Geology of Three Late Quaternary 

Stratovolcanoes on Sao Miguel, Azores 

U.S. GEOLOGICAL SURVEY BULLETIN 1900 

Prepared in cooperation with the 

Regional Government of the Azores 
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Ave. 
Geology of Three Late Quaternary 

Stratovolcanoes on Sao Miguel, Azores 

By RICHARD B. MOORE 

Prepared in cooperation with the 

Regional Government of the Azores 

U.S. GEOLOGICAL SURVEY BULLETIN 1900 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

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Box 25425 

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Library of Congress Cataloging-in-Publication Data 

Moore, Richard B., 1 945-

Geology of three late Quaternary stratovolcanoes on Sao Miguel, Azores I by 

Richard B. Moore. 

p. em. - (U.S. Geological Survey bulletin; 1900) 

"Prepared in cooperation with the Regional Government of the Azores." 

Includes bibliographical references. 

1. Volcanoes-Azores-Sao Miguel Island. 2. Geology, 

Stratigraphic-Quaternary. 3. Geology-Azores-Sao Miguel Island. I. Title. 

II. Series. 

QE75.B9 no. 1900 

[QE527] 

557.3 s-dc20 91-12012 

[551.2'1 '094699] CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Structure and morphology 2 

Sete ·Cidades volcano 2 

Pleistocene units 3 

Pre-caldera units 3 

Caldera-outflow deposit 3 

Post-caldera units 5 

Holocene units 5 

Trachytic deposits 5 

Mafic flank eruptions 7 

Agua de Pau volcano 7 

Pleistocene units 10 

Older caldera-outflow deposit 11 

Post -outer caldera units 12 

Younger caldera-outflow deposit 12 

Post-inner caldera units 12 

Holocene units 12 

Fogo A 12 

Post-Fogo A pumice deposits from vents within and near the inner 

caldera 13 

Eruptions at Lagoa do Congro 13 

Mafic flank eruptions 13 

Furnas volcano 15 

Pleistocene units 16 

Pre-caldera units 16 

Caldera-outflow deposit 18 

Post-caldera units 18 

Holocene units 19 

Trachytic deposits 19 

Mafic deposits 19 

Petrography 20 

Mafic rocks 20 

Intermediate to silicic rocks 20 

Syenite xenoliths 20 

Xenoliths of mafic and ultramafic rocks 21 

Major element chemical compositions 21 

Silica-variation diagrams 21 

Magnesia-variation diagrams 23 

AFM diagram 23 

Discussion 23 

References cited 25 

Contents Ill 
FIGURES 

1-3. Maps showing: 

1. Location of Azores fracture zone and other major structures 2 

2. Sao Miguel, Azores 3 

3. Generalized geology of Sete Cidades volcano 4 

4. Composite measured stratigraphic section, western flank of Sete Cidades 

volcano 6 

5. Measured stratigraphic section, western wall of Sete Cidades caldera 7 

6. Maps showing generalized geology of Agua de Pau volcano 8 

7. Graph showing radiocarbon ages of the Fogo A trachyte pumice 

deposit 13 

8. Map showing generalized geology of Furnas volcano 14 

9. Composite measured stratigraphic section, southern flank of Furnas 

volcano 17 

10. Measured stratigraphic section, western wall of Furnas caldera 18 

11. Silica-variation diagrams 27 

12. Magnesia-variation diagrams 38 

13. AFM ternary diagram 46 

TABLES 

1. Chemical analyses and normative compositions for representative rocks from Sete 

Cidades, Agua de Pau, and Furnas volcanoes, Sao Miguel, Azores 22 

2. Estimated volumes of different rock types on the three late Quaternary stratovolcanoes of Sao Miguel 25 

IV Contents 
Geology of Three Late Quaternary Stratovolcanoes on 

Sao Miguel, Azores 

By Richard B. Moore 

Abstract 

Three Quaternary trachytic stratovolcanoes-Sete 

Cidades, Agua de Pau, and Furnas-are located on the island 

of Sao Miguel, Azores. Mafic vent deposits and associated lava 

flows are between the stratovolcanoes and on their flanks. 

Each volcano consists of interbedded ankaramite, basanitoid, 

alkali olivine basalt, hawaiite, mugearite, and tristanite cones 

and flows, as well as trachyte domes, flows, and pyroclastic 

deposits. Detailed geologic mapping and new radiocarbon 

and K~Ar ages indicate that Furnas, constructed entirely within 

the past 100,000 years, is somewhat younger than the other 

two volcanoes. 

All three volcanoes have calderas that formed in the late 

Pleistocene as a consequence of voluminous eruption of 

trachytic pyroclastic flows and fall deposits. The outer caldera 

of Agua de Pau volcano is the oldest (26,500-46,000 years). 

The progression then is from west to east: the caldera of Sete 

Cidades is about 22,000 years old, the inner caldera of Agua 

de Pau is about 15,000 years old, and the caldera of Furnas is 

about 12,000 years old. Post-caldera activity has included 

emplacement of trachyte domes, associated with extensive 

Plinian and sub-Plinian pumice-fall deposits within the 

calderas, and eruption of more mafic lavas from vents on the 

flanks of the volcanoes. Each volcano has erupted during the 

past 500 years. Holocene eruptions have been most frequent 

on Sete Cidades and Furnas. Future eruptions present 

significant risk because a large population (150,000) now 

inhabits Sao Miguel. Furnas is particularly dangerous, partly 

because it has erupted five times during the past 1,100 years, 

most recently in A.D. 1630. 

The three volcanoes range in subaerial volume from 

about 60 to 80 km3 • They were constructed at rates of about 

0.02-0.03 km3/century on Sete Cidades, 0.04 km3/century on 

Agua de Pau, and 0.06 km3

/century on Furnas. Trachyte is 

volumetrically the most abundant rock type, ranging from 

about 72 percent of the exposed lavas on Sete Cidades to 

about 93 percent on Agua de Pau. Tristanites are most 

abundant on Furnas and reflect the generally higher KINa on 

that volcano. Potassic hawaiites and mugearites are most 

abundant (about 18 percent by volume) on Sete Cidades. 

Manuscript approved for publication January 31, 1991. 

New major element chemical and normative data for all 

mapped rocks from the three volcanoes demonstrate their 

alkalic (especially potassic) nature and indicate that virtually 

all mafic and many silicic rocks are nepheline normative. 

Peralkaline trachytes (including both quartz and nepheline 

normative types) are particularly abundant on Agua de Pau, 

where they constitute about 45 percent of the volume of all 

trachytes. They typically were erupted in large volume during 

short-lived events, most notably during the two calderaforming episodes. In contrast, peralkaline trachytes are rare on 

Sete Cidades and Furnas volcanoes and make up less than 1 

percent of their erupted volumes. 

Variation diagrams illustrate some chemical differences 

among the volcanoes and suggest that most of the diverse rock 

types are related by fractionation of parental basanitoid. 

Incorporation of silicic material in mafic melts locally has 

resulted in hybrid lavas, especially on Agua de Pau. 

INTRODUCTION 

The Azores straddle the Mid-Atlantic Ridge (fig. 1). 

Historic eruptions have occurred on five islands during the 

past 500 years (van Padang and others, 1967). Islands east 

of the Ridge, including Sao Miguel, are near the Azores 

fracture zone (called the Terceira Rift or Terceira Ridge by 

some workers), a seismically active spreading center 

(Krause and Watkins, 1970; Searle, 1980) near the triple 

junction of the African, Eurasian, and North American 

plates. The zone of plate divergence underlying the Azores 

archipelago changes to one of convergence· between the 

Azores and the Iberian Peninsula (Grimison and Chen, 

1986). 

This report summarizes information gathered during 

1:15,000- and 1:18,000-scale mapping (Moore 1983a, b, 

1986) of an area of about 475 km2 including Sete Cidades, 

Agua de Pau, and Furnas volcanoes on the island of Sao 

Miguel, Azores (figs. 1-3, 6, 8). New radiocarbon (<40,000 

years B.P.; Moore and Rubin, 1991) and K-Ar (>40,000 

years B.P.; E.H. McKee, U.S. Geological Survey (USGS), 

Introduction 
30"W 

39" N. 

37"N. 

25"W 

Graciosa ........ 0 -....._ Sao Terceira 

Faial ~Jorge 0 <QC:::;:;,~-...... 

Pico "l?i-....._-....._ _ . 1Y~s 1:. -....._ Sao M1guel 

IY.qc,_u~. ~<o ......._ 1'\t~ ........ 

Sant~Maria -....._ -....._ 

Figure 1. Location of Azores fracture zone and other major 

structures. Modified from Laughton and Whitmarsh (1975). 

written commun., 1986) ages augment the stratigraphic 

data; all unreferenced ages in this report are from these two 

sources. Silica- and magnesia-variation diagrams using 357 

new chemical analyses of rocks, representing every map 

unit, from the three volcanoes are presented. 

The volcanoes are composed of lava flows, domes, 

and pyroclastic rocks of the alkali basalt-trachyte suite 

(Irvine and Barager, 1971 ). Eruptions of both mafic and 

trachytic lavas have occurred on Sete Cidades and Agua de 

Pau during the past 1,000 years, and Furnas has erupted 

trachyte five times during the past 1,100 years. These 

volcanoes are the targets of geologic, geophysical, and 

geochemical investigations by members of the USGS, in 

cooperation with the Regional Government of the Azores, 

in an effort to identify and assess the geothermal energy 

resources of Sao Miguel. 

Zbyszewski and others (1958) and Zbyszewski and 

Ferreira (1959) published 1:50,000-scale geologic maps 

showing rock types on Sao Miguel. Moore ( 1990, 1991) 

presented a new geologic map (scale 1 :50,000) of all of Sao 

Miguel and summarized the geology of the island. Walker 

and Croasdale (1970) and Booth and others (1978) studied 

widespread trachyte pumice deposits from the three volcanoes. Chemical analyses are presented by Esenwein (1929), 

Jeremine (1957), Schmincke and Weibel (1972), Schmincke 

(1973), Flower and others (1976), and White and others 

(1979). White and others (1976) described strontium 

isotope variations, and Hawkesworth and others (1979) 

evaluated strontium and neodymium isotope data. Storey 

(1981) discussed the petrologic evolution of the magma 

reservoir beneath Agua de Pau volcano during the past 

4,600 years. 

Acknowledgments.-Edward W. Wolfe (USGS) 

aided considerably with computer generation of the 

variation diagrams and also critically reviewed the 

manuscript. Debby Kay (USGS) did the rapid-rock 

chemical analyses. A.M. Rodrigues da Silva (Laboratorio 

de Geociencias e Tecnologia, Ponta Delgada, Sao Miguel) 

gave considerable logistical support. Wendell A. Duffield 

and Donald A. Swanson (USGS) critically reviewed the 

2 Geology of Late Quaternary Stratovolcanoes, Azores 

manuscript. This work was supported by the Agency for 

International Development (U.S. Department of State), the 

Regional Government of the Azores, and the U.S. 

Geological Survey. 

STRUCTURE AND MORPHOLOGY 

Normal faults are common on Sao Miguel; 

widespread trachyte pumice deposits mask them in most 

areas. Locally, faults displace unconsolidated pumice 

deposits as young as 361 (Furnas volcano) and 428 (Agua 

de Pau volcano) years. Most faults strike northwest or 

west-northwest and are part of the Azores fracture zone, the 

axis of which crosses Sete Cidades volcano on the western 

end of the island. The location of Sete Cidades probably was 

controlled by the fracture zone; fault scarps about 60 m high 

on the northwestern and eastern rims of the caldera are 

truncated by the caldera and apparently experienced major 

displacement at the time of caldera formation about 22,000 

years ago. 

Landforms on the three volcanoes are dominantly 

constructional; significant erosion occurs only on sea cliffs 

and on pyroclastic deposits in deep canyons cut near 

contacts between large trachyte domes and lava flows. 

Oversteepened slopes characterize many of these canyons, 

and a high probability exists for destructive mudflows 

during periods of heavy rainfall. 

SETE CIDADES VOLCANO 

Sete Cidades volcano is located on the western end of 

Sao Miguel (figs. 2, 3). Its subaerial volume is about 70 

km3, and its lavas have accumulated at an estimated rate of 

0.02-0.03 km3/century. An almost circular caldera about 5 

km in diameter, with walls as high as 400 m, truncates its 

summit. Geologic mapping (fig. 3) indicates that precaldera rocks crop out chiefly in the caldera walls and in sea 

cliffs eroded into the flanks of the volcano. Post-caldera 

units are trachyte cones and pumice rings distributed across 

the caldera floor, and ankaramite, basanitoid, alkali olivine 

basalt, hawaiite, mugearite, and minor trachyte cones and 

flows erupted from flank fissures radial to the caldera. All 

trachytic vents on Sete Cidades are within a 7 by 4 km area, 

the smallest of the three stratovolcanoes, roughly centered 

on the caldera. 

Sete Cidades lacks hot springs, which are prominent 

on Agua de Pau and Furnas volcanoes. Only two warm 

springs are known, near Mosteiros on the northwestern 

coast and at Ponta da Ferraria on the western coast. The 

latter has a temperature of about 50°C. 
Area of 

figure 3 

Atlantic Ocean 

Area of 

figure 6 

Figure 2. Sao Miguel, Azores. Hachured lines mark calderas: S, Sete Cidades; A, Agua de Pau; F, Furnas; P, Povoa~ao 

(extinct). Major towns: M, Mosteiros; PD, Ponta Delgada; L, Lagoa; RG, Ribeira Grande; VF, Vila Franca do Campo; RQ, 

Ribeira Quente; N, Nordeste. Stippling indicates areas mapped at 1 :15,000 scale (figs. 3, 6, 8). 

Pleistocene Units 

Pre-Caldera Units 

The oldest subaerial rocks of Sete Cidades include 

trachyte and tristanite domes and flows exposed at the base 

of the caldera wall and in sea cliffs on the western and 

southern coasts. A trachyte flow at the base of the 

northwestern caldera wall (fig. 3) has a K-Ar age of 

21 0,000±8,000 years, and a tristanite flow at the base of the 

section at Ponta da Ferraria (fig. 4 ), on the western coast, 

has a K-Ar age of 74,000±6,000 years. Figure 4 is a 

composite stratigraphic section in the vicinity of Ponta da 

Ferraria. 

Flows of ankaramite, basanitoid, alkali olivine basalt, 

hawaiite, mugearite, and tristanite are interbedded with 

trachyte pyroclastic deposits that overlie the earlier domes 

and flows; these deposits form the major subaerial part of 

the stratovolcano. Trachyte pyroclastic deposits include 

pyroclastic flows, pyroclastic surges, mudflows, and 

pumice deposited during Plinian and sub-Plinian eruptions. 

Radiocarbon ages of charcoal recovered from beneath the 

flows and from within the pyroclastic units are greater than 

29,000 years. 

Caldera-Outflow Deposit 

The caldera of Sete Cidades volcano formed 

approximately 22,000 years ago after eruption of several 

cubic kilometers of trachyte pumice. The volume of the 

caldera is about 6 km3

• The volume of the caldera-outflow 

deposit cannot be determined with certainty because of 

burial by younger lavas. An unknown but probably 

significant volume of pumice fell at sea. This eruption 

produced pyroclastic flows, locally welded, and pyroclastic 

surges directed primarily southeastward. The resulting 

composite deposit is more than 60 m thick 13 km southeast 

of the caldera but only 5 m thick on the western and 

northern flanks of the volcano. The welded pyroclastic 

flows crop out chiefly on the western, northern, and 

southern walls of the caldera. 

A new roadcut on the western caldera wall provides 

the best exposure of the caldera-outflow deposit and 

overlying units (fig. 5). The caldera-outflow deposit and 

underlying and overlying deposits up to the Sete A pumice 

(terminology of Booth and others, 1978) are truncated by 

the caldera-bounding fault. Truncation of the beds that 

postdate the caldera-outflow deposit suggests that 

subsidence of the caldera continued incrementally (Walker, 

1984) for some time after the major collapse. 

Massive trachyte pumice deposited during an early 

Plinian phase of the caldera-forming eruption is exposed in 

the lowest 4 m of the roadcut. Strongly oxidized trachytic 

pyroclastic flows, 10-12 m thick and characterized by 

somewhat flattened black obsidian fragments in an orange 

fine-grained matrix, conformably overlie the Plinian 

pumice; a welded zone, here only 0.3 m thick and 

characterized by strongly flattened pumice fragments that 

are now obsidian, begins about 1 m above the base of this 

subunit. Above the caldera-outflow deposit is soil10-30 em 

thick that records perhaps several decades or centuries of 

weathering on the basis of comparison with modem soils 

developed on historic (A.D. 1563 Agua de Pau and A.D. 

1630 Furnas) pumice deposits on Sao Miguel. A 2-m-thick 

Plinian deposit, consisting of large (to 0.7 m), dark- and 

Sete Cidades Volcano 3 
Atlantic Ocean 

Atlantic Ocean 

1 KILOMETER 

Figure 3 (above and facing page). Generalized geologic map of Sete Cidades volcano. 

light-gray tristanite pumice fragments . that are partly 

agglutinated and probably close to their source, overlies the 

soil. A 25-30-cm-thick layer of soil overlies the lower 

tristanite pumice. The top subunit of this group is coarse 

tristanite pumice, similar to the lower subunit, that overlies 

charcoal in the soil dated at 21,160±180 years B.P. The 

thickness of the soils between the dated charcoal and the 

caldera.:.outflow deposit suggests that only a few centuries 

elapsed between these eruptions. The age of the calderaoutflow deposit thus is believed to be about 22,000 years. 

Voluminous ignimbrites were erupted from Pico Alto 

volcano on Terceira at about the same time, 23,000 and 

19,000 years B.P. (Self, 1976). 

Outside of the caldera the caldera-outflow deposit 

generally is not welded and is recognized chiefly by its 

4 Geology of Late Quaternary Stratovolcanoes, Azores 

stratigraphic position and by the common occurrence of 

large (1 0-40 em), slightly flattened, black obsidian 

fragments (formerly pumice) in a light-gray, fine-grained 

matrix. The thickest exposure of the deposit is about 13 km 

southeast of the caldera in almost inaccessible sea cliffs. 

Here, pumice fragments are not flattened to obsidian, the 

deposit is white to very light gray, and fine-grained crossbedded surge deposits are common. 

In most places (except in the new roadcut on the 

western caldera wall), the caldera-outflow deposit 

immediately underlies the Sete A pumice, a Plinian deposit 

probably from a trachyte cone in the northern part of Sete 

Cidades caldera (Booth and others, 1978). The calderaoutflow deposit is truncated by the caldera wall, whereas the 

Sete A pumice mantles it. 
EXPLANATION 

.. YYYYYYY 

YYYYYYY 

YYYYYYY 

YYY¥YYY 

~ y y y y y y PostTrachyte pumice- Trachyte pumice Trachyte domes 

fall deposits and rings and cones Mafic scoria cones 

f y y y y y y y~ 1 

Mafic flows caldera 

units 

minor alluvium 

.. 

Trachyte pyroclastic 

flows and surge 

deposits 

Trachyte and tristanite 

domes, flows, and 

pyroclastic deposits; 

mafic vent deposits 

and flows 

Calderaoutflow 

deposit 

Precaldera 

units 

Mafic rocks include basanitoid, ankaramite, alkali-olivine basalt, hawaiite and mugearite 

----Contact 

-----~'-- Fault-Bar and ball on downthrown side 

• • • Caldera rim-Partly mantled by post-caldera pumice-fall deposits 

- - - - Boundary of mapped area 

500 Mean radiometric age in years B.P. 

Post-Caldera Units 

At the new roadcut on the western wall of Sete 

Cidades caldera (fig. 5), the oxidized pyroclastic flows and 

overlying coarse tristanite pumice are separated from the 

Sete A pumice by about. 6 m of thinly bedded trachyte 

pumice deposits and soils interbedded with 6-8 m of alkali 

olivine basalt and potassic mugearite scoria and short flows 

from a nearby source. 

Elsewhere on Sete Cidades, the caldera-outflow 

deposit is separated from the Sete A pumice by only a few 

mafic flows and at least one trachyte pyroclastic flow 

deposit. A potassic mugearite flow near Feteiras (fig. 3) on 

the southern flank of the volcano has a radiocarbon age of 

20,890±240 years. A trachyte pyroclastic flow deposit on 

top of the sea cliff at Ponta do Escalvado, near the western 

end of the island, has a radiocarbon age of 17,160±130 

years. This deposit probably is related to post-caldera 

trachyte domes and flows about 1.5 km southeast of Ponta 

do Escalvado. 

Holocene Units 

Trachytic Deposits 

Sete Cidades volcano apparently was relatively 

inactive, except for the few eruptions noted above, during 

the approximately 17,000 years between formation of the 

caldera and deposition of the Sete A pumice. Such relative 

quiescence following caldera formation characterizes Agua 

de Pau and Furnas volcanoes as well. 

Booth and others (1978) studied the products of 12 

trachytic pyroclastic eruptions from Sete Cidades. They 

considered the age of the oldest eruption, Sete A, to be 

uncertain; the others, Sete B-L, rest on the 5,000-year-old 

Fogo A deposit from Agua de Pau volcano. I consider Sete 

A to be little older than 5,000 years, on the basis of the 

relatively thin ( <20 em) soil separating Sete A from Sete B. 

Six vents, the eruptions of which formed cones or 

pumice rings, occur on the floor of Sete Cidades caldera in 

a roughly circular pattern (fig. 3); perhaps magmas rose 

along a concentric fracture that formed at about the same 

time as the caldera. Other vents may be submerged beneath 

Lagoa Azul (Blue Lake) and Lagoa Verde (Green Lake). 

Four of the six vents were sites of voluminous Plinian and 

sub-Plinian eruptions of trachyte pumice (Booth and others, 

1978) that mantled the western part of Sao Miguel. The 

other two vents, Alferes and Freiras (fig. 3), erupted 

relatively degassed trachyte scoria of only local extent. 

Most of the young pumice deposits on Sete Cidades 

that Booth and others (1978) studied have not been dated. 

The peralkaline Sete J deposit has a radiocarbon age of 

1,860±120 years B.P. The next youngest deposit, Sete K, is 

older than the Carvao basanitoid cone and flow, dated at 

1,280±150 years B.P. 

Sete Cidades Volcano 5 
>40,000 years old 

Hawaiite, mugearite, and 

tristanite flows; minor 

intercalated trachyte 

pumice (total thickness 

45-70 m) 

Tristanite pyroclastic flow 

Potassic hawaiite flow 

Incipiently welded 

pyroclastic flow 

deposit and 

associated surge 

and mudflow 

deposits (trachytic); 

fills canyons 

UNCONFORMITY 

Pyroclastic surge deposits 

(trachytic) 

Trachyte pumice (air-fall) 

Alternating mudflow, >40,000 years old 

surge, and air-fall 

deposits (trachytic) 

Indurated mudflows >29,000 years old 

(trachytic) 

Tristanite flow 7 4,000 ± 6,000 years old 

BASE 

TOP 

Sete L trachyte pumice 

(<1m) 

, 

Pica das Camarinhas 

ankaramite cone and 

flow 

Sete A-K (Booth and 

others, 1978) trachyte 

pumice deposits 

Trachyte pyroclastic flow 

Ankaramite scoria 

and flow 

Trachyte pumice -- (pyroclastic flow and 

air-fall deposits) 

Ankaramite flow 

Trachyte pumice (air-fall) 

Ankaramite flow 

Trachyte pyroclastic flows 

intercalated with thin mudflow and air-fall deposits 

Ankaramite flow 

Ankaramite flow 

f 

Ankaramite scoria 

Ankaramite flow 

500 ± 100 years old 

840 ± 60 years old 

17,160 ± 130 years old 

Possibly correlative with 

the caldera-outflow 

deposit (approx. 22,000 

years old) 

>33,000 years old 

Figure 4. Composite measured stratigraphic section, sea cliff near Ponta da Ferraria on western flank 

of Sete Cidades volcano. Dips are approximately horizontal except on canyon-filling units. 

6 Geology of Late Quaternary Stratovolcanoes, Azores 
Sete A trachyte 

pumice 

Red-brown soil Lithic-rich 

pyroclastic flow deposit 

Trachyte pumice, ash, thin soils 

1-

Coarse basaltic scoria 

iMugearite, rootless 

flows and spatter 

Agglutinated tristanite pumice -

Agglutinated tristanite pumice --.... 

10-30-cm-thick soil 

Oxidized pyroclastic flow 

(trachyte) 

Welded zone j Oxidized pyroclastic flow 

Trachyte pumice 

(Plinian) 

BASE COVERED 

[:METERS 

Gray-green accretionary lapilli deposit 

Four thin trachyte pumice 

deposits with soils 

Lithic-rich pyroclastic flow deposit 

21,160 ±180 years old 

Soil with charcoal 

Caldera-outflow deposit 

Figure 5. Measured stratigraphic section, roadcut on 

western wall of Sete Cidades caldera. All units dip 

approximately 17°W. 

The youngest intracaldera eruption formed the Caldeira Seca pumice ring (fig. 3) and produced the peralkaline 

Sete L deposit. Shotton and Williams (1971) reported a 

radiocarbon age of 663±105 years B.P. for Sete L; a newly 

determined radiocarbon age of 500±100 years B.P. is 

similar to the previous determination and correlates with a 

reported eruption at the time of Portuguese discovery of the 

island in the middle of the fifteenth century (van Padang and 

others, 1967). 

Assuming that the Sete A deposit is about 5,000 years 

old, the average dormant interval for the 12 trachytic 

pyroclastic eruptions of Sete Cidades that Booth and others 

(1978) studied is about 409 years. At least 780 years 

elapsed, however, between the Sete K and L eruptions, and 

the present dormant interval has lasted at least 540 years. 

Mafic Flank Eruptions 

Sporadic eruptions of ankaramite, basanitoid, alkali 

olivine basalt, hawaiite, and mugearite have occurred on 

Sete Cidades since formation of the caldera. Most mafic 

vents are concentrated along the trace of the Azores fracture 

zone extending southeast from the caldera (volcanic zone 2 

of Moore, 1990, 1991 ). A few vents erupted along radial 

fissures on the flanks of the volcano. 

Four young eruptions on and near Sete Cidades (fig. 

3) have recently been dated. The ankaramite cone of Eguas, 

which forms the highest point on Sete Cidades about 2 km 

southeast of the caldera, is 2, 700±250 years old Pi co do 

Carvao, 1 km southeast of Eguas, and its flow have a 

radiocarbon age of 1,280±150 years B.P. The basanitoid 

spatter rampart at Ferrarias, 1 km west of Eguas, is 980±90 

years old, and did not form around A.D. 1444, as suggested 

by Zbyszewski (1961). The Pico das Camarinhas basanitoid 

cone and Ponta da Ferraria lava delta, products of the most 

recent subaerial mafic eruption on Sete Cidades, are 840±60 

years old. This date and detailed mapping indicate that 

reports of subaerial eruptive activity on Sete Cidades around 

A.D. 1713 and perhaps at other times (van Padang and 

others, 1967) are not correct. 

Surtseyan cones, remnants of which form the Ilheu 

dos (Islands of) Mosteiros near the northwestern coast of 

Sete Cidades (fig. 3), apparently erupted during the time 

that Sete A and younger trachyte pumice deposits 

accumulated. Hyaloclastite and deposits of basaltic 

accretionary lapilli from these vents are interbedded with 

trachyte pumice near the top of the sea cliff southwest of 

Mosteiros. 

Lagoa do Canario (fig. 3) is a lake within a basanitoid 

maar about 0.5 km southeast of the southeastern caldera 

rim. The vent is within 300m of a major fault marking the 

trace of the Azores fracture zone. Because the low cone 

surrounding the vent is thickly mantled by trachyte pumice, 

the composition of its juvenile material was unknown until 

it was exposed in roadcuts. These exposures also show that 

the maar is overlain only by the Sete 1-L deposits. 

Several vent deposits and associated flows, notably 

those of Eguas and Camarinhas, contain xenoliths of mafic 

and ultramafic rocks. 

The absence of young mafic deposits within the 

caldera and the occurrence of recent trachyte eruptions there 

suggest that trachyte magma may still underlie it. 

AGUA DE PAU VOLCANO 

Agua de Pau volcano, a stratovolcano whose summit 

is 947 m above sea level, is located in the central part of Sao 

Miguel (fig. 2). Its subaerial volume of about 80 km3 has 

increased at an estimated rate of 0.04 km3 /century. A 

prominent caldera 3 by 2.5 km across, with walls as high as 

300 m, truncates its summit and contains Lagoa do Fogo 

(Fire Lake). The topographic margin of an older caldera, 

about 7 by 4 km across, is preserved locally on the western, 

northern, and eastern sides of the volcano. Stratigraphic 

relations, especially within the calderas, are generally less 

obvious on Agua de Pau than on the other two volcanoes 

because of burial by widespread thick Holocene pumice. 

Deposits predating both calderas crop out on all flanks of 

the volcano and in sea cliffs on the northern and southern 

coasts (fig. 6A). At least five eruptions of trachyte pumice 

have occurred from vents mostly within the inner caldera 

during the past 5,000 years (Booth and others, 1978). Mafic 

and trachytic lavas were erupted from vents on all flanks of 

the volcano, before and after formation of the calderas. No 

Agua De Pau Volcano 7 
A 

I 

J 

~-®~ / ·.· 

/ 

/ 

I 

J \ I ,.. '-' 

( 

'-.. "" '/""-/ )_..;- \ 

\ 

\ 

\ 

37°45' \ "' C? 

\ 

\ 

\ 

2 KILOMETERS 

~ ~ WB ~ ]iiO~ 0: :r: Younger rocks, undivided 

\ 

\ 

I 

--

Trachyte pumice Tristanite 

.. 

flows 

rings and domes 

Trachyte pyroclastic 

flow deposits 

Trachyte domes, flows, 

and pyroclastic deposits 

Atlantic Ocean 

EXPLANATION 

Post-older 

caldera 

units 

Older 

calderaoutflow 

deposit 

Pre-older 

caldera 

units 

Atlantic Ocean 

\ 

Bfl o \ 

\ 

B \ 

I 

I 

I 

( 

\ 

\ 

( 

~ 

~ 

) 

I 

I 

\ 

\ 

\ 

I 

I 

\ 

~ 

\ 

I 

~ ....... ....... ....... 

----- Contact 

• • • Outer caldera rim-Partly mantled 

by pumice-fall deposits 

* * * Inner caldera rim-Partly mantled 

by p1,1mice-fall deposits 

- - - - Boundary of mapped area 

Figure 6 (above and following pages). Generalized geologic maps of Agua de Pau volcano. A, Units predating the younger 

caldera-outflow deposits. B, Younger caldera-outflow deposits. C, Units postdating the younger caldera-outflow deposits. 

8 Geology of Late Quaternary Stratovolcanoes, Azores 
B 

I\ ,.. 

( -....... "' ,/'"_/ )_.....-\ 

2 KILOMETERS 

\ 

\ 

\ 

\ 

" 0 \ 

\ 

\ 

\ 

\ 

I 

--

Younger rocks, undivided 

Trachyte domes, 

cones, and flows 

Trachyte pyroclastic flow 

and air-fall pumice deposits 

Older rocks, undivided 

Atlantic Ocean 

Atlantic Ocean 

EXPLANATION 

Post-younger 

caldera 

deposits 

Younger 

calderaoutflow 

deposit 

Pre-younger 

caldera 

deposits 

----- Contact 

• • • Outer caldera rim 

* * * Inner caldera rim 

- - - - Boundary of mapped area 

..,""" 

""" 

\ 

\ 

\ 

\ 

J 

I 

I 

( 

\ 

\ 

( 

""" 

~ 

) 

I 

I 

\ 

\ 

\ 

I 

I 

\ 

\ 

\ 

~ ' 

Figure 6. Generalized geologic maps of Agua de Pau volcano-Continued. 

Agua De Pau Volcano 9 
c 

Atlantic Ocean 

2 KILOMETERS 

Figure 6. Generalized geologic maps of Agua de Pau volcano-Continued. 

mafic vents are known within the calderas. Trachytic vents 

on Agua de Pau are within an area of about 17 by 14 km 

(fig. 6A), the largest of the three stratovolcanoes. 

Several hot springs, with temperatures commonly 

near boiling, are on Agua de Pau, mainly on its 

northwestern flank (Zbyszewski and others, 1958; Zbyszweski and Ferreira, 1959). They are near intersections of 

northwest-trending faults and the outer caldera boundary. In 

addition, A.M.R. da Silva (written commun., 1984) recently 

found an inactive hot spring deposit on the southern flank of 

the volcano. The hot springs suggest that hot rock or magma 

associated with the late Pleistocene and Holocene eruptions 

is close to the surface. Six holes have been drilled to 

evaluate the geothermal resource, and more are planned. 

1 0 Geology of Late Quaternary Stratovolcanoes, Azores 

Pleistocene Units 

The oldest subaerial rocks of Agua de Pau volcano 

(fig. 6A ) include a trachyte dome (181,000±15,000 years 

old; Gandino and others, 1985) on the southern coast, a 

trachyte flow (121,000±5,000 years old) near the summit, 

and trachytic welded tuffs (a representative one is 

103,000±7,000 years old; Gandino and others, 1985) and 

associated mudflows on the northern coast. Xenoliths of 

welded tuff possibly correlative with that on the northern 

coast are common in late Pleistocene and Holocene pumice 

deposits erupted from within the inner caldera. Muecke and 

others (1974) obtained K-Ar ages of 117,000± 24,000 years 

(57 m depth) and 280,000±140,000 years (950 m depth) for 
EXPLANATION 

Historical basalt cones and flows. 1563 and1652, 

years of historical eruptions 

, ....... , ~ YYYYYYY ..... .. ............ y y ...... 

... ... ... ... ... y ... 

... ... ... y ......... ... 

... ... ... y ....... ... 1111111111111111111111111111111111111 l 

Trachyte pumice-fall Trachyte of Lagoa Prehistoric mafic Hybridized mafic Tristanite cones deposits and minor do Congro cones and flows cones and flows and flows 

alluvium 

Q) 

s:: 

~ >< 1 "@ 

0:: Older rocks, undivided 

Mafic rocks include basanitoid, ankaramite, alkali olivine basalt, hawaiite, and mugearite 

v Vent for mafic lava 

----Contact 

• • • Outer caldera rim 

* * * Inner caldera rim 

- - - - Boundary of mapped area 

121,000 Mean radiometric age in years B.P. 

Figure 6. Generalized geologic maps of Agua de Pau volcano-Continued. 

samples of drill core on the northwestern flank of Agua de 

Pau. These deposits formed during a period when Agua de 

Pau may have been much more active than it has been 

during the past 100,000 years (Muecke and others, 1974). 

The main edifice of the volcano consists of trachytic 

lava flows, domes, and pyroclastic deposits that probably 

are about 40,000-100,000 years old (available K-Ar ages 

are 46,000-86,000 years; Gandino and others, 1985; E.H. 

McKee, written commun., 1986). Mafic eruptions occurred 

on the outer flanks of the volcano, outside the area of 

trachytic activity. One of the best exposures of the early 

trachytic deposits is in a sea cliff on the southern coast, 

where Booth and others (1978) recognized products of 65 

separate explosive eruptions. Charcoal from soil beneath a 

trachyte flow near the base of this section (between units 4 

and 5 of Booth and others, 1978) has a radiocarbon age 

greater than 40,000 years. Shotton and Williams (1973) 

obtained a radiocarbon age of more than 34,200 years for 

unit 14 of Booth and others (1978). 

Older Caldera-Outflow l)eposit 

Collapse of the summit area may have occurred many 

times, but only two deposits clearly record such catastrophic 

events. The outer caldera formed after eruption of several 

cubic kilometers of welded and nonwelded pumice, now 

exposed mainly on the northeastern and southeastern flanks 

of the volcano (fig. 6A). Any correlative deposits on the 

eastern or western flanks are deeply buried by younger 

rocks. The volume of the outer caldera is about 5.5 km3

, and 

the estimated subaerial volume of the caldera-outflow 

deposit is about 5 km3

• An unknown but probably 

significant volume of pumice fell at sea. 

The deposit fills canyons near the margins of the early 

trachyte flows. Welded tuff of the older caldera-outflow 

deposit overlies trachyte flows on the northern and 

southeastern flanks of Agua de Pau and underlies post-outer 

caldera, pre-inner caldera trachyte domes, pumice rings, and 

tephra on the northern flank. Along the road to Lombadas 

(fig. 6A), the welded tuff appears to be faulted at the caldera 

boundary. 

The youngest dated pre-caldera trachyte flow is 

46,000±6,000 years old, but stratigraphically younger trachyte flows underlie the older caldera-outflow deposit on 

the northern flank of the volcano. The deposit underlies a 

pumice ring and related central dome that have a radiocarbon age of 18,600±300 years B.P. Field relations suggest 

that the older caldera-outflow deposit predates a basanitoid 

flow dated at 26,500±500 years B.P., although that flow is 

outside the mapped areal extent of the caldera-outflow 

deposit. The deposit has been little eroded; incision of 

unconsolidated intracanyon pumice-fall deposits amounts to 

Agua De Pau Volcano 11 
less than 50 min a region of relatively heavy rainfall. The 

age of the older caldera-outflow deposit is thus uncertain 

but probably is about 30,000-45,000 years on the basis of 

the above described stratigraphic relations. The deposit may 

be correlative with a deep-sea tephra unit northeast of Sao 

Miguel dated by Huang and others (1979) at about 33,600 

years B.P. 

The recognizable part of the deposit is dominated by 

a welded zone, which locally exceeds 80 min thickness on 

the northeastern flank of the volcano. In a sea cliff on the 

northern coast, nonwelded Plinian pumice and associated 

pyroclastic flows 5 m thick conformably underlie the 

welded zone. Closer to the caldera, on the northern flank, 

the thickness of the lower nonwelded pumice increases to 

about 50 m. Mudflows, which by analogy with younger 

deposits from major eruptions of Agua de Pau might be 

expected to overlie the welded zone, have not been found 

because of either erosional stripping or, more likely, burial 

by younger pyroclastic deposits. 

Post-Outer Caldera Units 

No evidence exists of eruptions within the outer 

caldera for about 15,000-30,000 years; however, a few 

eruptions of tristanite (fig. 6A) and possibly mafic lavas 

occurred from vents on the flanks of the volcano. 

Three trachyte pumice rings, two with associated 

central domes, formed along a northwestern trend on the 

northern and northeastern flanks of the volcano (fig. 6A). 

The radiocarbon age of the northwestern ring is 18,600±300 

years B.P. Morphologic expression, stratigraphic relations, 

and relative lack of erosion suggest that the other two vents 

are approximately the same age. 

Younger Caldera-Outflow Deposit 

The inner caldera apparently formed about 15,200 

years ago when a large, southward-directed Plinian eruption 

resulted in deposition of extensive nonwelded pumice and 

locally welded pyroclastic flows. Emplacement of thick 

mudflows occurred near the end of this eruption. The 

welded pyroclastic flows, locally as thick as 3 m, crop out 

mainly along the southern sea cliff between Pisoes and 

Trinta Reis (fig. 6B). At Trinta Reis, xenoliths of syenite 

make up 10-20 percent of the rock. Elsewhere, the younger 

caldera-outflow deposit consists mainly of pumice-fall 

deposits more than 5 m thick and overlying mudflows 

commonly more than 50 m thick. Exposed remnants of the 

deposit are restricted to the southern flank of the volcano. 

The estimated subaerial volume of this unit is about 1.5 

km3

, slightly more than that of the inner caldera; an 

unknown, but probably significant volume of pumice fell at 

sea. Radiocarbon ages of the basal Plinian deposit and of the 

overlying welded tuff are 15,190±280 and 15,180±150 

12 Geology of Late Quaternary Stratovolcanoes, Azores 

years B.P., respectively. The caldera of Santa Barbara 

volcano on Terceira formed at about the same time (Self, 

1976). 

The evidence that a caldera formed at this time is less 

compelling than it is for the other three cases studied, 

chiefly because of burial by thick pumice deposits. The 

principal data suggesting that eruption of the 15,200-

year-old unit was responsible for formation of the inner 

caldera are the somewhat greater volume of the deposit 

compared to the inner caldera, the lack of any other known 

eruption as a candidate, and the observation that the next 

major pumice deposit, Fogo A, mantles the caldera wall and 

rim. 

Post-Inner Caldera Units 

No significant eruptions within the inner caldera 

occurred for about 10,000 years; however, six trachyte 

domes and associated short flows formed just outside the 

outer caldera rim on the western and northwestern flanks of 

the volcano (fig. 6B). No deposit older than Fogo A (Walker 

and Croasdale, 1970) overlies the domes, which therefore 

could be either latest Pleistocene or Holocene in age. 

Holocene Units 

Fogo A 

The inner caldera of Agua de Pau became active 

again about 5,000 years ago, when the voluminous Fogo A 

Plinian fall deposit (fig. 6C), pyroclastic flows, and late 

mudflows (Walker and Croasdale, 1970) were erupted. The 

Fogo A deposit rests directly on the 15,200-year-old 

caldera-outflow deposit on the southern flank of the 

volcano. 

The Fogo A deposit displays complexities not 

previously recognized by Walker and Croasdale (1970) that 

indicate Fogo A may be the product of at least five separate 

eruptions. An artificial exposure about 3 km south of Lagoa 

do Fogo reveals an approximately 8-m-thick section of 

syenite-bearing Fogo A that contains four internal soils 

ranging in thickness from 2.5 to 15 em, in addition to 

Plinian fall and pyroclastic surge deposits discussed by 

Walker and Croasdale (1970). The thicker soils in particular 

suggest that Fogo A formed over a significant period of 

time, perhaps several decades or centuries, rather than 

during a single short-lived event. 

Moore and Rubin (1991) reported nine new ages of 

charcoal collected from pyroclastic surge beds, pyroclastic 

flow deposits, and the base of the basal Plinian bed of the 

Fogo A deposit. These samples are from different locations 

on all flanks of the volcano. The results (fig. 7) show 

considerable variation. The average of all the ages is 4,936 
years, almost 400 years older than the average of two ages 

(4435, 4672 years) reported by Shotton and others (1968, 

1969). The range in mean ages is from 4,480 to 5,380 years, 

which in itself suggests that more than one eruption 

occurred. The new ages cluster around 5,000 years, and l 

suggest that that is a more correct mean age than the ages 

reported by Shotton and others (1968, 1969). Whatever its 

age, the Fogo A deposit is a valuable chronostratigraphic 

marker bed because no other major trachytic eruptions are 

known on central and eastern Sao Miguel at this time. 

FOGO A RADIOCARBON AGES 

5,700 r-------------------, 

5,600 

5,500 

5,400 

5,300 

5,200 

1- 5,100 

z 

w 

(/) 

w 

g: 5,000 

w 

a: 

0 

u.. 

~ 4,900 (/) 

a: 

<( 

w 

>-

~ 

w 

(.!) 

<( 

4,800 

4,700 

4,600 

4,500 

4,400 

4,300 

4,200 L__ ________________ _____J 

Figure 7. Radiocarbon ages of the Fogo A trachyte pumice 

deposit (Moore and Rubin, 1991 }. Brackets indicate one-sigma 

precision. 

Post-Fogo A Pumice Deposits From Vents Within and 

Near the Inner Caldera 

Booth and others (1978) and Walker and Croasdale 

(1970) described four post-Fogo A trachyte pumice fall 

deposits from Agua de Pau volcano, the latest in A.D. 1563. 

The average dormant interval between the five post-inner 

caldera eruptions was about 1,150 years (from about 5,000 

to about 400 years B.P.), longer than at either Furnas or Sete 

Cidades volcanoes. 

Eruptions at Lagoa do Congro 

Lagoa do Congro occupies a maar on the eastern 

flank of Agua de Pau volcano, about 5.5 km east of Lagoa 

do Fogo (fig. 6C). Initial activity built a trachyte dome. 

Explosive eruptions of trachyte pumice destroyed part of the 

dome and an underlying basalt flow and formed a pyroclastic deposit that is locally more than 13 m thick. An 

exposure of the lower part of the deposit 250 m west of the 

crater rim shows three lithic-rich, coarse pumice beds 

separated by ash and two thin soils, which indicate short 

time intervals between major explosive events. A quarry 1.5 

km southeast of the crater exposes the upper part of the 

deposit, which consists of 20 coarse pumice beds separated 

by 20 finer grained ash beds that include surge deposits 

(Booth and others, 1978). The basal pyroclastic deposit has 

a radiocarbon age of 3,800±400 years B.P., in agreement 

with the age inferred by Booth and others (1978) on the 

basis of soil development. 

The last eruptive event at Lagoa do Congro was 

extrusion of a small trachyte dome and short flows on the 

floor of the crater. Ankaramite aa from a vent 1 km north of 

Lagoa do Congro later flowed down the northwestern wall 

of the crater, mantling it with inward-dipping flow units. 

Mafic Flank Eruptions 

Ankaramite, basanitoid, alkali olivine basalt, hawaiite, and mugearite were sporadically erupted from vents on 

the flanks of Agua de Pau during the Holocene. Most 

eruptions occurred west of Agua de Pau, in the zone of 

dominantly mafic lavas (volcanic zone 2 of Moore, 1990, 

1991) that separates Agua de Pau from Sete Cidades 

volcano. A few eruptions occurred on the northern, eastern, 

and southern flanks. Dispersal of Holocene air-fall pumice 

mainly eastward by prevailing westerly winds resulted in 

thick mantles of pumice on mafic cones and flows (volcanic 

zone 4 of Moore, 1990, 1991) between Agua de Pau and 

Furnas volcanoes. A resulting illusion is that these cones are 

significantly older than the more prominent, thinly mantled 

cones and flows west of Agua de Pau. 

Vents for Holocene mafic lavas, like those of Pleistocene age, are outside the area underlain by the trachytic 

Agua De Pau Volcano 13 
\ 

I @.I 

\f&l 

----

1 KILOMETER 

'\ 

do 

Garajau 

Atlantic Ocean 

Figure 8 (above and facing page). Generalized geologic map of Furnas volcano. 

magma reservoir. As at Sete Cidades, the absence of mafic 

vents within the caldera suggests that trachytic magma may 

still underlie it. 

Little systematic spatial or temporal distribution of 

lava types exists, although concentrations of vents for 

certain compositions occur locally. For example, vents and 

flows of ankaramite are concentrated on the western and 

southwestern flanks of the volcano, although at least three 

are on the eastern and southeastern flanks. Vents of potassic 

hawaiite are concentrated on the eastern flank. 

Lagoa de San Bras, a maar about 7 km northeast of 

Lagoa do Fogo (fig. 6C), consists of palagonitic tuff 

overlying ankaramite spatter and rootless flows. Three other 

maars occur about 2 km southwest of Lagoa de San Bras. 

Two of these three vents are on the same west-northwestern 

trend as a large mafic cone at the eastern edge of the 

mapped area, and I believe that they formed at the same 

time in the latest Pleistocene or early Holocene. 

14 Geology of Late Quaternary Stratovolcanoes, Azores 

At least four vents erupted hybrid lavas that represent 

a mix of trachytic and more mafic magma (basanitoid at 

three vents and hawaiite at one vent). The lava ofbasanitoid 

parentage has about 58 percent Si02 and the lava of 

hawaiitic parentage (Monte Escuro; fig. 6C) about 55 

percent Si02• The basanitoid parentage is identified by 

titaniferous clinopyroxene in the groundmass, a high bulkrock Ti02 content, and ultramafic xenoliths. Three vents are 

within 1-2 km of the outer caldera boundary; possibly mafic 

magma rose along the margin of the trachytic magma 

reservoir and mixed with trachyte prior to eruption. 

Ascent of mafic magma has been postulated to trigger 

eruption of overlying, more silicic melts at many volcanoes 

(Sparks and others, 1977). The A.D. 1563 trachyte Plinian 

eruption of Agua de Pau possibly was so initiated because 

extrusion of basanitoid from a vent 5.5 km west-northwest 

of Lagoa do Fogo immediately followed the trachyte 

eruption from the caldera (Weston, 1964). An extensive 
EXPLANATION 

A.D. 1630 trachyte 

pumice rings and 

dome 

Trachyte pumice Tristanite cones rings and domes 

Trachyte pyroclastic 

flow deposits 

Trachyte and tristanite Mafic cones domes, flows, and and flows 

pyroclastic and 

lacustrine deposits 

Nordeste 

-

volcanic 

complex. Mafic 

flows and dikes 

and altered trachyte 

Hawaiite cones and flows 

Trachyte pumice 

fall deposits 

Postcaldera 

units 

Calderaoutflow 

deposit 

Precaldera 

units 

Mafic rocks include basanitoid, ankaramite, and hawaiite 

----Contact 

• • • Caldera rim-Partly mantled by pumice- fall deposits 

- - - - Boundary of mapped area 

93,000 Mean radiometric age in years B.P. 

hybrid flow from a vent about 4 km north of Lagoa do Fogo 

(fig. 6C) is interbedded with Fogo A trachyte pumice and 

has a radiocarbon age of 4,990±100 years (fig. 7). Ascent of 

this mafic magma may have triggered the Fogo A eruption. 

Several radiocarbon ages of Holocene mafic lavas on 

the western side (volcanic zone 2 of Moore, 1990, 1991) of 

Agua de Pau volcano (fig. 6C) have recently been 

determined (Moore and Rubin, 1991). An ankaramite cone 

about 4 km west of Lagoa do Fogo is 8,700±200 years old. 

An ankaramite flow on the western side of the village of 

Agua de Pau, 6 km southwest of Lagoa do Fogo, is 

6,500±100 years old. A potassic hawaiite flow that forms a 

lava delta in Ribeira Seca de Ribeira Grande, on the 

northwestern flank of the volcano, is 1,790±150 years old. 

The quarried basanitoid cone of Mata das Feiticeiras, about 

8 km west of Lagoa do Fogo, is 1,010±120 years old. Pico 

das Mos, a basanitoid cone dated at 1,250±150 years B.P., 

· is 2 km east of Mata das Feiticeiras. 

A young alkali olivine basalt cone and flow in 

Caloura, on the southern coast of the island, overlies Fogo 

A trachyte pumice but has not been dated. Its flow formed 

a lava delta that includes the southernmost part of Sao 

Miguel. 

Two eruptions of basanitoid have occurred near Agua 

de Pau volcano since the island was settled in the midfifteenth century. The A.D. 1563 eruption formed low 

spatter ramparts on top of Queimado, a relatively old 

trachyte dome about 6 km west-northwest ofLagoa do Fogo 

(fig. 6C). From that source, one small flow moved about 2 

km northwest. The main flow went north about 5 km and 

inundated part of the village of Ribeira Seca de Ribeira 

Grande (Weston, 1964). In A.D. 1652 (Weston, 1964), 

Strombolian activity built a cinder cone and extruded flows 

about 10 km west of Lagoa do Fogo. 

FURNAS VOLCANO 

Furnas volcano, a stratovolcano about 800 m above 

sea level, occupies the east-central part of Sao Miguel (fig. 

2). Its subaerial volume of about 60 km3 has increased at an 

estimated rate of 0.06 km3 /century. A region of mafic and 

minor trachytic vents and associated flows about 8 km long 

(volcanic zone 4 of Moore, 1990, 1991) separates Furnas 

and Agua de Pau volcanoes. The Povoa~ao caldera of the 

Furnas Volcano 15 
inactive Nordeste volcano adjoins the eastern side of Furnas. A caldera about 6 km in diameter, with walls as high as 

0.5 km, truncates the summit of Furnas volcano and 

contains Lagoa das Furnas (fig. 8). Eruptive units predating 

the caldera crop out chiefly in the caldera walls and in sea 

cliffs on the southern coast. At least 10 eruptions of trachyte 

pumice have occurred from intracaldera vents during the 

past 5,000 years (Booth and others, 1978); these eruptions 

formed incomplete pumice rings and associated trachyte 

domes distributed across the caldera floor. Pre- and postcaldera cones of ankaramite, basanitoid, and hawaiite are 

outside the caldera, chiefly on its northern and western 

sides; some pre-caldera mafic flows crop out in the caldera 

walls. Post-caldera cones, domes, and flows of tristanite 

erupted just west of the caldera boundary and on the eastern 

caldera rim. 

As at Sete Cidades and Agua de Pau volcanoes, no 

mafic vents are present within the caldera. The area that 

encloses only trachytic vents on Furnas volcano is about 8 

by 6 km, intermediate in size between Agua de Pau and Sete 

Cidades. Despite the relatively small size of the volcano, 

Furnas apparently has a larger subjacent magma reservoir 

than does Sete Cidades. The relation between the modem 

Furnas magma reservoir and the apparently extinct Nordeste 

magma reservoir is unknown; the two volcanoes adjoin, and 

a connection at depth may once have existed. 

Hot springs are prominent at Furnas and, along with 

its magnificent scenery, serve as the basis for a local tourist 

industry. The distribution of hot springs is shown by 

Zbyszewski and others (1958). Those on the northern shore 

of Lagoa das Furnas probably are related to the nearby 

caldera-bounding fault. Hot springs in the village of Furnas 

probably are associated with the Furnas C crater (Booth and 

others, 1978) or with radial fractures that formed during 

emplacement of the trachyte domes of Pico das Marconas 

(fig. 8). Hot springs along the upper Ribeira Quente (Hot 

River), 1 km southeast of the village, are associated with 

Pico das Marconas. The other principal location of hot 

springs is near the village of Ribeira Quente on the southern 

coast; here, they probably formed near faults associated 

with the southern caldera boundary. The hot springs suggest 

that a residual heat source associated with the frequent 

Holocene eruptions is close to the surface. 

Pleistocene Units 

Pre-Caldera Units 

The Quaternary Furnas volcano overlies the 1- to 

4-m.y.-old Nordeste volcanic complex (Fernandez, 1980), 

exposed near Ribeira Quente on the southern coast and 

locally at the base of the caldera wall. Roadcuts along the 

main highway between the villages of Furnas and Povoa~ao 

expose highly weathered mafic flows and altered trachyte of 

16 Geology of Late Quaternary Stratovolcanoes, Azores 

the Nordeste complex on the eastern wall of Furnas caldera. 

Xenoliths of Nordeste mafic rocks are locally common in 

Plinian fall deposits of Furnas volcano. 

The oldest known rocks of Furnas are late Pleistocene 

trachyte and tristanite flows and domes exposed at the base 

of the caldera wall and in sea cliffs near Ribeira Quente. A 

tristanite flow at the base of the section, directly overlying 

Nordeste basalt, near Ribeira Quente has a K-Ar age of 

93,000±9,000 years, and a trachyte flow overlying Nordeste 

basalt near the base of the southwestern caldera wall is 

48,000±4,000 years old. I conclude that Furnas volcano was 

built by almost exclusively subaerial eruptions during the 

past 100,000 years or less. 

Sea cliffs west of Ribeira Quente expose pyroclastic 

units displaying much of the eruptive history of Furnas (fig. 

9). Overlying the dated tristanite flow are 30 m of pyroclastic flows and mudflows whose radiocarbon ages are 

greater than 33,000 years. The next major deposit is a 

40-m-thick mudflow whose radiocarbon age is 22,060±250 

years B.P. The overlying section includes the calderaoutflow deposit, described below, and Holocene pumice-fall 

deposits. 

The western caldera wall exposes a 250-m-thick 

section of mainly flows and pyroclastic deposits, which, like 

the southern coastal section, spans much of the eruptive 

history of the volcano. The sources for many of these units 

were nearby. Most of the sequence here postdates the 

48,000±4,000-year-old flow. 

A small canyon eroded into the caldera wall on the 

southern side of Pico do Ferro (fig. 8) exposes a 

stratigraphic section (fig. 1 0) that provides key information 

about the latest Pleistocene history of Furnas volcano. A 

basanitoid flow, probably from a vent west of the caldera, 

crops out at the base of the section. Carbonaceous 

sedimentary rocks of lacustrine origin, interbedded with 

thin pumice-fall deposits and pillow lavas of basanitoid and 

trachyte, overlie the basal flow. The sedimentary rocks 

include many algal mats containing at least five species of 

diatoms and a dicot indicative of shallow fresh-water 

conditions (Chaves, 1908; J. Platt Bradbury, USGS, written 

commun., 1984). The sediments were deposited in a lake of 

unknown size that probably was confined within a pumice 

ring or crater similar to those on the floor of the modern 

caldera. Radiocarbon ages of three separate lake beds are 

23,000--27,000 years B.P. The basanitoid and trachyte 

pillow lavas apparently flowed into the lake from vents to 

the west. 

Overlying the lacustrine deposits is an 18-m-thick 

tristanite flow containing ultramafic xenoliths. The calderaoutflow deposit caps this tristanite. Three younger tristanite 

flows that may have cascaded into the caldera form the top 

of the Pleistocene section; they came from post -caldera 

vents to the west and southwest and are overlain by 

Holocene pumice. 
Mudflow with 

discontinuous 

lenses of dark 

(peralkaline 

trachyte) and light 

(peraluminous 

trachyte) pumice 

Clasts near base are 

dominantly welded tuff 

[:METERS 

22,060 ± 250 years old 

-1--Trachyte pyroclastic flow with charcoal 

1------------1-I--Trachyte pyroclastic surge with charcoal, 

1-------------1_ >34,000 years old 

Mudflow with locally thin 

discontinuous lenses of 

trachyte pumice 

Trachyte pyroclastic flow with charcoal, 

>33,000 years old 

-1- Lithic-poor trachyte pumice 

1------------1 (air-fall) 

Thinly bedded, locally 

lithic rich trachyte 

pumice deposits (air-fall) 

Tristanite flow 

Nordeste mafic flows 

BASE 

93,000 ± 9,000 years old 

TOP 

Trachyte pumice 

(air-fall) and trachyte 

pyroclastic flows 

Potassic hawaiite aa flow 

Mudflow 

Lithic-rich pyroclastic 

surge deposit 

Trachyte pumice (air-fall) 

-

Mudflow 

Mudflow 

Mudflow 

-

Trachyte pyroclastic flow 

Trachyte pyroclastic flow 

(welded tuff) 

Trachyte pyroclastic flow 

Mudflow with 50 percent 

lithic fragments and 

lenses of trachyte pumice 

Trachyte pumice (air-fall) 

Mudflow 

Trachyte pumice (air-fall) 

Mudflow 

Thick soil at top Trachyte 

pumice (air-fall) 

r---. A.D. 1630 

1--p 

c 

ossibly correlative with 

Furnas G deposit (Booth 

and others, 1978) 

oarse black trachytic pumice. 

Possibly correlative with 

Furnas F deposit (Booth 

and others, 1978) 

p yroclastic flow deposit 

T 

-I 

rachyte pumice (air-fall) 

Mudflow 

I rachyte pumice (air-fall) 

c aldera-outflow deposit 

(approx. 12,000 years old) 

Figure 9. Composite measured stratigraphic section, sea cliff west of Ribeira Quente on southern flank of Furnas 

volcano. Units dip 0°-26°W. 

Other lacustrine deposits, interbedded with potassic 

hawaiite pillow lava, crop out 2 km northeast of the section 

shown in figure 10, on the northern side of the village of 

Furnas. These deposits are older than the 2,900±120-

year-old Furnas C deposit (Shotton and Williams, 1971; 

Booth and others, 1978), whose crater rim truncates them. 

Furnas Volcano 17 
Trachyte pumice 

(Fogo A and younger) 

Tristanite flow 

Tristanite flow 

Tristanite flow 

' 

Trachyte pumice 

Welded tuff 

Trachyte pumice 

Tristanite flow with 

ultramafic xenoliths 

Trachyte pumice 

= = 30 m covered = = 

-f--

Lac~~ci~~fc ~:~;r~rth 

Trachyte pumice 

Lacustrine beds 

Lacustrine beds with 

organic lavers 

Basanitoid flow 

BASE COVERED 

[:METERS 

Caldera-outflow deposit 

(approx. 12,000 years old) 

Trachyte pillow lava 

Basanitoid flow (columnar jointed) 

Organic-rich clay (23,000 ± 1,000 

years old) 

24,000 ± 500 years old 

Figure 10. Measured stratigraphic section, western 

wall of Furnas caldera on southern side of Pico do Ferro. 

Dips are approximately horizontal. 

Other major pre-caldera units exposed on the western 

and northern caldera walls are chiefly flows of ankaramite, 

potassic hawaiite, and tristanite from vents west and north 

of the caldera. These flows are interbedded with trachytic 

deposits, chiefly mudflows and pumice of Plinian or subPlinian origin. 

Caldera-Outflow Deposit 

The caldera of Furnas volcano formed about 12,000 

years ago in response to ejection of trachyte pumice that 

18 Geology of Late Quaternary Stratovolcanoes, Azores 

formed a locally welded outflow deposit. This deposit is the 

uppermost unit truncated by the caldera wall in most places, 

except in the canyon whose stratigraphy is shown in figure 

1 0; younger pumice-fall deposits mantle the caldera rim and 

wall. The caldera-forming eruption produced pyroclastic 

flows directed primarily south and east. A small exposure of 

the deposit also occurs 10 km north of the village of Furnas. 

The caldera-outflow deposit is generally 25-50 m 

thick but ranges from 1 to 60 m. The welded zone is less 

than 1 to about 25 m thick. The estimated subaerial volume 

of the deposit is about the same as that of the caldera (7 

km3), but much tephra undoubtedly fell in the ocean. 

The outflow deposit is at or just below the western 

caldera rim and locally is plastered against ankaramite 

cones just northwest of the caldera (fig. 8). It fills canyons 

on the southern flank of the volcano, near Ribeira Quente, 

and covers about half of the older Povoa<;ao caldera. This 

unit is the "Povoa<;ao ignimbrite" discussed briefly by 

Booth and others (1978) and Schmincke and Weibel (1972). 

One of the best exposures of welded tuff is in an 

abandoned quarry on the western side of the Ribeira Quente, 

about 2 km northwest of the village of Ribeira Quente. The 

base of the deposit is concealed. The section consists of 9 m 

of densely welded tuff, 3 m of non welded pumice, and 14 m 

of densely welded tuff to the top of the quarry. Xenoliths of 

obsidian, trachyte, syenite, and basalt are present. Other 

good exposures are along trails west and east of the village 

of Ribeira Quente and at the beach in the village of 

Povoa<;ao. 

The age of the caldera-outflow deposit is not known 

directly, but an overlying cluster of trachyte domes at Pico 

do Ferro (fig. 8) has a radiocarbon age of 11,230±100 years 

B.P. The soil separating the outflow deposit from the Pico 

do Ferro trachyte is only 10-15 em thick and, by analogy 

with modem soils formed on historic pumice deposits of 

Sao Miguel, probably required only a few centuries to form. 

Post-Caldera Units 

Three tristanite cones, two with associated flows, and 

a cluster of seven trachyte domes postdate formation of the 

caldera; they were erupted from radial and concentric 

fractures that probably formed during caldera collapse. The 

cluster of domes, which includes Pico do Ferro, erupted 

along an arcuate fissure trending from N. 55° W. to due 

west; the fissure is approximately radial to the caldera but 

also is approximately parallel with a common regional 

fracture pattern on Sao Miguel. Truncation of the eastern 

dome at the caldera rim suggests either that caldera collapse 

was not yet complete when the dome formed or that part of 

the growing dome cascaded into the caldera. 

The cone of Pico do Canario consists of tristanite 

pumice that is locally welded to dense obsidian. It formed 

atop the eastern Furnas caldera rim, and pumice cascaded 

into both Furnas and Povoa<;ao calderas. 
Cones and flows of the Cedros area, about 1.5 km 

west of Lagoa das Furnas, consist of silica-rich tristanite. 

Five scoria cones erupted along fractures concentric to the 

caldera, and associated flows rafted scoria short distances 

from the vents. This unit overlies the caldera-outflow 

deposit in a deep canyon on the northeastern side of Cedros. 

Tristanite scoria and a thick flow were erupted at 

another vent 300 m west of the caldera rim. The flow may 

underlie Pico do Ferro, but the contact is covered. 

Except for the Pico do Ferro trachyte dome, the ages 

of these four post-caldera units are unknown but are either 

latest Pleistocene or Holocene. A potassic hawaiite cone 

that may overlie the Cedros tristanite has a radiocarbon age 

of 1 0,620±300 years B.P. (fig. 8). 

Holocene Units 

Trachytic Deposits 

No known eruptions of Furnas volcano occurred for 

several thousand years after formation of the caldera; 

however, the earliest intracaldera eruptions have not been 

dated. 

The oldest dated Holocene unit is a lake deposit about 

1 km east of the village of Furnas, 200 m northeast of the 

junction of the roads to Ribeira Quente and Povoa~ao. 

Fine-grained, thinly bedded, well-stratified lacustrine beds 

include lenses of black, fine-grained carbonaceous material. 

This deposit has not been recognized elsewhere, and it 

probably formed where a mudflow or pyroclastic flow 

dammed the ancestral Ribeira Quente. The radiocarbon age 

of one carbonaceous lens is 6,520±100 years B.P., an age 

not represented by known deposits elsewhere on the 

volcano. 

At least ten Plinian and sub-Plinian eruptions of 

trachyte pumice, four of which accompanied emplacement 

of domes, occurred within the caldera during Holocene 

time. The .stratigraphically oldest deposit forms an 

incomplete pumice ring, with a small central dome, on the 

northeastern side of Lagoa das Furnas (fig. 8). The lake 

occupies two adjoining shallow depressions that probably 

also mark vent locations. The Furnas A or B deposits of 

Booth and others (1978) possibly came from these vents. 

The largest known Holocene pumice eruption 

produced the Furnas C deposit (Booth and others, 1978) 

2,900±120 years B.P. (Shotton and Williams, 1971). The 

incomplete pumice ring that formed during this eruption 

crops out north and west of the village of Furnas. A late 

stage of the eruption produced thick mudflows that cover 

the northern part of the caldera floor. 

The next major eruption on the floor of Furnas 

caldera formed the two trachyte domes of Pico das Marconas and associated pumice rings. Booth and others (1978) 

suggested that these vents were the sources of their Furnas 

E and H pumice deposits. 

Pico do Gaspar is a trachyte dome about 1 km east of 

Lagoa das Furnas. Two nested incomplete pumice rings 

surround the dome. According to Booth and others (1978), 

these vents produced three widespread pumice deposits, 

Furnas F, G, and I. Charcoal from the F deposit in a quarry 

1.5 km east of Pico do Gaspar has a radiocarbon age of 

1, 1 00±60 years B.P ., and charcoal from the G deposit on the 

eastern caldera rim is 780±120 years old. 

The most recent eruption of Furnas, in A.D. 1630 

(Weston, 1964; van Padang and others, 1967), produced 

three partly nested, incomplete pumice rings and a central 

dome of trachyte on the southern caldera floor southeast of 

Lagoa das Furnas (fig. 8). The distribution and relative ages 

of the pumice rings indicate that the active vent moved 

west-northwestward about 1 km during the eruption, 

possibly along a concentric fracture related to collapse of 

the caldera. These vents lie near the southern caldera 

boundary, but the proximal deposits cover the rim. Vigorous 

mudflows swept down the Ribeira Quente and inundated the 

village at its mouth. Charcoal-rich pyroclastic flows 

characterize the base of the deposit at many localities. This 

charcoal yielded a radiocarbon age of 295±40 years B.P., in 

agreement with the known age of 320 years (before A.D. 

1950). Other pyroclastic flows occur higher in the section, 

where they are interbedded with Plinian fall deposits. 

At least 191 people reportedly were killed by the 

1630 eruption, but accounts (Weston, 1964; van Padang and 

others, 1967) vary as to the cause: earthquakes preceding 

the eruption, hot pumice fall (probably actually pyroclastic 

flows), or mudflows. All of these events undoubtedly 

occurred, but mudflows sweeping down the valley to the 

village of Ribeira Quente likely caused the most deaths. The 

area of Ribeira Quente has been inundated repeatedly by 

mudflows and pyroclastic flows, most notably those 

associated with the caldera-outflow deposit, the Furnas C 

eruption, and the 1630 eruption. 

Radiocarbon ages of the Furnas C and younger 

deposits permit estimation of the average dormant interval 

of Furnas volcano. Eight trachytic eruptions occurred 

between 2,900 and 320 years B.P., an average dormant 

interval of about 369 years. The interval decreased to only 

195 years, however, during the period from 1,100 (Furnas 

F) to 320 years B.P., when five eruptions occurred. The 

present quiet interval has now lasted 320 radiocarbon years 

and 361 calendar years, and Furnas may-be overdue for its 

next eruption. 

Mafic Deposits 

In contrast to Sete Cidades and Agua de Pau, postcaldera eruptions of mafic lavas have been rare on the flanks 

of Furnas. Potassic hawaiite cones (fig. 8), one dated at 

10,620±300 years B.P., are on the southern side of the 

Furnas Volcano 19 
Cedros tristanite cones. A potassic hawaiite flow, interbedded with pumice between the Furnas C and A.D. 1630 

deposits, is on the southern flank of the volcano. This flow 

overlies consanguineous cinders from a nearby vent, buried 

during A.D. 1630 or an earlier eruption, just south of the 

southern caldera boundary. 

PETROGRAPHY 

Mafic Rocks 

Mafic rocks on the three volcanoes include ankaramite, basanitoid, alkali olivine basalt, hawaiite, and 

mugearite. 

Ankaramite contains abundant olivine and clinopyroxene and minor plagioclase phenocrysts in an intergranular to intersertal groundmass of plagioclase, clinopyroxene, olivine, opaque oxides, and interstitial residuum. 

Xenocrysts of orthopyroxene and amphibole are present in 

many samples, and late-stage biotite commonly fills 

vesicles. 

Basanitoid has normative but not modal feldspathoids 

and thus is not termed basanite. Rocks are commonly 

aphyric but many contain scattered olivine and rare clinopyroxene and plagioclase phenocrysts in an intersertal to 

intergranular groundmass of the same minerals, opaque 

oxides, and interstitial residuum. Clinopyroxene in the 

groundmass is titaniferous (pleochroic brown or purple), 

reflecting the high bulk-rock Ti02 content (generally >3.5 

weight percent). 

Alkali olivine basalt is less common than the other 

mafic rock types and is transitional among primitive basanitoid, accumulative ankaramite, and differentiated hawaiite 

and mugearite. Vent deposits and flows typically have a few 

volume percent of olivine>clinopyroxene>plagioclase 

phenocrysts in an intergranular to intersertal groundmass of 

the same minerals, opaque oxides, and interstitial residuum. 

Alkali olivine basalt is not easily distinguished in the field 

from other mafic rocks but is defined here as having less 

than 5 percent normative nepheline and less than 15 percent 

olivine and clinopyroxene phenocrysts. 

Hawaiite typically has an intersertal to pilotaxitic or 

hyalophitic texture and is generally almost aphyric. A few 

samples contain less than 1 percent each of one or more of 

the following phenocryst minerals: olivine, clinopyroxene, 

plagioclase, amphibole, and biotite. Amphibole may be 

xenocrystic, for it invariably has rims of opaque oxides 

(opacite). Hawaiite is relatively rich in K20, as are most of 

the rocks on Sao Miguel (Schmincke and Weibel, 1972), 

and could be called trachybasalt. 

Mugearite is relatively uncommon, although it occurs 

in vent deposits and flows on all three volcanoes. It is most 

abundant on Sete Cidades, where relatively old (>30,000 

20 Geology of Late Quaternary Stratovolcanoes, Azores 

years) flows crop out chiefly in sea cliffs on the northern, 

western, and southern coasts. Mugearite appears completely 

transitional to hawaiite; samples of both generally have 

similar phenocryst populations and are indistinguishable in 

the field. Amphibole is an important phenocryst in the thick 

section of mugearite in the sea cliff south of Ponta da 

Ferraria (fig. 4). 

Intermediate to Silicic Rocks 

Tristanite, which generally has 0/Na 0~1 and 

Si02 of 55-58 percent, is relatively uncommon but occurs 

throughout the subaerial sequence on all three stratovolcanoes. Many tristanite vent deposits and flows are aphyric; 

others contain 1 percent or less of one or more of the 

following phenocrysts: olivine, clinopyroxene, sanidine, 

plagioclase, amphibole, biotite, and opaque oxides. Some, 

especially those containing olivine, may be hybrid mixtures 

of trachytic and more mafic magmas. A thick aphyric 

tristanite flow underlying the younger caldera-outflow 

deposit on the southern flank of Agua de Pau (fig. 6A) 

contains 5-10 percent late-stage biotite in vesicles. 

Most trachyte contains phenocrysts of alkali feldspar 

(generally sanidine or anorthoclase), plagioclase, biotite, 

opaque oxides, green clinopyroxene, and rare zircon and 

apatite. Amphibole that is possibly xenocrystic is in several 

samples. One trachyte flow on the southern flank of Agua 

de Pau contains 40 volume percent sanidine phenocrysts. 

Sphene microphenocrysts are unusually abundant (as much 

as 2-3 percent) in several trachyte flows and domes from 

Agua de Pau. Storey (1981) reported that clinopyroxenes in 

young pumice deposits from Agua de Pau are generally 

salite and have low Al20 3, Ti02, and Na20. Some trachyte 

is aphyric or almost so; most is in peralkaline domes on the 

western and northwestern flanks of Agua de Pau. 

Syenite Xenoliths 

Xenoliths of syenite as large as 0.5 m in diameter 

locally are common in trachyte pumice deposits of Furnas 

and Agua de Pau volcanoes. They are much rarer on Sete 

Cidades, although several dozen were observed in the sea 

cliff near Ponta da Ferraria. They are most readily found in 

alluvial deposits within the calderas and on the flanks of the 

volcanoes. I found xenoliths of syenite throughout the 

subaerial sequence on Agua de Pau, including ca. I 00,000-

year-old welded tuffs on the northern coast of Sao Miguel. 

They are particularly abundant in the 15,200-year-old 

younger caldera-outflow deposit, in which they locally 

make up 20 percent of the rock. 
Cann ( 1967) discussed the mineralogy of several 

samples of Agua de Pau syenite; they chiefly contain 

sanidine, arfvedsonite, quartz, and aegirine, as well as minor 

biotite and zircon or dalyite. Some lack quartz and contain 

sodalite. Xenoliths of syenite at Furnas volcano are 

generally similar to those at Agua de Pau, but most lack 

quartz and contain sodalite. 

E.H. McKee (Written commun., 1986) dated a syenite 

xenolith from a trachyte pumice deposit on Furnas at 

0.973±0.029 Ma. This relatively old age suggests that the 

syenite may have formed within Nordeste volcano. 

Cann (1967) and Schmincke (1973) noted that syenite 

of Agua de Pau may contain either normative nepheline or 

quartz, and many samples are peralkaline; thus, the syenite 

generally matches the compositions of the extrusive rocks 

and is probably cognate. 

Xenoliths of Mafic and Ultramafic Rocks 

Xenoliths of mafic and ultramafic rocks are in several 

vent deposits and flows of basalt, basanitoid, and ankaramite on each volcano. The most remarkable sites are on 

Sete Cidades. The ankaramite cone of Eguas (fig. 3) and its 

flows contain thousands of xenoliths, including spinel 

lherzolite and dunite. Some samples have a relatively fine 

grained; recrystallized groundmass surrounding megacrysts 

of clinopyroxene, orthopyroxene, and kink-banded olivine. 

No garnet is known in these rocks or elsewhere on the three 

volcanoes. Some of these xenoliths may have been carried 

up from the mantle. Many, however, have cumulate texture 

and sufficient Na20 to be nepheline normative; these may 

have formed in the conduit system of the volcanoes. 

Pico das Camarinhas, an 840-year-old ankaramite 

cone near the western end of Sao Miguel, and its flow 

contain abundant xenoliths of hornblende gabbro, hornblendite, and pyroxenite. 

The ankaramite cone of Mafra, 1 km east of the 

village of Mosteiros, and its flow contain common xenoliths 

of dunite (megacrysts of olivine in a finer grained, recrystallized groundmass of olivine and spinel) of possible 

mantle origin, wehrlite, olivine clinopyroxenite, green 

pyroxenite, black pyroxenite, websterite, and gabbro. 

One of the most unusual occurrences of xenoliths is in 

the relatively old tristanite flow that forms Ponta do 

Garajau, 0.8 km east of Ribeira Quente on Furnas volcano 

(fig. 8). Xenoliths of olivine clinopyroxenite and syenite are 

common in a hybridized host that has 59.6 percent Si02 and 

phenocrysts or xenocrysts of sanidine, plagioclase, olivine, 

clinopyroxene, amphibole, and biotite. The clinopyroxenite 

contains normative, but not modal, nepheline and leucite; 

this is the only example of normative leucite among the 

rocks analyzed for this study. 

MAJOR ELEMENT CHEMICAL 

COMPOSITIONS 

Lava flows and pyroclastic materials of Sete Cidades, 

Agua de Pau, and Furnas volcanoes belong to the alkali 

basalt-trachyte suite (Irvine and Barager, 1971). Variation 

diagrams (figs. 11-13) based on 357 new chemical analyses 

show, for the most part, a continuum in chemical 

compositions among these rocks. Analytical data and 

normative compositions of representative rocks from the 

three volcanoes are given in table 1. A sample from each 

map unit has been analyzed. 

Tholeiitic basalts are on the adjacent Mid-Atlantic 

Ridge but not on Sao Miguel (White and others, 1979). A 

few mafic rocks on all three volcanoes contain normative 

hypersthene and olivine, but these are either high-silica 

hawaiite, mugearite, tristanite, and hybrid rocks or ankaramite and basanitoid that have low alkali concentrations, 

possibly because of post-eruptive leaching. I consider them 

all to be alkali basalt or its derivatives, consistent with 

criteria of Poldervaart (1964). 

Silica-Variation Diagrams 

The A120 3 versus Si02 variation diagram (fig. 11A) 

shows generally increasing alumina with increasing silica 

through the basalt range, reflecting the lesser abundance of 

olivine in more silicic compositions. The curve is relatively 

flat from about 50 to 62 percent Si02• Alumina then 

decreases with increasing silica in the most silicic rocks, 

which include peralkaline trachyte. Below about 62 percent 

Si02, rocks of Agua de Pau have generally lower alumina 

than do those of Sete Cidades and Furnas volcanoes. 

The MgO versus Si02 variation diagram (fig. liB) 

shows a wide range in magnesia contents of basaltic rocks, 

in accord with olivine-controlled differentiation. Above 

about 50 percent Si02, the curve is almost straight; the few 

deviations represent hybrid rocks, mainly from Agua de 

Pau, that result from mixing of mafic and trachytic melts. 

The Fe0+0.9*Fe20 3 versus Si02 variation diagram 

(fig. llC) shows a fairly straight line. Basaltic lavas 

surrounding Agua de Pau have slightly higher iron contents 

than those on the flanks of the other two volcanoes. The 

greatest scatter is among the most silicic compositions on 

Agua de Pau, where magnesia abundances are undetectable. 

Likewise, the (Fe0+0.9*Fe20 3)/Mg0 versus Si02 

variation diagram (fig. liD) emphasizes the marked iron 

enrichment above about 62 percent Si02, most prominent 

among the trachytic rocks of Agua de Pau. 

The CaO versus Si02 variation diagram (fig. 11£) 

shows a fairly straight line. Deviations at the basaltic end 

are for xenoliths of gabbro and peridotite, as well as for 

olivine-controlled differentiation and resulting variations in 

Major Element Chemical Compositions 21 
Table 1. Chemical analyses and normative compositions for representative rocks from Sete Cidades, Agua de Pau, 

and Furnas volcanoes, Sao Miguel, Azores 

[In weight percent] 

2 3 4 5 6 7 8 

Chemical analysis 

Si02 44.2 45.0 50.8 54.0 58.2 59.6 61.8 63.4 .................. 

Al203 .................. 15.9 103 18.7 19.2 17.0 18.7 18.3 18.4 

F~OJ .................. 2.8 3.4 2.2 1.8 2.9 2.5 2.5 2.4 

FeO 10.0 7.4 6.5 4.9 2.8 1.8 1.2 0.76 

MgO 5.6 17.2 3.9 23 1.5 1.9 0.41 0.45 

CaO 10.7 10.0 13 4.8 3.0 2.9 0.87 0.68 

N~O 2.6 1.8 4.5 5.7 53 53 7.9 6.8 

K20 1.9 0.90 2.7 3.6 5.2 5.6 5.5 5.1 

H2o+·················· 0.34 0.08 0.19 0.18 0.48 0.19 0.66 0.06 

H 2o- 0.14 0.31 0.31 0.30 0.59 0.30 0.16 0.43 

Ti02 4.9 2.1 2.6 1.9 1.6 1.1 0.46 0.60 

P20s 0.87 0.44 0.81 0.57 0.51 0.23 0.05 0.08 

MnO 0.17 0.14 0.16 0.22 0.14 0.15 0.25 0.21 

C02 0.01 0.02 0.04 0.01 0.01 0.02 0.04 0.02 

Sum .............. 100.13 99.09 100.71 99.48 99.23 100.29 100.10 9939 

Nonnative composition 

Q ....................... 0.71 

c ........................ 0.71 

Or ...................... 11.29 5.38 15.96 21.51 30.31 33.27 32.86 30.55 

Ab ..................... 15.09 11.75 31.28 38.70 44.34 44.02 49.90 58.30 

An ..................... 26.25 17.67 22.85 16.34 9.52 10.76 2.75 

Ne ..................... 3.79 2.02 3.68 5.49 0.54 7.72 

Ac ...................... 1.10 

Ns ...................... 0.49 

Di-Wo .............. 8.91 12.38 3.27 1.62 2.04 0.87 1.58 

Di-En ................ 4.69 8.67 1.64 0.70 1.03 0.42 0.27 

Di-Fs ................. 3.96 2.66 1.56 0.92 0.97 0.43 1.44 

Hy-En ............... 1.18 1.15 

Hy-Fs ................ 1.10 4.00 

01-Fo ................ 6.54 24.43 5.66 3.57 2.74 3.04 0.53 

01-Fa ................ 6.09 8.26 5.94 5.12 2.83 3.37 3.11 

Mt ..................... 2.00 1.70 1.35 1.06 0.78 0.65 0.48 

11 ........................ 9.36 4.05 4.94 3.65 2.53 2.11 0.87 1.16 

Ap ..................... 2.06 1.07 1.92 1.37 0.64 0.54 0.12 0.19 

1. Basanitoid, basal flow on northwestern caldera wall, Furnas volcano. 

2. Ankaramite, basal flow on sea cliff below Feteiras, Sete Cidades volcano. 

3. Hawaiite, basal flow on sea cliff in Mosteiros, Sete Cidades volcano. 

4. Mugearite, flow on sea cliff below Camarinhas cone, Sete Cidades volcano. 

5. Hybridized basanitoid, flow on northern flank of Agua de Pau volcano. 

6. Tristanite, basal flow on sea cliff east ofRibeira Quente, Furnas volcano. 

7. Peralkaline trachyte, welded pumice of the Gaspar ring, Furnas volcano. 

8. Trachyte, flow in Feteiras, Sete Cidades volcano. 

phenocryst abundances. Ten samples of trachyte from Agua 

de Pau and one from Sete Cidades have no detectable CaO, 

another indication of their alkalic nature. 

The Na20 versus Si02 variation diagram (fig. llF) 

shows considerable variation throughout the compositional 

range. Three samples of trachyte from Furnas have low 

Na20 and probably are actually altered Nordeste rocks. The 

rest of the variation likely results from (1) olivine-controlled 

differentiation in the basaltic range, (2) differences in initial 

chemical composition or in degree of partial melting of the 

mantle source, (3) hybridization, (4) post-solidification 

leaching, or (5) some combination of these processes. 

22 Geology of Late Quaternary Stratovolcanoes, Azores 
The K20 versus Si02 variation diagram (fig. llG) 

resembles the Na20-Si02 plot, except that (1) the generally 

higher K20 in Furnas mafic and trachytic rocks is apparent 

and (2) above about 63 percent Si02 K20 begins to 

decrease, mainly in Agua de Pau rocks, because of 

crystallization and removal of potassium feldspar (Storey, 

1981). 

The Na20+K20 versus Si02 variation diagram (fig. 

llH), on which is drawn the line separating alkalic basalts 

from tholeiites in Hawaii (Macdonald and Katsura, 1964 ), 

shows that Sao Miguel rocks are strongly alkalic. The eight 

rocks in the tholeiitic field include an altered Nordeste 

basalt, an olivine clinopyroxenite xenolith from Furnas, and 

six nepheline normative mafic and ultramafic xenoliths 

from Sete Cidades. Figures 11£ and 11H can be combined 

to yield a Peacock ( 1931) index of about 49, further 

indication of the alkalic nature of the Sao Miguel rocks. 

The K20/Na20 versus Si02 variation diagram (fig. 

111) shows that, except for three highly potassic Furnas 

trachytes, the ratio increases only slightly with increasing 

silica. Higher K20 in Furnas mafic and silicic rocks is 

apparent. 

The Ti02 versus Si02 variation diagram (fig. 11.1) 

shows a generally straight line, except among basaltic 

compositions. Samples of basanitoid have typically high 

(>3.5 percent) Ti02 contents, shown modally in titaniferous 

clinopyroxene phenocrysts and groundmass grains. 

Fractionation of clinopyroxene, as well as olivine and 

perhaps iron-titanium oxides, probably accounts for the 

variable Ti02 contents in the basaltic range. 

The P20 5 versus Si02 variation diagram (fig. llK) 

shows wide scatter, especially below about 56 percent Si02• 

Two fields of Agua de Pau mafic rocks high and low in 

P20 5 can be distinguished. Apatite needles are present in 

most mafic rocks of Sao Miguel, and microphenocrysts of 

apatite are present in many trachytes. The reason for the 

wide scatter in figure IlK is not immediately apparent. 

Magnesia-Variation Diagrams 

The Al20 3 versus MgO variation diagram (fig. 12A) 

shows a relatively straight line throughout most of the 

compositional range. The exceptions are at the highmagnesia end, where some analyses are of ultramafic 

xenoliths that may not be cognate, and at the low-magnesia 

(trachytic) end, where alumina decreases somewhat in the 

most silicic rocks as feldspar is removed. 

The Fe0+0.9*Fe20 3 versus MgO variation diagram 

(fig. 12B) changes slope abruptly at about 12 percent iron 

and 5-10 percent MgO. This change reflects the decrease in 

iron as relatively forsteritic olivine is accumulated in 

ankaramites and ultramafic xenoliths. Departures from the 

general trend reflect, at the high-magnesia end, the analyses 

of ultramafic xenoliths and, in the basaltic range (about 

5-15 percent iron), the effects of hybridization of diverse 

melts. 

The CaO versus MgO variation diagram (fig. 12C) 

shows a general trend of increasing lime with increasing 

magnesia; the steep curve in the low-magnesia range 

flattens above about 8 percent MgO, where accumulation of 

olivine dominates. The diffuse pattern above about 15 

percent CaO represents analyses of gabbroic and ultramafic 

xenoliths. The small cluster of Agua de Pau mafic 

compositions at about 6 percent iron and 7 percent MgO are 

hybrid rocks. 

The Na20 versus MgO (fig. 12D) and K20 versus 

MgO (fig. 12£) variation diagrams are similar and show 

expected trends of decreasing Na20 and K20 with 

increasing magnesia. The Agua de Pau hybridized rocks are 

most obvious on figure 12.£. The K20/Na20 versus MgO 

variation diagram (fig. 12F) shows the generally higher 

K20 in Furnas rocks and the expected trend of slightly 

decreasing K20/Na20 with increasing magnesia. 

The Ti02 versus MgO variation diagram (fig. 12G) 

shows a peak in Ti02 contents (3.5->5 percent) at about 5-9 

percent MgO, corresponding to basanitoid compositions. 

The scatter of points in this range reflects the dominance of 

olivine-controlled fractionation, although fractionation of 

titaniferous clinopyroxen:e may also be important. The 

cluster of low-Ti02 hybrid rocks on the flanks of Agua de 

Pau is apparent. 

The P20 5 versus MgO variation diagram (fig. 12H), 

similar to the Ti02 versus MgO plot, shows a peak in P20 5 

(0.7-1.6 percent) in basanitoids. The measurable P20 5 

(commonly >0.1 percent) in the ultramafic xenoliths 

suggests they they may be cognate to the volcanoes because 

xenoliths representing fragments of mantle residuum 

probably would be depleted in incompatible P20 5• 

AFM Diagram 

The AFM plot (fig. 13) for the newly analyzed Sao 

Miguel rocks shows their alkalic nature, especially as 

compared to alkalic rocks from Hawaii (Macdonald, 1968). 

Most of the Sao Miguel mafic compositions are displaced 

toward the Na20+K20 comer relative to Hawaiian 

compositions. A small field of four hybrid lava flows on the 

flanks of Agua de Pau can be distinguished. Several samples 

of Furnas trachyte and one from Sete Cidades are somewhat 

more alkalic than other samples of trachyte, although Furnas 

mafic rocks fall in an intermediate position. No clear 

distinction among the volcanoes is apparent. 

DISCUSSION 

The major element chemical data indicate many 

similarities and some significant differences among the 

Discussion 23 
rocks of Sete Cidades, Agua de Pau, and Furnas volcanoes. 

One important similarity is that basanitoid of somewhat 

variable composition is the most likely parental magma 

from which the other magmas were derived. Perhaps the 

two most important differences are the common 

development of peralkaline compositions on Agua de Pau 

and the generally increasing relative abundance of K20 

from west to east. 

Figure 12G indicates that almost aphyric, high-Ti02 

basanitoid occupies a unique position among the analyzed 

rocks from the three volcanoes. From this composition, one 

trend leads, with decreasing MgO and Ti02, to the more 

silicic differentiates (hawaiite to trachyte). Another trend is 

defined by decreasing Ti02 and increasing MgO because of 

accumulation of olivine and low-titanium clinopyroxene in 

ankaramites. Ti02 is lower in all rocks that trend away from 

basanitoid and so must be incorporated in minerals that have 

been removed. The most compelling field and petrographic 

evidence regarding the distribution of Ti02 is that it is 

incorporated, under locally hydrous conditions, in biotite 

(Storey, 1981 ), basaltic hornblende-kaersutite, and ilmenitetitanomagnetite. These minerals are abundant and 

accompany plagioclase, pyroxene, and apatite in the 

common gabbro and hornblendite xenoliths in ankaramite 

of the Camarinhas cone. 

Magnesia variation diagrams (fig. 12) (Schmincke 

and Weibel, 1972) suggest that olivine fractionation was 

impor.tant in controlling mafic compositions. However, 

hawaiite and more silicic rocks have been fractionated 

beyond olivine control, although olivine commonly is 

present as scattered microphenocrysts (sometimes xenocrystic) in even the most silicic compositions. The slope of 

many magnesia variation curves changes near 5 percent 

MgO, presumably because variation departs from simple 

olivine control at about that point. The scatter of points at 

higher magnesia contents suggests, however, that olivine 

fractionation is riot the only process causing variation. The 

principal alternatives are (1) several parental magmas were 

generated beneath Sao Miguel that have different magnesia 

and other oxide contents resulting from different degrees of 

partial melting of a homogeneous mantle source or partial 

melting of an inhomogeneous mantle source, or (2) 

fractionation of other minerals (chiefly clinopyroxene and 

plagioclase but also orthopyroxene and opaque oxides) was 

important. Because microprobe data on mineral 

compositions are not yet available, quantitative modelling 

to test these hypotheses has not yet been attempted. 

Peralkaline trachyte was erupted throughout the subaerial history of Agua de Pau from vents along radial and 

possibly concentric fractures on the western, southern, and 

northern flanks of the volcano, as well as from intracaldera 

vents. The development of volumetrically important peralkaline magmas on Agua de Pau and not on the other two 

volcanoes may be related to the relatively large subjacent 

magma reservoir of Agua de Pau, which I infer from the 

24 Geology of Late Quaternary Stratovolcanoes, Azores 

distribution of trachyte vents (fig. 6) and syenite xenoliths 

(Booth and others, 1978). In addition, alumina contents 

generally are lower throughout the compositional range on 

Agua de Pau as compared to the other two volcanoes (figs. 

11, 12) (Schmincke and Weibel, 1972). Self and Gunn 

(1976) and Storey (1981) found that crystallization and 

removal of potassium feldspar, biotite, and clinopyroxene 

was largely responsible for producing peralkaline trachyte 

on Terceira and among the post-Fogo A pumice deposits on 

Agua de Pau; fractionation of plagioclase (Schmincke, 

1982) must also have been important. Stratigraphic and 

radiometric data indicate that Sete Cidades, Agua de Pau, 

and Furnas volcanoes commonly had lengthy repose 

following major eruptions, especially caldera-forming 

episodes. Only on Agua de Pau, however, did calderaforming eruptions produce peralkaline rocks in great 

volume. It is possible that peralkalinity typically 

characterized the late stage of magmatic cycles on Agua de 

Pau, as Storey (1981) found for the post-Fogo A pumice 

deposits. Perhaps potassium feldspar, biotite, clinopyroxene, and plagioclase were removed during early and 

intermediate episodes of a cycle, ultimately resulting in 

development of a peralkaline melt that erupted last in the 

cycle. Fisher and Schmincke (1984), however, presented 

evidence of common chemical zonation in silicic magma 

reservoirs, as shown by compositions of their erupted 

products; therefore, development of peralkalinity may be a 

relatively short-term process in large magma reservoirs. 

Figure 12F illustrates the general increase of K20 

from west to east across the three volcanoes. This increase 

somewhat mimics the increase in 87 Sr/86Sr found by White 

and others (1976) and Hawkesworth and others (1979) and 

suggests that increasing distance from the Mid-Atlantic 

Ridge or from the Azores fracture zone accounts for the 

increase in K20. This explanation does not hold, however, 

when older rocks are included. For example, Nordeste rocks 

(Fernandez, 1980) are not enriched in K20 relative to 

Furnas rocks. In addition, White and others (1976) found 

normal (low) 87Sr/86Sr in Pliocene basalt from Santa Maria, 

the next island southeast of Sao Miguel. I suggest that the 

higher abundance of K20, particularly in Furnas rocks, may 

result from partial remelting of previously erupted or 

intruded rocks from Nordeste volcano, such as the ca. 

1-m.y.-old syenite that McKee dated, during Furnas 

magmatic activity. Perhaps K20 is lowest on Sete Cidades 

because only a small volume of previously emplaced silicic 

rock was available for remelting, as suggested by the 

relative absence of syenite xenoliths. 

Preliminary study of available chemical data suggests 

that a mechanism similar to that proposed by Fernandez 

(1980) for the origin of Nordeste volcanic rocks produced 

the melts erupted by the three late Quaternary stratovolcanoes. Partial melting of 5-20 percent of mantle peridotite at 

depths of 35-70 km (Green and Ringwood, 1967) could 

produce parental basanitoid of somewhat variable 
composition. Crystallization and removal of olivine, pyroxene, and plagioclase in shallow (1-10 km) reservoirs leave 

more silicic melts (hawaiite, mugearite, tristanite, and 

trachyte). Accumulation of olivine and pyroxene results in 

ankaramitic magmas. The biotite- and amphibole-bearing 

magmas may have formed in reservoirs within 8 km of the 

surface (Eggler and Burnham, 1973). Evaluation of these 

processes awaits trace element, electron microprobe, and 

isotopic data. 

The volume of subaerial basanitoid and related basaltic rocks between Sete Cidades and Agua de Pau volcanoes 

is about 30 km3 and between Furnas and Agua de Pau 

volcanoes about 22 km3

• The volume of exposed trachyte on 

the three volcanoes is about 100 km3 (table 2). Intrusive 

complexes of basaltic material may underlie Sao Miguel in 

sufficient volume to account for the abundant silicic rocks 

observed on the volcanoes. 

Comparison of stratigraphic and chemical data for 

Sao Miguel with those for Terceira, the other Azorean 

island that has been studied in detail (Self, 1976; Self and 

Gunn, 197 6), shows that: 

1. Periods of particularly vigorous eruptive activity, 

commonly resulting in formation of calderas, occurred 

about 23,000-19,000 and 15,000-12,000 years B.P. 

2. Chemical bimodality (basalt and trachyte) 

characterizes both islands: trachyte dominates, and 

intermediate compositions, though present, are volumetrically low. 

Table 2. Estimated volumes of different rock types on the 

three late Quaternary stratovolcanoes of Sao Miguel 

Cubic kilometers 

Sete Cidades 

Ankaramite .............................. . 

Basanitoid ................................ . 

Basalt ........................................ . 

Hawaiite+ mugearite ............. .. 

Tristanite .................................. . 

Trachyte ................................... . 

1.14 

1.4 

0.9 

7.3 

0.5 

28.4 

Agua de Pau 

Ankaramite .............................. . 

Basanitoid ............................... .. 

Basalt ........................................ . 

Hybrid ...................................... . 

Hawaiite+ mugearite .............. . 

Tristanite .................................. . 

Trachyte (peralkaline) .......... .. 

Trachyte (peraluminous) ...... .. 

Furnas 

Ankaramite .............................. . 

Basanitoid ................................ . 

Basalt ....................................... .. 

Hawaiite+ mugearite .............. . 

Tristanite .................................. . 

Trachyte ................................... . 

1.0 

0.5 

1.0 

0.3 

0.35 

0.5 

20.0 

25.0 

2.0 

1.0 

0 

0.5 

2.0 

28.0 

Percentage 

3 

4 

2 

18 

1 

72 

2.1 

1.0 

2.1 

0.6 

0.7 

1.0 

41.1 

51.4 

6.0 

3.0 

0 

1.5 

6.0 

83.5 

The output of Azorean volcanoes in general is low, 

but Sete Cidades, Agua de Pau, and Furnas together have 

erupted at least five times as much lava as the volcanoes on 

Terceira (Self, 1976; this paper) during latest Pleistocene 

and Holocene time. In contrast, volcanoes of somewhat 

similar composition in the Miocene and younger Canary 

Islands (Schmincke, 1982) have been constructed at rates 

5-10 times greater than the Pliocene and younger Azores. 

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Mineralogico e Geologico da Faculdade de Ciencias, v. 10, 

fasc. 1, p. 3-18. 

White, W.M., Schilling, J.-G., and Hart, S.R., 1976, Evidence for 

the Azores mantle plume from strontium isotope geochemistry of the Central North Atlantic: Nature, v. 263, 

p. 659-663. 

White, W.M., Tapia, M.D.M., and Schilling, J.-G., 1979, The 

petrology and geochemistry of the Azores Islands: 

Contributions to Mineralogy and Petrology, v. 69, 

p. 201-213. 

Zbyszewski., G., 1961, Etude geologique de l'Ile de S. Miguel 

(Acores): Communications de la Service Geologique de 

Portugal, v. 45, p. 5-79. 

Zbyszewski, G., de Medeiros, A.C., de Almeida, F.M., and da 

Veiga Ferreira, 0., 1958, Carta geologica de Portugal na 

escala de 1:50,000 (Sao Miguel): Servicos Geologicos de 

Portugal, Lisbon. 

Zbyszewski, G., and da Veiga Ferreira, 0., 1959, Carta geologica 

de Portugal na escala de 1:50,000 (Sao Miguel): Servicos 

Geologicos de Portugal, Lisbon. 
A 

25,-.--.-.--.-.--,-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-. 

(") 

0 

N 

<i: 

• 

• 

• 

• 

5 • 

• • • 

• • 

Figure 11 (above and following pages). Silica-variation diagrams (in weight percent). Symbols: o, trachytes and tristanites ot 

Agua de Pau volcano;+, trachytes and tristanites of Sete Cidades volcano; x, trachytes and tristanites of Furnas volcano (e, 

ultramafic or mafic xenolith);'>, mafic rocks on the flanks of Agua de Pau volcano; 6, mafic rocks on the flanks of Sete Cidades 

volcano (.A., ultramafic or mafic xenolith);*, mafic rocks on the flanks of Furnas volcano; o, Nordeste volcano. 

Silica-Variation Diagrams 27 
... 

... 

... 

... 

40 

... 

A 

A 

... 

~ . 

50 

Figure 11. Silica-variation diagrams (in weight percent)-Continued. 

28 Geology of Late Quaternary Stratovolcanoes, Azores 
c 

20.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--.-.--,-,--,-,-,,-,--,-, 

M 

0 

N 

Q) 

u.. 

15 

J; 10 

ci 

+ 

0 

Q) 

u.. 

5 

L_~_L~~L_4~0~L_~_L~~L_~_L~~L_5~o--L_~-L~--L-~-L~--L-s~o--L-~~~--~~~~--~7o 

Si02 

Figure 11. Silica-variation diagrams (in weight percent)-Continued. 

Silica-Variation Diagrams 29 
D 

20.-.--.-.--.-~~--r-,--r~--~~~--~~~~.-.-~------~~--r-,--r~--~~----.-.--.-,,-, 

0 

C> 

~ 

-"M 

0 

N 

15 

~ 10 

en * 

ci 

+ 

0 

Q) 

LL 

5 

• 

• 

40 50 

Si02 

Figure 11. Silica-variation diagrams (in weight percent)-Continued. 

30 Geology of Late Quaternary Stratovolcanoes, Azores 

60 

X 

0 0 

0 

0 

0 

0 

0 

0 

70 
• • 

• 

• 

• 

• 

• 

Si02 

Figure 11. Silica-variation diagrams (in weight percent)-Contmued. 

Silica-Variation Diagrams 31 
F 

0 

[01 5 

z 

4 

3 

2 ... 

... 

50 

Si02 

Figure 11. Silica-variation diagrams (in weight percent)-Continued. 

32 Geology of Late Quaternary Stratovolcanoes, Azores 

60 70 
G 

10.-.--.-.-.--.-.--.-.-,,-.-.--.-.--,-.-.--.-.--.-.-.--.-.--.-.-.-~-.--.-.-.-~-.--~ 

Si02 

Figure 11. Silica-variation diagrams (in weight percent)-Continued. 

Silica-Variation Diagrams 33 
0 

N 

~ 

+ 

0 

N 

Ctl 

z 

* * 

0> 

0 

* ·~ 6 * ~ 6 A 

O.o 0 6 A 

Figure 11. Silica-variation diagrams (in weight percent)-Continued. 

34 Geology of Late Quaternary Stratovolcanoes, Azores 

X 

0 X X 
I 

0 

Figure 11. Sil ica-variaf . 5(] IOn dlagr ams (m . wei h s· 102 g t percent}-C ontmued. . 

Silica-Variati on D•agrams . 35 
0 0 

5 

* * 

Si02 

Figure 11. Silica-variation diagrams (in weight percent)-Continued. 

36 Geology of Late Quaternary Stratovolcanoes, Azores 
0 0 

0 

0 0 

0 0 ~ 

0 

0 oo oo oo 0 

It) 

~1 0 0 

a.. 

• • • • • • 

Si02 

Figure 11. Silica-variation diagrams (in weight percent)-Continued. 

0 

+ 

o8 0 

Silica-Variation Diagrams 37 
A 

25r~r~-.-,-,~-.-,-..-,-,-,-,-,-,-,,-,-,-.-.-.-.-..-.-.-.-.-.-,-, .. -.-.-.-.-.-.-.~ 

Figure 12. Magnesia-variation diagrams (in weight percent). Symbols as in figure 11. Symbols: o, trachytes and tristanites of 

Agua de Pau volcano;+, trachytes and tristanites of Sete Cidades volcano; x, trachytes and tristanites of Furnas volcano (e, 

ultramafic or mafic xenolith);<), mafic rocks on the flanks of Agua de Pau volcano; L, mafic rocks on the flanks of Sete Cidades 

volcano (A, ultramafic or mafic xenolith);*, mafic rocks on the flanks of Furnas volcano; 0, Nordeste volcano. 

38 Geology of Late Quaternary Stratovolcanoes, Azores 
B 

20~~~~~~~~~~~~~~~~~~~~~~~,-.-.-.-.-.-.-.-.-.-.-.-r-r-rlrlllll-.-.~ 

(Y) 

0 

N 

Q) 

LL. 

15 

d, 10 

0 

+ 

0 

Q) 

LL. 

• 

oO 

• 

• 

Figure 12. Magnesia-variation diagrams (in weight percent)-Continued. 

Magnesia-Variation Diagrams 39 
c 

25~~~~~~~~~~~.-.-.-.-.-.-,~-.-.-.-.-.-,-..-.-.-~~r-.-r~rt-,-.~~-r-r-rl 

Figure 12. Magnesia-variation diagrams (in weight percent)-Continued. 

40 Geology of Late Quaternary Stratovolcanoes, Azores 
D 

0 

A ~ A 

• 

10 20 MgO 

Figure 12. Magnesia-variation diagrams (in weight percent)-Continued. 

Magnesia-Variation Diagrams 41 
E 

10~~~~~-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.~~~.-.-.-.-.-.-.-.-.-.-.-.-.-.-~~~~~-.-.-.-.-. 

9 

8 

7 

6 

4 

3 

2 

o~~~~~~~~~~-L-L~~---L-L-L~~~~~~~~~._~~L-L-L-L-L-L-~~~~~~~_. 

0 10 20 

MgO 

30 40 

Figure 12. Magnesia-variation diagrams (in weight percent)-Continued. 

42 Geology of Late Quaternary Stratovolcanoes, Azores 
F 

3~~~~~~~~~~~~~~--~ro-.-.-..-.-.-r~~~-.-.-r.-r-r-r~-.~-r~r.-~~rr 

0 

N 

ctl 

z 

0 

N 

~ 

0 

Figure 12. Magnesia-variation diagrams (in weight percent). 

Magnesia-Variation Diagrams 43 
A 

<> <> 

A 

<> 

A 

A 

A 

A 

A 

• .. .. .. 

.. 

10 20 MgO 

Figure 12. Magnesia-variation diagrams (in weight percent)-Continued. 

44 Geology of Late Quaternary Stratovolcanoes, Azores 
H 

2~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.-.-,-,,,-,-,-,-.~ 

It) 

~1 a.. 

0 

<> <> 

<> 

<> <> 

~<> <> 

<> 

• • 

10 20 

MgO 

figure 12. Magnesia-variation diagrams (in weight percent)-Continued. 

• 

30 

Magnesia-Variation Diagrams 45 
F 

X 

A 

Hybrid 

rocks Hawaiian 

alkalic 

trend 

M 

Figure 13. AFM ternary diagram. A, Na20+K20; F, Fe0+0.9*Fe20 3 ; M, MgO. Symbols: o, trachytes and tristanites of Agua de 

Pau volcano;+, trachytes and tristanites of Sete Cidades volcano; x, trachytes and tristanites of Furnas volcano ( e, ultramafic or 

mafic xenolith);<), mafic rocks on the flanks of Agua de Pau volcano; D,, mafic rocks on the flanks of Sete Cidades volcano (.A, 

ultramafic or mafic xenolith);*, mafic rocks on the flanks of Furnas volcano; 0, Nordeste volcano. 

46 Geology of Late Quaternary Stratovolcanoes, Azores *U.S. GOVERNMENT PRINTING OFFICE: 1992·673-049/46027 


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