Earth Science docx

encyclopedia of
Earth Science

ENCYCLOPEDIA OF
Earth Science
Timothy Kusky, P
H
.D.
Department of Earth and Atmospheric Sciences,
Saint Louis University
Encyclopedia of Earth Science
Copyright © 2005 by Timothy Kusky, Ph.D.
All rights reserved. No part of this book may be reproduced or utilized in any form or by any
means, electronic or mechanical, including photocopying, recording, or by any information
storage or retrieval systems, without permission in writing from
the publisher. For information contact:
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New York NY 10001
Library of Congress Cataloging-in-Publication Data
Kusky, Timothy M.
Encyclopedia of earth science / T
imothy Kusky.
p. cm.
Includes bibliographical r
eferences and index.
ISBN 0-8160-4973-4
1. Earth sciences—Encyclopedias. I. Title.
QE5.K85 2004

550′.3—dc22
2004004389
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Printed in the United States of America
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This book is printed on acid-free paper.
Dedicated to G.V. Rao (1934–2004)
C
ONTENTS
Acknowledgments
xi
Introduction
xiii
Entries A–Z
1
Feature Essays:
“Coping with Sea-Level Rise in Coastal Cities”
81
“Gaia Hypothesis”
82
“Desertification and Climate Change”

116
“Earthquake Warning Systems”
131
“Loma Prieta Earthquake, 1989”
132
“Mississippi River Basin and the Midwest Floods of
1927 and 1993”
152
“Age of the Earth”
166
“Formation of the Earth and Solar System”
203
“Galveston Island Hurricane, 1900”
214
“Is There Life on Mars?”
262
“Lahars of Nevado del Ruiz, Colombia, 1985”
266
“History of Ocean Exploration”
302
“The World’s Oldest Ophiolite”
307
“Homo sapiens sapiens and Neandertal Migration and Relations
in the Ice Ages”
338
“Is Your Home Safe from Radon?”
350
“Why Is Seawater Blue?”
379
“Seismology and Earth’s Internal Structure”

383
“December 26, 2004: Indian Ocean Earthquake and Tsunami”
437
“Volcanoes and Plate Tectonics”
452
Appendixes:
Appendix I
Periodic Table of the Elements
477
Appendix II
The Geologic Timescale
479
Classification of Species
480
Summary of Solar System Data
480
Evolution of Life and the Atmosphere
480
Index
481
Many people have helped me with different aspects of preparing this encyclopedia.
Frank Darmstadt, Executive Editor at Facts On File, reviewed and edited all text
and figures in the encyclopedia, providing guidance and consistency throughout.
Rose Ganley spent numerous hours as editorial assistant correcting different ver-
sions of the text and helping prepare figures, tables, and photographs. Additional
assistance in the preparation was provided by Soko Made, Justin Kanoff, and
Angela Bond. Many sections of the encyclopedia draw from my own experiences
doing scientific research in different parts of the world, and it is not possible to
individually thank the hundreds of colleagues whose collaborations and work I

have related in this book. Their contributions to the science that allowed the writ-
ing of this volume are greatly appreciated. I have tried to reference the most rele-
vant works, or in some cases more recent sources that have more extensive
reference lists. Any omissions are unintentional.
Finally, I would especially like to thank Carolyn, my wife, and my children
Shoshana and Daniel for their patience during the long hours spent at my desk
preparing this book. Without their understanding this work would not have been
possible.
xi
A
CKNOWLEDGMENTS
I
NTRODUCTION
The Encyclopedia of Earth Science is intended to provide a broad view of some of
the most important subjects in the field of earth sciences. The topics covered in the
encyclopedia include longer entries on the many broad subdisciplines in the earth
sciences (hydrology
, structural geology, petrology, isotope geology, geochemistry,
geomorphology, atmospheric sciences, climate, and oceanography), along with
entries on concepts, theories and hypotheses, places, events, the major periods of
geological time, history, people who have made significant contributions to the
field, technology and instruments, organizations, and other subjects.
The Encyclopedia of Earth Science is intended to be a reference for high
school and college students, teachers and professors, scientists, librarians, jour-
nalists, general readers, and specialists looking for infor
mation outside their spe-
cialty. The encyclopedia is extensively illustrated with photographs and other
illustrations including line art, graphs, and tables, and contains 19 special essays
on topics of interest to society

. The work is extensively cross-referenced and
indexed to facilitate locating topics of interest.
Entries in the Encyclopedia of Earth Science are based on extensive research
and review of the scientific literature, ranging from the general science to very
specialized fields. Most of the entries include important scientific r
eferences and
sources listed as “Fur
ther Reading” at the end of each section, and the entries are
extensively cross-referenced with related entries. Some parts of the encyclopedia
draw from my collected field notes, class notes, and files of scientific reprints
about selected topics and regions, and I have tried to provide uniformly detailed
coverage of most topics at a similar level. Some of the more lengthy entries, how-
ever, go into deeper levels on topics considered to be of great importance.
xiii
E
NTRIES
A–Z
aa lava Basaltic lava flows with blocky broken surfaces.
The term is of Hawaiian origin, its name originating from the
sound that a person typically makes when attempting to walk
across the lava flow in bare feet. Aa lava flows are typically
10–33 feet (3–10 m) thick and move slowly downhill out of
the volcanic vent or fissure, moving a few meters per hour.
The rough, broken, blocky surface forms as the outer layer of
the moving flow cools, and the interior of the flow remains
hot and fluid and continues to move downhill. The movement
of the interior of the flow breaks apart the cool, rigid surface,
causing it to become a jumbled mass of blocks with angular

steps between adjacent blocks. The flow front is typically very
steep and may advance into new areas by dropping a continu-
ous supply of recently formed hot, angular blocks in front of
the flow, with the internal parts of the flow slowly overriding
the mass of broken blocks. These aa lava fronts are rather
noisy places, with steam and gas bubbles rising through the
hot magma and a continuous clinking of cooled lava blocks
rolling down the lava front. Gaps that open in the lava front,
top, and sides may temporarily expose the molten lava within,
showing the high temperatures inside the flow. Aa flows are
therefore hazardous to property and may bulldoze buildings,
forests, or anything in their path, and then cause them to
burst into flames as the hot magma comes into contact with
combustible material. Since these flows move so slowly, they
are not considered hazardous to humans.
See also
PAHOEHOE LAVA
;
VOLCANO
.
abyssal plains Flat, generally featureless plains that form
large areas on the seafloor. In the Atlantic Ocean, abyssal
plains form large regions on either side of the Mid-Atlantic
Ridge, covering the regions from about 435–620 miles
(700–1,000 km), and they are broken occasionally by hills
and volcanic islands such as the Bermuda platform, Cape
Verde Islands, and the Azores. The deep abyssal areas in the
Pacific Ocean are characterized by the presence of more
abundant hills or seamounts, which rise up to 0.6 miles (1
km) above the seafloor. Therefore, the deep abyssal region of

the Pacific is generally referred to as the abyssal hills instead
of the abyssal plains. Approximately 80–85 percent of the
Pacific Ocean floor lies close to areas with hills and
seamounts, making the abyssal hills the most common land-
form on the surface of the Earth.
Many of the sediments on the deep seafloor (the abyssal
plain) are derived from erosion of the continents and are car-
ried to the deep sea by turbidity currents, wind (e.g., volcanic
ash), or released from floating ice. Other sediments, known
as deep-sea oozes, include pelagic sediments derived from
marine organic activity. When small organisms die, such as
diatoms in the ocean, their shells sink to the bottom and over
time can create significant accumulations. Calcareous ooze
occurs at low to middle latitudes where warm water favors
the growth of carbonate-secreting organisms. Calcareous
oozes are not found in water that is more than 2.5–3 miles
(4–5 km) deep, because this water is under such high pressure
that it contains dissolved CO
2
that dissolves carbonate shells.
Siliceous ooze is produced by organisms that use silicon to
make their shell structure.
See also
CONTINENTAL MARGIN
.
accretionary wedge Structurally complex parts of subduc-
tion zone systems, accretionary wedges are formed on the
landward side of the trench by material scraped off from the
subducting plate as well as trench fill sediments. They typical-
ly have wedge-shaped cross sections and have one of the most

complex internal structures of any tectonic element known
on Earth. Parts of accretionary wedges are characterized by
numerous thin units of rock layers that are repeated by
1
A
numerous thrust faults, whereas other parts or other wedges
are characterized by relatively large semi-coherent or folded
packages of rocks. They also host rocks known as tectonic
mélanges that are complex mixtures of blocks and thrust
slices of many rock types (such as graywacke, basalt, chert,
and limestone) typically encased in a matrix of a different
rock type (such as shale or serpentinite). Some accretionary
wedges contain small blocks or layers of high-pressure low-
temperature metamorphic rocks (known as blueschists) that
have formed deep within the wedge where pressures are high
and temperatures are low because of the insulating effect of
the cold subducting plate. These high-pressure rocks were
brought to the surface by structural processes.
Accretionary wedges grow by the progressive offscraping
of material from the trench and subducting plate, which con-
stantly pushes new material in front of and under the wedge
as plate tectonics drives plate convergence. The type and style
of material that is offscraped and incorporated into the
wedge depends on the type of material near the surface on
the subducting plate. Subducting plates with thin veneers of
sediment on their surface yield packages in the accretionary
wedge dominated by basalt and chert rock types, whereas
subducting plates with thick sequences of graywacke sedi-
ments yield packages in the accretionary wedge dominated by
graywacke. They may also grow by a process known as

underplating, where packages (thrust slices of rock from the
subducting plate) are added to the base of the accretionary
wedge, a process that typically causes folding of the overlying
parts of the wedge. The fronts or toes of accretionary wedges
are also characterized by material slumping off of the steep
slope of the wedge into the trench. This material may then be
recycled back into the accretionary wedge, forming even
more complex structures. Together, the processes of offscrap-
ing and underplating tend to steepen structures and rock lay-
ers from an orientation that is near horizontal at the toe of
the wedge to near vertical at the back of the wedge.
The accretionary wedges are thought to behave mechani-
cally somewhat as if they were piles of sand bulldozed in
front of a plow. They grow a triangular wedge shape that
increases its slope until it becomes oversteepened and
mechanically unstable, which will then cause the toe of the
wedge to advance by thrusting, or the top of the wedge to
2 accretionary wedge
Cross section of typical accretionary wedge showing material being offscraped at the toe of the wedge and underplated beneath the wedge
collapse by normal faulting. Either of these two processes can
reduce the slope of the wedge and lead it to become more sta-
ble. In addition to finding the evidence for thrust faulting in
accretionary wedges, structural geologists have documented
many examples of normal faults where the tops of the wedges
have collapsed, supporting models of extensional collapse of
oversteepened wedges.
Accretionary wedges are forming above nearly every
subduction zone on the planet. However, these accretionary
wedges presently border open oceans that have not yet closed
by plate tectonic processes. Eventually, the movements of the

plates and continents will cause the accretionary wedges to
become involved in plate collisions that will dramatically
change the character of the accretionary wedges. They are
typically overprinted by additional shortening, faulting, fold-
ing, and high-temperature metamorphism, and intruded by
magmas related to arcs and collisions. These later events,
coupled with the initial complexity and variety, make identifi-
cation of accretionary wedges in ancient mountain belts diffi-
cult, and prone to uncertainty.
See also
CONVERGENT PLATE MARGIN PROCESSES
;
MÉLANGE
;
PLATE TECTONICS
;
STRUCTURAL GEOLOGY
.
Further Reading
Kusky, Timothy M., and Dwight C. Bradley. “Kinematics of Mélange
Fabrics: Examples and Applications from the McHugh Complex,
Kenai Peninsula, Alaska.” Journal of Structural Geology 21, no.
12 (1999): 1,773–1,796.
Kusky, Timothy M., Dwight C. Bradley, Peter Haeussler, and Susan
M. Karl. “Controls on Accretion of Flysch and Mélange Belts at
Convergent Margins: Evidence from The Chugach Bay Thrust
and Iceworm Mélange, Chugach Terrane, Alaska.” Tectonics 16,
no. 6 (1997): 855–878.
Adirondack Mountains The Adirondack Mountains occu-
py the core of a domal structure that brings deep-seated Late

Proterozoic rocks to the surface and represents a southern
extension of the Grenville province of Canada. The Late Pro-
terozoic rocks are unconformably overlain by the Upper Cam-
brian/Lower Ordovician Potsdam Sandstone, dipping away
from the Adirondack dome. The late Cenozoic uplift is shown
by the anomalous elevations of the Adirondack Highlands
compared with the surrounding regions and the relatively
young (Tertiary) drainage patterns. Uplift is still occurring on
the order of few millimeters per year.
Five periods of intrusion and two main periods of defor-
mation are recognized in the Adirondacks. The earliest intru-
sions are the tonalitic and calc-alkaline intrusions that are
approximately 1,350–1,250 million years old. These intrusions
were followed by the Elzevirian deformation at approximately
1,210–1,160 million years ago. The largest and most signifi-
cant magmatic event was the emplacement of the anorthosites,
mangerites, charnockites, and granites, commonly referred to
as the AMCG suite. This suite is thought to have been intruded
about 1,155–1,125 million years ago. This magmatism was
followed by two more magmatic events; hornblende granites
and leucogranites at approximately 1,100–1,090 million years
ago (Hawkeye suite) and 1,070–1,045 million years ago (Lyon
Mountain granite), respectively. The most intense metamor-
phic event was the Ottawan orogeny, which occurred
1,100–1,000 million years ago, with “peak” metamorphism
occurring at about 1,050 million years ago.
The Adirondacks are subdivided into two provinces: the
Northwest Lowlands and the Highlands, separated by the
Carthage-Colton mylonite zone. Each province contains dis-
tinct rock types and geologic features, both of which have

clear affinities related to the Canadian Grenville province.
The Northwest Lowlands
The Northwest Lowlands are located in the northwest por-
tion of the Adirondack Mountains. On the basis of litholo-
gies, the Lowlands are closely related to the Frontenac
terrane of the Canadian metasedimentary belt and are
thought to be connected via the Frontenac Arch. The North
-
west Lowlands are smaller in area, have lower topographic
relief than the Highlands, and are dominated by metasedi-
mentary rocks interlayered with leucocratic gneisses. Both
lithologies are metamorphosed to upper amphibolite grade.
The metasedimentary rocks are mostly marbles but also con-
tain units of quartzites and mica schists, suggesting a plat-
form sedimentary sequence provenance. The protoliths of the
leucocratic gneisses are controversial. Some geologists consid-
er the leucocratic gneisses to be basal rhyolitic and dacitic
ash-flow tuff deposits that have been metamorphosed, based
on geochemical signatures and the absence of xenoliths in the
formations. However, others question this interpretation and
suggest that the leucocratic bodies are intrusive in nature,
based on crosscutting field evidence and geothermometry.
The geothermometry on the leucocratic gneiss yields a tem-
perature of 1,436°F–1,490°F (780°C–810°C). This is an
anomalously high metamorphic temperature compared with
other rocks in the region, suggesting that they may be igneous
crystallization temperatures.
The Highlands
The Highlands are correlative with the central granulite terrain
of the Canadian Grenville province. The Green Mountains of

V
ermont may also be correlative with the Highlands, although
other Proterozoic massifs in the northern Appalachians such as
the Chain Lakes massif may be exotic to Laurentia. The High-
lands are dominated by meta-igneous rocks, including abun-
dant anorthosite bodies. The largest anorthosite intrusion is
the Mount Marcy massif located in the east-central Adiron-
dacks; additional anorthosite massifs are the Oregon and
Snowy Mountain domes that lie to the south-southwest of
Mount Marcy. The anorthosite bodies ar
e part of the suite of
rocks known as the AMCG suite; anorthosites, mangerites,
Adirondack Mountains 3
charnockites, granitic gneisses. Between the Marcy massif and
the Carthage-Colton mylonite zone is an area known as the
Central Highlands. Here, the rock types consist of AMCG
rocks and hornblende gneisses, both of which exhibit variable
amounts of deformation. The Southern Highlands are com-
prised of granitic gneisses from the AMCG suite with infolded
metasedimentary rocks that are strongly deformed. Within the
Southeastern Highlands, metasedimentary rocks are found;
4 Adirondack Mountains
Structural map showing axial traces of folds in the Adirondack Mountains: AMA: Arab Mountain antiform; G: Gore Mountain; LM: Little Moose Mountain
synform; OD: Oregon Dome; SD: Snowy Mountain Dome; WM: Wakeley Mountain nappe
these metasedimentary rocks may be correlative with rocks in
the Northwest Lowlands. The following sections briefly review
the important Highland suites.
T
ONALITIC
S

UITE
The tonalitic suite outcrops in the extreme
southern Adirondacks where they are highly deformed. These
tonalitic rocks are one of the oldest suites in the Adirondacks
and have been dated at circa 1.3 billion years. The tonalitic
gneiss is thought to be igneous in origin based on the presence
of xenoliths from the surrounding rock and the subophitic tex-
tures. Strong calc-alkaline trends suggest that these rocks are
arc-related; however, this geochemical signature does not dif-
ferentiate between an island-arc and an Andean arc-type set-
ting. This suite may be correlative with tonalitic rocks in the
Green Mountains of Vermont based on age relations and pet-
rographic features. They are also similar in composition with
the somewhat younger Elzevirian batholith (1.27–1.23 billion
years old) in the central metasedimentary belt. Consequently,
the tonalitic suite in the Adirondacks is thought to have been
emplaced in the early intraoceanic history of the Elzevirian arc,
prior to collision at circa 1,200 million years ago.
AMCG S
UITE
The circa 1,555–1,125-million-year-old
AMCG suite occurs predominantly in the Adirondack High-
lands and central granulite terrain of the Canadian Grenville
province. Though highly deformed, the AMCG suite has been
characterized as igneous in origin based on the presence of
relict igneous textures. Several geologists, pioneered by Jim
McLelland, have suggested that the post-collisional delamina-
tion of the subcontinental lithospheric mantle generated gab-
broic melts that ponded at the mantle-crust boundary. This
ponding would have provided a significant source of heat,

thereby affecting the lower crust in two ways: it created melts
in the lower crust, thus producing a second generation of
more felsic magma. This model is supported by the bimodal
nature of the AMCG suite. The second effect was weakening
of the crust, which provided a conduit for the hot, less dense
magmas to ascend to the surface. This hypothetical emplace-
ment model is supported by the AMCG suite’s anhydrous
nature in conjunction with the shallow crustal levels the
magma has invaded.
Large-Scale Structural Features
The structure of the Adirondack Mountains has puzzled geol-
ogists for decades. This is due to the polyphase deformation
that complexly deformed the region during the Ottawan
orogeny (1.1–1.0 billion years ago). In 1936 J. S. Brown was
one of the first investigators who recognized that the stratig-
raphy of the Northwest Lowlands is repeated by a series of
folds. Later workers, including Ynvar Isachsen, suggested
that there are five sets of large-scale folds that occur thr
ough-
out the Adirondacks. In addition, rocks of the central and
southern Adirondacks are str
ongly foliated and lineated. The
large-scale folds and rock fabrics suggest northwest directed
tectonic transport, which is consistent with other kinematic
indicators in the rest of the Grenville province.
Even the most generalized geologic maps of the Adiron-
dacks reveal that this region possesses multiple large-scale
folds. Delineating the various fold sets is difficult, due to the
fold interference patterns, but at least five sets of folds are
recognized. The timing of these fold sets has remained

obscure, but at least some are related to the Ottawan oroge-
ny. It is also not clear whether these folds formed as a pro-
gressive event or as part of distinct events.
Fold nomenclature, i.e., anticline and syncline, is based
on structural evidence found in the eastern parts of the
Adirondacks. The shapes of igneous plutons and orientation
of igneous compositional layering have aided structural geolo-
gists to determine fold superposition in this region. The earli-
est fold set (F
1
folds) are reclined to recumbent folds. Mainly
minor, intrafolial F
1
folds have been documented, with rare
outcrop-scale examples. The presence of larger F
1
folds is sus-
pected based on rotated foliations associated with F
1
folding
in the hinge areas of F
2
folds. Many F
1
folds may have eluded
detection because of their extremely large size.
The F
2
folds are the earliest mappable folds in the
Adirondacks, an example being the Wakely Mountain nappe.

In general the F
2
folds are recumbent to reclined, isoclinal
folds. The F
2
folds are coaxial with the F
1
folds and have fold
axes that trend northwest to east-west. Both of these fold sets
have been suggested to be associated with thrust nappes.
The F
3
folds are large, upright-open folds that trend west-
northwest to east-west. Therefore, they are considered coaxial
with F
1
and F
2
folds. F
3
folds are best developed in the south-
central Adirondack Highlands. Examples of these folds are
the Piseco anticline and the Glens Falls syncline. Northwest
trending F
4
folds are best developed in the Northwest Low-
lands and are rare in the Highlands, except in the southern
regions. North-northeast trending F
5
folds are open, upright

folds except near Mount Marcy where they become tight. F
5
folds are better developed in the eastern parts of the Adiron-
dacks. Due to the spatial separation of F
4
and F
5
folds, distin-
guishing relative timing between the two is difficult.
See also G
RENVILLE PROVINCE
;
PROTEROZOIC
;
STRUC
-
TURAL GEOLOGY
;
SUPERCONTINENT CYCLE
.
Further Reading
Brown, John S. “Structure and Primary Mineralization of the Zinc
Mine at Balmat, New York.” Economic Geology 31, no. 3 (1936):
233–258.
Buddington, Arthur F. “Adirondacks Igneous Rocks and Their Meta-
morphism.” Geological Society of America Memoir 7 (1939):
1–354.
Chiarenzelli, Jeffrey R., and Jim M. McLelland. “Age and Regional
Relationships of Granitoid Rocks of the Adirondack Highlands.”
Journal of Geology 99 (1991): 571–590.

Adirondack Mountains 5
Corrigan, Dave, and Simon Hanmer. “Anorthosites and Related
Granitoids in the Grenville Orogen: A Product of the Convective
Thinning of the Lithosphere?” Geology 25 (1997): 61–64.
Davidson, Anthony. “Post-collisional A-type Plutonism, Southwest
Grenville province: Evidence for a Compressional Setting.”
Geological Society of America Abstracts with Programs 28
(1996): 440.
———. “An Overview of Grenville province Geology
, Canadian
Shield.” In “Geology of the Precambrian Superior and Grenville
provinces and Precambrian Fossils in North America,” edited by
S. B. Lucas and Marc R. St-Onge. Geological Society of America,
Geology of North America C-1 (1998): 205–270.
Hoffman, Paul F
. “Did the Br
eakout of Laurentia T
urn Gondwana-
land Inside-Out?” Science 252 (1991): 1,409–1,411.
Kusky
, T
imothy M., and Dave P
. Loring. “Structural and U/Pb
Chronology of Superimposed Folds, Adirondack Mountains:
Implications for the Tectonic Evolution of the Grenville
province.” Journal of Geodynamics 32 (2001): 395–418.
McLelland, Jim M., J. Stephen Daly
, and Jonathan M. McLelland.
“The Grenville Orogenic Cycle (ca. 1350–1000 Ma): an Adiron-
dack perspective.” In Tectonic Setting and Terrane Accretion in

Precambrian Orogens, edited by Timothy M. Kusky, Ben A. van
der Pluijm, Kent C. Condie, and Peter J. Coney. Tectonophysics
265 (1996): 1–28.
McLelland, Jim M., and Ynvar W
. Isachsen. “Synthesis of Geology of
the Adir
ondack Mountains, New Y
ork, And Their Tectonic Set-
ting within the Southwestern Grenville province.” In The
Grenville province, edited by J. M. Moore, A. Davidson, and Alec
J. Baer. Geological Association of Canada Special Paper 31
(1986): 75–94.
———. “Structural Synthesis of the Southern and Central Adiron
-
dacks: A Model for the Adirondacks as a Whole and Plate Tec
-
tonics Interpretations.” Geological Society of America Bulletin 91
(1980): 208–292.
Moores, Eldredge M. “Southwest United States-East Antarctic
(SWEAT) Connection: A Hypothesis.” Geology 19 (1991):
425–428.
Rivers, T
oby. “Lithotectonic Elements of the Grenville province:
Review and T
ectonic Implications.” Precambrian Research 86
(1997): 117–154.
Rivers, Toby, and Dave Corrigan. “Convergent Margin on Southeast-
ern Laurentia during the Mesoproter
ozoic: Tectonic Implica-
tions.” Canadian Journal of Earth Sciences 37 (2000): 359–383.

Rivers, T
oby, J. Martipole, Charles F. Gower, and Anthony David-
son. “New Tectonic Subdivisions of the Grenville province,
Southeast Canadian Shield.” Tectonics 8 (1989): 63–84.
Afar Depression, Ethiopia One of the world’s largest,
deepest regions below sea level that is subaerially exposed on
the continents, home to some of the earliest known hominid
fossils. It is a hot, arid region, where the Awash River drains
northward out of the East African rift system, and is evaporat-
ed in Lake Abhe before it reaches the sea. It is located in eastern
Africa in Ethiopia and Eritrea, between Sudan and Somalia, and
across the Red Sea and Gulf of Aden from Yemen. The reason
the region is so topographically low is that it is located at a tec-
tonic triple junction, where three main plates are spreading
apart, causing regional subsidence. The Arabian plate is moving
northeast away from the African plate, and the Somali plate is
moving, at a much slower rate, to the southeast away from
Africa. The southern Red Sea and north-central Afar Depres-
sion form two parallel north-northwest-trending rift basins,
separated by the Danakil Horst, related to the separation of
Arabia from Africa. Of the two rifts, the Afar depression is
exposed at the surface, whereas the Red Sea rift floor is sub-
merged below the sea. The north-central Afar rift is complex,
consisting of many grabens and horsts. The Afar Depression
merges southward with the northeast-striking Main Ethiopian
Rift, and eastward with the east-northeast-striking Gulf of
Aden. The Ethiopian Plateau bounds it on the west. Pliocene
volcanic rocks of the Afar stratoid series and the Pleistocene to
Recent volcanics of the Axial Ranges occupy the floor of the
Afar Depression. Miocene to recent detrital and chemical sedi-

ments are intercalated with the volcanics in the basins.
The Main Ethiopian and North-Central Afar rifts are
part of the continental East African Rift System. These two
kinematically distinct rift systems, typical of intracontinental
rifting, are at different stages of evolution. In the north and
east, the continental rifts meet the oceanic rifts of the Red Sea
and the Gulf of Aden, respectively, both of which have propa-
gated into the continent. Seismic refraction and gravity studies
indicate that the thickness of the crust in the Main Ethiopian
Rift is less than or equal to 18.5 miles (30 km). In Afar the
thickness varies from 14 to 16 miles (23–26 km) in the south
to 8.5 miles (14 km) in the north. The plateau on both sides
of the rift has a crustal thickness of 21.5–27 miles (35–44
km). Rates of separation obtained from geologic and geodetic
studies indicate 0.1–0.2 inches (3–6 mm) per year across the
northern sector of the Main Ethiopian Rift between the
African and Somali plates. The rate of spreading between
Africa and Arabia across the North-Central Afar rift is rela-
tively faster, about 0.8 inches (20 mm) per year. Paleomagnet-
ic directions from Cenozoic basalts on the Arabian side of the
Gulf of Aden indicate 7 degrees of counterclockwise rotation
of the Arabian plate relative to Africa, and clockwise rota-
tions of up to 11 degrees for blocks in eastern Afar. The initia-
tion of extension on both sides of the southernmost Red Sea
Rift, Ethiopia, and Yemen appear coeval, with extension start-
ing between 22 million and 29 million years ago.
See also
DIVERGENT OR EXTENSIONAL BOUNDARIES
;
RIFT

.
Further Reading
Tesfaye, Sansom, Dave Harding, and Timothy Kusky. “Early Conti-
nental Breakup Boundary and Migration of the Afar Triple Junc-
tion, Ethiopia.” Geological Society of America Bulletin 115
(2003): 1,053–1,067.
agate An ornamental, translucent variety of quartz, known
for its spectacular colors and patterns. It is extremely fine-
grained (or cryptocrystalline), and mixed with layers of opal,
which is another variety of colored silica that has combined
6 Afar Depression, Ethiopia
with variable amounts of water molecules. Opal is typically
iridescent, displaying changes in color when viewed in differ-
ent light or from different angles. Agate and opal typically
form colorful patterns including bands, clouds, or moss-like
dendritic patterns indicating that they grew together from sil-
ica-rich fluids. Agate is found in vugs in volcanic rocks and is
commonly sold at rock and mineral shows as polished slabs
of ornamental stone.
See also
MINERALOGY
.
air pressure The weight of the air above a given level. This
weight produces a force in all directions caused by constantly
air pressure 7
Landsat Thematic Mapper image of the area where the Ethiopian rift segment of the East African rift meets the Tendaho rift, an extension of the Red Sea
rift, and the Goba’ad rift, an extension of the Gulf of Aden rift system. Note the dramatic change in orientation of fault-controlled ridges and how internal
drainages like the Awash River terminate in lakes such as Lake Abhe, where the water evaporates.
moving air molecules bumping into each other and objects in
the atmosphere. The air molecules in the atmosphere are con-

stantly moving and bumping into each other with each air
molecule averaging a remarkable 10 billion collisions per sec-
ond with other air molecules near the Earth’s surface. The
density of air molecules is highest near the surface, decreases
rapidly upward in the lower 62 miles (100 km) of the atmo-
sphere, then decreases slowly upward to above 310 miles (500
km). Air molecules are pulled toward the Earth by gravity and
are therefore more abundant closer to the surface. Pressure,
including air pressure, is measured as the force divided by the
area over which it acts. The air pressure is greatest near the
Earth’s surface and decreases with height, because there is a
greater number of air molecules near the Earth’s surface (the
air pressure represents the sum of the total mass of air above a
certain point). A one-square-inch column of air extending
from sea level to the top of the atmosphere weighs about 14.7
pounds. The typical air pressure at sea level is therefore 14.7
pounds per square inch. It is commonly measured in units of
millibars (mb) or hectopascals (hPa), and also in inches of
mercury. Standard air pressure in these units equals 1,013.25
mb, 1,013.25 hPa, and 29.92 in of mercury. Air pressure is
equal in all directions, unlike some pressures (such as a weight
on one’s head) that act in one direction. This explains why
objects and people are not crushed or deformed by the pres-
sure of the overlying atmosphere.
Air pressure also changes in response to temperature and
density, as expressed by the gas law:
Pressure = temperature × density × constant (gas constant,
equal to 2.87 × 10
6
erg/g K).

From this gas law, it is apparent that at the same temper-
ature, air at a higher pressure is denser than air at a lower
pressure. Therefore, high-pressure regions of the atmosphere
are characterized by denser air, with more molecules of air
than areas of low pressure. These pressure changes are caused
by wind that moves air molecules into and out of a region.
When more air molecules move into an area than move out,
the area is called an area of net convergence. Conversely, in
areas of low pressure, more air molecules are moving out than
in, and the area is one of divergence. If the air density is con-
stant and the temperature changes, the gas law states that at a
given atmospheric level, as the temperature increases, the air
pressure decreases. Using these relationships, if either the tem-
perature or pressure is known, the other can be calculated.
If the air above a location is heated, it will expand and
rise; if air is cooled, it will contract, become denser, and sink
closer to the surface. Therefore, the air pressure decreases
rapidly with height in the cold column of air because the
molecules are packed closely to the surface. In the warm col-
umn of air, the air pressure will be higher at any height than
in the cold column of air, because the air has expanded and
more of the original air molecules are above the specific
height than in the cold column. Therefore, warm air masses
at height are generally associated with high-pressure systems,
whereas cold air aloft is generally associated with low pres-
sure. Heating and cooling of air above a location causes the
air pressure to change in that location, causing lateral varia-
tion in air pressure across a region. Air will flow from high-
pressure areas to low-pressure areas, forming winds.
The daily heating and cooling of air masses by the Sun

can in some situations cause the opposite effect, if not over-
whelmed by effects of the heating and cooling of the upper
atmosphere. Over large continental areas, such as the south-
western United States, the daily heating and cooling cycle is
associated with air pressure fall and rise, as expected from
the gas law. As the temperature rises in these locations the
pressure decreases, then increases again in the night when the
temperature falls. Air must flow in and out of a given vertical
column on a diurnal basis for these pressure changes to occur,
as opposed to having the column rise and fall in response to
the temperature changes.
See also
ATMOSPHERE
.
Aleutian Islands and trench Stretching 1,243 miles
(2,000 km) west from the western tip of the Alaskan Peninsu-
la, the Aleutian Islands form a rugged chain of volcanic
islands that stretch to the Komandorski Islands near the
Kamchatka Peninsula of Russia. The islands form an island
arc system above the Pacific plate, which is subducted in the
Aleutian trench, a 5-mile (8-km) deep trough ocean-ward of
the Aleutian Islands. They are one of the most volcanically
active island chains in the world, typically hosting several
eruptions per year.
The Aleutians consist of several main island groups,
including the Fox Islands closest to the Alaskan mainland,
then moving out toward the Bering Sea and Kamchatka to
the Andreanof Islands, the Rat Islands, and the Near Islands.
The climate of the Aleutians is characterized by nearly con-
stant fog and heavy rains, but generally moderate tempera-

tures. Snow may fall in heavy quantities in the winter
months. The islands are almost treeless but have thick grass-
es, bushes, and sedges, and are inhabited by deer and sheep.
The local Inuit population subsists on fishing and hunting.
The first westerner to discover the Aleutians was the Dan-
ish explorer Vitus Bering, when employed by Russia in 1741.
Russian trappers and traders established settlements on the
islands and employed local Inuit to hunt otters, seals, and fox.
The Aleutians were purchased by the United States along with
the rest of Alaska from Russia in 1867. The only good harbor
in the Aleutian is at Dutch Harbor, used as a transshipping
port, a gold boomtown, and as a World War II naval base.
See also
PLATE TECTONICS
.
alluvial fans Fan- or cone-shaped deposits of fluvial grav-
el, sand, and other material radiating away from a single
8 Aleutian Islands and trench