Forms of Basaltic 

Rocks in Hawaii

GEOLOGICAL SURVEY BULLETINJ994

STRUCTURES AND FORMS OF BASALTIC ROCKS 

IN HAWAII

By CHESTER K. WENTWORTH and GORDON A. MACDONALD

ABSTRACT

The Hawaiian volcanic mountains are broad shields, built by eruption of many 

thousands of thin lava flows from zones of fracturing known as rift zones. Most 

of the summits are indented by calderas formed by collapse. Smaller collapse 

craters, known as pit craters, lie along some of the rift zones, and the calderas 

have developed at least partly by the coalescence of pit craters. Grabens lie 

along parts of the rift zones, and collapse of the edges of the shields has produced 

high fault scarps facing the ocean.

The vents are marked by spatter ramparts, spatter-and-cinder cones, and lava 

cones on the basaltic volcanoes. On the andesitic volcanoes the cones are mostly 

true cinder cones. Littoral cones, closely resembling the cinder-and-spatter cones 

at vents, are formed where aa flows enter the sea.

Most flows consist of several thin flow units, extruded at slightly different times, 

but during the same period of eruption. Near the vents the lavas are predominantly pahoehoe, but aa becomes more abundant at greater distances. Pahoehoe 

flows are characterized externally by smooth, hummocky, ropy, or festooned 

surfaces, and internally by lava tubes and nearly spherical vesicles. Tumuli, 

pressure ridges, pressure plateaus, and slump scarps form principally on ponded 

pahoehoe flows.

Aa flows are characterized by a central massive phase lying between clinkery 

fragmental top and bottom layers. The fragments are exceedingly irregular and 

spinose. Aa vesicles are less regular in outline than those of pahoehoe. The 

central river feeding an aa flow moves through an open channel through most of 

its course. Spines and accretionary lava balls are common surface features 

on aa flows.

Basaltic cinder-and-spatter cones consist largely of agglutinated spatter and 

driblet, with some cow-dung bombs, and more rarely spherical cored bombs. 

Reticulite and Pele's hair are formed by lava fountains. In the andesitic cones 

the material generally is little welded, and the bombs are generally bipolar, 

fusiform bombs.

Ash and tuff are not abundant in Hawaii. Most of the ash is vitric magmatic 

debris, commonly palagonitized. Accretionary lapilli are common in it in the 

vicinity of Kilauea caldera. Tuff cones with broad craters are formed by explosions where hot rising lava encounters water-saturated rock. Bomb sags 

and vegetation molds are common in the ash in some areas.

1
2 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

Erosion has exposed hundreds of narrow dikes, forming dike complexes along 

the rift zones of several of the older volcanoes. Sills and intrusive necks and 

stocks are less common. Most Hawaiian eruptions are of the fissure type, and 

congelation of the conduit filling produces a dike.

INTRODUCTION

PURPOSE AND SCOPE

The purpose of this paper is to list and describe the physical 

features of basaltic rocks found in Hawaii and to discuss their origins, 

insofar as is practical. The features described are chiefly primary, 

formed in the course of the eruption and emplacement of these 

rocks. Even in works devoted to igneous structures (Balk, 1937) 

there is little systematic reference in geologic literature to the physical 

features of effusive or pyroclastic rocks, possibly owing to the preponderant concern of geologists with tectonic or petrographic 

approaches.

With little exception, the whole mass of the Hawaiian Islands 

consists of basaltic lavas containing small amounts of pyroclastic 

rocks of the same chemical composition. This paper is a synopsis of 

the structures and forms of such rocks, based on extensive field studies 

on all the major islands. The mode of treatment is systematic and 

only such limited reference to areal or historical findings is made as 

may be required for clarity. Areal and petrographic studies have been 

reported elsewhere, as listed in the literature cited. For additional 

references see the bibliography for the island of Hawaii (Macdonald, 

1947).

TERMINOLOGY

Many of the features described are objects of common observation 

in volcanic regions and the names have been taken from the common 

usage of these areas. In some instances it seems best to follow usage 

long established in Hawaii, or most applicable to the local features. 

Terms such as structure, texture, and formation, are used in the 

general, rather than the strict stratigraphic or other geologic sense. 

We have not attempted to follow a strict pattern but have adopted 

and defined the usage that offers the clearest understanding.

ACKNOWLEDGMENTS

Geologists and other visitors have noted many of the features 

described and have often recognized, their significance and origin. In 

other instances, like early workers in other fields, they have offered 

interpretations that have been shown by a better knowledge of the 

usual or normal volcanic process in Hawaii to be erroneous or exaggerated. Often the early travelers have emphasized the features
VISCOSITY AND EXPLOSIVENESS OF ERUPTING MAGMA 3

that were novel to the visitor, but have not treated them systematically 

or described them in the terms we now use.

James D. Dana visited Hawaii hi 1840 as geologist of the U. S. 

Exploring Expedition and observed many volcanic features, that he 

vividly describes in papers and books on Hawaii (Dana, 1849, 1887, 

1890), and in the successive editions of his Manual of Geology, 1863, 

1875, 1880, 1895. Brigham's visit in 1868 also led to publication of 

his extensive observations on volcanic features (Brigham, 1868, 

1887, 1909). Both Dana and Brigham came into contact with many 

lay observers whose observations covered the period from 1820 to 

1900, and whose contributions were reported by Dana or Brigham 

or through their encouragement. Notable among these was the 

Reverend Titus Coan, who lived on the island of Hawaii from 1835 

to 1882, and was the chief observer of a wealth of volcanic episodes 

(Coan, 1852-81).

In 1884, Dutton spent several months in Hawaii studying the volcanic terrane preparatory to mapping the Cascade Range of the 

Pacific Northwest for the United States Geological Survey. Worldwide use by geologists of the terms pahoehoe and aa dates from Dutton's (1884) report. He made many other valuable observations at 

that time. W. L. Green, Sereno Bishop, C. H. Hitchcock, W. Lindgren, and W. H. Dall published important papers from 1887 to 1903.

Since 1900, studies of the geology of Hawaii, chiefly in relation to 

water supply, have been by the United States Geological Survey 

with the collaboration of the Territory of Hawaii, by the Hawaiian 

Sugar Planters' Association, by other agencies of the sugar and pineapple industries, and by the Honolulu Board of Water Supply. 

Other studies have been made under auspices of the Bishop Museum, 

the Hawaiian Volcano Observatory, and the University of Hawaii. 

The data presented in this paper, whiJe based more particularly on 

work done for the Geological Survey and the Board of Water Supply, 

has benefited largely by access to the results of many years of discussion and exchange of observations between all the geologists of 

Hawaii. We are. especially indebted to H. A. Powers and R. H. 

Finch, whose many years in Hawaii have made their criticism of this 

paper particularly helpful. J. Y. Nitta of the Territorial Division of 

Hydrography prepared the drawings for this report.

FACTORS INFLUENCING VISCOSITY AND EXPLOSIVENESS 

OF ERUPTING MAGMA

Hawaiian lavas range in composition from ultrabasic nepheline 

basalts and picrite-basalts, to trachyte verging toward rhyolite 

(Macdonald, 1949b). This paper deals primarily, however, with 

those structures occurring in rocks of a general basaltic nature. The
trachytes and some of the oligoclase andesites, being more viscous and 

richer in gas than the basaltic lavas, build large cinder cones, steepsided domes (tholoids), and thick flows similar to those formed by 

rhyolitic, dacitic, and sometimes by andesitic lavas in the continental 

zones of volcanism (Stearns and Macdonald, 1942, p. 173-178; 1946, 

p. 161, 180). These forms are not characteristic of Hawaiian 

volcanism.

The dominant lavas of Hawaii, from the eruption of which result 

the structures typical of Hawaiian volcanoes, are the olivine basalts 

and basalts, and the closely related picrite-basalts and basaltic andesites. Generally, the forms developed are those due to great fluidity 

 in the erupting lava. Generally speaking, the features influencing 

fluidity of lava flows are chemical composition, gas content, and 

temperature.

The silica content of the common Hawaiian lavas ranges from 

about 42 percent in the picrite-basalts containing abundant phenocrysts of augite, to about 51 percent in the andesine andesites. In 

the melilite-nepheline basalts the silica content drops as low as 36 

percent; in the common olivine basalts it averages about 48 percent. 

Typical chemical compositions of the common Hawaiian lavas are 

shown in the accompanying variation diagram (fig. 1).

The degree of explosiveness of the erupting magma does not correlate directly with the increase of silica content or with the change in 

proportion of any other oxide. Rather, there is a general increase in 

both directions from a middle point at olivine basalt. In other 

words, assuming that the parent magma is the olivine basalt, the 

explosiveness of the erupting magma increases with the degree to 

which differentiation has altered it from the parent magma. This is 

true among the augite-rich picrite-basalts as well as among the 

nepheline basalts, although in both there are many exceptions in 

which the erupting lava is not appreciably more explosive than the 

ordinary olivine basalt. An example of comparatively great explosiveness in erupting nepheline basalts is the large cinder cone' and 

widespread ash showers produced by the Tantalus eruption on Oahu. 

Similar examples among the augite-rich picrite-basalts are large 

cinder cones on Haleakala and Mauna Kea. The picrite-basalts of 

oceanite type which lack abundant phenocrysts of augite do not 

share in the greater explosiveness, but closely resemble the ordinary 

olivine basalts in their mode of eruption. It is probable that the 

greater explosiveness of the erupting nepheline basalt and augiterich picrite-basalt magma, as compared with the olivine basalt, is 

the result of a greater gas content, possibly coupled with somewhat 

lower temperature.
VISCOSITY AND EXPLOSIVENESS OF ERUPTING MAGMA

35

30

(D 

LJ 

Q

X

o

(T 

LJ

I 

ho

25

20

15

LJ 

U

cr

LJ 10 

Q.

I I I I I I T

O- C3 O 0"

w ." to O ">

O V, D O,D

o)._-- a)-- a>--

I i I l _

w

0) o

c 

o

O. 

O>

O< OQO

o 

o

H -{

35 40 45 50 55

PERCENT SiCU

60 65

FIGURE 1. Diagram showing composition of Hawaiian lavas.

Viscosity of the erupting magma generally increases from olivine 

basalt toward the more sodic members of the series. As already 

stated, thick lava flows and steep-sided domes are common among 

the andesites and trachytes. The nepheline basalts and augite-rich 

picrite-basalts, although they are commonly more explosive than the 

olivine basalt, do not show a correspondingly greater viscosity. That 

the greater explosiveness of those magmas was the result probably of
6 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII .

greater gas content, not of greater viscosity, is shown by the numerous 

pancake or cow-dung bombs, and other ejecta expelled in a fluid state, 

that lie on the flanks of the cinder cones. In contrast, the bombs on 

the andesite cones are more generally well formed spindle bombs and 

other Strombolian-type ejecta, suggesting a greater viscosity of the 

erupting lava.

The actual gas content of erupting Hawaiian lavas has never been 

measured, nor is it likely that it can be measured directly in the near 

future. The volume percentage of gas is undoubtedly large, although 

the weight percentage is probably small. Explosive eruptions are 

rare at Hawaiian volcanoes, and in typical eruptions of olivine basalt, 

the gas, which is present, is liberated quietly. The only violent 

explosions during historic times were those of 1924 at Kilauea volcano 

which were of phreatic, not magmatic origin (Jaggar and Finch, 1924; 

Stearns, 1925). Similar phreatic explosions have occurred at other 

times on Kilauea volcano, on Mauna Loa, and probably on Hualalai 

volcano, but they are comparatively uncommon and not typical of 

Hawaiian volcanism. As pointed out, many eruptions of the more 

highly differentiated lavas, such as the augite-rich picrite-basalts and 

andesites, were moderately explosive, building large cinder cones, but 

the largest of those cones are small compared with many of the cinder 

cones of continental volcanoes. The common basalts of Hawaii erupt 

quietly, building only low spatter ramparts and diminutive spatterand-cinder cones.

The effects of falling temperature are apparent in any basaltic flow 

in its gradual passage from thin fluidity through increasing viscosity 

to eventual immobility. It is evident, also, that as fluidity increases 

with gas content, the loss of gases through flow, agitation and reduction of temperature would in addition to temperature decline alone 

result in increased viscosity. Temperature alone is not the governing 

factor in the behavior of lava flows. Although the magmatic temperatures are about the same, the lava of Paricutin forms thick flows 

and a large cinder cone, whereas, the lava of Mauna Loa and Kilauea 

volcanoes form thin flows and low cinder-and-spatter cones (Zies, 

1946, p. 178-189: Jaggar, 1917a, p. 397-405).. The temperature of 

erupting basaltic lava at Hawaiian vents generally ranges from 900° 

to 1100° C.

The fluid basaltic lavas of the Hawaiian Islands build broad, gently 

sloping cones of flat domical shape. These are commonly referred to 

as domes, and more specifically by Daly (1933) as exogenous lavadomes. However, the term dome is commonly used for other types
MAJOR VOLCANIC STRUCTURES 7

of geological structures. Volcanic dome is restricted by Williams 

(1932) and others to the steep-sided accumulations of viscous lava at 

vents on volcanoes erupting more viscous lava. Therefore, it seems 

preferable to use the term shield volcano (Schildvulkan of German 

writers) (Rlttmann, 1936) for the Hawaiian and Icelandic types of 

structures.

Typically, Hawaiian shield volcanoes are broad and flat in profile. 

The general form and dimensions of the five major volcanoes of the 

island of Hawaii are shown in the accompanying table (Wentworth, 

1938, p. 13). The lengths and widths are measured at sea level. 

The actual dimensions of the base of the mountains, at the level of 

the surrounding ocean floor, are much greater. Thus the area covered 

by Mauna Loa, at the minus 15,000-foot level, is probably of the 

order of 5,000 square miles compared with 2,000 square miles at sea 

level. Some conception of the enormity of the huge mountain mass 

of Mauna Loa, which rises over 30,000 feet above the surrounding 

ocean floor, may be gained from a comparison of its volume, of the 

order of 10,000 cubic miles above the minus 15,000 foot-level, with 

that of 80 cubic miles above the 4,500 foot-level for Mount Shasta 

in California (Williams, 1934).

Dimensions and form of the shield volcanoes of the island of Hawaii

Name

Kohala... ......

Mauna Loa.-.-

Kilauea.... ....

General outline

Elliptical.. .....

...--do...... ........

Three-pointed... ....

Sigmoid belt. .......

Slope of shield surface

Maximum 

(degrees)

9 

21 

11 

15 

5

Minimum 

(degrees)

4 

5 

3 

3 

1

Length 

(miles)

22 

51 

75 

24 

51

Width 

(miles)

15 

25 

64 

20 

14

Area 

(square 

miles)

234 

919 

2,035 

290 

552

Summit 

altitude 

(feet 

above 

sea level)

5,505 

13, 784 

13, 680 

8,251 

4,090

The slopes of the basaltic shields generally range from about 4° to 

10°. Locally, steeper slopes may result from the mantling of fault 

scarps or buried cinder-and-spatter cones. Thus, the steep western 

slope of Mauna Loa and the southern slope of Haleakala (East Maui) 

volcano may have resulted from the burial of a series of normal faults 

along which the seaward blocks have been down thrown. Such faults 

have been found on the flanks of both volcanoes (Stearns and Macdonald, 1942, p. 73; 1946, p. 34-42), and others may be entirely 

hidden by later flows. Elsewhere, steeper slopes are partly or entirely 

the result of a capping of andesitic or trachytic lavas erupted in a 

more viscous condition; or the presence of more abundant pyroclastic 

material, probably also resulting from an increase in viscosity or gas 

content, or both, accompanying some differentiation of the magma.
8 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

Owing to their characteristic mode of construction, by flows erupted 

from an elongate zone of fissures, the ideal form in plan of the Hawaiian 

shields is a more or less elongate ellipse. Departures from this ideal 

form arise principally from interference with the growth of the structure by neighboring mountains. Thus the three-lobed plan of Mauna 

Loa was caused by interference with the outward growth of the 

mountain by Mauna Kea on the north, Hualalai volcano on the 

northwest, and Kilauea volcano on the southeast. However some 

shields, such as West Maui, are nearly circular in plan, because of an 

approach to radial arrangement of the eruptive fissures, or because of 

more effective domination by the central vent (fig. 2).

oNiihau

15 30 Miles

FIGURE 2. Outlines of Hawaiian shields. The inner contour for each is 1,000 feet 

below supposed original summit; the second is 3,000 feet below.

The major shield volcanoes of the Hawaiian Islands occur in definite 

groups. The five major volcanoes that comprise the island of Hawaii 

are united above sea level, but separated from the Maui group of 

volcanoes to the north by water 8,000 to 10,000 feet deep. The six 

volcanoes of the Maui group form the islands of Maui, Molokai, Lanai, 

and Kahoolawe, separated by channels 250 to 1,300 feet deep, but 

generally less than 900 feet deep. The Maui group is separated from 

the two volcanoes of the island of Oahu by water 2,300 feet deep and 

the Oahu group from the island of Kauai by water 9,000 to 10,000 feet
MAJOR VOLCANIC STRUCTURES 9

deep. Water more than 2,000 feet deep separates Kauai from the Niihau 

mass, and a channel more than 5,000 feet deep separates the Niihau 

mass from the volcanic mass carrying the tuff cone of Kaula Island 

on its summit. Although separated from each other by moderately 

deep water, the islands of Kaula, Niihau, and Kauai form a fairly 

definite group of three major volcanoes.

Within each group the volcanoes have merged, partly by overlapping, and partly by interfingering of their lavas at the periphery 

of the shields. Although the order of extinction of the several volcanoes within a group can be demonstrated with more or less accuracy, 

there is no evidence of the order of initiation of activity. Within the 

Hawaii group, Mauna Kea, Hualalai volcano, Kilauea volcano, and 

Mauna Loa apparently were active during the same general period. 

The active period of Mauna Kea almost certainly commenced while 

the earlier Kohala was still active. In other groups also, neighboring 

volcanoes were probably active simultaneously during part or most 

of their lives. Volcanic masses such as the island of Hawaii, formed 

by the overlapping and interfingermg of a group of shield volcanoes, 

may appropriately be termed volcanic shield-clusters.

PHYSICAL COMPOSITION OF HAWAIIAN SHIELD VOLCANOES

The Hawaiian volcanoes are composed largely of lava flows. Pyroclastic material comprises certainly less than 5 percent and probably 

less than 1 percent of the mass. Among the basaltic flows aa and 

pahoehoe types are approximately equal in abundance. Pahoehoe 

predominates on the upper parts of the volcanic mountains and near 

the rift zones. Farther downslope aa increases in abundance until it 

predominates. Sections in the walls of the calderas of Mauna Loa 

and Kilauea volcanoes (Macdonald, 1949a, p. 60, 64-65) contain 

respectively over 98 and 92 percent pahoehoe. At Hilina Pali, 9 

miles from the center of Kilauea caldera, the lavas of the Hilina 

volcanic series are 76 percent pahoehoe. Of the lavas exposed hi the 

shaft of the Olaa well, 37 miles east of the caldera of Mauna Loa and 

about 20 miles from the lowest recent vents on the northeast rift 

zone, 71 percent are aa. The lavas exposed in the Paauilo well on the 

northeastern flank of Mauna Kea, 17 miles from its summit, are 

entirely aa.

The sections in figures illustrate well the composition of the Koolau 

shield volcano on Oahu, 4 to 5 miles from the center of the former 

rift zone. Wentworth and Winchell (1947, p. 62) report about 75 

percent aa lava flows in 2,000 feet of core-drilled sections in the Red 

Hill and Honolulu areas 1 or 2 miles from the leeward coast.
10 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

EXPLANATION

No exposures 

Talus and alluvii

Poor exposures

HALAWA VALLEY

300 -

No exposures

WAIMALU VALLEY

100

FIGURE 3. Sections showing proportions of lava-flow types exposed in the walls of 

Halawa and Waimalu valleys, Oahu.
MAJOR VOLCANIC STRUCTURES 11

RIFT ZONES

The Hawaiian shield volcanoes have been built by innumerable 

fissure eruptions. The fissures are generally concentrated within 

certain narrow zones, known as rift zones. The rift zones range from 

a few hundred feet to more than 2 miles in width, and extend down 

the flanks from the summit of the mountain to below sea level. 

Typically, each volcano has two principal rift zones extending outward from the summit of the mountain. Some, such as Haleakala 

volcano, have a third minor rift zone, also extending outward from 

the summit. The two major rift zones generally meet at the summit 

with an angle of 130° to 180°. They may represent a single continuous straight or gently curved zone of fissuring. The third minor 

rift zone generally approximately bisects the external angle between 

the major rifts. Localization of most eruptions along the major rift 

zones is the cause of the common elongate shape of the shield 

volcanoes (fig. 4).

At the surface the rift zones are marked by lines of cinder-andspatter cones and spatter ramparts, pit craters, lava cones, and on 

the active volcanoes where erosion has not appreciably modified the 

surface, by many open fissures (fig. 5). The fissures range in width 

from a few inches to about 10 feet. They frequently can be sounded, 

or are visible to a depth of 50 feet, and many probably are open to a 

much greater depth. The great irregularity of their walls and the 

narrowing of the fissures downward make it difficult to lower a sounding weight to any great depth. The most spectacular of these surface fissures, the Great Crack along the southwest rift zone of 

Kilauea volcano, extends continuously for more than 8 miles, with 

a width in places 30 to 40 feet and a visible depth of as much as 40 

to 50 feet. The extreme widths are generally the result of local 

minor graben collapses. Where the walls have not been too much 

modified by collapse, projections on one wall commonly correspond 

with concavities in the opposite wall. The opening of the fissures 

has resulted largely from tension; there is little or no evidence of 

shearing movements along them.

At depth the rift zones are occupied by hundreds of nearly vertical 

dikes, most of which represent the feeders of the surficial eruptions 

(see Dike complex). In places they are so abundant that the intervening rock nearly or entirely disappears, leaving only the intrusive 

dike-rocks. The distension of the volcanic structure represented 

by these dike complexes is enormous and, although difficult to form 

a reliable estimate of its magnitude, it probably amounts in some 

places to several thousand feet.

Although most of the fissures that have given vent to lava flows 

are radial with respect to the mountain summit, a few are concentric.

253279 53 2
12 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

FIGURE 4. Sketch map of island of Hawaii, showing cracks, vents, cones, and margins of the five component

shields.

The concentric fissures are generally associated with a caldera, and 

may be faults bounding it. Lava escaping from a concentric fissure 

0.7 mile southwest of the boundary of Kilauea caldera built a line of 

small spatter cones (Stearns and Clark, 1930, p. 52; Finch, 1942). 

During the 1942 eruption lava issued from concentric fissures at 

several places just northeast of the rim of the North Bay of Mokuaweoweo (Mauna Loa) caldera, as well as from radial fissures in the 

same area (Macdonald, 1945b, p. 2). On Mauna Kea, eruption 

occurred at the surface from radial fissures, but localization of some 

vents appears to be the result of intersection of the surficial radial 

fissures with concentric intrusions (ring-dikes ?) at depth (Macdonald, 

1945a, p. 210-217).
MAJOR VOLCANIC STRUCTURES 13
14 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

The origin of the rift zones is uncertain. However, evidence indicates that subterranean magma pressure preceding and during eruptions somewhat exceeds the hydrostatic pressure of the overlying 

rocks. Both tumescence of the volcanoes preceding and during eruptions, and subsidence after eruptions, are indicated by tilting of the 

ground surface that is easily measurable by means of tiltmeters. 

Differential leveling has shown that the elevation of the mountain 

summit during tumescence probably amounts to several feet. Lava 

fountains issuing from the rifts of Mauna Loa may reach heights of 

several hundred feet above the ground surface, even at the summit 

of the mountain. During the 1949 summit eruption the fountains 

attained heights of at least 800 feet (Macdonald and Orr, 1950, p. 9). 

If the pressure in the magma exceeds the weight of the overlying 

rocks, the upward thrust applied in a relatively small area beneath 

the summit portion of the volcano may be expected to produce an 

opening of the overlying structure in the form of three principal 

radiating fracture zones. Considering the nonhomogeneous structure 

of the mountain, it also is expectable that two of the radiating rifts 

be better developed than the third. That is the pattern actually 

found in the rifts of most Hawaiian volcanoes, and consequently this 

origin of the rifts appears most probable.

Whatever may be the original cause of the rift zones, the opening of 

the fissures, allowing magma to rise to the surface, probably results 

at least partly from the tumescence of the mountain previous to 

eruptions that has been shown to take place at Kilauea (Jaggar, 

1920, p. 201-249; Jaggar and Finch, 1929, p. 38-51). In addition 

there may be some tendency for the cone to split open under the 

influence of gravity, the sides moving downward and outward toward 

the ocean deeps (Williams, 1941, p. 287).

RIFT-ZONE OBABENS

Along some of the rift zones, particularly the southwest rift zone of 

Kilauea volcano, collapse has resulted in broad shallow grabens at the 

surface. There is no reliable evidence of the magnitude of the 

collapse, however. It may be represented in its entirety by the 

existing topographic depressions a few feet deep, or the subsidence may 

have amounted to scores or even hundreds of feet, the trough having 

been filled by related lava flows more or less contemporaneously with 

its formation.

CALDERAS

The summits of both Mauna Loa and Kilauea volcanoes are indented 

by calderas. Similarly, calderas appear to have existed during 

earlier stages of some of the other volcanoes, including the Waianae and 

Koolau volcanoes on Oahu, and those of East Molokai, West Maui,
MAJOR VOLCANIC STRUCTURES 15

Lanai, Kahoolawe, and Kauai. The calderas of Kilauea and Mauna 

Loa are oval in outline. That of Kilauea is 2.7 miles long, 1.9 miles 

wide, and 490 feet deep at the highest point of the boundary cliff. 

Mokuaweoweo caldera, on Manua Loa, is 3.3 miles long (including 

the North Bay, but not the independent pit crater South Pit) and 

1.6 miles wide. The boundary cliff is about 600 feet high at its highest 

part, but its height decreases almost to zero in North Bay (fig. 6).

FIOURE 6. Mokuaweoweo caldera and adjacent pit craters on the summit of Mauna Loa. View from the 

southwest. The large caldera is about 9,000 feet from left to right. (Drawn from photo taken by 

U. S. A. A. F.)

Neither of these figures for depth represents total collapse because of 

the unknown amount of fill to form the present floor in each caldera. 

The boundary cliffs slope at an average angle of 60° to 70°. The 

walls of the calderas comprise a series of step-fault scarps. The 

floors of both calderas slope outward very gently toward their peripheries. In Kilauea caldera, the high part of the floor is the remnant 

of the summit of a broad flat lava cone built up by overflows of 

Halemaumau lava lake and partly destroyed by the great collapse at 

Halemaumau during the 1924 explosions. In Mokuaweoweo caldera, 

the floor inclines outward from the apex of a gently sloping lava cone, 

crowned by a large cinder cone, at the source of the long-continued 

1940 lava flow. .

The calderas of some Hawaiian volcanoes may have been larger 

than those of Mauna Loa and Kilauea. Stearns (1946, p. 81-86) 

believed that of East Molokai to have been 4 miles long and that of
16 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

Kauai to have been some 10 miles across. There is no positive 

evidence that calderas ever existed on Hualalai volcano and Mauna 

Kea, although it is not unlikely that they did.

The caldera of Mauna Loa, and probably that of Kilauea, apparently 

developed at least partly by the gradual coalescence of pit craters 

(Stearns and Macdonald, 1946, p. 29-33). During the century and a 

quarter of written history in Hawaii, the calderas appear to have 

alternated between deepening, by collapse of the central part of the 

caldera, and infilling by lava flows! Williams (1941, p. 246, 286-292) 

places the Hawaiian calderas in a special class, called the Kilauea type. 

He believes that rapid effusion of lava on the flanks of the volcano or 

intrusion of magma as dikes or sills drains the central conduit and 

causes foundering at the central vent. The principal evidence for this 

belief is the observed collapse that accompanied the great lowerings of 

magma level at Halemaumau pit in Kilauea caldera. There is no 

doubt that collapse at Halemaumau may, at least at times, be the 

direct result of lowering of the magma level. However, large as was 

the volume of engulfment at Halemaumau during and following the 

1924 explosions, it was minute compared with the total subsidence 

that formed the great depression of the entire caldera. More comparable in scale are the great collapses that occurred in the Kilauea 

caldera early in the 19th century. In 1840 the collapsed portion 

occupied more than half the area of the inner caldera floor (Stearns 

and Macdonald, 1946, p. 32). Finch (1940, p. 2; 1941, p. 1) estimated 

that in 1823 and 1832 the volume of the depressions formed by collapse 

exceeded 19 and 20 billion cubic feet respectively.

Records are too incomplete to permit more than a presumptive 

correlation of the collapse that took place about 1823 with the flank 

eruption of 1823. The collapses of 1832 and 1840 are known to 

have been closely associated in time with the eruptions of these 

years. However, during the eruption of 1832 the extrusive vents 

were approximately 275 feet above the caldera floor before the collapse 

and probably more than 1,000 feet above the floor after the collapse. 

It therefore appears unlikely that simple drainage by the eruption 

could have caused the collapse. Furthermore, the volumes of lava 

extruded during the 1823, 1832, and 1840 eruptions were only a 

small fraction (0.1 to 6 percent) of the volume of the associated 

collapse. Thus, allowing for considerable error in the estimates of 

volumes, unless the volume of magma intruded subterraneously 

vastly exceeded that extruded, the eruptions were totally inadequate 

to account for the associated collapse by drainage of magma from 

beneath the caldera. That the volume of intruded magma exceeded 

that extruded appears improbable from what is known of the relatively
MAJOR VOLCANIC STRUCTURES 17

small proportion of intrusive material in the exposed internal structure 

of older Hawaiian volcanoes.

A large collapse in the caldera of Mauna Loa during the middle 

part of the 19th century may have been associated with one or more 

of the flank outflows of that volcano, but knowledge is too fragmentary 

to permit any definite correlation. Since that time the history of 

Mauna Loa caldera has been one of gradual infilling. Several large 

flank eruptions, including that of 1950 during which more than 600 

million cubic yards of lava was liberated, have not been accompanied 

or followed by any collapse in the caldera.

Stearns and Macdonald (1946, p. 33), have advanced as an alternative explanation the suggestion that the Hawaiian calderas resulted 

from weakening of the summit area by large-scale stoping and cauldron 

subsidence, as in the calderas of Williams Glen Coe type. The 

sinking may have resulted from the greater density of the surficial 

rocks compared with the underlying magma, and certainly any withdrawal of the underlying magma or reduction in its pressure, for 

whatever cause, would greatly abet the collapse. Williams accepts 

stoping and melting as the primary cause of pit craters, and extends 

it to the early stage of formation of Hawaiian calderas. It appears 

most probable that the calderas of Mauna Loa and Kilauea owe their 

origin to a combination of the two processes.

PIT CRATERS

The term pit crater was proposed by Wilkes (1845, p. 180), and 

defined as "that description of crater of which there is no appearance 

whatever until one is close upon it, and which never throws out 

lava". These features are circular or ellipsoidal pits sunk below the 

gently sloping surface of the volcano and surrounded by no mound 

of accumulated lava. The pit craters are collapse structures that 

have never been filled to overflowing with molten lava. In many 

pit craters, however, lava flows have broken out along the walls, at 

the base or part way up in the talus banks that mantle the lower part 

of the boundary cliffs, and formed ponds of lava in their bottoms.

Many pit craters occur along the rift zones of Mauna Loa and 

Kilauea (fig. 7), and filled pit craters have been found on other 

Hawaiian volcanoes. They range in size from 100 feet or less to 

more than half a mile across. The largest single pit crater is Napau 

Crater, on the east rift zone of Kilauea volcano, which is 0.7 mile 

long and 0.6 mile wide. The neighboring Makaopuhi Crater is 0.9 

mile long, but is in reality a double pit crater. Devil's Throat, on 

the east rift zone of Kilauea, when originally described was only 30 

feet across at the surface, and 250 feet deep. It is remarkable in
18 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII
MAJOR VOLCANIC STRUCTURES 19

having overhanging walls, the width at the bottom being 125 feet 

(Stone, 1926a, p. 50). Since 1926 collapse^has doubled the diameter 

of the opening. The pit craters range in depth from less than 50 

feet to 800 feet. Except where talus banks are present, their walls 

are essentially vertical. Their margins are in general broad curves, 

but in detail they are highly irregular, with numerous angular projections and re-entrants, indicating that the actual detailed course of 

the boundary fractures was controlled by older joints (fig. 8).

FIQUKE 8. Block diagram showing pit craters along the east rift zone of Kilauou volcano. Contour

interval, 50 feet.

Pit craters undoubtedly are the result of collapse, but. the immediate cause and manner of the collapse are conjectural. Wilkes 

(1845, p. 180) thought them to be collapsed lava tubes. Some small 

ones undoubtedly do originate in that way. A large lava tube is 

visible at the bottom of the two small pit craters near Cone Crater 

on the southwest rift of Kilauea, and at the bottom of a very small 

pit crater near the Kulani Prison road on the east slope of Mauna 

Loa. However, the large size of many of the pit craters precludes 

that origin. It is unlikely that a crater such as Makaopuhi, 1 mile 

long and 0.6 mile wide, could have formed by collapse into a lava 

tube (fig. 9).

Stearns and Clark (1930, p. 127) attribute the pit craters to "stoping and fluxing action of the magma surging at a particular place 

along a crack. With later subsidence of the lava, collapse occurs." 

Stone (1926a, p. 54) also suggests that the collapse results from the 

removal of support by withdrawal of underlying magma. The 

suggestions of Stearns and Stone are accepted by Williams (1941, 

p. 228). Stone believes that the collapse occurs by the sinking, not 

of a single block, but of many small blocks "such as occurs when a 

plug is drawn from the bottom of a box of loose sand and a funnel-
20 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

20' I55°00'

FIGURE 9. Sketch map of Kilauea and adjacent rift zones.

shaped depression is formed," but cites no particular evidence to 

support his conclusion. Subsequent dilapidation of the walls has 

produced large banks of talus within the pit craters, but this does not 

prove that the pits were formed by piecemeal collapse. On the 

contrary, there appears to be evidence that many if not all pit craters 

were formed essentially by the subsidence of single cylinders of rock. 

Eruptions occurring within the pit craters, such as those of 1840 at 

Alae Crater, 1868 at Kilauea Iki, and 1922 at Makaopuhi and Napau 

Craters, have broken out high on the walls, as though the magma 

were guided to the surface by cylindrical bounding fractures. Moreover, as Stearns and Clark (1930, p. 128) pointed out, there are all 

gradations from pit craters to grabens. Heake Crater on Kilauea 

volcano is surrounded by a broad area that is subsiding en masse 

along concentric fault planes. In Lua Poholo, on Mauna Loa, a 

large block of the floor has sunk essentially intact, although much 

tilted (Stearns and Macdonald, 1946, pi. 18).
MAJOR VOLCANIC STRUCTURES 21

The shape of Devil's Throat conforms well to that deduced by 

Anderson (1924, p. 11-12) as the shape that should theoretically be 

exhibited by ring fractures resulting from the withdrawal of support 

by an underlying magma body. However, if the overlying roof is 

relatively thin in comparison to the breadth of the magma body, the 

part of the roof above the projectile-shaped ring fractures would be 

unable to support itself. There would then result a roughly cylindrical vertical-walled subsiding block, or piece-meal collapse resulting 

in a projectile-shaped dome of equilibrium. Following subsidence 

of a central block, the walls of the crater would undoubtedly to some 

extent tumble in under the impulse of gravity and earthquakes, 

building at their foot banks of talus which in some craters would 

coalesce at the center to form funnel-shaped depressions.

Pit craters may be enlarged by explosion (Stearns and Clark, 1930, 

p. 128). Phreatic explosion debris was ejected from both Alae and 

Puulena Craters (on the east rift zone of Kilauea volcano) in prehistoric times. However, the volume of such explosive material is 

small compared with that of the pit crater from which it came.

It is concluded that large pit craters are formed by collapse resulting from the withdrawal downward of the surface of underlying 

magma. Probably in some cases they are formed by the sinking en 

masse of a cylinder bounded by ring fractures approximately vertical 

at the surface. In others the collapse may have been piece-meal. 

Small pit craters may be formed in the same way, or by collapse into

lava tubes.

LATERAL FAULTING

On the flanks of several Hawaiian shield volcanoes normal faults 

occur, dipping seaward generally at moderate angles, along which the 

peripheral parts of the shield are slipping downward and seaward. 

The best known of these are the faults of the Hilina system, on the 

southern slope of Kilauea volcano, and those of the Honuapo and 

Kaoiki systems on the southeastern side of Mauna Loa. Similar 

faults have been found on the western flank of Mauna Loa and on 

the eastern flank of West Molokai. Apparently these faults occur 

also on the southern flank of Haleakala volcano, and at the southwest point of the island of Lanai.

The Hilina fault system on the southern flank of Kilauea volcano is 

typical. It consists of a dozen or more normal faults, with subparallel en echelon arrangement. The displacement on each fault 

ranges from a maximum near the center to zero at the ends. The 

principal scarp, at Hilina Pali, reaches a height of 1,500 feet. The 

dip of the fault planes cannot be measured directly, but judging 

from the slope of the scarp it is probably about 40°. The angularity 

of the fault pattern suggests that the position of the faults was prob-
22 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

ably influenced by earlier joint patterns. The fault planes may 

flatten to become parallel with bedding planes at depth.

STRUCTURES AT PRIMARY VENTS

The common structures at vents of Hawaiian lava flows are spatterand-cinder cones, spatter ramparts, and lava cones. True cinder cones 

are rare at the vents of basalt flows, but common at the vents of 

andesite flows. Cinder cones and tuff cones as products of exclusively 

pyroclastic eruptions are discussed in detail in the section on pyroclastic features.

SPATTER RAMPARTS

During the early hours of a typical Hawaiian eruption, fountains of 

very fluid basalt magma erupt from a fissure or fissure-zone that in 

some instances may attain a length of 3 or 4 miles (Schulz, 1943, p. 

741; Macdonald, 1943, p. 245, 256). These fountains build low 

walls of coagulated spatter, known as spatter ramparts, along the 

eruptive fissure. Typically, a wall is built along each side of the 

fissure, although locally copious outwelling of lava may carry away 

the spatter as rapidly as it falls and prevent the building of the wall. 

Locally also, abundant spatter may unite the two parallel walls at 

their tops, forming a bridge across the fissure. The bridges may be 

several feet thick, and it is possible to cross the hot and still fuming 

fissure by means of these bridges within a few hours after the cessation 

of fountaining. The ramparts range in height from a few inches to 15 

or 20 feet and in thickness from 1 to 10 feet. They merge into spatter 

cones. The ramparts are composed of coagulated and welded spatter 

or agglutinate (Tyrrell, 1931, p. 66). The forms of individual ejecta 

are described in the section on types of materials and fragments 

(p. 73).

SPATTER-AND-CINDER CONES

After a few hours of eruption the activity typically becomes restricted to a short stretch of the fissure, about a quarter of a mile in 

length. Lava fountains continue for days or weeks, and a spatterand-cinder cone is built (Macdonald, 1943, p. 247, 256). The great 

bulk of the material comprising the cone is fluid spatter, the glassy 

skins of which adhere to form agglutinate (fig. 10). True cinder and 

Strombolian bombs also may be present and there generally is some 

pumice and Pele's hair. Small driblet flows are common on the cone 

flanks. The resulting cone is commonly elongate and seldom more 

than 50 feet high. The summit of the cone is indented by pits that 

mark the sites of the principal lava fountains. During activity, a lava 

river generally issues at the lower end of the cone, and that end of the
STRUCTURES AT PRIMARY VENTS 23

FIGURE 10. Detail of driblet lava in road-cut outcrop, Chain of Craters road, Hawaii. Width shown is 

about 24 inches. Photo by C. K. Wentworth.

cone remains partly or entirely open, the cinder and spatter falling 

there is carried away on the surface of the lava flow. The sides of the 

cones generally slope at an angle approximating the angle of repose 

of loose debris, that is about 30°, but some cones are much steeper 

because of the large proportion of agglutinate. Small steep-sided 

cones composed very largely of spatter arc common on the rift zones

of Hualalai volcano.

CINDER CONES

True cinder cones are rarely formed during eruptions of olivine 

basalt and basalt in Hawaii, but examples arc known on both Mauna 

Loa and Kilauea volcanoes. They are usually larger and more symmetrical than the spatter-and-cinder cones described above. The 

cone formed in the caldera of Mauna Loa during the 1940 eruption is 

about 250 feet high, 700 to 1,200 feet across at the base, and quite 

symmetrical. In its top is a deep crater, breached by a shallow 

notch toward the north. It is composed largelv of typical cinder 

(fig. 11).
24 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

i 8

! 3

- 9 2 8

 S3

I!II

§ 0

3 £

23 fc

o> 3 O

-0 S 

^ f M

S >->' s s

O ^ (J J3 >. S

^ 2 H<
STRUCTURES AT PRIMARY VENTS 25

True cinder cones are characteristic of the vents of flows of andesite 

and augite-rich picrite-basalt and are abundant on the flanks of 

Mauna Kea, and Haleakala, where they range jto several hundred 

feet in height, and almost a mile across at the base. The cones are 

commonly asymmetrical, built higher on the side which was downwind 

during the eruption, commonly southwest. Some have well preserved 

bowl-shaped craters but many vent cones are breached, usually on 

the downhill side, either through removal of falling ejecta by concurrent lava streams or by subsequent burrowing of a lava flow.

The cones are composed largely of scoria from half an inch to an 

inch across, together with some larger fragments from 8 to 12 inches 

in diameter. Some contain abundant finer lapilli and ash, but others 

do not. More commonly in andesitic cones spindle-shaped bombs are 

found whereas pancake, or cow-dung type bombs are characteristic of 

augite-rich picrite-basalt cinder cones of Hawaii. Where the later 

ejecta are more spattery than the earlier, a layer of agglutinate may 

form around the crater rim. Because it is more resistant than the 

cinder layers it will stand out with the appearance of a ruff on slightly 

eroded cones. In Hawaii these cones commonly bear the native 

name "Pohaku o Hanalei", which means "wreath of stones".

LAVA CONES

Lava cones are built around some vents. These cones are miniature 

shield volcanoes composed largely of lava flows with very little 

pyroclastic material. Excellent examples of these are Maunaiki, 

the broad low cone at the vent of the 1920 lava flow on the southwest 

rift zone of Kilauea volcano, Kane Nui o Hamo and Heiheiahulu on 

the east rift zone, as well as Kaimuki in Honolulu. These lava cones 

generally range in width from half a mile to 1.5 miles, and in height 

from 100 to 350 feet. Their sides slope at an average angle of 2° to 

5°. They are formed largely by fissure eruption, but at the summit 

some have small craters formed principally by collapse. Thus 

Heiheiahulu cone contains a crater about 200 feet wide and 30 feet 

deep. An unusually large crater, 1,000 feet wide and 207 feet deep, 

with nearly vertical walls, indents the summit of Kane Nui o Hamo. 

This crater was the result of collapse caused by lava draining back into 

the conduit at the end of the eruption. The floor of Kilauea caldera is 

largely occupied by a gently sloping lava cone with the collapse crater 

Halemaumau at its apex.

Rarely the lava cones are steep sided. The small lava cone formed 

during the late stages of the 1949 eruption of Mauna Loa (Macdonald 

and Finch, 1949) by repeated overflow of a small lava pond is about 

100 feet high and 300 feet across at the base. Its sides slope at an 

average angle of about 35°.
26 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

LAVA RINGS

The lava ring is a very specialized structure that sometimes develops around the margin of an active lava lake (Daly, 1914, p. 135). 

This feature is a wall of spatter built up by the ejecta from the many 

small lava fountains that play on the lake surface. It is similar in 

structure and general nature to the ramparts formed along erupting 

fissures. Lava rings range from a few inches to several feet in height. 

At times they may act as dikes to hold the liquid lava of the lake at a 

level slightly higher than the surrounding surface. In August 1892 

the Halemaumau lava lake is reported to have been enclosed in a 

narrow wall, with the lake surface about 40 feet above the surrounding 

floor (Dodge, 1893). A beautifully developed but lower ring was 

present in 1894 (fig. 12). A similar but still lower ring was formed 

around a small lava pond during the 1934 eruption of Kilauea (Jaggar, 

1934a, p. 1-2; 1934b, p. 1).

FIGURE 12. Lava ring at Halemaumau, Kilauea, in March 1894 (after Brigham). The lava lake stood 

about 40 feet above its surroundings, and was about 1,200 feet in greatest diameter.

ROOTLESS VENTS

The term "rootless vent" is applied to vents not directly associated 

with the conduits that brought the magma to the surface from the 

deep-seated magma reservoir. Three varieties of rootless vents have 

been observed in Hawaii. One variety includes the hornitos and small 

accumulations of lava overflow on the surface of pahoehoe flows, fed 

through openings in the flow crust from liquid lava in an underlying 

tube. Hornitos are discussed on page 52.

The second variety occurs on a much larger scale, and produces 

major outflows of lava at a considerable distance, sometimes several
ROOTLESS VENTS 27

miles, from the true vents of other flows of the same eruption. 

It apparently results from the lava entering tubes in older pahoehoe, 

and flowing long distances underground before it breaks through to the 

surface. In 1935, the primary vents were located along fissures on the 

northeast rift zone of Manna Loa above an altitude of 11,700 feet 

and were marked by rows of lava fountains. The lava flow originating at those vents extended 9 miles down the northern slope of the 

mountain. At two places near the source fountains streams of lava 

plunged into an open fissure and a small pit crater (Jaggar, 1935, 

p. 3). A week after the commencement of the eruption near the summit, lava broke out at a point 5 miles from the rift zone vents, at an 

altitude of 8,800 feet. This vent on the northern slope of Mauna 

Loa, was far off the course of the northeast rift zone. This fact 

alone is not significant, as other scattered vents on the northern 

slope are on fissures that spread out from the upper part of the rift 

zone. It is, however, significant when coupled with the facts that the 

lava issued from a tubelike opening in older pahoehoe, and that the 

strong fountain activity characteristic of Mauna Loa vents was 

lacking. It is believed that the lava flowed underground for 5 miles, 

through one or more old lava tubes, before it broke through to the 

surface to form the rootless vent at an altitude of 8,800 feet.

In 1942, the principal fountaining vents were situated at an altitude 

of 9,200 feet, on the northeast rift zone of Mauna Loa. Within 2 

days after the outbreak at 9,200 feet, another vent opened 3 miles 

farther east-northeast, at an altitude of 7,800 feet. The locality is on 

the northeast rift zone, but the lava issued from somewhat arcuate 

cracks that trended northwesterly, approximately at right angles to 

the fissures of the rift zone. This lower vent exhibited none of the 

characteristics of the usual vents on Mauna Loa. The lava issued 

quietly from beneath the toe of an older aa flow that overlies a still 

older brown pahoehoe. Fume was liberated in much less volume 

than at the vent at 9,200 feet, and apparently no lava fountains were 

present. Only a very small amount of dense spatter was formed, 

and the lava was denser and much poorer in gas than that at the 

higher sources. This lower, outbreak probably represents lava 

draining through an older pahoehoe tube that intersected the eruption 

fissure at some point higher up the mountain (Macdonald, 1943, p. 

246). The gas-poor nature of the lava at this lower, rootless vent 

probably resulted partly from gas loss at the lava fountains of the 

primary vents, and partly from progressive loss of gas as it flowed 

through the subterranean tube.

The third type of rootless vent is the littoral cone, discussed in 

the following section.

253279 53 3
28 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

LITTORAL CONES

Where an aa lava flow enters the sea there commonly occurs a 

series of steam explosions that hurl into the air large amounts of ash, 

lapilli, and small bombs. The debris is derived entirely from the 

new lava. Accumulation of this debris on the stagnant top of the 

flow results in a littoral cone (Wentworth, 1938, p. 22) that in many 

ways resembles the cinder-and-spatter cones at vents. So far as 

known, the cones are built exclusively by aa flows. The heated 

interior portion of a pahoehoe flow is largely protected from the 

water by its poorly conducting glassy crust, whereas the open clinkery 

surface of an aa flow permits easy access of the water to the hot inner 

portion (Macdonald, 1944a, p. 181).

Littoral cones reach heights up to 300 feet and diameters up to 

half a mile. They are commonly double, a hillock developing on 

each side of the flowing lava river, while the debris falling on the 

river itself is carried away. The cones lack craters, and are generally 

better bedded than typical cinder cones. They consist^-largely of 

vitric ash of sand and silt grades, enclosing irregular lapilli and bombs 

up to 2 feet across. Many of the bombs show ribbon and spindle 

shapes. The matrix differs, however, from that typical of cinder 

cones built by lava fountains at primary vents. The latter is highly 

inflated, many of the lapilli are pumice, and the small ash fragments 

show the typical arcuate shard outlines resulting from fragmentation 

of pumiceous material. In contrast, the small fragments of the 

littoral cones are dense or only moderately vesicular. The shards are 

angular and the arcuate forms are absent or comparatively rare. On 

the surfaces of some of the littoral cones there are scattered angular 

blocks of lava resembling those typical of vulcanian explosion (Finch, 

R. H., personal communication). The differences result from the 

fact that the gases accompanying lava fountains at vents are of 

internal origin, the lava undergoing active inflation during the explosions, whereas the steam that atomizes the liquid lava in the littoral 

explosions is of external origin (Macdonald, 1949a, p. 72).

MARGINAL EXPLOSION CRATERS

Lava flows entering the wet jungle cause many minor explosions 

by generation of steam from the underlying swampy soil and distillation of combustible gases from inundated vegetation. The odor of 

marsh gas is often distinct near the flow boundary. Explosion craters 

3 to 20 feet across and 2 to 5 feet deep have been observed at the flow 

margin and as much as 200 or 300 feet distant from it. Blocks of 

rock as much as a foot in diameter are scattered around the craters 

for a distance of 15 or 20 feet. The explosions apparently result from
FUMAROLES 29

the combustion of hydrocarbon gases accumulated in lava tubes in the 

underlying older pahoehoe (Jaggar, 1917c, p. 274; 1926, p. 37).

FTJMAROLES

An extended discussion of fumaroles is not warranted here. Hawaiian fumaroles are of both primary and secondary (rootless) types 

(Lacroix, 1906, p. 650). The primary fumaroles issue from cracks 

in and near the calderas or along the rift zones, and are continuous 

for long periods of time. Those at Kilauea caldera have been active 

continuously since the first visits by Caucasians, in 1823. The 

liberated gases are largely water vapor, derived principally by the 

heating of ground water, but include also magmatic gases presumably 

rising from underlying magma reservoirs or slowly cooling intrusives. 

Alteration of the rocks by fiimarolic gases has been described elsewhere (Macdonald, 1944c, p. 496-505).

Secondary or rootless fumaroles are short-lived gas vents that 

exist on the surface of lava flows only while the flow'ns still hot and 

liberating gas. The gas is derived largely from the" underlying lava, 

but at least in some cases is partly steam from underlying wet ground 

and volatiles distilled from vegetation burned by the flow. A typical 

secondaiy fumarole observed on the 1942 lava flow 7 miles below 

the vents and 2 days after the flow ceased moving, consisted of a 

hot glowing area liberating a small amount of sulfurous fumes, and 

encrusted with a thin deposit of sulfur. This fumarole was situated 

at the edge of one of the aa channels, a common location for secondary 

fumaroles.

LAVA FLOWS

GENERAL FEATURES 

DIMENSIONS

The basaltic lava flows of the Hawaiian volcanoes vary greatly in 

all dimensions. On the flanks of the volcano, where they can spread 

freely, they range in thickness from a few inches to 40 or 50 feet, 

averaging about 5 feet near the mountain summit and 20 feet near 

the shoreline (Stearns and Macdonald, 1946, p. 28). The increase in 

thickness on the lower slopes is largely the result of decreasing temperature and gas content, with concomitant increase in viscosity.

It is accompanied by a general transformation in the prevalent 

type of lava flow, from pahoehoe to aa, pahoehoe flows averaging 

considerably thinner than those of aa. Where they are ponded on 

the floors of the calderas, or in craters or other depressions, or where 

the slopes are very low as in the saddles between the mountains, the 

thickness is greatly increased. Some flows ponded In pit craters or 

calderas have thicknesses of 200 feet and more.
30 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

The lengths of flows also vary greatly. Some flows travel only a 

few feet. Thus the lava of 1868 along the southwest rift zone of 

Kilauea volcano in places hardly flowed away from the feeding fissure, 

and at other places only reached to within a few feet of the surface 

level at the fissure. Other flows travel many miles from their vents. 

The 1880 lava flow of Mauna Loa (fig. 13) reached a point in the

FIGURK 13. Map of Hawaii showing historic lava flows. The 1950 flows of Mauna Loa arc shown in solid 

black. Older historic flows are stippled.

outskirts of the city of Hilo, 30 miles from its source. In 1859, the 

lava flow entered the ocean more than 31 miles from its vents, and 

continued for an unknown distance below sea level.

The breadth of lava flows ranges from a few feet, or at the very 

upper end of source 'fissures sometimes even less than a foot, to
LAVA FLOWS 31

several miles. Where it spread in the saddle between Mauna Loa 

and Mauna Kea, the 1843 lava flow is nearly 3 miles wide. On slopes 

where they flow freely, however, the flows are seldom more than 1.5 

miles wide, and average between 0.5 and 1 mile.

The volume of lava extruded during a single eruption ranges from 

a few to several hundred million cubic meters. The range in length, 

average width, area, and approximate volume of historic lava flows of 

Mauna Loa is shown in the accompanying table. Prehistoric flows 

apparently did not differ appreciably from the historic flows. For 

the flows that entered the sea, the dimensions given are those of the 

part of the flow above sea level.

Historic flank flows of Mauna Loa and Kilauea 

[Asterisk (*) denotes flows that reached the sea]

Date

Length » 

above 

sea-level 

(miles)

Approximate 

average 

width 

(miles)

Area b 

above 

sea-level 

(square 

miles)

Approximate 

volume ° 

above 

sea-level 

(million 

cubic

yards)

Mauna Loa

1843.. ._._._._._._. ....... .-.._....__. _.

1851 

1852... .................................................

1855 

1859 

1868....--....-........ ....... ....................

1880-81..-.. .................................... ....... .

1887..................... ............................

1899................................................

1907.......................... ..........................

1916 .__ 

1919 . . _ ........

192G .. . ... . . 

1935-.....-..--...--.-.---.-...--.......-...-.-. . .

1942........................................... ...

1950 

13.5

10.0

13.8

 =20.0

*31.7

 11.1

30.2

*15.0

15.0

13.6

7.5

*11.8

*13. 2

16.8

15.7

*14.8

1.5

.5

.6

.5

.8

.5

.5

.7

.9

.4

.3

.3

.5

.5

.6

.5

20.2

6.9

11.0

12.2

32.7

9.1

24.0

11.3

16.2

8.1

6.6

9.2

13.4

13.8

10.6

35.6

250

90

140

150

600

190

300

300

200

100

80

350

150

160

100

491

Kilauea

1790(?). ...... 

1823...............-........----.--.---...-.. .

1840 

1919-20...-...-.-.-----.---------.-.-.-.--..-...... . .

1922..... ........................................... ...

1923................................................ . .

5.6 *6.3

*6. 9

7.1

.4

.1

0.3

.4

.7

.5

.1

.2

3.0

3.9

6.6

5.0

.04

.2

38

15

281

62

.3

.1

a Where the flow had more than one branch, the length given is that of the longest branch. 

b Data on area and volume from Stearns and Macdonald, 1946, p. 69, 111.

c The upper end of the 1855 lava flow has been lost iu the complex of recent flows along the northeast 

rift zone. The figures given are for the part of the flow below where it is overlapped by the 1880 lava.

ATTITUDES OF LAVA FLOWS

The attitudes of lava flows are governed largely by the conformation 

of the underlying terrane. Owing to the fluidity of the prevailing 

lava types, the slopes of large volcanic accumulations in Hawaii are 

usually gentle, and accordingly the newly erupted lava beds on the
32 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

flanks of the volcanoes have gentle dips. Wherever the flow is 

ponded its attitude becomes essentially horizontal. On the other 

hand, where lava flows have poured down the sides of cinder cones 

or over fault scarps or sea cliffs, the dips increase to approximately 

equal the underlying slopes. Flows on the flanks of cinder cones dip 

as much as 35°. Those which pour over cliffs commonly form a 

veneer adhering to the cliff face, with dips in some places approaching 

the vertical.

FLOW UNITS

Many lava flows of the Hawaiian volcanoes comprise two or more 

parts poured one over the other during the course of a single eruption. 

For these nearly contemporaneous subdivisions of flows Nichols 

(1936) proposed the name flow units. In Hawaii, the flow units 

commonly range from a few inches to about 10 feet in thickness. 

Their extent is variable, ranging from local gushes that move a few 

feet over an earlier surface of the same major flow, to sheets that 

flow for thousands of feet or even miles partly or entirely over earlier 

lava of the same eruption. Repeated flow units occur in both aa 

and pahoehoe, but are seen most commonly in pahoehoe. In some 

sections, scores of pahoehoe units, none more than 2 to 4 inches 

thick, aggregate 10 or 20 feet in thickness. In some instances a 

later flow unit of pahoehoe may move over an earlier flow unit of aa, 

or vice versa. In prehistoric lavas it is commonly difficult or impossible to ascertain whether successive beds of similar lava, without 

intervening soil, are merely different flow units of the same major 

flow, or independent flows extruded during different eruptions.

CLASSIFICATION OF FLOWS

The common types of lava flows in Hawaii are pahoehoe and aa. 

Although these two forms are intergradational, they are characteristically distinct. Pahoehoe may be defined as the variety typified by 

a smooth, billowy, or ropy surface, and aa as that typified by a rough, 

jagged, spinose, clinkery surface. Not only the surfaces, but in 

general the internal structures of pahoehoe and aa flows are distinct, 

and the mode of advance of the two types is quite different. They 

are discussed in detail in the following paragraphs. The terms 

dermolith and aphrolith have been proposed for the two types by 

Jaggar (1917a) ; but have not been generally accepted. A third type, 

block lava, resembles aa in the fragmental character of the flow surface, but differs in that the fragments are not spinose and clinkery 

but are relatively smooth angular blocks (Finch, 1933b, p. 769-770). 

It is intergradational with true aa. Block lava is rare in Hawaii, 

but occurs in some flows of andesite (fig. 3).
LAVA FLOWS 33

PAHOEHOE 

GENERAL FEATURES

Palioehoe lava flows are characterized by a smooth, hummocky, 

ropy, or entraillike surface, and the presence of lava tubes. After 

the initial burst of lava the top becomes crusted over by congealed 

rock, and the advancing fluid lava entirely fills the resulting lava 

tubes. The large tube divides into many smaller ones, each of which 

feeds a lobe of lava along the advancing flow front. The entire front 

advances by the protrusion of successive lobate toes, somewhat in 

the manner of a moving amoeba. Large tubes may be several tens of 

feet in diameter, but are commoniy^not more than 10 or 15 feet. 

The smaller tubes are generally 1 t%; "3 feet across. Lava may congeal in the tube, the position of the tube being marked only by the 

concentric arrangement of vesicles; 'or the fluid lava may drain away, 

leaving the tube partly or entirely empty. In cross section a flow 

unit often consists of a large number of small, filled tubes. The 

ellipsoidal shape of the tubes and the way in which they are packed 

together give the outcrop on superficial examination the appearance 

of pillow lava (Stearns and Macdonald, 1942, p. 24-25). Flows of 

pahoehoe are commonly a succession of flow units, from a foot to 

15 or 20 feet in thickness. Each flow unit represents a separate 

period of lava spreading, a few hours, days, or weeks apart, during 

the same eruption.

Thin pahoehoe flows, and the upper parts of thick flows, are characterized by very abundant vesicles making the rock almost spongy. 

Extremely gas-rich flows, especially in the summit region of Manna 

Loa, develop a layer from less than 1 inch to about 2 inches thick, 

of typical basaltic pumice (Finch, Powers, and Macdonald, 1.948). 

The lower and middle parts of thick flows, which remain fluid long 

enough to lose much of their gas, are dense. The vesicles of pahoehoe 

are typically spherical or spheroidal, composed of clusters of spheroids 

or moderately distorted spheroids. Their outlines are smooth, regular 

cut'ves, as contrasted with the typically very irregular outlines of aa 

vesicles (fig. 14). Although this difference in vesicle shape does not 

always hold true, and gradations in vesicle shape from aa to pahoehoe 

are recognized it is nevertheless so general that it constitutes a 

very useful criterion for the recognition of aa and pahoehoe in cross 

section or in hand specimen.

At the surface of pahoehoe flow units there is generally a glassy 

skin, ranging from a mere film to a layer an inch or more thick. The 

glassy crusts weather more readily than the rest of the rock, and in 

cross sections of old lavas the flow tops are commonly marked by 

undulating bands of red or yellow. In detail the surfaces of pahoehoe 

flows may be smooth, or pockmarked by innumerable broken blisters.
34 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII
LAVA FLOWS 35

Most commonly, however, they are covered with innumerable tiny 

spicules and spines produced by the escape of gas from the lava 

surface, each bubble dragging with it a filmaent of the enclosing 

liquid. The liquid dragged upward by the escaping bubble quickly 

chills, sometimes leaving a small upright spine a millimeter or more 

high. The resultant minutely spinose surface feels exceedingly rough 

to the hands. This type of pahoehoe surface has been termed sharkskin pahoehoe by T. A. Jaggar and his associates at the Hawaiian 

Volcano Observatory (fig. 15). It generally is developed where quick 

chilling of the flow surface, as by heavy rain, has formed a tachylite 

crust.

More commonly the filaments carried upward by the escaping gas 

bubbles bend partly or entirely over before they freeze to immobility. 

Most of the threads fall back onto the flow surface, and commonly are 

alined in the direction of flowage. The surface of this type of 

pahoehoe has a lacy or filamented appearance, and the flows may be 

termed filamented pahoehoe (fig. 16). It is the commonest type of 

pahoehoe surface, and generally is superimposed on the ropy, hummocky, or entraillike forms.

On a larger scale, many pahoehoe surfaces are marked by a series 

of small cordlike ridges, commonly aimed parallel to the direction 

of flow (figs. 17 and 18). These small cords, usually an inch or less 

in diameter, may be superimposed on still larger ropelike convolutions of the crust. This type of surface may be termed corded pahoehoe. Less commonly, liquid lava flowing beneath the plastic 

crust drags it into ropy festoons, which are convex in the direction 

of motion. This type of surface is best termed festooned pahoehoe. 

These ropy or festooned surfaces are often considered characteristic 

of pahoehoe flows, but actually such surfaces are comparatively 

limited in their distribution.- Sjt.ilI another type of pahoehoe surface 

consists of innumerable small contorted toes of pahoehoe, intricately 

jumbled together, and generally associated with festooned surfaces. 

It is usually found on the flanks of minor vents or rootless vents such 

as hornitos and ruptured tumuli. Its entraillike appearance has 

been noted by several writers, and may be appropriately termed 

entrail pahoehoe (fig. 19).

In a broad sense, a field of pahoehoe is generally irregular and 

hummocky. Some of the hummocks are typical tumuli (p. 45). 

Others are broad swells and ridges, that range from less than 1 foot 

to more than 10 feet in height (fig. 20). When viewed from a 

distance and especially from the air the hummocks and ridges 

sometimes have a wrinkled and draped appearance that has given 

rise to the term elephant-hide pahoehoe (Stearns and Macdonald, 

1946, p. 20).
36 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII
FIGURE 10. Filamented pahoehoe, surface of lava flow of 1919, Kilauea caldera. Photo by G. A. Macdonald. CO
38 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII
f

oI

FIGURE 18. Detail of surface of 1935 Mauna I-oa How, near II uinuula. -Motion from right to Mt. i'hoto by C. K. Wcntwmth.

OOCO
40 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII
I

CO

I'd.- smiacc of Ih4;i .Manila Lou lava How , souUuvest ol JIuniuula, view to northward, with -Mauna Koa slope and cinder eones in diakinee. This flow is marked 

by tumuli and intervening collapse depressions on a large scale. Tumulus at right is 25 feet high. Photo by G. K. Wcntworth.
42 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

On some pahoehoe flows, draining of the liquid from beneath a 

thin crust, followed apparently by some movement of the flow as a 

whole, has resulted in fracturing of the crust and tilting and heaping of the fragments. The fragments of crust are plates, 1 to 4 or 5 

feet in length and width, and 2 to 6 or 8 inches thick. Their upper 

surfaces exhibit the minor features characteristic of any pahoehoe 

flow, but their lower surfaces are exceedingly rough and spinose. 

The plates are tilted, jumbled, or imbricated, in a manner similar to 

the plates of ice in an ice pack. This type of pahoehoe surface has 

been described from several other localities, including the McCartys 

flow in New Mexico (Nichols, 1938, p. 601-603). It has been termed 

slab-lava by Jones (1943, p. 267), and block-fields of pahoehoe by 

Cotton (1944, p. 125). Because it is more slablike than blocklike, 

it is suggested that the name slab pahoehoe be used for this form of 

lava.

The rare variety known as arborescent or dendritic lava develops 

along fractures in the crust of both pahoehoe and aa flows close to 

the condition of transition from one to the other. It is formed at 

points where the crust sags leaving one side of the crack higher than 

the other (Stearns and Macdonald, 1946, p. 80). The lava is glassy 

but apparently otherwise entirely normal in composition, covered by 

a red oxidized shell less than 1 millimeter in thickness. Its beautifully dendritic character is shown in figure 21. The exact reasons 

for its formation and mode of development are obscure, but it apFIGURE 21. Dendritic luvti, on asmall scarp in the!920 now ofKilauca. The sculu is shown by the pencil.

Photo by O. A. Macdonald.
LAVA FLOWS 43

pears likely that it is genetically related to the spinose and sproutlike 

development, of equally obscure origin, that typifies the surface of 

some aa flows.

Near some vents,. especially on the upper slopes of Mauna Loa, 

floods of gas-rich pahoehoe have developed a plexus of small tubes 

and blisters. Some were originally filled with fluid lava that drained 

away, but others apparently were never entirely filled with liquid but 

were inflated by gas pressure (Macdonald, 1943, p. 249). The crust 

above these small tubes and blisters ranges from about 2 inches to 1 

foot in thickness. It usually will not support the weight of a man 

and when crossing these lava fields a person repeatedly undergoes the 

startling and unpleasant experience of falling through the crust into 

the openings beneath, a distance of 1 to 4 or 5 feet. This type of 

lava has been termed shelly pahoehoe (Jones, 1943, p. 265-268).

PAHOEHOE CHANNELS AND TUBES

During early stages of the eruption, and sometimes near the vent 

during later stages, the pahoehoe river flows in an open channel. As 

it leaves the vent, the surface of the river is largely incandescent, of 

yellowish orange to red. Thin lead-gray crusts quickly form and are 

borne along on the moving river, repeatedly fracturing, sometimes 

turning on edge and sinking, and continually reforming. Drag of 

the crust at the edges of the river results in the formation of ropy 

festoons convex downstream. Occasional overflows build up low 

levees of congealed lava along the river margins. Spatter from the 

river often forms a thin veneer on the levee, particularly near the 

vent.

A short distance below the vent, the pahoehoe river commonly 

starts to develop a continuous roof, resulting in a typical lava tube, 

through which the river moves during most of the duration of the 

eruption and for most of the length of the flow. The main tube 

forms a myriad of distributary tubes that feed the active front and 

margins of the flow. Tubes form in two ways: by the chilling of a 

skin around protruded pahoehoe toes, and by the crusting over of 

the main lava river (fig. 22). The toes form by the breaking open 

of the crusted front of the flow, permitting the escape of "a new 

tongue, which emerges as a rounded bulk encased in a newly formed 

skin" (Jaggar, 1930, p. 2). This skin is plastic at first, stretching as 

the toe elongates. It is possible to poke a stick into the end of the 

toe, releasing liquid lava from within, or even to cause the liquid to 

spurt from the end by jumping on the top of the toe. The crust 

quickly thickens, forming a rigid shell over the still fluid interior, and 

repeated outbursts lengthen the toe and develop a small tube.

253279 53 4
44 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

' ""' ' '
LAVA FLOWS 45

The mam tube is formed by crusting over of the main lava river. 

The crust is at first thin and unstable, repeatedly breaking up and 

floating downstream. Eventually, however, floating crusts form a 

jam across the river, more crusts pile against the jam, and a permanent roof is established that is gradually extended upstream. This 

process was witnessed during the 1935 eruption of Mauna Loa by 

H. T. Stearns (personal communication), and again during the 1942 

eruption by Macdonald. Once the crust is established, it thickens 

downward as the lava congeals against its under side.

At the end of an eruption the tubes may remain filled with congealed lava (fig. 23), or the lava may drain away partly or entirely, 

leaving an open tunnel. The superficial resemblance of cross sections 

of small filled lava tubes to pillow lavas (fig. 24) has already been 

mentioned. Open tubes range in diameter from less than 1 foot to 

50 feet, or more in extreme cases. In Hawaii they seldom exceed 20 

feet. The larger tubes may extend for miles, but seldom if ever are 

they unbroken, even during the eruption. Typically each tube has a 

series of openings in its roof, where a permanent roof either failed to 

form or was broken. Through these openings, during the eruption, 

the stream of molten lava can be seen -below. Drained tubes common^ exhibit a series of shorelines left on their walls as the level 

of the lava dropped during closing stages of the eruption. The 

floor is generally the congealed essentially flat surface of the dwindled 

lava stream, and commonly this very last lava in the tube congealed 

as aa. The walls and roof of the tube are commonly veneered with 

spatter and glazed by gas fusion, and hung with stalactites. Larger 

tubes in old lava formations often have their floors cluttered by piles 

of blocks fallen from the roof, both floor and roof being convex upward. The walls of most large lava tubes are composed of several 

thin flow units. These are formed by repeated overflows of the lava 

river before it crusts over to form the roof of the tube. Several thin 

flow units commonly are visible in the roof of the tube also, and are 

formed by repeated small overflows from the openings in the tube 

roof.

TUMULI AND PRESSURE RIDGES

Tumuli (fig. 25) (also known as pressure domes or schollendomes) 

are common on pahoehoe flows that have been ponded in depressions 

or poured out on very gentle slopes. Tumuli are domical upbowings 

of the flow surface, typically elliptical in plan, and grading into much 

elongated structures that are best termed pressure ridges. Tumuli 

in Hawaii range from 1 or 2 to 10 or 15 feet in height, 10 to 30 feet in 

width, and in length up to three or four times their width. Extremely large examples, such as those of the Stony Rises near Lake
46 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

FIGURE 23. Section of filled lava tube, Makapuu Head, Uahu. Vertical ciiini'iisiun about 14 feet. 

Photo by C. K. Wentworth.
f

f 

o

FIGURE 24. Section of pahoehoc lava flows, Makapuu Head, Oahu. Photo by C. K. Wentworth.
48 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII
LAVA FLOWS 49

Corangamite in Western Victoria (Skeets and James, 1937, p. 245- 

278), are not found in Hawaii. Tumuli are generally cracked open 

along the summit, the crack paralleling the long axis of the dome. 

In some instances the crack extends to a depth of several feet, and 

it is possible to climb down into it to examine the structure of the flow. 

Other less regular cracks extend down the flanks. On tumuli with a 

subcircular ground plan the cracks may be disposed radially from the 

summit. Commonly, a little viscous lava has been squeezed up from 

the deeper parts of the flow partly filling the cracks and forming 

driblets on the flanks of the tumulus.

Pressure ridges generally resemble tumuli except in their greater 

elongation. In Hawaii their height seldom exceeds 15 feet although 

Schulz has recorded some 30 feet high formed in the caldera of Mauna 

Loa during the eruption of 1940 (Schulz, 1943, p. 745). Commonly 

they are fractured to a much greater degree than tumuli, and some 

examples are elongate heaps of crust blocks, tilted, jumbled, and 

overturned. Some resemble asymmetrical anticlines, with one gently 

dipping limb and the other limb steep or even overturned.

Dana (1887, p. 356) and others have explained tumuli as great 

blisters raised by the pressure of gas in the flow. Careful examination 

of many tumuli in the field indicates, however, that they are not 

formed in that way. Tumuli are usually not hollow, like blisters, 

although in some an open space is left when lava drains out. Some 

certainly were formed by hydrostatic pressure of the underlying fluid 

lava lifting the congealed crust after the manner of a laccolith (Daly, 

1914, p. 135). Some also may have been formed by differential collapse of the flow crust owing to the draining away of the underlying 

fluid portion (Skeets and James, 1937). However, many of them, 

particularly those of highly elongate form, and the great majority of 

pressure ridges, apparently were formed by the horizontal thrusting 

and budding of the flow crust caused by the pressure of fluid lava. 

Such buckling was actually observed in Mokuaweoweo caldera dining 

the eruption of 1940 (Waesche, 1940, p. 8). The small open space 

beneath the crust in some tumuli is probably the result of upbuckling 

of the crust under horizontal compression.

PRESSURE PLATEAUS

Pressure plateaus are broad areas of lava, generally pahoehoe, that 

have been bodily elevated by the intrusion of new fluid lava into the 

lower, still uncongealed part of the flow without the addition of 'any 

new lava at the surface. They are, therefore, generally formed on 

thick ponded flows, in which the lower parts may remain fluid for 

many weeks. The amount of uplift may range from less than a foot 

to 15 or 20 feet. During the 1940 eruption of Mauna Loa the solidi-
50 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

fied lava surface in the North Bay of the caldera was raised as a pressure plateau 15 feet above its original position at the southern edge 

of the bay (nearest the source fountains) and 2 to 3 feet at its northern 

edge (Schulz, 1943, p. 745-746). The edge of the pressure plateau 

was not at the caldera walls, rather a narrow moat or a broader area 

of unelevated lava generally lay between the edge of the plateau and 

the wall of the caldera. The limit of intrusion, and consequent 

elevation, was probably determined by the extent of congelation of the 

interior of the flow.. Even where the flow was thick, chilling of the 

lava against the confining wall would have resulted in congelation of 

the entire thickness of the lava for a few feet from the wall.

Pressure plateaus are generally associated with pressure ridges, and 

indeed the spectacular pressure ridges in the 1940 lava of Mauna Loa 

were probably largely the result of the intrusion of the new lava that 

caused the pressure plateaus. According to Schulz, each plateau was 

bordered on all sides by a pressure ridge of small height.

Similar sill-like shallow intrusions into slightly older lavas of the 

same eruption have been described at Paricutin. There during 

February and March 1945, the surfaces of the Zapicho flow of late 

1943 and the San Juan flow of 1944 were raised and arched as much 

as 60 feet by subsurface intrusions (Krauskopf and Williams, 1946, 

p.. 406), and during August 1944 the solidified surface of a flow ponded 

in an old crater was elevated 30 to 40 feet in a similar manner (Bullard, 

1947, p. 437).

SLUMP SCARPS

Lava flows ponded in depressions, and those that have flowed down 

valleys, commonly exhibit along their margins low inward-facing 

cliffs, formed by subsidence of the general flow surface. These cliffs 

are known as slump scarps (Finch, 1933a, p. 647-649). Similar 

"high-lava" marks left by subsidence of the lava stream after flood 

stage are found around the sides of hills that had been surrounded 

by lava flows.

The subsidence is probably in small part the result of decrease in 

volume of the lava owing to cooling and loss of gas. In many of the 

flows occupying valleys it is largely due to draining away of fluid 

lava from beneath the congealed crust, as the still fluid lower part of 

the lava continues down the valley (fig. 26). Even in flows that apparently were confined in craters and other depressions, it seems that 

some sort of drainage of the underlying liquid portions of the lava 

must have taken place. Using Daly's estimates of the coefficients of 

expansion of crystalline and vitreous basalt, Schulz (1943, p. 743) 

calculated that the decrease in volume owing to cooling of the 1940 

lava in the North Bay of Mauna Loa should not have exceeded about 

2.45 percent. The volume decrease necessary to produce the observed
5

>fo

FIGURE 26. Successive high-lava marks on 50-foot cone east of Pahahi on Kilauea slope. Fhoio by (.'. K. \\ riu\\ onh.
52 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

slump scarps would have been nearly 18 percent. Allowing for 

volume decrease as a result of loss of gas, it is unlikely that cooling 

and gas liberation alone can account for the decrease in volume. 

Many flows confined in depressions, and even many of those in valleys, 

do not develop slump scarps. Yet the loss of heat and volatiles by 

those flows must be of comparable magnitude with the flows that 

develop slump scarps. It is probable that where prominent slump 

scarps are present on flows ponded in craters or calderas, there has 

been some drainage of the fluid lower part of the flow back into the 

source vents or other fissures.

LAVA BLISTERS

The surfaces of pahoehoe flows commonly exhibit small blisters 

formed by gas bubbles puffing up the still viscous flow crust. Ballooning crusts frequently have been observed on pahoehoe rivers near the 

vents, where the lava is losing large volumes of gas. The gas-inflated 

toes in the gas-rich shelly pahoehoe of the early stages of Mauna Loa 

eruptions have been described at the end of the section on general 

features. Blisters are present on all types of pahoehoe, but are most 

conspicuous on the type with a relatively smooth tachylite crust. 

They range in diameter from less than an inch to about 3 feet, and in 

height to about 2 feet. The shell ranges in thickness from about 1 

millimeter to 2 centimeters. Blisters up to 10 feet in diameter have 

been observed forming and bursting with minor explosions on aa 

flows at Paricutin (Billiard, 1947, p. 438).

Domical eminences as much as 30 feet high and 60 feet across on a 

pahoehoe flow at Byaduk, Western Victoria, have been interpreted 

by Skeats and James (1937, p. 273-275) as huge blisters formed by 

steam generated where the lava overflowed marshy ground. Nothing 

approaching this size has been observed in Hawaii. The hollowness 

of the mounds at Byaduk has not been demonstrated, and from the 

descriptions, it appears not impossible that these structures are 

actually nearly circular tumuli.

HORNITOS

Hornitos, also known as driblet spires and driblet cones, are small 

mounds of spatter built at rootless vents on the backs of lava flows. 

The typical hornito is a rounded, more or less beehive-shaped mound 

(fig. 27), whereas the typical driblet spire is a thin column or spine. 

Both may conveniently be included under the general designation of 

hornitos. Both commonly have an open pipe at the center, although 

the pipe may be clogged or sealed over by the last ejecta. They are 

built by the gradual accumulation of clots of lava ejected through an 

opening in the roof of an underlying lava tube. In Hawaii most of the
tr*

1 to

FIGURE 27. Hornito on 1919 lava flow. Kilauea caldera. Photo by G. A. Macdonald. Or 

CO
54 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

ejecta are still partly fluid when they hit the ground, and the mounds 

are composed of agglutinate. In other parts of the world some are 

composed of scoria or ash, and are built by secondary (rootless) 

fumaroles.

In Hawaii, typical hornitos range from less than 2 feet to about 6 

feet in height, and up to about 10 feet in diameter. Driblet spires 

range from less than 1 foot to about 2 feet in diameter, and up to 

about 12 feet in height, although most of them are less than 6 feet 

high. Bullard (1947, p. 442) has used the term hornito for cones as 

much as 50 feet high, apparently built over eruptive fissures, but it 

seems preferable to use the terms "spatter cone", "volcanello", or 

"vulcancito" for such structures, reserving hornito for the small 

mounds built at rootless vents.

LAVA STALACTITES AND STALAGMITES

Lava stalactites are two different types. One type forms simply 

by the dripping of fluid lava from ,the roof and walls of a lava tube or 

open lava river where, for any reason, the level of the fluid in the 

conduit falls, or in tubes or on the walls of spatter cones where spatter 

is flung against the roof or walls. Such stalactites generally resemble 

icicles, although they may be much more irregular. They commonly 

attain lengths of 6 to 12 inches, and rarely as much as 3 feet. Only 

seldom are they accompanied by stalagmites, and when stalagmites 

are present they generally are poorly developed.

The second type of lava stalactite is formed by gas-heating of the 

roof of a lava tube or the inner walls of a cone (Jaggar, 1931, p. 1-3). 

The intense heat, at temperatures up to about 1,200° C., fuses the 

exposed surfaces to a vitreous glaze that is sufficiently fluid to trickle, 

forming stalactites. The latter are typically very slender, attaining 

lengths as great as 4 feet (Stearns and Clark, 1930, p. 114) with a 

thickness of only % to % inch. Some are fairly regular smooth rods, 

but many resemble slender elongate bunches of grapes, or contorted 

worms. Others are icicle shaped. Jaggar has observed them forming 

on the yellow-hot incandescent walls of tubes and spatter cones, and 

in grottoes at the wall of lava lake in Halemaumau Crater. While 

they are still glowing hot, the}^ are sufficiently plastic to sway freely 

in blasts of gas.

In lava tubes, drip from the stalactites to the floor builds up lava 

stalagmites. The latter are mounds of agglutinated droplets, from 

1 or 2 inches to 1 foot high, and 1 to 4 inches in diameter. Their 

position directly under the stalactites shows that the drip must have 

occurred after the lava stream comprising the floor of the tube stopped 

moving.
LAVA FLOWS 55 

SHARK-TOOTH PROJECTIONS

At breaks in pahoehoe crust where one side has pulled away, the 

viscous lava may be dragged out into pointed projections resembling 

flattened icicles. They were found by Schulz (1943, p. 743-745) in 

the 1940 lava of Mauna Loa, on the underside of the remnant shelf 

that adhered to the caldera wall when the general flow surface slumped 

away from it. There the still plastic material on the underside of 

the crust had been pulled into projections as much as 10 inches long, 

tapering to hairlike points. They were termed "pulled stalactites" 

by Schulz, but because they were not formed by drip the name appears 

inappropriate. Similar projections along cracks in the 1823 lava 

flow of Kilauea were termed "shark teeth-slickensides" by Stearns 

(1926, p. 343). Those in lavas of Ascension Island (Daly, 1925, p. 

21) and the McCartys basalt of New Mexico (Nichols, 1939c, p. 

188-194) have been called "shark's-tooth projections," and that name 

is adopted here. Examples as large as those in the 1940 lava are 

rare, but small ones are fairly common.

LAVA TREE MOLDS

Pahoehoe lava surrounding the trunk of a tree is chilled against it, 

preserving a mold of the trunk and sometimes of branches. The 

tree itself burns, leaving an upright tubular opening in the lava. 

The mold is often sufficiently perfect to preserve the impression of 

the checks in the charred wood, and rarely details of the bark. Draining away of the lower portion of the flow may result in a lowering of 

the surrounding flow surface, leaving the lava that congealed against 

the tree standing in relief above the flow surface (fig. 28). These 

projecting hollow columns have also been called lava tree casts (Finch, 

1931, p. 1-3) or simply lava trees. The tree molds may have any 

diameter, up to that of the largest tree trunk. Tree molds reach 

heights as much as 15 feet above the surrounding lava surface. The 

shell of lava surrounding the internal tube is generally 4 inches to 

1 foot in thickness.

Similar structures have been found in many parts of the world. 

Those in Craters of the Moon National Monument, Idaho, were first 

termed by Stearns (1924 p. 370) "lava mortars", and believed to 

have originated by the accumulation of spatter around minor vents. 

He later recognized them to be lava tree molds (Stearns, 1928, p. 

29-31).

SQUEEZE-DPS

Many flows, both of pahoehoe and aa, exhibit dikelike autointrusions of fluid matter from the lower part of the flow into fractures 

in the crust. Many of these reach the surface. In pahoehoe flows
56 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

FJOUKE 28. Tree mold, 1823 lava flow of Kilauca volcano. The mold Is about 4 roet high. 

Hawaii National Park photo by D. IT. Hubbard.
LAVA FLOWS 57

some reach the surface in moderately fluid condition, and spread out 

as pahoehoe toes for a few inches or a few feet on the flow surface. 

These are the "bulbous squeeze-ups" (Nichols, 1939b, p. 421-423). 

Others, of more viscous nature, rise as wedges into the fissures or 

project a few inches to a footer two above the surface (fig. 29). 

These are Nichols' "linear squeeze-ups" (1939b, p. 423). Many of 

them are grooved on the sides (see below). They grade into the spines 

that occur 011 a few aa flows. Closely similar in origin but on a smaller 

scale, are the so-called "barnacle stalactites" that are found occasionally on the walls of lava tubes, formed by squeezing of viscous 

lava through cracks in the tube wall (Jaggar, 1931, p. 3). Also 

related to the "bulbous squeeze-ups" in origin, but formed by still 

more fluid lava, are the "lily-blossom" forms that result where veiy 

liquid pahoehoe wells up through a hole in the roof of a lava tube, 

spreads out a few feet, and then partly drains back into the tube 

(Stearns and Macdonald, 1946, p. 80, pi. 26). This leaves a broad, 

nearly flat, smooth subcircular disc, with a funnellike depression in 

the center.

GROOVED LAVA

Wherever moderately to highly viscous but still plastic lava, either 

aa or pahoehoe, moves past an irregular edge or surface of more rigid 

lava or other rock, or is rubbed by such rigid rock moving past it, the 

grooves and striations formed in the plastic lava may become preserved (fig. 30). The striations resemble glacial striations or slickensides on a fault surface, but generally may be distinguished from 

those by the minutely rough, granular, or spinose character of the 

surface. Small shark-tooth projections commonly are present. 

"The grooves vary from mere scratches up to marks an inch deep. 

They may be straight, or the whole series may be slightly curved" 

(Nichols, 1938, p. 604).

The grooves may develop on the under side of the crust blocks in 

slab pahoehoe, where the still plastic under side of the block is pushed 

across a more rigid jagged edge or upper surface. These grooves arc 

very common on the sides of linear squeeze-ups, on spines that 

develop on some flows of aa and block lava, and the still more numerous 

and much larger spines characteristic of viscous domes (tholoicls).

AA

GENERAL FEATURES

Aa lava flows are characterized by exceedingly rough, jagged, 

clinkery surfaces. Much of the surface is covered, to a depth of 

several feet, with loose fragments of clinker. Not all the jagged 

spinose surface material is loose clinker, however. As Jaggar (1930, 

p. 3) has pointed out, many of the jagged boulderlike masses "are
58 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII
LAVA FLOWS 59

FIGURE 30. Plastic strutions in lava, southwest slope of Mauna Kea. Photo by C. K. Wentworth.

not loose at all, but are sprouts and crags connected with the continuous ledge beneath." The sprouts and loose clinker fragments 

are almost indescribably rough and jagged, cutting stout boots to 

ribbons within a few days. The clinker fragments range from a 

fraction of an inch to several feet in diameter, although most are less 

than 1 foot. The character of the flow surface and of the clinker 

fragments is best shown in photographs (figs. 31, 33).

The almost universally clinkery surface of aa flows gives rise to 

the misapprehension in the minds of many geologists, and oven some 

volcanologists, that aa flows consist of loose clinker throughout. 

That impression is generally false (Macdonald, 1945c, p. 1179-1180). 

Certainly hundreds, and probably thousands of aa flows in the Hawaiian Islands have been examined in cross section by the writers, in cliffs 

and canyon walls, road cuts, well shafts, tunnels, and diamond-drill 

cores. Almost all have a central massive phase. Rarely, a flow may 

locally consist of clinker through its entire thickness, but all such 

flows that have been traced laterally in cliff's or canyon walls have 

passed into a massive central layer within a few feet. The margins 

of many recent aa flows appear to be solid banks of clinker, but digging 

into the bank 1 to 6 feet exposes the massive phase,

253279 53 5
60 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

FIGURE 31. Surface of aa flow, on trail from Kilauea to Puu TJlaula. Photo by 0. K. "VVentworth.

FIQUEB 32. Section of an aa h:va Min\, Wanl.u' quarry, Oahu. Dense part about 5 feet thick. 

Photo by C. K. Wcntvvorth.
LAVA FLOWS 61

Typically, the central massive part of the flow is both overlain and 

underlain by layers of clinker (fig. 32). As pointed out above, the 

upper surface layer of clinker is essentially continuous throughout the 

extent of the flow. It ranges in thickness from less than a foot to as 

much as 8 or 10 feet, being in some degree proportional to the total 

thickness of the flow. The lower clinker layer is commonly thinner 

and less continuous than the upper clinker layer. It may be absent 

over considerable areas. Locally, layers or irregular masses of clinker 

are found within the massive phase. In 11 carefully measured 

sections, from 150 to 400 feet thick, on the walls of South Halawa, 

Moanalua, and Waimalu valleys, on Oahu, the proportion of clinker 

to total aa ranges from 15 to 45 percent (fig. 3). In the shaft of the 

skimming well at Paauilo, on the northeastern slope of Mauna Kea 

on Hawaii, a section 267 feet thick consists entirely of aa. Twenty 

flow units from 4 to 28 feet thick are exposed, and in the individual 

flow units the proportion of clinker to the total thickness of the flow 

unit ranges from 17 to 66 percent, averaging 39 percent. Such 

measurements demonstrate that, far from composing the entire 

thickness of an aa flow, clinker generally comprises well under half 

of the thickness.

The central massive phase of an aa flow is generally not dense. 

In exceptionally thick flows, especially those which were ponded, the 

lower part may be quite dense with very few vesicles. However, the 

upper part of thick flows and all thinner flows are commonly mark-

. Contrast between pahoehoe lava (foreground) and aa lava (distance). The cam 

the edge of the aa flow is 6 Inches across. Photo by Q. A. Macdouald.
62 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

edly vesicular. The massive phase of aa generally is less vesicular 

than pahoehoe, but there is so much variation in the vesicularity of 

both types that there are many exceptions to the rule. Vesicularity 

of aa may reach as much as 50 percent, but generally it is less than 30 

percent. In contrast to the regularity of outline of pahoehoe vesicles, 

those of aa are typically irregular, and commonly much stretched in 

the direction of flowage. Their character is shown in figure 14.

An aa lava flow is fed by a main lava river that flows down the 

mountainside in an open channel. On either side of the main river, 

and particularly at the advancing toe of the flow, the pasty central 

layer of the flow pushes outward. Locally and for short distances, 

the movement may be quite rapid, but generally the most active 

front portion of the flow advances only a few tens or at most a very 

few hundreds of feet a day. There are many exceptions, but generally aa flows advance much less rapidly than pahoehoe flows.

The advance of the flow front generally occurs much as follows. 

The black clinkery front of the flow may appear temporarity as a 

steep, relatively immobile bank. Its liveness is shown only by 

moderate fume liberation, the typical "foundry" odor, the incessant 

grating and cracking of surficial rock resulting from cooling and slight 

shifting, an occasional boulder tumbling down the flow front, and 

especially at night by innumerable viciously red eyes glaring out 

through the clinker from the deeper parts of the flow. During such 

times, radiant heat often is so low that it is possible to walk directly 

up to the flow front. Gradually, however, the front steepens and 

bulges, sometimes imperceptibly but at other times quite noticeably. 

Eventually the bulging results in instability of the flow front, and a 

slab of black clinkery rock peels off the front, breaks up, and tumbles 

to the foot of the bank with a sound like the clinking of breaking 

crockery. The large block commonly is accompanied and followed 

by a slide of finer incandescent debris. Where the block separates 

from the flow front it reveals a glowing hot face of pasty lava in the 

interior of the flow. It is in this lava paste that the flow movement 

principally occurs, material in the pasty layer moving toward the front 

or sides of the flow beneath a relatively much less mobile surface 

layer of bristling clinker.

The top of the flow generally moves also, but less rapidly, dragged 

along by the flow of the pasty material beneath. The movement 

differs from that of pahoehoe principally in the degree of fluidity. 

On exposure of a large area of the glowing pasty flow interior to the 

air, radiant heat is suddenly greatly increased, making it decidedly 

uncomfortable to remain close to the flow margin. The exposed 

glowing paste is quickly chilled to a black clinkery flow front, which 

again remains relatively immobile until bulging makes it unstable.-
LAVA FLOWS 63

The flow thus tends to advance over a layer of its own debris, formed 

by collapse of the repeatedly bulging front. The spasms of bulging 

may occur every few seconds or at intervals of several minutes, depending on the activity of the flow. In rapidly advancing flows it is 

essentially continuous. Occasionally, particularly in thin flows, the 

pasty flow layer may glide forward with little or no collapse of the 

front, chilling to form a spinose clinkery top, but developing no basal 

clinker layer.

AA CHANNELS

The lava river that feeds an aa flow moves down the mountainside 

in an open channel. Near the vent, in early stages of the eruption, 

the river may be as much as 50 feet wide, but in the late stages of the 

eruption it generally dwindles to 15 feet or even less. Commonly, on 

gentle gradients, the lava river may repeatedly divide and reunite, like 

a braided stream. The volume of lava fluctuates considerably, and 

the channel is at times full to overflowing, whereas at other times the 

surface of the stream is several feet below the banks. During times of 

overflow, blocky material and congealing lava along the banks build 

up walls, or natural levees, that may confine the stream several feet 

above the level of the surrounding land surface. As the level of 

liquid lava in the channel drops, spatter from the surface of the 

stream commonly forms a veneer on the walls,.hung with lava stalactites. From the behavior of aa rivers, and the common discrepancies 

between the estimated depth of the freely flowing lava stream and the 

observed depth of the empty channel after cessation of the eruption, 

Jaggar (1919, p. 136) and Finch (1943a, p. 1-2) have concluded that 

the flowing aa river is actually dual in nature, consisting of an upper 

rapidly flowing portion and a lower, more viscous, slower-moving 

portion.

At the end of activity, the upper fluid portion of the lava river 

and part of the lower less fluid portion drain away, leaving an open 

channel, 10 to 50 feet wide and 2 to 15 feet deep. The channel is 

floored with typical spinose aa, part of which may, however, show 

festoons resembling in both general form and origin those typical of 

pahoehoe, but with the spinose granulated surface of aa.

SPINES

The surfaces of some aa flows, particularly the thick massive 

flows of andesite, exhibit monolithic crags and spines of solid lava 

rising above the general flow surface. These are similar in origin to 

the squeeze-ups of more fluid flows and to the spines which form in 

great number on the surface of many viscous domes (tholoids) 

(Williams, 1932, p. 51-146). They are protrusions of lava squeezed 

up in viscous condition from the underlying part of the flow, through
64 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

openings in the solidified upper carapace. They have been found on a 

jagged aa flow on the floor of the Koolau Gap in the summit depression 

of Haleakala (East Maui) volcano, and on andesite flows on Mauna 

Kea. They range in height from 3 or 4 feet, commonly to 15 or 20 

feet, and may rarely reach as much as 100 feet (Stearns and Macdonald 

1942, p. 161). The monolithic core is generally surrounded by a 

talus bank of angular debris formed by crumbling of the outer part of 

the spine on cooling.

Accretionary lava balls are the balls found on the surfaces of many 

aa flows, formed by the rolling up of viscous lava around some fragment of solidified lava as a center, in much the same way as a snowball rolling down hill gains in diameter through the accretion of 

additional snow (Macdonald, 1943, p. 253-254). These balls have 

been called "bombes a roulement", but the name is inappropriate, 

as they are not at all like true volcanic bombs in origin. Jaggar 

(1926, p. 27, 45) has referred to them as concretionary balls. They 

range from approximate spheres to spheroids in which the long axis 

is nearly twice as long as the short axis. In diameter they range from 

about 3 inches to 15 feet (fig. 34). The core may be a fragment of 

clinker from the flow top, a massive block which caved off the bank 

of the channel, or even a fragment of older lava torn off the walls 

of the conduit by the rising magma (fig. 35). The surface of the 

ball may be quite smooth, but is more commonly very irregular.

A closely related form is the ball lava, or spheroidal lava, in the 

flow of 1823 along the Great Crack on the southwest rift zone of 

Kilauea volcano (Stone, 1926b, p. 434-440; Stearns, 1926, p. 336-351). 

The flow, where it is exposed in cross section in the walls of the fissure, 

contains great numbers of spheroidal lava balls, from 3 inches to 4 

feet in diameter, in a matrix of vesicular pahoehoe. Each ball consists 

of a core made up of one or more angular fragments of lava, enclosed 

in a shell of the 1823 pahoehoe. The shells range in thickness from 

1 inch to 8 or 10 inches. The core fragments may be chilled clots of 

1823 lava, hurled into the air by fountaining along the fissure, to fall 

back into the liquid lava; or fragments of older lava picked up from 

the walls of the eruptive fissure. Both types became wrapped in a 

coating of viscous pahoehoe. Similar balls, in smaller number, have 

been observed in other flows. In origin they resemble the cored 

bombs (see p. 83).

JOINTS

Both pahoehoe flows and the massive part of aa flows are generally 

broken by joints into innumerable irregular blocks. Well-developed 

columnar jointing is rare in Hawaii, but has been observed at a few
LAVA FLOWS 65

- [GTJtK 0-t. -UTK'f M iv.i i.oll, oliuvvu iii aecUun, Kiamoku lava flow from Mauna Loa, west of 

Kllauea caldera. Photo by Q. A. Macdonald.

FIGURE 35.- Ball lava in the How of 1823, exposed along the edge of the Great Crack on the southwest rift 

zone of Kllauea. Each ball contains a nucleus of older lava. Width of foreground about 10 feet. 

Photo by O. A. Macdonald.
66 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

localities, generally where the lava either certainly or possibly overflowed wet ground. The columns, where present, arc approximately 

normal to the cooling surface.

More commonly the joint blocks are highty irregular. Two or 

more sets of nearly vertical joints are roughly at right angles to a set 

of nearly horizontal joints, the latter being approximately parallel to 

the surface of the lava flow. The joint blocks are irregularly cubical 

or polygonal, and less commonly platy. They range in size from 1 

inch or 2 to several feet across. Jointing is generally much more 

prominent in dense lavas than in highly vesicular lavas.

Thick masses of the ultra basic rocks of the Honolulu volcanic 

series are commonly broken by cooling joints into great blocks, 10 or 

more feet across, with somewhat curved faces and roughly similar 

dimensions in all directions. The major faces of these blocks are 

often marked by a secondary system of cooling joints with a mesh of 

a few inches or a foot, indicating that they were still very hot when 

separation took place along the primary joint surfaces and provided a 

new focus of cooling (fig. :>>(}}.

-Fiui.uk ;iii. Secondary cooling joints on joint surfaces of large block of ultrabasic lava of the Ntiuanu 

volcanics. Height about 5 feet. Photo by C. K. Wentworth.

ORIGIN OF AA AND PAHOEHOE

The reasons for the formation of aa and pahoehoe have been discussed at length in another paper (Macdonald, 1953). Only a brief 

summary of the conclusions is given here.
PYROCLASTIC FORMATIONS 67

It is believed that the formation of aa instead of pahoehoe is the 

result of greater viscosity, resulting from greater loss of dissolved 

gases in the lava, and greater degree of crystallization. Both loss of 

gas and crystallization progress as the flow continues downslope, and 

consequently it is common to find pahoehoe in the portion of the flow 

near the vents changing into aa farther down the flow. Both loss of 

gas and crystallization are promoted by stirring, and consequently a 

flow is likely to change from pahoehoe to aa where it plunges over any 

steep escarpment. Violent fountaining at the vents also promotes 

the change of the lava into aa. On the other hand, if the lava erupts 

quietly, and flows quietly, crusting over and the formation of lava 

tubes may so retard the loss of gas that the lava remains pahoehoe to 

great distances from the vents. Loss of gas and crystallization 

eventually produce a degree of viscosity beyond which continued 

flowage results in breaking up of the upper portion of the flow into 

fragments. The spinose character of typical aa fragments results 

partly from pulling apart of still plastic surfaces, and partly from 

granularity caused by crystallization. The fragments continue to be 

modified in shape so long as the flow is moving by slight plastic 

deformation and by attrition against each other in the moving flow top.

PYROCLASTIC FORMATIONS 

GENERAL DEFINITION

The term pyroclastic is an adjective applying to detrital material 

which has been projected into the air from a volcanic vent (Wentworth 

and Williams, 1932, p. 24-25). Among volcanic vents we would 

include rootless vents such as those induced where a lava flow reaches 

the sea or other body of water (see p. 26). On the other hand the 

scope of the term p3a'oclastic is not synonymous with fragmental because parts of aa flows are fragmental without being in any significant 

sense aerial. Adherence to the criterion of aerial projection is justified because the shapes and textures of fragments and their sorting 

and distribution are characteristically related to such projection, and 

are often diagnostic of distinctive volcanic episodes, or relationships.

We learn especially from Hawaiian observations that significant 

amounts of pyroclastic material are commonly produced in the course 

of volcanic action that is predominantly nonexplosive. True explosive 

activity occasionally takes place in Hawaii and in addition pyroclastic 

material is ejected from nearly all lava flow vents to form crescentic 

source cones, ramparts of spatter material, or widely disseminated 

Pele's hair, and pumiceous cinders. The amount of material so expelled aerially, despite being a small fraction of the total output, may 

amount to a million cubic yards in the course of a day or two and
68 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

be sufficient to produce distinctive topographic effects. Terms for 

the various types of pyroclastic ejecta will be defined under the 

suitable headings below.

SIZES AND FORMS OF PYBOCLASTIC CONES

Conical pyroclastic accumulations found in Hawaii may be discriminated as tuff cones, cinder cones, and driblet cones. The tuff 

cones (tufa cones of Dana) occur notably on southeastern Oahu and 

are typified by Diamond Head at Honolulu (Wentworth, 1926). A 

few tuff cones occur elsewhere, such as Molokini near Maui and 

Lehua and Kaula near Kauai (Palmer, 1927, 1930, 1936), and all 

occur near the coast or offshore from the larger, basaltic islands. 

Cinder cones occur at all elevations and all distances from the sea 

and commonly mark rift zones or appear in groups near the summits 

of major shields such as Mauna Kea, and Hualalai, Kohala, or 

Haleakala volcanoes (Stearns and Macdonald, 1942, p. 20-23; Wentworth, 1938; Stearns and Macdonald, 1946, p. 162-164).

The tuff cones are the saucer type, the rim is the form of a wide 

circle, and the central depression often nearly as low as the surrounding country. The in-dipping beds and the out-dipping beds of all 

the higher runs approach an angle of repose of about 35°. The 

structure indicates clearly that the form has resulted from preponderant growth at a rather fixed radius by progressive mantle bedding. 

These tuff cones consist chiefly of palagonitized ash of grain size less 

than 1 millimeter and in many instances show maximum growth on 

the southwest side, leeward in respect to the prevalent trade winds. 

Occurrence of the tuff cones at or near the coast shows the agency 

of sea water in producing these phreatic explosions and in achieving 

the fineness of grain which in turn has promoted extensive alteration 

to palagonite. The same fineness of grain with violence of explosion 

has been responsible for the wideness of the circular rims and the 

marked effect of trade wind drift, especially in such cones as Koko 

Crater on Oahu. All these characteristics, plus the narrowness and 

symmetry of rims, give strong support to the doctrine of brevity of 

tuff cone eruptions first voiced by Sereno Bishop in 1901 (Wentworth, 

1926, p. 48-54).

The cinder cones, on Mauna Kea, Haleakala volcano and elsewhere, 

are higher than the tuff cones in relation to the diameter of the rim, 

and the bottom of the central bowl is usually higher above the external 

base. It appears that cones of this type were produced by explosive 

action having less tendency to spread and thus producing a smaller 

rim circle. These cones show much less effect of trade wind drift, 

though such is not entirely lacking. This may be due to the coarseness of the material, probable lesser height of explosion, and also that
PYROCLASTIC FORMATIONS 69

many of these cones in the summit areas lie at altitudes where the 

trade wind is weak or infrequent. Tantalus cone on Oahu, unlike 

Diamond Head and others of the Honolulu ultrabasic series, consists 

of cinders and ball agglomerate and is a fairly typical cinder cone. 

This is due to the absence of phreatic explosion, probably because of 

its inland position. The cinder cones give the impression of having 

steeper slopes, perhaps as much owing to their more commonly reaching angles of repose as to slightly higher inherent angles of repose of 

the coarser materials. Finally it should be noted that at higher elevations, where chemical weathering is less active and where the loose, 

uncemented pyroclastic material is resistant to physical weathering,- 

the summit cinder cones are so porous as to be very little subject to 

erosion. Aside from slight displacement of cinder slopes by glacial 

ice and slow creep due to snow and frost, producing types of rock 

stripes or polygonboden, the forms of these cones have suffered very 

little change.

Cinder-and-spatter cones associated with lava flows are common 

along the rift zones of Mauna Loa and some other shields. These 

cones consist of ill-sorted material ranging from cinders to bombs, and 

including spatter and driblet material reconstructed while still hot 

and plastic. Because of the eccentricities of eruption at a given vent 

and because of the cementing of larger masses of driblet, neither the 

expulsion nor the accumulation of the ejecta tends in these cones to 

such symmetry as is shown in true cinder or tuff cones. The cinderand-spatter cones are commonly breached on the downslope side and 

show other irregularities. Because of the cementing of part of the 

material, the walls may not collapse but may preserve the configuration of the upper part of the vent (Wentworth, 1938, p. 25-28).

Cones of cinder and ash formed along the coast by contact of lava 

flows with the sea are common along the coast of southern Hawaii 

(Wentworth, 1938, p. 102-104). With few exceptions, such littoral 

cones are now partly cut away by the sea, so that only part of the 

form can be seen. They appear to be piles of steeply dipping beds 

of ash or cinders, and may not have been built symmetrically around 

a fixed vent. Commonly a heap of cinder accumulated on each side 

of the principal lava stream where it entered the sea, resulting in twin

cones.

ASH BEDS

The most extensive pyroclastic formation in Hawaii is the Pahala 

ash which shows several different phases, with maximum thicknesses 

of more than 50 feet reaching 10 feet over at least two or three hundred square miles. Except for the red tuff lenses of the Koolau 

volcanic series on Oahu, this is the chief ash formation not closely 

continuous with known ash or cinder cones. The bedded deposits of
70 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

tuff comprising the Pahala ash and Ninole volcanic series have been 

described elsewhere (Stearns and Clark, 1930, p. 61-68; Wentworth, 

1938, p. 37-86; Stearns and Macdonald, 1946, p. 71-76). The distribution and form of such deposits is a product of then- mantle bedding from the air and there is an important contrast between such 

distribution and that of lava flows that must move downslope continuously from the source. Tuff formations may result from single 

eruptions, or they may in some places be accumulated from many 

eruptions whose products elsewhere may be interbedded throughout 

a great aggregate thickness of lava flows. In the wetter sections of 

the island of Hawaii, the Pahala ash is banded and includes many 

thin layers of recognizable tuff interbedded with equally thin layers 

of soil which because of their similar wide extent can hardly be other 

than weathered ash or tuff (Wentworth, 1938, p. 55-56).

Many if not all the lava-flow formations of Hawaii locally have 

thin lenses of ash or tuff produced by subordinate explosive phases 

of some eruptions. Those in the Koolau volcanic series of eastern 

Oahu..range up to several feet thick and may extend laterally for one 

or two thousand feet. They contain some accidental detritus but 

consist mostly of primary, palagonitized, fine-grained glass fragments. 

Such lenses are much more abundant in some areas than others. 

Relatively small masses of coarser and more steeply bedded agglomerate are found in some of the lava formations, on Oahu particularly 

in the Waianae volcanic series (Stearns and Vaksvik, 1935, p. 79-86).

Coarse "black sand" or volcanic-cinder beds extend over several 

square miles in the Honolulu area lying east of Punchbowl. The 

deposits were derived from the Tantalus and Sugar Loaf eruptions 

and the mantle bedding reaches a thickness more than 10 feet on 

parts of the low ground due south of Punchbowl and more than 

2 miles to leeward of the vents. Other smaller black sand formations 

are found elsewhere.

STRUCTURE AND TEXTURE OF MATERIALS 

DYNAMICS OF DISTRIBUTION

In respect to the origin of their size, composition, and their stratification, pyroclastic materials are sedimentary rocks. Their distributions in given thicknesses and sizes are primarily fixed by a very 

definite, if not easily definable, event or a series of events. In any 

single explosion, the fall and dispersal of the fragments of various 

sizes through quiet or drifting air gives rise to a symmetrical and 

somewhat systematic pile of material. The sizes of particles influence 

both the height of trajectory and the rates and paths of fall through 

the air.

If the slope on which the ash has fallen is steep, fragments may roll,
PYROCLASTIC FORMATIONS 71

giving rise to change in shape of particle, to growth by accretion, and 

to sorting of particles by size. Secondary transportation by water 

or wind is not considered here, but gravity effects on a slope built 

to the angle of repose must be considered as a necessary part of the 

original pyroclastic event. No more than these brief suggestions 

can be offered.

MANTLE BEDDINO

Because of distribution from the air with little or no influence by 

topography, the initial deposition of pyroclastic particles on a land 

or under-water surface produces a continuous mantle. According to 

the form of explosion, deposition at one point or in one zone may be 

maximum, but the amount does not change abrupt!y, the maximum 

position being like the mode of a frequency-distribution graph. 

Away from the mode the thickness and the coarseness of particles in 

a given layer ma3r change slowly with distance, but there is a continuity and sequence of layers in the section that is only equalled in 

marine deposits (fig. 37).

On a nearly level surface, the continued accumulation of ash or 

cinders may build the maximum pile or rim so that the slopes reach 

the angle of rest for the material. Such a condition may be produced 

also on original steep slopes or at the edge of a cliff. Because of 

differences in rate of accumulation, or irregularities in the surface on

FIOUKE 37. Mantle bedding in volcanic cinders, Tantalus Roud, Oahu. Photo by C. K. Wentworth.
72 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

which pyroclastics are deposited, monoclines, synclines, or anticlines 

may be formed that are occasionally misinterpreted as due to deformation. Rapidly accumulated pyroclastics may suffer contemporaneous 

slumping and form small normal faults or other distortions. Channels 

may be promptly eroded during rain showers that often accompany 

such eruptions. Thus, during continuous deposition small unconformities may be produced which, though valid, are of very limited 

significance, both in time and space.

Compared with most sediments, successive layers in pyroclastics 

are commonly less sharply set off from those adjacent because of the 

overlapping of each sort of material by those coming before and after. 

Thus from phase to phase there is a gradual transition in both coarseness, shape, and composition that produces a distinct lamination but 

often does not form clearly marked strata. In tuffs, the laminations 

are often accentuated by alteration or deposition of cementing 

materials deposited in layers of favorable texture or permeability.

ANGLES OF REPOSE

Subaerial angles of repose of beds in tuff cones most commonly 

range up to about 31°, depending on shapes and sizes of fragments, 

moisture content, and other factors. The angle of repose in cinders

FIGURE 38. Section of black cinders showing "reversed coarseness," Tantalus, Oahu. These lapilli range 

from about 2 to 16 millimeters. Photo by C. K. Wentworth.
PYROCLASTIC FORMATIONS 73

reaches 33° or 34° maximum. True angles of repose due to gravity 

commonly grade smoothly into lower angles of slope determined by 

effects of moisture, frost, wind, and plant growth. In some cinder 

beds at the angle of repose the fragments are finer at the base and 

grade upward to coarser material at the top, giving way again to the 

finer material of the base of another such layer (fig. 38). This is the 

reverse of what might result from winnowing of heterogeneous products 

of a single explosion. It is possible that the mechanics of explosion 

produces more finely comminuted fragments at the beginning of a 

spasm than toward the end. It has also been suggested that this 

effect may be due to progressive accumulation by rolling and sliding, 

on the premise that coarse material will roll on finer, but not the 

reverse.

TYPES OF MATERIAL AND FRAGMENTS 

DRIBLET

Driblet is a term applied to fragments larger than lapilli and showing 

forms assumed while plastic at the time of detachment from the 

magma or when landing after flight (Wentworth and Williams, 1932, 

p. 46). The term spatter is essentially synonymous (Stearns and 

Macdonald, 1946, p. 17). Deposits are not uncommon in which the 

components lie pancaked, sagging over one another while still showing 

their discrete character. A considerable part of the Mauumae cone 

inland from Kaimuki, Oahu, consists of driblet beds, as shown both in 

outcrops and also in the tunnel recently driven through its flank. 

The component cakes range in size from 2 to 10 inches across and are 

molded over one another to form a complexly interlocked although 

still quite porous mass (fig. 10). The individual masses are somewhat 

vesicular but may be fairly dense between the vesicles.

Occasionally small masses of flow lava are interbedded with the 

detrital material of such accumulations, often at steep angles, and 

these may be derived from overflow, or may be built from recombined 

driblet that is still liquid. Driblet is capable of building up to very 

steep slopes, vertical or slightly overhanging in places, and small 

driblet cones or hornitos ma.y be formed around a persistent vent. 

These are commonly well bonded through being kept near melting 

temperature and the surfaces of the vent tubes are often strikingly 

glazed by remelting.

AG GLOMERATE

The term agglomerate has been applied in Hawaii to poorly sorted 

and somewhat indurated pyroclastic material coarser than the palagonitic tuff. In the Honolulu volcanic series of Oahu, the chief 

agglomerate is that near the Sugar Loaf and Tantalus vents. Some
74 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

of this material contains cored bombs (Brady and Webb, 1939) 

several inches in diameter, probably formed by growth within the vent.

CINDERS

This term is applied to glassy, vesicular ejecta coarser than sand 

sizes and ranging up to an inch or more in diameter. Such material is 

light in weight and the particles show glazed, pulled, or freshly broken 

surfaces that are often iridescent when fresh. Some cinders are 

extremely light, ranging to reticulite (see below) but the more common 

texture is pumiceous, with irregular, complete vesicles. More rarely 

the lapilli may be dense glass having but few vesicles. The cinder 

formations in place are commonly very porous. Some representative 

porosities are given below.

Specific gravity and porosity of cinders from island of Hawaii

Specimen 

no.

544a ...

626. 

623.......

744.......

813. .

Specific gravity

Gross

0.287 

.548 

1.29 

.306 

1.533

Of lapilli

0.467 

1.00 

2.07 

.919

Of 

powder

2.562 

2.58 

2.99 

2.435

2.84

Porosity

Gross 

(percent)

89 

79 

57 

87.5 

46

Aggregate 

(percent)

39 

45 

38 

67

Of lapilli 

(percent)

82 

61 

31 

63

Description

Pumiceous cinders erupted from 

Halemaumau, July 1929. 

Cinder beds, near Kalapana, island of Hawaii. 

Ash, from cone near Kalapana, 

island of Hawaii. 

Basaltic cinders, Puu Waawaa, 

island of Hawaii. 

Black ash sand, Punaluu, island 

of Hawaii.

Specific gravity and porosity of black ash from island of Oahu

Specimen no.

Diamond Head: 

50 . 

85 

161 

Specific gravity

Gross

1.30 

1.08 

1.08

.99 

1.00 

1.00 

.82 

.83

1.02

Of grains

2.44 

2.37 

. 2.57

2.39 

2.40 

2.47 

. 2.25 

2 41

2.41

Porosity 

(total 

percent)

0.470 

.544 

.581

.586 

.585 

.597 

.637 

.655

.577

The cinders of the 1929 eruption of Kilauea volcano (544a) thrown 

and wafted from the 1,000-foot pit of Halemaumau, were very light, 

probably because of selective winnowing. The units are rounded, 

mostly 4 to 12 millimeters in diameter, with a glassy, dark-brown, 

iridescent skin. The cinders of Tantalus, Oahu, are closer to the
PYROCLASTIC FORMATIONS 75

dense, vitreous end of the series. Many particles are broken sections 

of pulled masses. From most descriptions it appears that some very 

light pumiceous cinders and reticulite are generally produced at the 

sources of lava flows (Stearns and Clark, 1930, p. 112-113). On the 

contrary, the lapilli of littoral cones are usually a more dense glass 

with a few larger vesicles; because the lava of such flows has lost much 

of its magmatic gas, the explosions are due to external action of 

steam (Macdonald, 1949a, p. 72).

No sharp line can be drawn between volcanic cinders and volcanic 

sand, though intermediate material is more likely to be designated 

cinders if the grains are vesicular and attenuate, and sand if they are 

equidimensional and dense. The Tantalus-Sugar Loaf pyroclastics 

consist mainly of material coarse enough not to have been conspicuously palagonitized, but they include both sand and cinders. Material that is mostly % to 4 millimeters in diameter and fairly dense 

is commonly called "black sand"; parts that are coarser and more 

vesicular are called cinders. If still coarser, usually partly lithic, and 

not vesicular, the aggregate is more commonly called agglomerate. 

In practical excavation and construction usage in Hawaii, volcanic 

detritus coarser than black sand has often been called "gravel".

Despite the objection that is sometimes voiced in regard to the 

term "ash", no better one has been proposed. In a descriptive sense 

there is also much justification for the occasional use of terms sand, 

dust, and grit, in reference to particular pyroclastic beds. Juvenile 

pyroclastic detritus of grain sizes from 4 millimeters down to dust 

occurs in several different units of the Honolulu volcanic series in 

southeastern Oahu, and as parts of cinder cones, and littoral cone 

formations on Hawaii and other islands of the group. Accumulations 

consisting chiefly or wholly of sand or dust grains derived by explosion 

from earlier rocks, and hence accessory, are shown in some variety in 

the area surrounding the Kilauea vent, and to a limited extent 

elsewhere.

Occasional layers of juvenile black ash may be found in some of the 

cones formed chiefly of palagonitic tuff, though such nonpalagonitized 

layers more commonly consist of cinders. They apparently result 

from some condition of lesser moisture or heat that did not favor the 

alteration to palagonite. In such places, most of the black ash is 

slightly coarser or more uniform in grain than other formations that 

were altered to palagonite on the grain surfaces. The two best examples of black ash on Oahu are the Diamond Head ash of the Black 

Point area and the black ash of the Punchbowl area which is a fine253279 53 6
76 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

grained phase of the Tantalus-Sugar Loaf basalt. The characteristics of the Diamond Head ash have been set forth by various observers 

and were analysed in detail in substantiating the origin of this formation by explosion of magmatic material, rather than by marine abrasion of volcanic rocks as had been suggested (Hitchcock, 1909, p. 

35-36; Wentworth, 1926, p. 99-100, 1937, p. 91-103; Stearns and 

Vaksvik, 1935, p. 138, 142).v

Such juvenile ash consists chiefly of more or less vesicular glass 

with many of the grains showing pulled, twisted, or droplet forms and 

including crystals of olivine, magnetite, and occasional other minerals 

both within the glass and free. Nearly everywhere such ash beds are 

marked by layers and joint or plant-stem fillings of calcium carbonate. 

Such calcification may be aided by leaching from accidental calcareous 

grains or fragments deposited at the time of the explosion and derived 

from underlying reef limestone. However, it is also common where no 

such source of calcium carbonate is evident, and it is logically 

presumed to have been provided through weathering of the ash 

itself.

Where the ash beds or similar cinder beds are thick and even only 

slightly cemented or interlocked, they often stand in vertical exposures and in such places extraordinarily deep and narrow rill clefts 

may be cut, even to vertical depths of 50 or more feet with a width at 

the top less than 2 feet. These are especially characteristic of the 

saddle areas on the uphill sides of small ash and cinder cones on the 

flanks of Mauna Kea, and in the head of Pauoa Valley, Honolulu.

Among the conspicuous black sand deposits of littoral origin are 

those near the east cape of Hawaii, and at Kalapana and Punaluu on 

the southern coast of Hawaii, where they give rise to well-known 

black sand beaches. The vitric grains of these littoral cones are 

fairly dense glass and become abraded black beach sand grains, contrasting strikingly with the gray tone of the equally abraded but 

larger lithic pebbles of basalt associated with them. Puu Hou 

(New Hill), a littoral cone formed where the Kahuku lava flow 

reached the coast of Hawaii west of South Point in 1868, carries in its 

Hawaiian name the equivalent of the similar, well-known Monte 

Nuovo near Naples.

Some pyroclastic beds in the Kilauea caldera area are composed of 

wholly essential material, such as the reticulite beds, or the mantles of 

pumiceous lapilli like that thrown out in 1929. The material of others 

is totally accessory, gray or lavender dust, or multicolored grit and 

lapilli, such as the deposits of 1790 and those of earlier explosions 

(Powers, 1948). Various intermediate sorts occur, especially olivegreen sandy layers (Wentworth, 1938, Spec. 552a, p. 150).
PYROCLASTIC FORMATIONS 77 

TUFF

The principal application of the term tuff in Hawaii is to the brown, 

tan, or mottled and lavender tuffs of the southeast Oahu variety that 

are generally palagonitized, although to a limited extent the term 

may be required in Hawaii for certain indurated beds of cinders or 

other pyroclastics that are not palagonitic. These rocks were called 

tufa by Dana, and considered by him as having been deposited as a 

mud (Dana, 1890, p. 292). More extensive study fails to reveal 

structures in the rock indicating movement as a mud, so all recent 

workers have recognized the aerial placement of the material as 

discrete grains, even though moisture and heat probably aided the 

alteration to palagonite.

The palagonite tuff is a compact rock of chalklike texture and 

variable grain size. Fragments of basalt, essential or accessory, are 

commonly found in it, but the bulk of the rock consists of dust or 

coarser grains, or aggregates of grains, of basaltic glass, of which the 

outer portion has been altered to palagonite. Extremely little, if any, 

fresh glass is exposed in the fracture surface of the tuff. In thin 

section, the interiors of nearly all grains and a large fraction of the 

whole appear as clear, fresh glass with only narrow rims of palagonite.

The tuff breaks into irregular blocks of various sizes with little 

tendency to form slabs parallel to the lamination. The tuffs of 

Hawaii generally show more variation from bed to bed, are more 

subject to natural separation into small blocks, and do not show the 

massive, nearly unbedded appearance characteristic of occurrences of 

palagonite tuff in some other parts of the world, such as Iwo Jima.

The dominant structure of palagonite tuff of Hawaii is mantle 

bedding, identical with that shown in the black cinders and black 

ash. Some calcareous cementation and joint filling, including some 

well-banded travertine zones, is general, but the induration of the 

mass is principally achieved by the formation of the palagonite 

(fig. 39). Occasional beds have the firmness of a medium brick but most 

are more friable than brick and do not permit dressing of well-shaped 

hand specimens. Hawaii tuff is generally not now used in construction of any sort.

Owing to the wide range of grade sizes, the interlocking of the 

shards and irregular pellets of glass, and the formation of palagonite, 

the tuff is a comparatively impervious rock. In the Honolulu artesian 

area the tuff is part of the cap rock and is never an aquifer. In the 

Pahala region of Hawaii, tuff beds intercalated with lava flows serve 

as water perching layers (Meinzer, 1930), and successful water tunnels 

have been driven on their upper surfaces. Beds of red tuff in the 

Koolau volcanic series of the Honolulu area perch water in the same 

way, but in areas too small to attract exploitation.
78 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

FIGURE 39. Megascopic texture of palagonite tuff, Salt Lake Crater, Oahu (X2). 

Photo by C. K. Wentworth.

KBTICULITB

Dana (1890, p. 163-166) described this remarkable material as 

"thread-lace scoria." The name reticulite was proposed by Wentworth and Williams (1932) to avoid certain objections to the use of 

the word scoria in Dana's term. Well-developed reticulite is found 

chiefly in the vicinity of Kilauea caldera, although small amounts of 

open-textured pumice may be found in certain lava-fountain deposits 

elsewhere in Hawaii. The true reticulite is remarkable in that practically all the walls between the cells have burst and essentially no 

closed space exists in the thread network. Pumice, on the other 

hand, typically consists of closed gas-formed cells. Pumice commonly 

floats, but reticulite, while weighing very much less than pumice, is 

open to nearly complete penetration by water and does not float for 

long. In a genetic series reticulite lies between pumice and ash.

The notable occurrence of reticulite at Kilauea caldcra is a stratum 

10 to 16 inches thick over 2 or 3 square miles north and east 

of the caldera. This layer, which thins rapidly away from the caldera, 

forms the basal member of a series of pyroclastic beds antedating the 

ash of 1790. Reticulite also is found in lesser amounts in other parts 

of the Kilauea pyroclastics, both older and younger. The reticulite
PYROCLASTIC FORMATIONS 79

beds consist of masses of typical glass network partly enclosed in a 

thin, brown or iridescent membrane. The masses are commonly 1 

inch in diameter and occasionally reach 3 inches. Much of the 

accumulation consists of broken material, probably crushed in place. 

The structure of reticulite has been thoroughly described by Dana 

and others. It is important, however, to emphasize the completeness 

with which the walls of vesicles have been resorbed into the threads 

which join the solid angles and the failure of those threads to assume 

a round cross section. They retain the cross section of a triangle 

with recurved sides and extremely sharp edges, indicating prompt 

cooling after the walls of vesicles had burst. Probably the rapid 

expansion of gas and the change in mechanical conditions induced 

this nearly instantaneous freezing.

FIGURE 40. Detail of texture of reticulite, photomicrograph of immersed section. The larger rings have 

a diameter of about one-fourth millimeter. Photo by C. K. Wentworth.

It is apparent that such a change must have taken place at some 

stage in the explosive rise of expanding foam. The general network 

is made up of polygonal rings % to 2 millimeters in diameter and the 

threads are about 1/20 millimeter across the whole triangular section 

(fig. 40) (Wentworth, 1938, p. 149). The porosity of reticulite specimens has been determined at 98.3 percent by Dana and 98 percent 

by Wentworth. The amount of water required to account for the
80 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

expansion of the reticulite has been calculated to be about 1.1 percent 

by weight (Dana, 1890, p. 67).

PELE'S HAIR

The threads of rock glass drawn out by flying clots of molten lava 

during bubbling or fountaining stages of volcanic activity have long 

been called Pele's hair (Dana, 1890, p. 160), and have been described 

by various observers (Brigham, 1887, p. 22; Krukenberg, 1877). 

These threads commonly have nearly uniform cross section for 

several inches and may occasionally reach a yard in length. The 

diameter mostly ranges from 0.01 to 0.10 millimeter but may be 

greater. The smaller threads are clear, light-green glass; larger 

threads carry crystallites and bubbles, the latter sometimes expanded 

to form glass bulbs one-half millimeter in diameter. Some of the 

threads show a pulled, minutely striated structure and configuration 

and show slight anisotropism between crossed nicols, owing to strain 

in the glass.

Pele's hair is often carried high into the air during strong fountaining and wafted for several miles by the wind. Probably the hot 

updraft of active fountains permits spinning of such glass during the 

period of several seconds corresponding to the trajectory of the 

higher fountains, and the branching and complex forms are probably 

due to collision and adherence while still plastic. Given certain 

temperatures and velocities of pulling, the diameter of the spun 

threads is probably a measure of the contemporary viscosity, but no 

practical formulation of these conditions has yet been offered.

BOMBS

Bombs are fragments of lava that were liquid or plastic at the time 

of ejection and assumed characteristic forms as a result of forces 

acting during flight. The mechanics causing various forms have 

been discussed in detail by several students (Reck, 1915, p. 39-61; 

Stearns and Clark, 1930, p. 115; Wentworth, 1938, p. 111-115; Macdonald, 1949a, p. 81-82). There is much variety in the forms and 

commonly those of a given locality are of one kind only, leading to 

misapplication of names or misinterpretation of mode of origin.

Bombs of the Hawaiian volcanoes may be classified according to 

their shape, which in turn appears largely to depend on the viscosity 

of the lava. The very fluid ejecta of Mauna Loa and Kilauea volcanoes commonly flatten out on striking the ground to produce the 

so-called "Hawaiian type" (cow dung or pancake) bombs. Blobs 

ejected during the 1790 and late prehistoric explosive eruptions of 

Kilauea were sufficiently fluid to draw into spherical shapes during
PYROCLASTIC FORMATIONS 81

flight, but solid enough to retain their shape on striking the ground 

(Macdonald, 1949a, p. 82). Less fluidity in the ejected blobs apparently results in the fusiform (almond-shaped or spindle-shaped) 

bombs. The solidified and broken up strings of magma ejected with 

the fusiform bombs, and to some extent drawn out from them, constitute the ribbon bombs.

In Hawaii bombs are found most abundantly on many of the cinder cones of Mauna Kea, Kohala, Hualalai, and Haleakala volcanoes, 

and-of a few other localities. These are chiefly of the fusiform and 

ribbon types, and range from less than 1 inch to 6 feet long, some of 

them weighing more than a ton (fig. 41). It is noteworthy that 

bombs appear to be much more abundant in the surficial layers of 

some cones than in the interior of the cones. The formation of 

bombs apparently is favored by increasing viscosity and waning violence of explosion toward the end of an eruption. The shapes and 

structure of the Mauna Kea bombs have been described elsewhere 

(Wentworth, 1938, p. 111-115).

It appears that the fusiform bombs are typically due to modification, during flight, of a blob of lava detached from other lava at both 

ends. The chief factors involved in shaping the bomb are the surface tension, which tends to produce the globose form, and the fluid 

drag of surrounding air that tends to cause skin flowage and develop 

a contrast between the forward and following sides. No basis is 

seen for "accepting the view that the shapes, particularly the end 

projections, are due to rotation during flight (Stearns and Clark, 

1930, p. 115).

The concept that the "ears" or ends are due to centrifugal force is 

unsound in a dynamic sense, there being no dynamic reason for genesis 

of two protuberances. Stearns states that "the spiral motion causes 

the bomb to develop earlike projections on the ends of its axis of 

rotation." The dominant elements of shape are the continuity of 

comparable cross section from end to end caused by pulling, the 

thickening of the middle brought about by surface tension while aloft, 

and the surface drag and contrast between stoss and lee sides caused 

by air resistance and drag while holding a nearly steady orientation 

during the flight. These elements do not indicate or require spinning and the latter denies it. Breadcrust bombs, that is, bombs showing shrinkage cracks in the skin, are found sparingly on Mauna Kea 

and elsewhere. Ribbon bombs are filaments of lava without medial 

swellings. This type commonly shows continuity of ribbing from 

one end to the other, and passes uninterruptedly into fusiform bombs. 

Juvenile lava fragments are found in the tuff craters of southeast 

Oahu but distinctive shapes that would be called bombs are rare.
82 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

Kp: 11. 

the south 

size range.

- Pile of almond-shaped bombs gathered within a 100-foot radius at the eone Kalopeainoa, on 

slope of Mauna Kea, to illustrate relative abundance and similarity of form through a large 

Photo by C. K. Wontworth.
PYROCLASTIC FORMATIONS 83 

BLOCKS

Accessory or accidental blocks are found in the tuff cones of the 

Honolulu volcanic series of southeast Oahu, and accessory blocks are 

common in the several formations that surround Kilauea caldera. 

Both lava blocks and blocks of reef limestone are found in the tuffs 

of the Honolulu volcanic series. The limestone fragments range 

upward from single small shells to reef blocks of several hundred 

pounds weight. They occur throughout the tuff section, in some 

layers more markedly than in others, and up to the tops of cones 1,000 

feet or more above sea level. From the occurrence of these marine 

forms, some observers have wrongly deduced emergence of the land 

without recognizing the process of emplacement by explosive eruption 

and opening of vents through the lava and reef formations.

At Kilauea the caldera floor immediately surrounding the pit of 

Halemaumau is strewn with blocks of various types of basalt thrown 

out during the explosive eruption of 1924. Approximately 60 million 

cubic feet of material was thrown out over the surrounding country 

(Finch, R. H., in Jaggar, 1924, p. 117), and the blocks, all sizes and 

ranging up to several tons weight, represent only a very small part of 

the total. They came from the walls of Halemaumau, which was 

collapsing by progressive engulfment, and were thrown out by spasmodic steam explosions. A block estimated at 8 tons was thrown 

3,500 feet from the center of Halemaumau, and a still larger block 

was thrown out onto the edge but fell back into the pit with a wall 

avalanche 6 years afterward. All these are angular blocks of wall rock 

and show no signs of fusion, though some were incandescent when 

ejected.

CORED BOMBS

Bombs containing xenolithic nuclei have been termed "cored 

bombs" by Brady and Webb (1943). Bombs of this type are fairly 

common in cinder cones on Kilauea and Mauna Kea volcanoes, and 

in the deposits of the 1790 and late prehistoric explosive eruptions in 

and near Kilauea caldera, and are occasionally found at other localities 

in Hawaii. The nucleus may be either an accessory fragment picked 

up from the walls of the conduit, or an essential fragment of clinker, 

produced during the same eruption as the bomb. The rock fragment, 

or piece of clinker which fell back into the vent, becomes coated with 

liquid magma and is ejected as a bomb. Most of the cored bombs 

are nearly spherical, but some are distinctly fusiform. In size they 

range from about 1 foot in diameter to lapilli about half an inch in 

diameter. The vent of the 1801 lava flow of Hualalai volcano threw 

out many lapilli and small bombs that consist of cores of bright-green 

dunite surrounded by a shell of black glassy lava.
84 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

ACCRETIONARY LAPELM

Volcanic pisolites from Kilauea vent, with diameters of 3 to 10 

millimeters, were described by Ferret (1913). Because the etymology 

of the word "pisolite" has nothing to do with the volcanic variety 

and because the usage is not very firmly established, the term accretionary lapilli has been suggested (Wentworth and Williams, 1932, 

p. 37), and is used here.

Accretionary lapilli are formed by rain drops, which fall through 

ash-laden air, and also by rolling of wet nuclei on a surface covered 

with fresh ash (Macdonald, 1949a, p. 70). The occurrence of some 

in steeply dipping strata suggests the latter genesis. Accretionary 

lapilli 5 to 20 millimeters in diameter have been found in steeply 

dipping layers in the Koko Crater region and elsewhere on Oahu, and 

these are thought to result from accretion while rolling down the 

slopes of the cones. Other larger lapilli found near Sugar Loaf vent 

in the Honolulu area are probably formed by accretion and milling 

during repeated tossing within a volcanic vent.

MINOR STRUCTURES IN PYROCLASTIC FORMATIONS 

BOMB SAGS

These structures are displayed chiefly in the fine-grained palagonite 

tuff of various cones where a bomb or block has landed on ash already 

fallen and is presently buried in the later ash. In section, the laminae 

below are slightly depressed or deranged and the laminae above are 

mantle bedded over the bomb in the form of a shallow dome (fig. 42). 

The effect is confined to two or three diameters of the bomb laterally 

and even a shorter distance below and above. Where the bomb lies 

exposed on an eroded bedding surface, the bomb sag appears as a 

sort of nested structure of layers dipping under the bomb from all 

sides.

TREE AND OTHER MOLiDS

External molds of tree trunks or of other parts of plants are found 

in tuff formations or in cinder or black sand beds. In the latter case 

the form is often preserved through filling and cementation b}7 calcium 

carbonate. Though this may occur in the tuff the more usual condition is a simple opening. A number of such openings, both vertical 

and prostrate, some branched, are especially well preserved in the 

upper tuff of the road cut a few yards toward Red Hill saddle from 

the South Halawa bridge, on Oahu. Leaf molds have been found in 

tuff in the Kalae area, Hawaii, and probably elsewhere, but none but 

modern plant forms are known to have been identified.

JOINTING

Several kinds of jointing are seen in the pyroclastic rocks, especially 

the palagonitic tuffs. The major joints are commonly vertical and
PYROCLASTIC FORMATIONS 85

JJ'IUUKE 42. Outcrop i>[ lull showing bomb sag, where a. block or bomb depresses the underlying strata 

and causes a domed structure in the mantling beds above. Photo by C. K. Wentworth.

transverse to the greatest dimension of the mass. In a crater rim they 

are radial to the vent and cut across the circular rim. Spacing in some 

parts of the Diamond Head tuff cone is not more than 3 or 4 feet; 

wider spacing is found in some other masses. Commonly these joints 

are filled by calcium carbonate.

In some places where lava flows have cooled immediately over tuff 

beds, the upper few inches of the tuff beds have a columnar structure 

with joints spaced only 2 or 3 inches apart. A third type of jointing 

is displayed in many places where the palagonite tuff crumbles on 

dry, bare rock surfaces. To a slight extent the tuff splits along bedding 

surfaces but more commonly it crumbles to small, rudely cuboidal or 

compact polyhedral masses, generally from a fraction of 1 to 2 or 

3 inches in diameter, from which corners tend to spall off rather 

neatly. This crumbling is probably induced by the volume expansion 

associated with wetting followed by drying. It has some features in 

common with the spheroidal weathering that takes place in deeply 

weathered basalt, but forms are less pronounced.

CEMENTATION

Most of the pyroclastic accumulations are only weakly cemented 

but there is a considerable range in this respect. Occasionally black
86 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

sand or cinder beds are without any bonding and slump down to an 

angle of rest as freely as dry dune sand. More commonly such beds 

are lightly and almost imperceptibly cemented b}7 calcium carbonate, 

or in the coarser sizes so interlocked by irregular shape that the mass 

will stand in a vertical cut 10 or 20 feet high. Only rarely and locally 

are black sand or cinders sufficiently cemented to require a hammer for 

collecting any particular bed, although a few beds come out in slabs 

that can be prepared for very careful handling as hand specimen!?.

Masses of driblet often are rather tightly interlocked and in hornitos 

may become fully welded. Some bombs are so plastic when they hit 

the ground that fragments of rock or other bombs may be inbedded 

in them. 'f;, .....'

Palagonitic tuff commonly carries an appreciable amount of cementing calcium carbonate, but even without this it would be moderately 

well cemented by the palagonite shells. This rock commonly has about 

the strength of chalk.

Beds of the finer grained accessory volcanic dust from such eruptions as 1790 and 1924 at Kilauea caldera often are slightly indurated. 

This induration is probably due to bonding that takes place in the 

drying of many fine-grained materials.

INTRUSIVE FORMATIONS 

DIKES

No large intrusive masses are exposed in Hawaii; erosion has reached 

only two or three thousand feet downward into any of the lava shields 

and there is no positive knowledge that large intrusive masses exist 

at any depth.

Dikes locally follow the jointing at a few degrees off vertical but 

in the over-all course tend to offset to approximate a vertical attitude. 

They are found sparingly in most of the lava formations of Hawaii; 

the concentration over much of the area is of the order of one dike 

per thousand transverse feet or fewer. The most detailed and systematic mapping has been done in the Honolulu area, where dikes are 

found to increase in abundance from the periphery of the shield 

toward the rift axis along which the Koolau dike complex is developed 

(Wentworth and Jones, 1940). Subordinate dike swarms are found 

in this area and there are some variations in the decrease of frequency 

of dikes away from the rift zone. Because of the greater numbers, 

the character of the dikes has mostly been observed in the area of 

dike complexes. Columnar jointing is general but the polygonal sections of the columns are often quite irregular (fig. 43). Many of the 

larger dikes show three phases of jointing. They are shown in the 

central part where the finely crystalline texture is most fully developed, 

on the two sides where the rock is less perfectly crystalline and where
INTRUSIVE FORMATIONS 87

FIGURE 43. Dike in channel of Wailupe Stream, Oahu, showing principal columnar jointing. Near the 

end of each column is splinter jointing that marks the dike margin. Photo by C. K. Wentworth.

incipient splintering invades from the sides, and finally, in the finemeshed checking of the glassy selvage, often with a spacing of not 

over yt to }'i inch. These patterns are related to thickness and tensile properties of the respective masses; dikes several feet thick have 

columns a foot across in the central phase (fig. 44).

In the Honolulu area the prevailing thickness of dikes is 2 to 3 feet, 

with very few dikes more than 6 feet thick. In parts of the dike 

complex, a few thicker dikes are found, but commonly these are compound. Studies of the Koolau mass indicate that most of the dikes 

have resulted from the passive rise of lava toward the surface and 

that commonly no more pressure was required than the hydrostatic 

pressure needed to reach the surface. In a close pattern of dikes 

and sills, pressure of intrusion may have somewhat exceeded load 

pressure. It was concluded that the thickness of dikes was the amount 

of shrinkage of the lava flows represented in the rather moderate distance along which horizontal displacement could take place (Wentworth and Jones, 1940, p. 985).

Observations in the Honolulu area show a systematic, if somewhat 

irregular, relationship between the texture of dikes and the depth 

below the surface at which they were probably intruded. At depths 

300 feet or more below the surface of the volcanic mass, the dikes are 

commonly dense rock with few vesicles and conspicuous columnar 

jointing, as if formed under pressure and with an orderly history of 

cooling in relation to the surrounding rock. On the other hand,
88 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

FIGURE 44. Detail of checking in the glass selvage layer of a dike. The pattern of the larger columns, as 

well as the intermediate splintering of these units, is shown. Photo by C. K. Wentworth.

dikes intruded at depths of 200 feet or less show imperfect columnar 

jointing and commonly show bands of vesicles parallel to the contact with the country rock. There are many differences in individual 

dikes, due to thermal and other differences in the adjacent rock, and 

these figures give an approximate view of the prevailing condition inferred from data collected mainly by A. E. Jones (Wentworth and 

Jones, 1940, p. 990-993).

SILLS

Most sills in Hawaii are inclined less than 15° to the horizontal, 

conformable to the lava-flow bedding. Commonly they follow one 

bedding plane for a few tens of feet, then break across the bedding to 

a new horizon and resume their sill character. Sills are commonly 

offset or bifurcated as seen in section and are commonly associated 

with irregular dikes. They are less abundant than dikes and probably represent a more aggressive intrusive action. A few large sills 

have been traced with only a few breaks for 1,000 feet or more but 

probably most extend for much shorter distances. The jointing and 

the selvages of sills are similar to those of dikes. Sills are sufficiently 

impermeable to play a part in the perching of ground water. Small 

springs or seeps commonly emerge from above the outcrops of sills. 

However, we know of no instance in Hawaii where substantial amounts 

of water are perched by a sill without a preponderant confining of 

ground water in the same area by associated dikes.
INTRUSIVE FORMATIONS 89

NECKS, PLUGS, AND BUDS

Fillings of tubular conduits are not abundant in exposed sections in 

Hawaii. A few are found on the island of Maui in the Kula volcanic 

series (Stearns and Macdonald, 1942, p. 83-84, 164). The Palolo 

quarry mass of the Koolau Range on Oahu is an expanded complex 

dike, which shows in various parts columnar joints, platy joints, and 

irregular polyhedral masses according to local placement and cooling 

history. It is not evident that this mass at depth has any form other 

than that of a thick dike.

In several localities in the southeastern Koolau Range of Oahu 

masses of intrusive rock have an oval ground plan with the long axis 

in many instances coinciding with a known dike. These masses occur at or very near the elevation of the original surface of the shield 

and are believed to be thickened parts of the dikes that have taken 

a somewhat bulbous form near the point of pressure relief at the surface. They have been called buds and show centripetal jointing and 

concentric selvage and texture zones comparable to those of dikes 

(Wentworth and Jones, 1940, p. 988-990).

The rounded domes (tholoids) of trachyte and related rocks that 

occur on West Maui are considered to be partly intrusive. They 

range from 100 to 600 feet high and 1,000 to 3,000 feet in diameter 

and are believed to have been squeezed from a small central opening 

when in a viscous condition (Stearns and Macdonald, 1942, p. 21-23).

DIKE COMPLEXES

The term dike complex was first applied by Stearns in his description of the rift zone formation of the Koolau Range (Stearns and 

Vaksvik, 1935, p. 77, 95). As a general term it is quite satisfactory 

but with increasing knowledge of the occurrence of such formations 

further attention to definition may be required. A dike complex is 

characterized by a notable concentration of dikes. In some instances 

they make up almost the whole mass with only small amounts of normal lava flows between them (fig. 45). Stearns and Vaksvik refer 

to the impracticability of mapping the individual dikes, and comment 

on the rapidity with which the frequency of dikes falls off outside 

the limits of the dike complex proper.

The most extensively exposed dike complexes are those of the 

Koolau Range and Waianae Mountains on Oahu and that of East 

Molokai. Others are mostly less deeply eroded.

The essential characteristic of the dike complexes of Hawaii is 

that they represent recurrent intrusion in parallel or subparallel 

pattern along the line of a source rift zone. The concentration of 

dikes necessary to justify the use of the term dike complex must vary 

somewhat with the nature of the normal surrounding lava-flow
90 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII
LITERATURE CITED 91

formation and in practice will be difficult to define precisely. Parts 

of the well-exposed dike complexes of the Moknlua Islands off Oahu, 

of the Kolekole Pass area, and of the Waikane-Waiahole area show 

dikes in excess of 100 to the mile, measured transversely, and locally 

approaching 1,000 to the mile.

It may be profitable to consider what number of dikes might be 

required to build the upper part of such a shield as the Koolau mass. 

Assume that in any given section transverse to the rift zone each lava 

flow reaches from its vent to the coast or beyond and averages 10 feet of 

increased height to the range on one side. Assume another flow to be 

required for the same gain on the other side and that each has come 

from a separate dike. Assume the lengths of dikes to be comparable to 

the widths of the resulting flows. This would mean two dikes for each 

10 feet or 200 for each 1,000 feet of rise. If the dike complex is 2 miles 

wide and the horizontal section be considered at 1,000 feet below the 

attained crest, the dikes would on these assumptions average 100 to 

the mile. This is consistent with the earlier suggestion that concentration of 100 to the mile is common in the recognized dike 

complexes. Apparently a limit between 10 and 100 dikes to the mile 

as a threshold for dike complexes is consistent with the depth of 

dissection and other conditions found in much of Hawaii.

Where the lava flows are comparatively permeable and where the 

dikes do not constitute more than a minor part of the whole mass, 

perhaps nearer the lower limit mentioned above, the dike complex 

may under suitable rainfall conditions serve an important function 

in confining high-level water bodies and it is in such areas that are 

somewhat dissected that tunneling for water has yielded the most 

direct and quantitative information of the attitudes and distribution 

of dikes. On the other hand, definitely effective water development 

has not been carried out in sections showing a concentration more 

than 100 to the mile.

LITERATURE CITED

Anderson, E. M., 1924, Dynamics of cone-sheets and ring-dikes: in Bailey, E. B.,

and others, Tertiary and post-Tertiary geology of Mull, Loch Aline, and

Oban, Geol. Survey Scotland Mem., p. 11-12. 

Balk, Robert, 1937, Structural behavior of igneous rocks: Geol. Soc. America,

Mem. 5. 177 p. 

Bemmelen, R. W. van, 1930, De Tengger .strijdvraag: Nat. Tijdschr. V. Ned.-

Indie, v. 90, p. 96-161.

Bishop, S. E., 1901, Brevity of tuff-cone eruptions: Am. Geologist, v. 27, p. 1-5. 

Brady, L. F., and Webb, R. W., 1939, Volcanic bombs from northern Arizona

cinder cones (abstract): Geol. Soc. America Bull., v. 50, p. 1947. 

 1943, Cored bombs from Arizona and California volcanic cones: Jour.

Geology, v. 51, p. 398-410.

253279 53 7
92 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

Brigham, W. T., 1868, Notes on the volcanic phenomena of the Hawaiian Islands: 

Boston Soc. Nat. Hist. Mem., v. 1, p. 341-472.

 1887, Kilauea in' 1880: Am. Jour. Sci., 3d ser., v, 34, p. 19-27.

 1909, The volcanoes of Kilauea and Mauna Loa: Bishop Mus., Mem. 2,

no. 4, 222 p. 

Bullard, F. M., 1947, Studies on Paricutin Volcano, Michoacan, Mexico: Geol.

Soc. America Bull., v. 58, p. 433-450. 

Coan, Titus, 1852-81, Numerous papers, chiefly in the Am. Jour. Sci. [See

Macdonald, 1947, p. 33-39.] 

Cotton, C. A., 1944, Volcanoes as landscape forms, 416 p., Wellington, New

Zealand. 

Dall, W. H., 1900, Notes on the Tertiary geology of Oahu: Geol. Soc. America

Bull., v.. 11, p. 57-60. 

Daly, R. A., 1914, Igneous rocks and their origin, 563 p., New York, McGrawHill.

 1925, The geology of Ascension Island: Am. Acad. Arts Sci. Proc., v. 

60, p. 1-80.

1933, Igneous rocks and the depths of the earth, 598 p., New York,

McGraw-Hill: 

Dana, J. D., 1849, Geology: U. S. Exploring Exped. Rept., v. 10, 756 p.

 1887, History of the changes in the Mount Loa craters: Am. Jour. Sci., 

3d ser., v. 34, p. 349-364.

 1890, Characteristics of volcanoes, 399 p., New York.

1895, Manual of geology, 4th ed., 1087 p., Philadelphia.

Dodge, F. S., 1893, Kilauea in August, 1892: Am. Jour. Sci., 3d ser., v. 4.5, p.

241-246. 

Dutton, C. E., 1884, Hawaiian volcanoes: U. S. Geol. Survey, 4th Ann. Rept.,

p. 75-219. 

Emerson, 0. H., 1926, The formation of aa and pahoehoe: Am. Jour. Sci., 5th

ser., v. 12, p. 109-114. 

Finch, R. H., 1931, Lava tree casts and tree molds: Volcano Latter, no. 3.1.6,

p. 1-3.

 1933a, Slump scarps: Jour. Geology, v. 41, p. 647-649.

 1933b, Block lava: Jour. Geology, v. 41, p. 769-770.

 1940, Engulfment at Kilauea volcano: Volcano Letter, no. 470, p. 1-2.

 1941, The filling-iu of Kilauea crater: Volcano Letter, no. 471, p. 1-3.

 1942, The surface ash deposits at Kilauea volcano: Volcano Letter, no. 

479, p. 1-2.

 1943a, Lava rivers and their channels: Volcano Letter, no. 480, p. 1-2.

1943b, The seismic prelude to the 1942 eruption of Mauna Loa: Seismol.

Soc. America Bull., v. 33, p. 237-241. 

Finch, R. H., Powers, H. A., and Macdonald, G. A., 1948, Flow-pumice on

Hawaiian basalts: Volcano Letter, no. 502, p. 1-2.

Green, W. L., 1887, Vestiges of the molten globe, pt. 2, 377 p., Honolulu. 

Hitchcock, C. H., 1900, Geology of Oahu: Geol. Soc. America Bull., v. 11, p.

15-57.

 1901, Volcanic phenomena on Hawaii: Geol. Soc. America Bull., v. 12, 

p. 45-56.

 1909, Hawaii and its volcanoes, 314 p., Honolulu.
LITERATURE CITED 93

Jaggar, T. A., 19l7a, On the terms aphrolith and derrnolith: Wash. Acad. Sci. 

Jour., v. 7, p. 277-281.

 1917b, Thermal gradient of Kilauea lava lake: Wash. Acad. Sci. Jour., 

v. 7, p. 397-405.

 1917c, Lava flow from Mauna Loa, 1916: Am. Jour. Sci., 4th ser., v. 43, 

p. 255-288.

 .1919, Activity of Mauna Loa: Hawaiian Volcano Observatory Bull., v. 7, 

p. 136-142.

 1920, Seismometric investigation of the Hawaiian lava column: Seisinol. 

Soc. America Bull., v. 10, p. 155-275.

 1924, Volume relations of the explosive eruption 1924: Hawaiian Volcano 

Observatory Bull., v. 12, p. 117-122.

 1926, Journal of Mauna Loa eruption: Hawaiian Volcano Observatory 

Bull., v. 14, p. 24-29, 31-47, 54-57.

1930, Distinction between pahoehoe and aa or block lava: Volcano Letter,

no. 281, p. 1-4.

 1931, Lava stalactites, toes, and "squeeze-ups": Volcano Letter, no. 

345, p. 1-3.

 1934a, Kilauea report for September 1934 [account of Kilauea eruption]: 

Volcano Letter, no. 415, p. 1-2.

 1934b, Kilauea report for October 1934 [account of Kilauea eruption, 

continued]: Volcano Letter, no. 416, p. 1.

 1935, Journal Mauna Loa eruption, December 1935: Volcano Letter,

no. 480, p. 8. 

Jaggar, T. A., and Finch, 11. H., 1924, The explosive eruption of Kilauea in Hawaii,

1924: Am. Jour. Sci., 4th ser., v. 8, p. 353-374.

 1929, Tilt records for thirteen years at the Hawaiian Volcano Observatory: Seismol. Soc. America Bull., v. 19, p. 38-51.

Jones, A. E., 1943, Classification of lava surfaces: Am. Geophys. Union Traus. 

for 1943, pt. 1, p. 265-268.

Krauskopf, K., and Williams, H., 1946, The activity of Paricutin during its third 

year: Am. Geophys. Union Trans., v. 27, p. 406-410.

Krukenberg, C. F. W., 1877, Mikraphie der Glasbasalte von Hawaii, 38 p., 

Tubingen.

Lacroix, A., 1904, La Montague Pelee et ses eruptions, 662 p., Paris.

 1906, Contribution a F etude des breches et des conglomerate volcanic|iie:

Soc. Geol. de France Bull., ser. 4, v. 6, p. 650. 

Lindgren, W., 1903, The water resources of Molokai, Hawaiian Islands: U. S.

Geol. Survey Water-Supply Paper 77. 

Macdonald, G. A., 1942, Petrography of Mani: Hawaii Div. of Hydrog. Bull. 7,

p. 276-334.

 1943, The 1942 eruption of Mauna Loa, Hawaii: Am. Jour. Sci., v. 241, 

p. 241-256.

 1944a, The 1840 eruption and crystal differentiation in the Kilauean 

magma column: Am. Jour. Sci., v. 242, p. 177-189.

 1944b, Unusual features in ejected blocks at Kilauea Volcano: Am. 

Jour. Sci., v. 242, p. 322-326.

 1944c, Solfataric alteration of rocks at Kilauea Volcano: Am. Jour. 

Sci., v. 242, p. 496-505.

 1945a, Ring structures at Mauna Kea, Hawaii: Am. Jour. Sci., v. 243, 

p. 210-217.
94 STRUCTURES AND FORMS OF BASALTIC ROCKS IN HAWAII

MacDonald, G. A., 1945b, Summit and northeast rift zone of Mauna Loa: Volcano Letter, no. 489, p. 1-3.

 1945c, Structure of aa lava flows (abstract): Geol. Soc. America Bull., 

v. 56, p. 1179-1180.

 1947, Bibliography of the geology and water resources of the island of 

Hawaii: Hawaii Div..of Hydrog., Bull. 10, 191 p.

 1949a, Petrography of the island of Hawaii: -U. S. Geol. Survey, Prof. 

Paper 214-D, p. 51-96.

 1949b, The Hawaiian petrographic province: Geol. Soc. America Bull.,

v. 60, p. 1541-1596.

 1953, Pahoehoe, aa, and block lava: Am. Jour. Sci. v. 251, p. 169-191. 

Macdonald, G. A., and Finch, R. H., 1949, Activity of Mauria Loa during April,

May, and June, 1949: Volcano Letter, no, 504, p. 1-5. 

Macdonald, G. A., and Orr, J. B., 1950, The 1949 summit eruption of Mauna

Loa, Hawaii: U. S. Geol. Survey, Bull. 974-A, p. 1-31. 

Meinzer, 0. E., 1930, Ground water in the Hawaiian Islands: U. S. Geol. Survey,

Water-Supply Paper 616. 

Nichols, R. L., 1936, Flow units in basalt: Jour. Geology, v. 44, p. 617-630.

 1938, Grooved lava: Jour. Geology, v. 46, p. 601-614.

 1939a, Pressure ridges and collapse depressions on the McCartys basalt 

flow, New Mexico: Am. Geophys, Union Trans. for 1939, pt. 3, p. 432-433.

 1939b, Squceze-ups: Jour. Geology, v. 47, p. 421-425.

 1939c, Surficial banding and shark's-tooth projection in the cracks of

basaltic lava: Am. Jour. Sci., v. 237, p. 188-194. 

Palmer, H. S., 1927, Geology of Kaula, Nihoa, Necker, and Gardner Islands and

French Frigates Shoal: Bishop Mus. Bull. 35, 35 p.

 1929, A fossil lava tube: Jour. Geology, v. 37, p. 272-274.

 1930, Geology of Molokini: Bishop Mus. Occasional Papers, v. 9, no. 1, 

18 p.

1936, Geology of Lehua and Kaula Islands: Bishop Mus. Occasional

Papers, v. 12, no. 13, 36 p. 

Perret, F. A., 1913, Some Kilaueau ejectarnenta: Am. Jour. Sci., 4th ser., v. 35,

p. 611-618. 

Powers, H. A., 1948, A chronology of the explosive eruptions of Kilauea: Pacific

Sci., v. 2, p. 278-292. 

Reck, H., 1915, Physiographische Studie liber vulkanische Bornben: Zeits. fur

Vulkanologie, Erganzungsband, 124 p.

Rittmann, A., 1936, Vulkane und ihre Tatigkeit, 188 p., Stuttgart. 

Schulz, P. E., 1943, Some characteristics of the summit eruption of Mauna Loa,

Hawaii., in 1940: Geol. Soc. America Bull., v. 54, p. 739-746. 

Skeats, E. W., and James, A. V. G., 1937, Basaltic barriers and other surface

features of the Newer Basalts of western Victoria: Royal Soc. Victoria

Proc., v. 49, pt. 2, p. 245-278. 

Stearns, H. T., 1924, Craters of the Moon National Monument: Geog. Rev.,

v. 14, p. 62-372.

 1925, The explosive phase of Kilauea Volcano, Hawaii in 1924: Bull, 

volcanologique, nos. 5-6, p. 193-208.

 1926, The Keaiwa or 1823 lava flow from Kilauea Volcano, Hawaii: 

Jour. Geology, v. 34, p. 336-351.

 1928, Craters of the Moon National Monument, Idaho: Idaho Bur. Mines 

Bull. 13, 57 p.
LITERATURE CITED 95

Stearns, H. T., 1938, Pillow lavas in Hawaii (abstract): Geol. Soc. America, 

Proc. for 1937, p. 252-253.

 1946, Geology of the Hawaiian Islands: Hawaii Div. Hydrog., Bull. 8,

106 p. 

Stearns, H. T., and Clark, W. 0., 1930, Geology and water resources of the Kau

District, Hawaii: U. S. Geol. Survey, Water-Supply Paper 616. 

Stearns, H. T., and Macdonald, G. A., 1942, Geology and ground-water resources

of the island of Maui, Hawaii: Hawaii Div. Hydrog., Bull. 7, 344 p.

 1946, Geology and ground-water resources of the island of Hawaii: Hawaii

Div. Hydrog., Bull. 9, 363 p. 

Stearns, H. T., and Vaksvik, K. N., 1935, Geology and ground-water resources of

the island of Oahu, Hawaii: Hawaii Div. Hydrog., Bull. 1, 479 p. 

Stone, J. B., 1926a, The products and structure of Kilauea: Bishop Mus. Bull.

33, 59 p.

 1926b, The Keaiwa flow of 1823, Hawaii: Am. Jour. Sci., 5th ser., v. 11,

p. 434-440.

Tyrrell, G. W., 1931, Volcanoes, 252 p., London. 

Waesche, H. H., 1940, Mauna Loa summit crater eruption, 1940: Volcano Letter,

no. 468, p. 1-9. 

Washington, H. S., 1923, Petrology of the Hawaiian Islands; IV. The formation

of aa and pahoehoe: Am. Jour. Sci., 5th ser., v. 6, p. 409-423. 

Wentworth, C. K., 1926, Pyroclastic geology of Oahu: Bishop*Mus. Bull. 30,

121 p.

 1937, The Diamond Head black ash: Jour. Sed. Petrology, v. 7, 

p. 91-103.

1938, Ash formations of the island Hawaii: Hawaiian Volcano Observatory, 3d Spec. Rept., 183 p. 

Wentworth, C. K., and Jones, A. E., 1940, Intrusive rocks of the leeward slope of

the Koolau range, Oahu: Jour. Geology, v. 48, p. 975-1006. 

Wentworth, C. K., and Williams, H., 1932, The classification and terminology of

the pyroclastic rocks: Natl. Research Council, Bull. 89, p. 19-53. 

Wentworth, C. K., and Winchell, H., 1947, Koolau basalt series, Oahu, Hawaii:

Geol. Soc. America Bull., v. 58, p. 49-77. 

Wentworth, C. K., Carson, M. H., and Finch, R. H., 1945, Discussion of the

viscosity of lava: Jour. Geology, v. 53, p. 94-104. 

Wilkes, C., 1845, Narrative of the United States Exploring Expedition during

the years 1838-42, v. 4, 539 p., Philadelphia. 

Williams, Howel, 1932, History and character of volcanic domes: California

Univ. Pub. Bull. Dept. Geol. Sci., v. 21, p. 51-146.

 1934, Mount Shasta, California: Zeitschr. fur Vulkanologie, v. 15, 

p. 225-253.

 1941, Calderas and their origin: California Univ. Pub. Bull. Dept. Geol.

Sci., v. 25, p. 239-346. 

Winchell, Horace, 1947, Honolulu series, Oahu, Hawaii: Geol. Soc. America Bull.,

v. 58, p. 1-48. 

Wood, H. 0., 1915, The seismic prelude to the 1914 eruption of Mauna Loa:

Seismol. Soc. America Bull., v. 5, p. 39-50.

 1917, Notes on the 1916 eruption of Mauna Loa: Jour. Geology, v. 35,

p. 322-336, 467-488. 

Zies, E. G., 1946, Temperature measurements at Paricutin Volcano: Am. Geophys.

Union Trans., v. 27, p. 178-189.

INDEX

Page 

Aa, abundance-.....--.-.....-..-....-.....-- 9

central massive phase..........-.--. 59,01,62,87

crystallization...-.......-...---.-.-...... 67

definition--.--------------..-.---.----.-.- 32

Accretionary lava balls..-....-..-.......-..-. 64

Acknowledgments........------.-..---.--.-.. 2-3

Agglomerate.... 69,73,75

Agglutinate... .-. .- .. .__.- 22,23,25,54

Aloe Crater. See Pit craters.

Altitude of vents...-.-....................... 27

Aphrolito.. ..- -- . -. 32 

Ash beds-.----..-............................ 69

Attitude of lava flows-.---------.-.......---- 31-32

''Barnacle stalactites"-..----- ------------- 57

Black sand--..-..---.--------......----.----. 75,86

Block lava, accessory......-.--.--........---- 83

description.....-------......-.----.......- 32

occurrence...-----.-.------....-..--.-.-.. 32.83

size...----...--..-..------.--.--.---.----- 83

Bombs, breticlcrust..-------------...-.-----.. 8.1

classification... -..---------...-----------. 80

cored .. - - - 64,74,83 

dimensions.-.----.---------------.------. 81

fusil'orm................................. 81,83

"Hawaiian type" (cow clung or pancake). 6,80 

ribbon_................................ 81

sags.. - - 84, fig. 42

spindle. -. --- -.-.---------------- -. - 6

'' Bulbous squceze-ups"_......--.-.....--... 57

Cinder cones, composition................. 23-24,25

dimcnslons..-- ..---.....-..-----.-.- 4, 23-24

explosi vcness.---------------------------- 4

occurrence...--._-. . .._. - 23

Cindci-and-spattor cones--.----.--. ...- 6,7,1.1,69

Collapse, magnitude..-----------.-------. 14-16,25

structures. See Pit craters. 

Columnar jointing....-.-----.---.-- 64,86-89, fig. 43

Composition of lava, chemical. 3, fig. 1

gas con ten t. 4,6,7

physical.__.......... -.... . ....... 9

Concretionary balls. . - - 64

Dermollth 32

Devil's Throat....--------------......... 17,19,21

Domes, exogenous lava..- . ..-....-..-.-.- 6

pressure 45

tholoids..---..--..-.---.......-.. ..- 4

Driblet, cones......-.------------------- 52,68,73

 definition...-.-.-.----------.-.....------- 73

spires---..--------.-----.---------- 52,54,68

Dunlte. . 83

Exogenous lava._............

Explosi veness -----.-...-.-....

6

4, 5, 6, 28

Faults, clisplaccment. ....... ............

ssarps. .. .-.

systems..................................

Features influencing fluidity.. . . 

Fissures, concentric.-.........................

dimensions...............................

erupt! vo-.. _ -. .. _-_

radial... ......

surface...................................

Flow units, range In thlcknoss...........__-

Fluidity. ........................ 4,6,31,

Formation of tubes, range In diameter........

Fountains, activity . ... ........ . 22,

Fumaroles, composition of gases..............

length of activity. ....... ...

.primary... . 

rootless.... 

secondary.

Page

12,21

7,15

21

4

11,12 

12

8,11

11,12

11,14

32

57,81

43,45

27, 67

29

29

26

29

29

Great Crack........-.---.......-.-.-......- 11,64

Historic flows._-._- ----- 31

Hornitos, general features__................ 31,52

range in size.-...--..-.-.--.---.-.... ... 54

Kauai Island..........

Kaula Island. ---------

Koolau volcanic serles.

Lapllll.... - 25,73,74

Lava, balls... -- - -- - -----.

cones. --. -.. ----------.. -.---.--

fountains... .-. . - 

gas con tent. - --- -- --- 

lakes.-_ -.. - ....... ..

rings..---. ...... .. _... 

stalactites...... .........-.-.---.----...

stalagmites.. ----------... - - 

tree molds__...--.-.-...........--. 55,

tubes....--....-.-.....-...-.- 19,29,33

viscosity 5,6,7, 29, 57,64; volume extruded .........^............ 16,

Length of flows_.- --------- ...-

"Lily blossom" forms................._..-.

Littoral cone, composition......- ...........

explosions-.-----------..- _-----------

general features.----.---.--....-.---.-- 2S,

height 

Makaopuhi Crater. Ser Pit craters.

Mantle bedding .....-..-..-......-....-.-..

Marsh gas.----.--..-.----- - ----.-- 

Maul group of volcanoes.......--.-- .-..

9

9

69

, 76.83 

64

11,25 

14

6,29 

26 

26

54,55 

54,55 

fig. 28 

, 54, 57 

,66,81 

,21,31 

30 

57

28,75 

28

, 69,76 

28

Napau Crater. See Pit craters. 

Niihau Island...-..-___._..... 

Ninole volcanic series_.........

97
98 INDEX

Reticulate. ..........-.-----.-....-.

Page 

.......... 8

.----.._.. 14

.......... 69-70

.-.-...... 9

..-.--..-. 42

...._..... 35

......_-.. 32

--........ 35

---------- 35

.-----..._ 33

---..-... 35

.......... 66

.......... 43

.......... 42

.... 22,57,67,80

..----..._ 6,21

.......... 17

.......... 19-20

.......... 17

.......... 25

.......... 33

.......... 74-75

.......... 68

.......... 68-69

.... 74.75.78-79

Tholoids. -_.--........-.--.... .

Tiltmetcr. ................... ...

Tumuli....-..--....---.--.-....

"Wreath of stones".. . ..........

Page 

__-_------__-. 21

......-.-..-. 27

.............. 27

............. 9

......--...-. 7,8

......------- 4

 ...- - 6,22,23

........... 57.C3-64

........----._ 28,83

............. 28

---...-..... 4,6,29

............. 78

......... 4,57,63,89

--.-.-.----.. 14

............. 22,68

............. 14

........ 35,45,49,03

... 33,62,74, fig. 14

.. 5,6,7,57,64,66,81

............. 54

-.....-..---- 7

.............. 81

..... ........ 25

o