one may speak of an average 

discrepancy here of 0.10 percent. Another useful measure of the 

chemical feasibility of the graphically simulated process is the amount 

of divergence from the actual "trend" between starting and end 

materials, and in this example the divergence for the four tested 

oxides may be expressed as 0.03 percent per percent change of sil~ca 

between analyses 10 and 19. · 

The results of testing the olivine-plagioclase-xenolith variables 

two at a time are shown in figure 105, and those of the complete 

combination in figure 106, all tests using analysis 10 as the starting 

point. In both figures the open circles represent the plotted pomts 

of the bulk arinlyses, and the filled circles represent the compositions 

of materials derived graphically from no. 10 by using the different 

combinations. Soda and potash have been left out of the diagrams 

on the assumption that, no matter what combinations are used, 

their test points will fall near the actual points because the soda and 

potash contents of all the materials, lavas as well as phenocrysts 
PETROLOGY OF PARICUTIN VOLCANO 333 

and xenoliths, are low and nearly in line with each other on the 

diagram. 

On figure 105 the closest conformance to the actual plotted points~is 

obtained in the test of the olivine-xenolith combination (points con-

... 

z 

la.l 

~ la.l 

Q, 

19 

Al203 

18 

17 

8 

FeO 

7 

6 

5 

CoO 8 and 

MgO 

7 

6 

0 

0 Bulk analysis 

Points -obtained by addition or subtraction 

of xenolith ond olivine 

Points obtained by addition or subtraction of plagioclase and olivine 

FeO 

--;;······ .......... ()······················-.. ... 0 0 0 oo 

................ '------....__ •.... Xenolith and plagioclose 

xenolith and olivine 

2 -·--·- Points obtained by 

addition or subtraction of 

.. plagioclase and xenolith 

55 

PERCENT SILICA (CONTROL) 

60 

19 

18 

17 

8 

7 

I 

s 

8 

7 

4 

FIOURE 105.-Silica-variation diagram comparing actual bulk compositions (open circles) with trends 

developed by graphical addition or subtraction of olivine, plagioclase, and xenolithic material two at a time, from analysis 10 (tables 2 and 3), using Si02 and AhOa as controls. Dotted lines connect points obtained by fractional crystallization involving olivine, Foae, and plagioclase, An70. Dashed lines con- nect points obtained by assimilation of xenolithic material and fractional crystallization of plagioclase, An,.. Solid lines connect points obtained by assimilation of xenolithic material and fractional crys- tallization of olivine, Fo10. 
334 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

7 

6 

0 0 

r o 

\~ FeO 

6c~~:t-o............ . 

EXPLANATION 

0 

Bulk analysis 

Points obtained by 

addition or subtraction 

of xenolith, olivine, and plagioclase 

55 

6-..q 0 0 

.......:.~ oo ·-......... ..... 

PERCENT SILICA (CONTROL) 

• 

f 

6 

8 

FWURE 106.-Silica-variation diagram comparing actual bulk compositions of Parlcutin lavas (open circles) 

with trends developed by graphical addition or subtraction of olivine, plagioclase, and xenolithic material 

from analysis 10 (table 2), using Si02, AbOs, and MgO as controls. 

nected by solid line). Even here the discrepancies are considerable 

and erratic at the low-silica end of the diagram. Although the discrepancies reaeh nearly the same magnitude at the high-silica. end, 

for iron and n1agnesia the rough parallelism over certain portions of 

the silica range appears to be significant. The olivine-plagioclase 
PETROLOGY OF PARIC~ VOLCANO 335 

-combination, on the other hand, gives trends (dotted lines) from no. 

10, which diverge widely from the actual trends of magnesia and iron 

-at the low-silica portion and from that of magnesia at the high-silica 

~nd. The divergence is so marked that one must conclude that addition and subtraction of olivine and plagioclase could. not have been 

11n effective process by itself in producing the lava series. 

The test points of the plagioclase-xenolith combination (connected 

by dashed lines in fig. 105) diverge somewhat more from the actual 

points of magnesia and iron than do those of the olivine-xenolith combination. In the high-silica range they fall on the opposite side of the 

-actual points from the test points of the olivine-plagioclase conbination. Although the graphic test is valid in itself, the plagioclasexenolith combination is of dubious physical significance, because it 

excludes olivine, which is much more abundant as phenocrysts in the 

lava and seemingly more subject to gravitational movement in the 

magma than plagioclase. This and the appreciable individual diserepancies of the plagioclase-xenolith test points furnish a basis for 

-strong doubt that addition or subtraction of only plagioclase and xenolithic material could account for the chemistry of the lava series. 

Figure 106, at the same scale as the preceding figure, shows there-

-sults of simultaneous additions or subtractions of all three variables, 

olivine, plagioclase, and average xenolith from analysis 10. The disprepa.ncies in FeO are generally less than in any of the paired-variable 

tests, most of them less than one-fourth of 1 percent and probably 

approaching the combined margin of error of analysis, graphical plotting, and spurious effects. As in the previous tests, the discrepancies 

in CaO are generally negligible, at least for the high-silica portion. 

The conformance is sufficiently good to justify the conclusion that 

combined action of both fractional crystallization and assimilation 

processes-which involve olivine, Fo80 ; plagioclase, An70 ; ap.d "average" xenolith-could have been the major cause of chemical differences between the Ia va of analysis 10 and the higher silica members 

of the series. From the standpoint of bulk of material involved, assimilation would appear to have been the predominant process. 

While the results of the foregoing tests appear to argue against 

fractional crystallization as the only process involved, they are based 

on but one set of compositions for the participating intra telluric olivine 

and plagioclase, Fo80 and An70• In view of the uncertainty surrounding the exact compositions of the intratelluric crystals, it is desirable 

to test other combinations of olivine and plagioclase, as well as the 

. possible effect of participation of orthopyroxene and magnetite. Such 

tests have been made, first for all combinations of Fo70, Fo7151 Fo8o, and 

Fo85 with An80, An70, and An80 ; then for combinations of magnetitebearing olivine with plagioclase; for combinations of magnetite as a 
. 336 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

free variable with olivine and plagioclase; and finally for combinations 

of orthopyroxene with plagioclase and with olivine 

The results of these graphic tests and others are summarized in 

table 9, where average "divergences" of test trends from actual trends 

are tabulated. · In calculating the average divergence for a particular 

oxide the ordinate distance between each test point and actual point 

was divided by the abscissa distance between test point a.ad starting 

point (analysis 10) without regard for algebraic sign. The resulting 

quotients for points to the left of the starting point were averaged 

separately from those to the right and listed for each oxide. Test 

points for analyses 6, 8, and 9 were not included because of the close 

approach .of these analyses to analyses 7 and 10. The left and right 

divergetices of all four oxides were then averaged and listed, as were 

the over-all average divergences for the whole series. The numbers 

so obtained express the average percent divergence per percent·chooge 

of silica content through the series and thus measure the ability of 

each tested combination to reproduce the observed chemical trends. 

Although it would be difficult to prescribe limits, an over-all average 

divergence of greater than 0.15 percent per percent change of silica 

would seem to be too great to permit acceptance of the particular 

combination u,s feasible, while one between 0.10 and 0.15 would cast 

doubt on the feasibility of the combination. A combination whose 

over-all average divergence is less than 0.10 might be regarded as quite 

promising, and an over-all average divergence of less than 0.05 might 

be regarded as indicating an exceptionally close match, in view of the 

possible unsystematic variables involved. 

In table 9 the over-all average divergences for all combinations of 

olivine and plagioclase tested range from 0.18 to 0.38, all too high 

by the above standards to allow acceptance of these combinations as 

feasible. The over-all average divergences of the magnetite-bearing 

olivine (magnetite as inclusions in olivine in ratio of 5:95 by weight) 

in combination with plagioclase are 0.20 and 0.24, likewise too high. 

Those of the freely variable plagioclase-olivine-magnetite combinations 

are 0.13, making this combination possibly feasible from the standpoint 

of the chemical variation alone. In attaining this low-value of divergence, however, the differential movement of from one-quarter to onehalf as much magnetite as olivine is implied by the graphical construction. This is regarded as definitely unrealistic in view of the 

observed lack of significant amounts of movable magnetite in the rockS 

themselves and in view of the physical chemical arguments against 

such large amounts of magnetite being able to crystallize from a 

magma of this type. 
PETROLOGY OF PARI CUTIN VOLCANO 337 

TABLE 9.-Average divergences of test trends from actual 

silica-variation diagram 

trends of the Parf.cutin 

[Expressed as percent divergence pllr percent change of silica content. Starting point in all cases is 

analysis 10] 

Average divergence of individual oxides Average divergence 

of the four oxides 

Combination tested graphically 1 AlaOa FeO CaO MgO 

to to over-all to to to to to to to to left right 

left right left right left right left right 

An7o alone •• ---------------------- 2.21 1. 79 1. 56 1.10 1.02 0.95 0 91 1.23 1.42 1.27 1.34 Foao alone_ ________________________ 1.21 .98 .27 .41 .64 .43 2.18 1.27 1.08 .77 .92 

Xenolith :alone ••. _____ ------------ .37 .14 .19 .06 .06 .16 .39 .26 .23 .16 .20 

Fo,o-Anee. ------------------------ 0 0 .26 .19 .12 .12 .88 .20 .32 .13 .22 

Fo7o-An7o------------------------- 0 0 .25 .19 ;·19 .ro· . 75 .18 .30 .12 .20 

Fo,o-Anao. __ -_ -------------------- 0 0 .24 .17 .15 .10 .69 .12 .27 .10 .18 

Fo,a-Anae. --- ___ -------.---------- 0 0 .31 .10 .12 .12 .92 .32 .34 .13 .23 

Fo1s--An78. ------------------------ 0 0 .29 .11 .15 .10 .87 .28 .33 .12 .22 

Fo,r Anao.------------------------ 0 0 .27 .11 .14 .10 .84 .22 .31 .11 .21 

Fo!O""Anao. _ ----------------------- 0 0 .45 .17 .12 .13 1.12 .47 .42 .19 .30 

Foae-An7o. ----------------·------- 0 0 .41 .14 .20 .10 1.03 .38 .41 .15 .27 Foae-Anao. __ ---______________ ----- 0 0 .38 .12 .14 .10 .93 .32 .36 .13 .24 

Fosa-Anee. ____________ • ----- _ ----- 0 0 .64 .31 .13 .13 1.29 .61 .51 .26 .38 

Fosa-An7o. ------------------------ 0 0 .53 .27 .16 .10 1.24 .56 .48 .24 .35 

Fosr.A.nso. ------------------------ 0 0 .50 .22 .17 .10 1.16 .46 .46 .20 .32 

An7o-(Mgt Fo,s)* ----------------- 0 0 .25 .16 .25 .10 .77 .16 .32 .10 .20 

.An,a-(Mgt Foao)* ----------------- 0 0 .35 .10 .25 .10 .92 .25 .38 .11 .24 

En7s--An70------------------------- ------ 0 .10 .16 . 74 .25 

En7o-Foao. _------_---------------- ------ 0 .33 .12 .33 .19 

En,s-Foao. __ ---------------------- ------ 0 .12 .10 .16 .09 

Enae-Foao. ____ -------------------- ------ 0 .05 .09 ~11 .06 

X en-An7o------------------------- 0 0 .27 .15 .29 .12 .32 .20 .22 .12 .17 

X en-Fo,o. ------------------------ 0 0 .26 .17 .26 .11 .40 .14 .23 .11 .17 

X en-Foso. __ ---------------------- 0 0 .18 .10 .26 .11 .55 .14 .25 .09 .16 

Xen-Foss.-----·------------------ () 0 . 21 .08 .23 .14 .58 .11 .26 .08 .16 

X en t-Foao.--------------------·-- 0 0 .25 .06 .26 .09 .60 .09 .28 .06 .16 

An7o-Foso-Mgt ____ ---------------- 0 0 .48 .24 .22 .10 0 0 .15 .09 .13 

.A.n,o-Fosa-MgL •. --- ____ --------- 0 0 .53 .21 .17 .10 0 0 .17 .08 .13 

Xen-Foso-Mgt._ __ ---------------- 0 0 .54 .06 .31 .15 0 0 .21 .05 .18 

Xen-Foso-ADBO-------------------- 0 0 .20 .10 .21 .12 0 0 .10 .05 .07 

Xen-Fo~MrAn7o-------------------- 0 0 .23 .09 .21 .13 0 0 .11 .05 .08 

1 An=anorthite; Fo=forsterite; En=enstatite; Mgt= magnetite; Xen=average xenolithic material. 

*Ratio of olivine to magnetite, 95 to .5 by weight. 

tFeO 1 percent higher, Alila 1 percent lower than in "average" xenolith. 

The divergences of trends of orthopyroxene-plagioclase combinations 

are similar to or somewhat greater than those of the corresponding 

olivine-plagioclase combinations, at least for that portion of the lava 

series to the right of analysis 10 on the variation diagram. Because 

of the proximity of orthopyroxene to the lavas lower in silica than no. 

10 (see fig. 103), the graphical constructions must be performed with 

great care to a void large errors; and if the silica content of the orthopyroxene is the same as that of the lava being compared, the graphical 
338 GEOLOGIC INVESTIGATIONS IN THE PARlCUTIN AREA 

procedure becomes indeterminate with silica as the abcissa. The lowsilica portion of the lava series has been left out of consideration here 

because of this and because the lack of orthopyroxene megaphenocrysts in those lavas make it unlikely that movement of orthopyroxene 

crystals was involved in their derivation. As an example of there· 

suits to be expected in tests of the orthopyroxene-plagioclase combinations, the combination, En75-An70 is listed in table 9 and shows a 

4-oxide average divergence of 0.25 percent per percent change of silica 

to the right of analysis 10. Exploratory tests of the other combinations of En70, En75, and En80 with An80, An70, and An80 indicate that 

their 4-oxide divergences would be similarly large. It is therefore 

concluded that the orthopyroxene-plagioclase combinations cannot be 

used to account for the chemical relationships of the high-silica portion 

of the lava series. 

The results of tests of three orthopyroxene-olivine combinations are 

given in table 9 for that part of the lava series to the right of analysis. 

10 on the diagram. The 4-oxide average divergence of the combination En80-Fo80 is 0.06, that of En7rFo80 is 0.09 and that of En70-Fosois 0.19 percent per percentchange of silica, indicating that the first 

two combinations permit tolerably good matches of the observed 

trends. But the graphical operations by which such good matches 

are obtained imply events in the magma that are extremely complicated. To derive a late member of the lava series from no. 10, orthopyroxene must be added in each case while olivine is being subtracted. 

Starting with 100 grams of no. 10, for instance, one must add 26 grams 

of En70 while subtracting 22 grams of Fo80 to approach the composition 

of no. 19. Tests with the combihations En75-Fo80 and En80-Fo80 

indicate that similar amounts of the two minerals would be involved~ 

To furnish enough magnesia for continued crystallization of olivine, 

the orthopyroxene must be continuously fed to and dissolved in the 

liquid; and the dissolution of the orthopyroxene "xenocrysts" must 

be complete, for corroded forms are not found in the resulting lavas~ 

Upon approaching the composition of no. 19, the process must be 

stopped and erystallization of some orthopyroxene phenocrys.t~ accomplished before eruption. It may be assumed furthermore that 

the trend of intermediate bulk compositions follows something like 

the trend of the lava series in passing from no. 10 (or from a more 

distant parent) to no. 19 by this method of differentiation. Thusr 

after the approximate composition of no. 15 is reached, for instance, 

some 10 grams of additional olivine must be crystallized and removed 

while dissolving some 12 grams of orthopyroxene. But this would 

appear to be impossible, for the petrographic characters of the lavas 

indicate that olivine was not able to crystallize in the magma of this 

composition and that orthopyroxene was crystallizing rather than dis-
PETROLOGY OF PARICUTUN VOLCANO 339 

solving. These and similar difficulties lead to the conclusion that 

addition and subtraction of combinations of olivine and orthopyroxeno 

alone, while able to reproduce fairly well the bare chemical trends, 

cannot be reconciled with the observed petrographic characters of the 

lava series. 

With these tests, all the reasonable combinations that might have 

been involved in fractional crystallization of the Parfcutin magma 

appear to have been considered and all appear to be infeasible, either 

from the standpoint of the chemical requirements alone or from the 

standpoint of the petrography and physical chemistry. The conclusion is drawn therefore that fractional crystallization could not have 

been the only process producing the lava suite of Parfcutin volcano. 

The tests of the two combinations of xenolithic material with 

olivine and plagioclase listed at the bottom of table 9, show over-all 

average divergences of 0.07 and 0.08, both quite acceptable values, 

which allow the conclusion that some such combination of assimilation 

and fractional crystalliza.tion can account for at least the chemical 

requirements imposed by the series.. There likewise appears to be 

nothing in the petrographic characters of the series that would make· 

this combination infeasible. It remains to consider whether the 

thermal requirements of the implied assimilation could have been met 

by the Parfcutin magma, and this aspect will be discussed in the final 

section. 

The average over-all divergences listed in table 9 may perhaps be 

taken as the most basic measure of the adequacy of the tested combinations to meet the chemical requirements, since they roughly 

represent what one might obtain if, instead of arbitrarily using certain 

oxides as control, an over-all control could be imposed to best suit the 

combination and the oxide relationships. Nevertheless, the other 

averages listed are of interest, most notably the consistently greatet· 

average divergences to the left of the starting point as compared to 

those to the right. This is interpreted here as meaning that all the 

combinations tested account less well for the low-silica lavas than they 

do for the high-silica lavas. It may be, therefore, that some additional 

factor has been involved in producing the low-silica rocks even in the 

case of the xenolith-olivine-plagioclase combination in which the overall divergence is small. It should be mentioned here also that the 

individual divergences of certain of the analyses, especially no. 11 

and to a lesser extent no. 12, are much larger than those of the other 

analyses in the tests of many of the combinations, including those of 

xenolith-olivine-plagioclase. If these combinations are accepted as 

feasible to account for the series as a whole, an additional factor or 

factors may be involved in the production of the lavas of analyses 11 

and 12. 
340 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

THERMAL REQUIREMENTS 

The graphical tests in the preceding section indicate that assimilation and fractional crystallization acting together could have produced 

the chemical features of the erupted lavas. It remains to examine 

whether sufficient heat energy might have been available to, 'the 

magma to enable the appreciable amount of assimilation implied by 

the results of the graphical investigation of the chemistry of the lava 

series. 

The approach to the thermal problem will be to determine the 

amount of xenolithic material added in changing from the composition of a low-silica member of the series to that of a high-silica member, 

and from this, the amount of heat demanded by the assimilation of the 

added material. Because no superheat may be presumed to have been 

present, the source of heat for assimilation must be looked for in the 

heat of crystallization of minerals from the magma. Because they are 

presumed to have already crystallized in its magma, the megaphenocrysts in the starting lava had already spent their heat of crystallization. Any useable heat for assimilation must therefore come from 

crystallization of the still liquid portion of the magma. As an approximation of the composition of the magmatic liquid, the calculated 

composition of the groundmass (table 2) will be accepted here, and as 

a starting material, that of the groundmass of the least silicic lava, 

analysis 1, will be chosen. The same considerations do not carry the 

same weight when applied to the end product, nor does it make a great 

deal of difference in the cases of the high-silica members of the Paricutin series, for their content of megaphenocrysts is very small. 

The graphical procedure indicates that addition of 25.4 grams of 

"average" xenolithic material and subtraction of 2.9 grams of olivine, 

Fo80, and 9.6 grams of plagioclase, An70, from 100 grams of the 

groundmass material of no. 1 will give 112.9 grams of material closely 

approximating the composition of the groundmass of no. 19. Thus, 

25.4 grams of xenolithic material must be assimilated, and the heat 

required may conveniently be divided into two parts: that necessary 

to raise the temperature of the quartz monzonitic country rock to that 

of the magma and that necessary to convert the country rock to a 

liquid and incorporate it into the magma. It is of not great concern 

here that the two parts must overlap somewhat in actual operation. 

For the first step let it be assumed that the chosen 25.4 grams of 

quartz monzonite to have been at a depth of about 6 kilometers in the 

crust, with an initial temperature of about 200° C, determined by a 

thermal gradient of about 30° C per kilometer and that its temperature is to be raised to 1,100° C. The average heat capacity of quartz 

monzonite over this indicated temperature range may be assumed to 
PETROLOGY OF PARlCUT~ VOLCANO 341 

be about 1.1 joules per gram on the basis of data listed for granite by 

Goranson (1942, table 16-2). Thus, to heat the 25.4 gram mass from 

200° 0 to 1,100° 0 would require about 25,000 joules. For the second 

step-that of actually melting and dissolving the _granitic material 

once its temperature has been raised to the melting rang&-we find 

that appreciably less heat would be required. The heats of fusion of 

the common granitic minerals, according to Goranson (1942, table 

16-3) are 203 joules per gram for albite, 418 joules per gram for orthoclase, and 244 joules per gram for quartz. For quartz monzonite 

rock, of the type we are dealing with in the observed xenoliths, a rough 

value of 250 joules per gram would seem indicated for its heat of 

fusion. Heats of mixing are of such a low order of magnitude, both 

positive and negative, that they probably may be neglected. The 

fusion and solution of 25.4 grams of granitic material, already at its 

fusion temperature, would thus require about 6,350 joules, and the 

total heat required to assimilate 25.4 grams of cold quartz monzonite 

would be about 31,350 joules. 

To obtain this amount of heat from the original100 grams of magma 

would require the crystallization of so large a proportion of it that its 

mobility would be lost. From the data listed by Goranson (1942, 

table 16-3), for instance, the latent heat of crystallization of plagioclase, An70, may be estimated as about 370 joules per gram, and that 

of olivine, Fo80, as about 425 joules per gram. Those of the more acid 

members of the reaction series are generally lower but of the same 

order of magnitude. To get 31,350 joules, the crystallization of 85 

grams of plagioclase alone or 7 4 grams of olivine alone would be required. Any combination of these or the lower members of the reaction series would not significantly change the total amount of crystallization needed to furnish the desired heat so that one could not expect 

to have more than about a. fifth of the original mass remaining as 

liquid. This situation, of course, is entirely unrealistic; therefore, the 

bulk of the required heat must be looked for outside the particular 

100 gram unit of liquid chosen here as a starting point. 

A mechanism proposed by Holmes (1931) for the upward penetration of sima tic and sialic rocks by the fluxing action of magma cupolas 

(overhead stoping and fluxing of Daly, 1914, p. 194-208; 1933, p. 

267-286), which he applied as an explanation of the acidic-basic volcanic complexes of Scotland, appears to offer a basis for the availability of adequate heat, as well as an explanation of other observed 

relationships at Paricutin volcano, perhaps also of those of the whole 

Paricutin region. Holmes emphasizes more than Daly the probability 

that thermal convection can furnish the great quantities of heat needed 

for the fluxing and stoping of the country rock at the cupola roof. 

Holmes (1931, fig. 3) first supposes a deep-seated, broad basaltic 
342 GEOLOGIC INVESTIGATIONS IN THE PARiCUTIN AREA 

magma chamber with only slight local irregularities in the roof. 

Thermal convection currents will tend to start under any slight arch, 

and once started, will tend to bring hotter magma up to the central 

part of the arch. Thus, the fluxing action at the apex may begin, 

slowly at first, but becoming more effective as the arch becomes 

steeper by solution and removal of the country rock at the apex. 

Once under way this process should continue to assimilate the roof 

rock, and the magma would dissolve out a high-arched cupola in the 

overlying rock, regardless of the composition of the rock. With a 

sufficiently large main chamber of the magma below, the cupola may 

be considered able to flux its way upward through the overlying thick 

layer of generally basic and intermediate rock and into the sialic 

layer. While yet in the simatic layer, no great modification of the 

composition of the cupola magma may be expected by incorporation 

of roof rock. Once in the sialic layer, however, a tendency towards 

acidification of the magma of the cupola may be caused by the progressive incorporation of granitic material. Initially much of this 

granitic material may be expected to be carried down with the basaltic 

magma along the flanks. But Holmes considers that a point may be 

reached when the granitic material will not all sink, and at that stage 

a secondary circulatory system is set up in the upper portion of the 

cupola that will tend to allow the development of a definitely more 

granitic phase of the magma separate from the generally basaltic 

magma below (Holmes, 1931, fig. 4). 

It should be remarked that the slow, upward solution-penetration 

into the country rocks and the development of the slim cupola can be 

accomplished by the magma even though it be crystallizing and therefore not possessing any superheat in the usually accepted sense. 

Thus, there is an indefinitely large amount of heat being brought by 

convection from the hotter main magma chamber below, and one can 

reason that, because of the great mass of hotter magma available in 

the main chamber, an entirely adequate amount of heat can be furnished to the cupola as long as the convection is effective. The heat, 

brought in by convection, is made available by crystallization. 

It is used in raising the temperature of the country rock and, where 

mechanically favorable, in incorporating wall rock material into the 

magma as a liquid. 

The limits beyond which such a process ceases to operate may 

involve the eventual exhaustion of the heat of the main chamber or 

perhaps, before this, the increase of viscosity of the cupola magma 

and its acidic "roof magma" to the point that convection is no longer 

effective. The closer the cupola approaches the surface, the more 

rapid will he .its loss of heat; and with no disturbing factors, it would 

seem that an economic limit of upward migration would be reached. 
PETROLOGY OF PARICUT~ VOLCANO 343 

Perhaps more effective, however, would be the increasing likelihood of 

eruption as the cupola roof draws nearer to the surface. The tapping 

of the cupola and, exhaustion of the pent up volatiles, as well as bodily 

removal of the. hot material, must certainly have a cooling effect 

that, if the cupola development is slowing down anyway, may serve 

to stop it altogether. 

To apply Holmes' mechanism to the explanation of the Paricutin 

rock suite, the writer has sketched (with some indefiniteness as to 

actuad dimensions) the situation of the imagined magma cupola below 

the volcano- (fig. 107). The cupola is shown slightly offset from a 

position directly below the vent for somewhat indirect reasons which 

will be discussed later. The cupola is visualized as a thin upward 

extension of a much larger, basaltic magma body, the main lateral 

development of which would be at an appreciable depth, say 20 to 30 

kilometers. It is represented as having penetrated upward into the 

sialic rocks, the base of which, in the absence of data, is assumed to 

be at a depth of about 12 kilometers. What little regional evidence 

is available indicates that the sialic rocks here consist chiefly of quartz 

monz()nitic rock~ and acidic effusives overlain by the andesitic and 

basaltic effusives. 

The rates of abstraction of heat for the two stages, heating of 

country rock and dissolution of xenoliths, conceivably would be 

different and would lead to different rates of crystallization. The 

heating of the country rock must necessarily be a very slow process, 

controlled largely by the small thermal conductivity of the rock. 

The crystals so produced in the magma should be large and capable 

of gravitative migration. Once the preheated wall rock has been 

broken off and enters the mass of the magma, it may cause a relatively 

rapid chilling of the immediate magma and a speeding up of the rate 

of crystallization, hence production of fine crystals incapable of 

significant gravitative migration. One does not expect that the subsequently -erupted lavas nepreserrt the portions of the magma that 

preheated the country rock, for much of that magma would have 

moved on past in the convection system. Its large, slowly formed 

crystals would likewise have moved with it or ahead of it down the 

flank of the cupola. At least some of the smaller crystals of the lavas, 

the microphenocrysts for instance, might represent crystals whose 

formation was caused by the more rapid extraction of heat owing to 

the presence finally in the magma of the xenolithic and xenocrystic 

material. To distinguish them petrographically from the crystallization owing to cooling on extrusion of the lava might be difficult or 

impossible. 

The Paricutin magma chamber, or cupola as we will now regard it, 

is shown in the sketch of figure 107 as offset from the site of the 
344 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

Paricutin vent on the surface. The reasons for showing it thus areindirect; and such a postulated position must, of course, be regarded 

as merely conject11ral. . In the first place, the results of the airborne, 

magnetic surYey of the ·region by the U. S. ·Geological Survey· in 

Decem.ber 1947 (J. R. Balsley, personal communication) have shown a 

strong negative magnetic anomaly of some 200 gamm.as centering 

about a point about 3 kilometers NNW. of the volcano, and the 

closure of the anomaly has a radius of about 3 kilometers. This 

anomaly is much larger than any other in the vicinity of the volcano 

and is not to be accounted for by any topographic effect. It implies 

that a body of abnormally low m.agnetism must be located at depth 

below and somewhat north of the surface expression of the anomaly. 

It could be a boss of solid rock of low m.agnetism, such as granite or 

acid effusives, protruding upward into the generally m.ore m.agnetic 

basaltic and andesitic surface mantle. Or it might be the therm.al 

aureole of the Paricutin cupola, the apex of which lies not far below thebase of the basaltic and andesitic mantle. 

Another reason for showing the Paricutin cupola in an offset 

position is that such a situation might furnish the explanation for the 

remarkably continuous and generally steady rate of eruption at the 

vent during the 9 years of its life. Were a gas-charged cupola tapped 

at its apex, it seems reasonable to suppose that the consequent eruption would be intense but brief, probably discharging all its pent up 

pressure in the space of a few months. But were a gas-charged 

cupola tapped on its flank, as sketched on figure 107, the apex por'tion 

of the cupola could continue to act for some time as a reservoir of 

pressure, the rate of eruption being controlled chiefly by the equilibrium between pressure and the resistance owing to the viscosity. 

Here the origin of the supposed gas pressure and its possible maintenance at a fairly constant level comes into consideration. There is no 

doubt, from the inflated character of the majority of the granitic 

xenoliths, that they originally contained water, whether interstitial or 

water of constitution of the hydrous minerals, and that it was volatil--

ized and has escaped, either into the enclosing magma during eruption 

or into the air aftel.' eruption. The notably low content of water in the 

xenoliths after eruption is similar to that of the normal Paricutin lavas. 

(see analyses, tables 2 and 3). The question arises as to whether, at 

the level of the supposed apex of the cupola, say at 5 or 6 kilometers 

depth, this inflation of xenoliths and expulsion of volatiles could have 

been sufficient to produce a build-up of pressure in the cupola. Cer-· 

tainly the tendency would exist, for the granitic (and tuffaceous) 

m.aterial must have contained more water originally than it could hold 

after being heated to the temperature of the magma, and the pressure 

build-up could have taken place over a long period of time prior to 
PETROLOGY OF PARICUTIN VOLCANO 345 

:FmuRE 107.-Schematic cross section of Paricutin volcano and its supposed magma cupola. Arrows 

represent character of slow thermal convection. Area within dotted line represents cross section of 

approximate volume of magma erupted during 1943-52. Depth of top of gabbroic-type rocks assumed 

to be about 12-15 kilometers. 

outbreak. The surge of especially strong activity from July 1951 

until final cessation in March 1952 (Fries and Gutierrez, 1952a and 

1954), WO'tlld be explainable as flushing of the upp.er part of the 

magma column with the escape of pent up reserv·oir gas when the 

magma surface had been forced down to the level of the o1fset outlet. 

An offset magma cupola m.ay also account for the apparent lack of 

-effec.ts of fluctuations of atm.ospheric pressure and tidal force on the 

behavior of the activity. (See History of the Eruption.) Changes in 
346 GEOLOGIC INVESTIGATIONS IN THE PARICUTIN AREA 

atmospheric pressure are so slight compared to the supposed pressure 

in the cupola reservoir that they would have little affect on the rate or 

type of discharge from a cupola outlet located below the upper surface 

of the magma in the cupola. Likewise, such, a. speci&l arrangement 

would be less affected by daily fluctuations in tidal force tlian, for 

instance, a laccolithic magnia· l:>ody. 

All this does not necessarily suppose that there would have been a 

strong concentration of solid or partially melted xenolithic material in 

the upper portion of the cupola. It is only necessary that the volatile 

material be released from the xenoliths or immediate portions of the 

wall rock and rise through the magma without complete resolution in 

the magma before reaching the apex of the cupola. In this connection, 

however, it seems worthwhile to examine the possible behavior and 

movement of the xenolithic material. The specific gravity of the 

xenoliths of granitic composition should be less tha.n that of the magma 

especially after having been heated to the magma temperature and 

perhaps partially inflated. It seems doubtful that even initially there 

would be any tendency for a xenolith to sink' in the magma after its 

detachment from the wall. Rather there would be a consistent tendency to move upward in relation to the magma; and if the rate of 

relative upward movement were greater than the rate of the postulated 

downward convectional current of the peripheral zones of the magma, 

there would result an absolute upward migration of the xenolith and 

a tendency to enrich the magma of the upper portion of the cupola in 

salic rna terial. 

A mechanism such as outlined above, while furnishing adequate heat 

and fitting into the general pattern of the behavior of Paricutin, would 

seem to leave the chemical and petrographic trends of the successively 

erupted lavas as a fortuitous relationship. It implies that the ch~mical 

differences had already existed in the magma· of the cupola, (')nly being 

arranged in space in such a way that the successively withdrawn· 

samples would show the trend from fe'mic towards salic rock that we 

now observe in the chronologie series of lavas at the surface. The 

volume of material actually erupted is such a small fraction of the 

volume of the supposed cupola (see fig. 107), that, had nonhomogeneities existed in the magma, it would seem just as possible that the 

sequence of withdrawal could have furnished a series of lavas trending 

in composition in just the opposite direction, namely, from more salic 

towards more fernie or that the trend of the first few years could be 

reversed later. This, of course, would be no contradiction of the 

general progressive development of more salic magma by the combined 

action of assimilation and crystal fractionation. It only illustrates 

that the sequence in which the successive portions of the magma were 
PETROLOGY. OF PARICUT~ VOLCANO 347 

erupted was not necessarily: the sequence in which they were initially 

formed. . 

The striking randoln scatter of young cinder cones and associated 

lavas over the region has been noted and ilnplies that most of the 

eruptions have been short.:.lived and that, once interrupted, have 

seldom been renewed froni the same vent. In contrast, . the old 

volcanic pile of Cerros de .Tandtaro must have been built up by 

eruptions from closely spaced vents, some of which were repeatedly 

active. The transition Jnay be represented by the smaller volcanic 

piles of the Cerros de San Marcos, Aguila, Angahuan and Los Hornos 

(see Williams, 1950, pl. 8), finally to the scattered short eruptions 

represented by the young cinder cones. Whether this type of eruption, 

which has continued intermittently to the present, can be regarded as 

a final and decadent phase of the grand cycle of eruptivity from the 

Michoacan magmas is, of course, a matter of conjecture. 

In conclusion, it is of ·interest to speculate on the possible future 

activity of Paricutin, admitting the risk involved in predicting the 

behavior of any volcano. On the basis of the inferred· behavoir of 

the other young volcanoes of the area, it would seem improbable that 

significant renewal of activity would take place from the Paricutin 

vent. If the presumption of an offset outlet from the Paricutin magma 

cupola is well founded, renewal of activity at the Paricutin vent would 

seem even more improbable, although this does not rule out the possibility of future outbreaks in the area above the cupola apex . 

. LITERATURE CITED 

Bowen, N. L., 1928, The evolution of the igneous rocks: Princeton, p. 1-334. 

Chayes, F., 1949, A simple point counter for thin-section analysis: Am. Min., 

v. 34, p. 1-11. 

Daly, R. A., 1914, Igneous rocks and their origin: New York, p. 1-563. 

--1933, Igneous rocks and the depths of the earth: New York, p. 1-598. 

Fosha.g, W. H., and Gonzalez R., J., 1954(inprepamtion), U.S. Geol. Survey Bu11,965-:-D. 

Fries, C., Jr., 1953, Volumes and weights of pyroclastic material, lava and water 

erupted by Parfcutin Volcano, Michoacan, Mexico: Am. Geophys. Union 

Trans., v. 34, p. 603-616. 

Fries, C., Jr., and Gutierrez, C., 1950a, Activity of Paricutin Volcano from 

August 1, H~48, to June 30, 1949: Am. Geophys. Union Trans. v. 31, p. 

406-418. 

--1950b, Activity of Paricutin Volcano from July 1 to December 31, 1949: 

Am. Geophys. Union Trans., v. 31, p. 732-740. 

--1951a, Activity of Parfcutin Volcano from January 1 to June 30, 1950: 

Am. Geophys. Union Trans., v. 32, p. 212-221. 

--1951b, Activity of Paricutin Volcano from July 1 to December 31, 1950: 

Am. Geophys. Union Trans., v. 32, p. 572-581. 

--1952a, Activity of Paricutin Volcano from January 1 to June 30, 1951: 

Am. Ge~phys.·Union Trans.; v. 33; p. 91-100. 
348 GEOLOGIC . INVESTIGATIONS IN THE PARICUTIN AREA 

Fries, C., Jr., and Gutierrez, C., 1952b, Activity of Paricutin Volcano from July 

1 to December 31, 1951: Am. Geophys. Union Trans., v. 33, p. 725-733. 

--- 1954, Activity of Paricutin Volcano during 1952: Am. Geophys. Union 

Trans. v. 35. 

Gonzalez R., J., and Foshag, W. H., 1947, The birth of Parfcutin: Smithsonian 

Inst. Ann. Rept. for 1946, Washington, D. C., p. 223-234. 

Goranson, R. W., 1942, Heat capacity; heat of fusion, (in F. Birch, J. F. Schairer, 

and H. C. Spicer: Handbook of Physical Constants): Geol. Soc. America 

Special Paper no. 36, p. 223-242. 

Harker, A., 1900, Igneous rock-series and mixed igneous rocks: Jour. Geology, 

v. 8, p. 389-399. 

--- 1909, The natural history of igneous rocks: New York p. 1-384. 

Hess, H. H., and Phillips, A. H., 1940, Optical properties and chemical composition of magnesian orthopyroxenes: Am. Mineralogist, v. 25, p. 271-285. 

Holmes, A., 1931, The problem of the association of acid and basic rocks in central 

complexes: Geol. Mag., v. 68, p. 241-255. 

Johannsen, A., 1937, A descriptive petrography of the igneous rocks: v. 3, The 

intermediate rocks: Chicago, p. 1-361. 

Kennedy, G. C., 1946, Activity of Paricutin Volcano from April 12 to May 3, 

1946: Am. Geophys. Union Trans., v. 27, p. 41Q-411. 

--- 1947, Ch,arts for .correlation of optical properties with chemical composition of some common rock-forming minerals: Am. Mineralogist, v. 32, p. 

561-573. 

Kohler, A., 1949, Recent results of investigations on the feldspars: Jour. Geology, 

v. 57, p. 592-599. 

Krauskopf, K. B., 1948a, Mechanism of eruption at Paricutin Volcano, Mexico: 

Geol. Soc. America Bull., v. 59, p. 711-731. 

---1948b, Lava movement at Paricutin Volcano, Mexico: Geol. Soc. America 

Bull., v. 59, p. 1267-1284. 

Krauskopf, K. B., and Williams, H., 1946, The activity of ~aricutin Volcano during 

its third year: Am. Geophys. Union Trans., v. 27, p. 406-410. 

Kuno, H., 1950, Petrology of Hakone Volcano and the adjacent areas, Japan: 

Geol. Soc. America Bull., v. 61, p. 957-1019. 

Larsen, E. S., Jr., 1938, Some new variation diagrams for groups of igneous rocks: 

Jour. Geology, v. 46, p. 505-520. 

Larsen, E. S., Jr., and Switzer, G., 1939, An obsidian-like rock formed from the 

melting of a granodiorite: Am. Jour. Sci., v. 237, p. 562-568. 

Larsson, W., 1941, Petrology of interglacial volcanics from the Andes of northern 

Patagonia: Upsala Univ. Geol. Inst. Bull., v. 28, p. 191-405. 

Milton, C., 1945, Notes on volcanic rocks from Paricutin, Mexico: Am. Geophys. 

Union Trans., v. 25, p. 618-621. 

Morey, G. W., and Bowen, N. L., 1922, The melting of potash feldspar: Am. Jour. 

Sci., 5th ser., v. 4, p. 1-21. 

Niggli, P., and Beger, P. J., 1923, Gesteins- und Mineralprovinzen, I: Berlin, p. 

1-602. 

Ord6ilez, Ezequiel, 1947, El Volcan de Paricutin: Editorial Fantasia, Mixcoac, 

D. F., p. 1-181. 

Peacock, M. A., 1931, Classification of igneous rock series: Jour. Geology, v. 39, 

p. 54-67. 

Poldervaart, A., and Hess, H. H., 1951, Pyroxenes in the crystallization of basaltic 

magma: Jour·. Geology, v. 59, p. 472-489. 

Rittmann, A., 1929, Die Zonenmethode: Schweizer. min. pet. Mitt., Band 9, p. 

1--46. 
PETROLOGY. OF PARICUT~ VOLCANO 349 

Rosenfeld, J. L., 1950, Determination of all principal indices of refraction on 

difficultly oriented minerals by direct measurement: Am. Mineralogist, v. 

35, p. 902-905. 

Schmitter, Eduardo, 1945, Estud:ia petrografico de lavas y productos piroclasticos; 

determinaci6n de las formas cristalinas e indices de refracci6n de algunos de los 

sublimados del volcan, in El Parlcutin: Inst. geol. Mexico, Univ. Nac. 

Autonoma, p. 111-131. 

Segerstrom, K., 1950, Erosion studies at Paricutin VolCano, State of Michoacan, 

Mexico: U. S. Geol. Survey .Bull. 965-A, p. 1-164. 

Troger, W. E., 1952, Tabellen zur optischen Bestimmung der gesteinbildenen 

Minerale: Stuttgart, p. 1-147. 

Wilcox, R. E., 1944, Rhyolite-basalt complex on Gardiner River, Yellowstone 

Park, Wyoming: Geol. Soc. America Bull., v. 55, p. 1047-1079. 

--- 1947a, Activity of Parfcutin Volcano from September 18 to November 30, 

1946: Am. Geophys. Union Trans., v. 28, p. 567-572. 

--- 1947b, Activity of Parfcutin Volcano from December 1, 1946, to March 

31, 1947: Am. Geophys. Union Trans., v. 28, p. 725-731. 

--- 1948a, Activity of Paricutin Volcano from April 1 to July 31, 1947: Am. 

Geophys. Union Trans., v. 29, p. 69-74. 

--- 1948b, Activity of Paricutin Volcano from December 1, 1947 to March 31, 

1948: Am Geophys. Union Trans., v. 29, p. 355-360. 

Wilcox, R. E., and Gutierrez, C., 1948, Activity of Paricutin Volcano from April 

1 to July 31, 1948: Am. Geophys. Union Trans., v. 29, p. 877-881. 

Wilcox, R. E., and Shoup 0., S., 1948, Activity of Parfcutin Volcano from August 

1 to November 30, 1947: Am. Geophys. Union Trans., v. 29, p. 74-79. 

Williams, H., 1950, Volcanoes of the Parfcutin region: U. S. Geol. Survey BulL 

965-B, p. 165-279. 

Winchell, A. N., 1951, Elements of optical mineralogy: Part 2, 4th edition, p. 1-551, 

New York. 

Zies, E. G., 1946, Temperature measurements at Parfcutin Volcano: Am. Geophys. 

Union Trans., v. 27, p. 178-180. 

INDEX 

A Page 

Abstract ..••• ____ . __ -----------._---_. __ ----__ 281 

Acidic-basic volcanic complexes, Scotland ••.• _ 341 

Activity, future, of Paricutin.................. 347 

Acknowledgments____________________________ 283 

Ahu~n------------------------·-· 285,287,295, fig. 96 

Airborne magnetic survey----------------·-·· 344 

Alkali-lime index •.••.. -----·-------------·-·· 315 

Analyses, data on specimens. 294-295, 296, tables 2, 3 

method of plotting........................ 314 

lavas .•••••.•••••••• ------------· 29()-306, table 2 

See also Lava specimens and Xenolith 

specimens. xenoliths ________________________ 30&-311, table 3 

Anorthite contf.n:ts in central portions of plagioclase microphenocrysts ••••• _ table 4 

Area covered by lavas of Parlcutin, end of 

1944 •••••.•• ---------------------- fig. 96 

end of 1952.. .. ·-------------------------- fig. 97 Assimilation ________________ 282,319,326,327,341-342 

Atmospheric pressure •. -------------------•· 289,319 

B 

Basaltic andesite.-·-·------------···------___ 290 

Birth of Paricutin •••.• -------------------···· 282 

Boninite .•••••••••• __ •• --------••••• •••••••••• 322 

Brazo del Viento, Patagonia, volcanic rocks in. 322 

Bulk aualysis'Uf lava_ ______________________ table 2 

silica-variation diagrams of _____________ :tlg.l04 

c 

Calculated groundmass compositions ________ table 2 

Cerro de Cutzato·------------------------·--- 322 

Cerros de JabalL............................. 322 

Cerros de Tancftaro ••• -------------------··-· 322 

Chadbourn, Erma, analyst................... 313 

Chemical analyses ••••• --------------------- table 2 

recalculation for plotting.................. 314 

Chemical and petrographic trends, origin..... 281, 

319-347 

Chemical compositions, lavas _______________ table 2 

minerals, from optical characters.......... 29<>-

291,335 

Chemical relationships.-------------------- 326-339 

Cinde:r-0011,es, random scatter .of............... 347 

clliiop"yroxene __________________________ 304,323,324 

Compositions, calculated for groundmasses... 315 

mineral, inferred from optical characters.. 290-

219,335 

Cones, cinder, random scatter of______________ 347 

Page 

Constituents of lava, individuaL ••••••••••• 300-306 

clinopyroxene............................. 304 

glass...................................... 306 

olivine .•••••••• --------·.--------------- 302-304 

opaque and accessory minerals •••••••••• 305-306 

orthopyroxene ••••• -----------~--------- 304-305 

plagioclase •••••••••••••••••••••••••••••• 300-302 

groundmass constituent.............. 302 

megaphenocrysts ________ 300, 302, figs. 98, 99 

Country rock, assimilation ••••••••• 282,319,326,327 

Crystallization, fractional ••••••••••• 282,319, 326,327 

rates·----------------------------------- 324,343 

Cupola, development. •••.••••••••• 341--346, fig. 107 

gas pressure in............................ 344 

offset position of •••••••••••••••• 343,345, fig. 107 

schematic cross section of................. 107 

tapping of.----------------------------- 343-344 

Cutzato, Cerro de............................ 322 

D 

Daly, hypothesis of cupola development..... 341 

Divergences of test trends ••••••• 322, 336-339, table 9 

E 

Engleman, Edythe, analyst.................. 313 

Eruptions, history·········----------------- 284-289 

pyroclastic .••••••••••••• _____ ------------- 288 

affect of atmospheric pressure on •••• 289, 345 

samples of analyzed ••••••••••••• 290-313, table 2 

strength •••••••••• _......................... 288 

total weight of.mateda.l.of •••• ------------- 289 

Extinction angles, plagioclase ••••••••••• 291, table 4 

· clinopyroxene............................. 304 

Ferric iron, recalculated as ferrous oxide...... 314 

First lava :flow................................ 284 

Fractional crystallization ••••••••••• 282,319,326,327 

Future activity of Parlcutin.................. 347 

G 

Gas pressure in cupola........................ 344 

. (}lass •••••••••••••••• _----••••• .:~3Y.f1,~11, 325-326 

Granodiorite and .xenoliths, photomicrographs 

of·-------------------------------- pl.11 

Granodiorite specimen........................ 311 

Graphic tests using silica-variation diagram... 328-

339, figs. 104-106 

351 
352 INDEX 

Page 

Groundmass, calculated composition_________ 315, 

fig. 100, table 2 

olivine _____ ---_------------------_________ 303 orthopyroxene __________________________ 305,326 

plagioclase _______________ ----- _____ ------- 302 

volume percentage of. __________________ table 1 

Groundmass minerals in ejecta _____________ fig. 99 

H 

Hakone volcano, Japan_______________________ 326 

Harker variation diagram_____________________ 327 

Heat, loss of by magma to country rock______ 342 

required for assimilation.___ _ _ _ _ _ _ _ _ __ 340-343 

History of eruption _________________ ----- 284-289 

Holmes, A, hypothesis of cupola development. 341-

342 

Huanliracua, Mes,a de________________________ 322 

Hyman, Harry H., analyst___________________ 313 

I 

Intra telluric crystals, compositions __ .________ 335 

Iron, ferric, recalculated as ferrous oxide _____ • 314 

J 

Jabali, Cerros de ______________ ·---------------- 322 

Jorullo volcano, Mexico----------------------- 322 

K 

Kerr, James, analyst. __ ---------------------- 313 

L 

Larsen variation diagram ___________ ---------- 328 

Lava, analyses or_ ________ ._ 29o-306, 32i-328, table 2 

composition .• 282.313-318, figs. 100. 101, table 2 

crystallization _____________________ ------_ 325 

phenocrysts and xenoliths, silica variation 

diagram oL___________________ fig. 103 

silica-variation diagram of _______________ fig. 101 

specific gravity------------------------- table 8 

Lavas and pumice, photomicrographs _______ pl. 10 

Lavas and xenoliths, transition zones between_ 312-

313 Lava eruption ______________________________ 284-·288 

Lava fiow, first_______________________________ 284 

Parrcutin, area covered to end of 1944 ___ fig. 96. 

area covered to end of 19fl2 _________ fig. 97 

San Juan _______________________________ 285,293 

Lava specimens, representative ____________ 292-306, 

table 2, pl. 10 

analysis 1.-------------------------------- 292 

analysis 4---------------------- 293; pl. 10, fig. 2 

analysis 12-------------------.• 297; pl. 10, fig. 3 analysis 13 _____________________ 298; pl. 10, fig. 6 

analysis 15·------------------------------- 298 

analysis 16-------------------------------- 298 

analysis 18-------------------------------- 299 

analysis 19--------------------- 299; pl. 10, fig. 4 

analysis 22-------------------· _ 300; pl. 10, fig. 5 

erupted March 1943 ____________ 293; pl. 10, fig. 1 

Lava vents, new ___________________________ 284-289 

See also Vents. 

Litera'l;l]re cited_______________________________ 347 

Loss of heat by magma_______________________ 342 

M PageMagma, cooling by cupola tapping_---------- 343 

loss of heat by---------------------------- 342 

loss of olivine crystal stability in__________ 322· 

Magma cupola, cross section ________________ fig. 107 

Magnetic survey, airborne ______________ ------ 344 

Megaphenocrysts ______________ 321-323, figs. 98, 102· 

clinopyro)rene ____________________ ---- __ __ _ 304 

occurrence .. --------------------------- fig. 102" olivine ______________________ 302-304,322, table 5· 

orthopyroxene reaction rims on. ____ . _ 323' 

orthopyroxene _____ ----___________________ 304 

clinopyroxene flanking plates on ___ 304-305, 

325. pl. 10, fig. 6· 

plagioclase _______________ 300,302,323, figs. 98,99 

scarcity of mega phenocrysts_------- 300, 322· 

relationships __________________________ ·--- 323 

volume percentages _________________ 291, table 1 

Melting of xenoliths ________________________ 306-313· 

Mesa de Huanaracua_________________________ 322· 

Mesa de los Hornitos_________________________ 285 

Mesa de Zirim6ndiro_________________________ 315· 

Mesostases, glassy, refractive indices oL ____ table 7 

Microlites.-------------------------- 325-326, fig. 99 

Microphenocrysts. ------------- 323-325, figs. 99, 102 

clinopyroxene _____________________ ------ 323, 324 

occurrence.----------------------------- ·fig .. 102· 

olivine ___________________________ ------- 323, 324 

in groundmass________________________ 303~ 

orthopyroxene ______________________ 305, 323-325· 

plagioclase ______________________________ 323,324 

anorthite contents in central portions of _______________________________ table 4 

relationships ______ --- __ ------ _______ -- __ 323--325· 

Milton, Charles, analyst ------------------- 292,313: 

Minerals, determinative methods ___________ 290-291 paragenesis ____________ .:.________________ 320-321 

Modal analyses _________________________ 291, table 1 

Murata, K. J., spectrographer________________ 314 

N 

Neo-Volcanic zone of Mexico_________________ 315 

Niggli variation diagram ____________________ 327-328 

Normative minerals _________________________ table 2" 

Northeallt lava vent..----------~----------- 288, 297' 

0 

Obsidian xenolith __________ ------------------- 310· Olivine, lava constituent_ __________________ 302--304 

instability------------·------------------- 322' 

Olivine megaphenocrysts _______ 302-304,322, table 5 

orthopyroxene reaction rims on _______ .• 303, 3z:t 

Olivine microphenocrysts _______________ 303,323,324 

in groundmass____________________________ 303 

Opaque and accessory minerals _____________ 305--306-

0ptic angles of orthopyroxene _______________ table 6· 

Optical data, applying to curves ____________ 290-291 

Orthopyroxene in groundmasses ____________ 305, 326· 

megaphenocrysts_________________________ 304 

optic angles ..... --------------------- 290, table 6 

Orthopyroxene reaction rims on olivine 

megaphenocrysts _______________ 303,323: 
INDEX 353 

p Page 

Faricutin lava flows, area covered by, to end 

of 1944.·------------------------- fig, 96 area covered by, to end of 1952 ___________ fig. 97 

Parfcutin volcano, birth and location. 282, fig. 95, 97 

Partcutin region lavas __________________ 326, fig. 101 

Patagonia, volcanic rocks_____________________ 322 

Paragenesis of minerals _____________________ 32o-321 

Peck, Lee 0., analyses supervised by_________ 313 

Petrochemistry ________ ------ ________ ------_ 313-319 

Petrographic and chemical trends, origin •• _ 319-347 

Petrography---------------·-·------------·- 289-313 

determinative methods _________________ 290-291 

progression irr character of lavas •. ------ 282,289 of lavas _________________________ 292-293,297-306 

of xenoliths----------------------------- 806-313 

Phenocrysts, lavas, and xenoliths, silicavariation diagram of. ___________ fig. 103 

Phenocrysts, compositions _________________ 300-305, 

335, fig. 103, tables 4-6 

Photomicrographs of lavas and pumice _______ pl. 10 

Photomicrographs of xenoliths and granodi- orite _____________________________ pl. 11 

Plagioclase, "high temperature"------- 291, tahle 4 

lava constituent. _______________________ 30o-302 

megaphenocrysts ______ 300, 302, 323, 1!igs. 98, 99 

microphenocrysts _______________ -------- 323,324 

anorthite contents' in central portions of. table 4 

scarcity _______ --------------------------- 322 

Pressure, atmospheric________________________ 345 

Progression of petrographic character_________ 289 

Puertecito _______________________________ --- 285, 286 

Pumice and lavas, photomicrographs________ pl. 10 

Pyroclastic eruptions.------------------------ 288 

effect of atmospheric pressure on__________ 289 

R 

Reaction rims, orthopyroxene, on olivine 

megaphenocrysts _______________ 303,323 

Refractive indices of glassy mesostases _______ table 7 

Refractive indices of olivine megaphenocrysts ___________________________ table 5 

Rittmann procedure__________________________ 291 

Roofrock, assimilation of by magma__________ 342 

s 

San Juan .flow------------------------------ 285, 293 

Sanukite. ___ -------------------------- _ ------ 322 

Saplcbn ___________ ------------- 284,285,2186,287, 288 

Scotland, acidic-basic complexes______________ 341 

Silica-variation diagram______________________ 327 

addition and subtraction of materials from 

bulk compositions _____________ 33o-332, 

:figs. 105, 106 

results of. •• 332--339, table 9, figs. 105, 106 

comparing bulk compositions by adding or 

subtracting olivine, plagioclase, 

and xenolithic materiaL _____ figs. 104, 

105,106 

lavas of Paricutin and nearby volcanoes.. 3 22 

• fig 101 

lavas, phenocrysts, and xenoliths of PartPage 

Southwestern lava vent_______________________ 298 

Specific gravity of lavas ________________ 315, table 8 

Specimens analyzed, data on ____________ 294-295, 296 

See al.,o Lava specimens, 

Spectrographic analyses______________________ 314 

Stoping, magmatic ••• ---------··------------- 341 

SuperheaL.--------------------------·- 282,304,342 

T 

Taqui. ---·----------------- ___ ---------------- 285 

Tancitaro, Oerros de__________________________ 322 

Tarrant, Lucile, analyst______________________ 313 

Temperature, lava____________________________ 324 

Tests, graphic, on silica-variation diagram____ 328-

339, figs. 104-106, table 9 

Thermal requirements---------------------- 34o-347 

Tidal force·--------------------------·------ 289,345 

Time-variations in lava compositions______ 281-282, 

289, 314, 319. :fig, 100 

Transition zones between lavas and xenoliths. 311-

313 

specimen of..____________________ pl. 11, fig. 5 

Tzintzungo volcano, xenoliths of._____________ 311 

v 

Variation diagram_______ 327-328, figs. 101, 103-106 

Vents, lava, new ____________________________ 284-289 

Ahiian _____________________________ 285,287,298 

Me<>a de los Hornitos_____________________ 285 

northeast_ ______________________________ 288,297 

Puertecito ----------------------------- 285,286 

Sapichu •• ------------------ 284,285,286,287,288 

southwestern •.. _--·--------------------·- 298 

TaquL ____________________ -------- _ ------ 285 

Vesicles, volume percentage of_________ 291, table 1 

Volume of megaphenocrysts in ejecta_________ 291, 

table 1, fig. 98 

w 

Water content in xenoliths___________________ 344 

Weight of material erupted, total_____________ 289 

X 

Xenolith specimens representative _________ 306-312; 

pl.ll 

analysis X-L __________ 306-307; pl. 11. fig. 1 

analysis X-2------------------------------ 310 

analysis X-3---------------------------- 307-308 

analysis X-4------------------------------ 308 

obsidian ••. ________ : __ ----_--------------- 310 

phenocrysts and lavas, silica-variation 

diagram of..___________________ fig. 103 

photomicrographs oL---------------·-··· pl. 11 

water content____________________________ 344 

Xenoliths and lavas, transition zones between. 312-

313; pl. 11, fig. 5 

z 

cutin __________________________ fig, 103 Zirimondiro, Mesa de .•• --------------------- 315 

0