Gates A.E., Ritchie D. Encyclopedia of Earthquakes and Volcanoes

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Plate Margins

A plate margin, or plate boundary, occurs where lithospheric plates meet. Adjacent

plates interact at plate margins in one of three ways:

1. They move away from each other in a divergent margin;

2. They move toward or into each other in a convergent margin; or

3. They slide past each other in a transform margin.

But why should they move at all? Why don’t they just remain in one spot? The

answer is convection. There are hotter and cooler areas in the mantle. The hot-

ter mantle is less dense and tends to rise, just like hot air or hot water. The cooler

mantle is more dense and tends to sink. When mantle is heated in the hotter areas,

it rises to the upper mantle and spreads out, cooling as it moves away from the

heat source and sinking back to the lower mantle where it can be reheated. In this

way the mantle circulates in a convection cell similar to a boiling pot of soup. The

lithospheric plates float on the flowing, circulating mantle.

Divergent Margins

Divergent margins typically begin on continental crust but quickly wind up on oce-

anic crust. There are several distinct stages in these margins. The process of pulling

crust apart is called rifting. The initiation of rifting appears to involve the forma-

tion of triple junctions. Mantle plumes strike the underside of the plates, leaving

a hole with three cracks emanating from it at 120° angles. Two of the cracks on the

Earth will become active rift zones and mature divergent margins that connect with

other triple junctions, while the third crack will start to rift but eventually fail. The

failed crack is called an aulocogen. The best example of a triple junction is at the

southern tip of the Arabian crustal plate. The Gulf of Aden and Red Sea repre-

sent the two active cracks, and the East African rift system is the aulocogen.

The early stage of rifting is short-lived, if present at all, and involves bulging

of the continental crust and uplift. The second stage involves thinning of the con-

tinental crust. The lower crust is ductile and stretches thinner, whereas the upper

crust is brittle. Brittle deformation includes the development of active normal

faults and consequently horsts and grabens (similar to the modern Basin and

Range Province of the southwestern United States). Eventually, mafic magma from

the upper mantle reaches the surface in fissure eruptions. flood basalts cover

the landscape with massive flows (such as the Laki eruptions of Iceland or the

Columbia River Plateau of Washington). These lava plateaus are the largest accu-

mulations of lava on the continents. In some cases rhyolite volcanoes may also

be produced by the melting of continental crust from the elevated heat flow from

the basalt. These volcanoes can be violent, producing huge eruptions, such as at

Yellowstone National Park.

The next stage involves the development of a narrow ocean basin such as

the Red Sea. Volcanic activity continues, but it is submarine and purely basalt.

Earthquakes can continue on land, but they are less common and less intense. Most

of the earthquake activity is also submarine. The final stage is the mature ocean

basin such as the Atlantic Ocean. The coasts are passive margins with no volcanic

activity and rare seismic activity. All of the volcanic and seismic action is subma-

rine and occurs at the mid-ocean ridge at the center of the basin. A mid-ocean

ridge is where new ocean crust is continuously being formed. The Mid-Atlantic

Ridge is a huge submarine mountain range that extends almost from pole to pole.

It surfaces at Iceland, which provides a glimpse of the intense volcanic and seismic

activity associated with these margins.

Convergent Margins

There are three types of convergent margins depending upon the type of crust on

the colliding plates:

1. ocean-ocean convergent margins

2. ocean-continent convergent margins

3. continent-continent convergent margins

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It is at convergent margins where ocean crust is consumed at the exact rate as it

is being produced in the mid-ocean ridge. This balance is necessary or the Earth

would be constantly changing size.

At an ocean-ocean convergent margin, ocean crust is driven beneath facing

ocean crust in a feature called a subduction zone. On the ocean floor, the top

edge of the subduction zone is marked by a trench that contains the deepest ocean

depths on Earth. The downgoing or subducting plate drives deeper into the asthe-

nosphere, where it first partially melts and then is absorbed. Earthquakes occur

all along the surface of the subducting plate and are thus of progressively deeper

focus away from the trench. The melted ocean crust forms magma of interme-

diate composition that rises through the overlying crust eventually to form sub-

marine volcanoes. These volcanoes continue to grow until they breach the ocean

surface to become volcanic islands. This chain of islands forms an arcuate map

pattern called an island arc. The Aleutian Islands are a good example of an

island arc. These andesite volcanoes are explosive and among the most danger-

ous on Earth. They are also very seismically active primarily as the result of move-

ment on reverse and thrust faults. Krakatoa is a subduction zone volcano.

Subduction zones are the source of megathrusts that produce such disasters as

the 2004 Banda Aceh tsunami.

Ocean-continent margins are geometrically similar to ocean-ocean margins.

The difference is that the overriding plate is continental rather than oceanic crust.

Instead of island arcs, there are volcanic mountain ranges called magmatic arcs.

The Andes Mountain range is the best example of a magmatic arc. These margins

also have subduction zones and Benioff zones with both seismic and volcanic

activity. The volcanoes are at higher elevations than those in island arcs, and the

magma chambers are much larger.

Connected to the subducting ocean crust, somewhere far behind, is another

continent. Eventually, the ocean crust is completely consumed in the subduction

zone, and the two continents face each other and collide. The subducting plate

attempts to follow the ocean crust down the subduction zone, but it is too buoy-

ant to enter the asthenosphere. Instead, the two plates collide. The subducting

plate plows beneath the overriding plate and then large thrust faults develop on

the subducting plate, moving large amounts of rock away from the collision zone

accompanied by a series of massive earthquakes. The overriding plate crumples

while building huge mountains that progressively grow away from the collision

zone. This crumpling involves intense seismic activity. The only potential vol-

canic activity is leftover from the subduction zone activity on the old magmatic

arc.

There is an odd effect that occurs in continent-continent collisions called

extrusion or escape tectonics. If one or both sides of the overriding plate

(at a high angle to the collision zone) faces an open ocean basin, it is said to

have a “free face.” As the continental collision proceeds, chips of the overrid-

ing plate are shoved laterally and out of the way of the intense collision zone

along large strike-slip faults. Basically, land is being squirted sideways, away

from the zone of high force. The strike-slip faults accommodate up to 621 miles

(1,000 kilometer) offset and produce regular, intense earthquakes. An example

of extrusion tectonics is Turkey, which is being squirted westward into the

Mediterranean Sea along the strike-slip North Anatolian fault. Some of the more

devastating earthquakes of the 20th century have occurred along this fault as the

result of extrusion tectonics.

Transform Margins

Transform Margins or faults are strike-slip faults that separate two litho-

spheric plates. Furthermore, they must connect two other types of plate margins.

Therefore, there are three types of transform margins based upon the margins con-

nected. There are:

1. divergent-divergent transform margins;

2. convergent-convergent transform margins; and

3. divergent-convergent transform margins.

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Because they are plate tectonic margins, they are constantly active, potentially

for very long periods of time. They are some of the most active faults on Earth.

Luckily, more than 99 percent of transform faults are on the ocean floor. The vast

majority of these are divergent-divergent transform margins that accommodate

bends and offsets on the mid-ocean ridges. These faults are also called geofractures

or fracture zones and can be seen on bathymetric maps of the ocean floor at high

angles to the mid-ocean ridges. The offsets give them a dentate appearance with the

high density of transform faults along the ridge. Convergent-convergent transform

margins accommodate jogs and offsets in island arcs, but they are not common.

Divergent-convergent transform margins are also uncommon and are usually asso-

ciated with smaller plates such as the Caribbean plate.

Transform faults on continental crust are very dangerous because of their per-

sistent activity. The best example is the San Andreas Fault in California. Every

few years it produces an earthquake with a magnitude of 7.0 or greater, many

of which cause great damage. Another example is the South Alpine Fault in New

Zealand, which also produces numerous strong earthquakes.

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My thanks to the Department of Earth and Environmental Sciences at Rutgers

University–Newark for support and resources; to the students in my natural disas-

ters classes for leading me to new resources for earthquakes and volcanoes; and

to reference librarian Veronica Calderhead at Dana Library for efforts above and

beyond the call of duty in finding obscure references for me.

To Dr. Tobi Zausner for her excellent work in scanning slides and improv-

ing images, and to Colin Gates for producing several of the tables. To Dr. David

Valentino at SUNY–Oswego for his suggestions to improve this volume; to execu-

tive editor Frank K. Darmstadt for his suggestions, encouragement, and patience;

to Alana Braithwaite for her invaluable help in getting the manuscript into shape;

and to literary agent Max Gartenberg for his sage advice.

I would also like to acknowledge the phenomenal resources of the U.S.

Geological Survey and National Oceanographic and Atmospheric Administration,

without which the production of this book would have been impossible.

ACKNOWLEDGMENTS

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The volcano on the island of Thira (now Santorini) in the Aegean Sea exploded

in about 1500 b.c. As a result, it sent a tsunami now estimated at over 200

feet (61 m) high across the eastern Mediterranean Sea that destroyed the domi-

nant Minoan civilization on the island of Crete. The hill people of Greece

thereby formed the dominant culture, eventually heralding the Golden Age of

Greece. The huge tsunami caused an extensive retreat of seawater along the

Mediterranean shoreline for up to 10 minutes before it came crashing back. It is

possible that this opening of the sea allowed Moses and the Israelites to escape

the Egyptians in 1500 b.c., as described in the Bible.

It is to this astounding degree that earthquakes and volcanoes have affected

humans’ history, culture, and civilization. In ancient times, earthquakes sometimes

tipped the scales in power struggles, allowing one kingdom to defeat another. It

is no wonder that earthquakes are sometimes said to be connected to punishment

imposed by a displeased deity in ancient and even modern religions. Volcanoes

have even been credited with altering the Earth’s climate, thereby causing some of

the major famines and even plagues in history. Earthquakes and volcanoes are the

most powerful and destructive natural phenomena on the planet, fascinating people

both young and old.

It was this fascination that inspired David Ritchie to write the first edition

of the Encyclopedia of Earthquakes and Volcanoes. Untrained in science, he con-

centrated on historical accounts of the disasters. When I revised the first edition,

I added the missing scientific aspect to the encyclopedia. I also expanded the cov-

erage of volcanoes as well as updated information on recent geological phenom-

ena. In addition, new photos and maps were added to show where the events took

place, and diagrams were provided to illustrate the processes.

One would think that with all the human advances in monitoring technology

and earthquake engineering that earthquakes and volcanoes would become less

damaging and thus less important with every year. However, the still young 21st

century is showing humans that this is not the case. The horrifying recent disas-

ters associated with the Banda Aceh, Indonesia, tsunami and the Bam, Iran, and

Muzaffarabad, Pakistan, earthquakes are among the worst in history. The burgeon-

ing world population in geologically hazardous areas is clearly growing faster than

the disaster-reduction technology.

These new events made it clear that the second edition of the encyclopedia

lacked the resources to put these disasters into proper historical context. This third

edition not only includes the new events but also many examples of historical

earthquakes for contrast or comparison. The third edition has earthquake informa-

tion for Italy, Greece, Egypt, Iran, Pakistan, China, India, and several other areas.

It is especially timely, with greatly expanded coverage in the Middle East, where

the world’s focus has rested in recent years. Because many of these disasters involve

destruction by landslides and avalanches, entries related to such events and pro-

cesses are significantly expanded. Tsunami research and technology is also updated

in the new edition to answer questions about the Banda Aceh disaster, and many

INTRODUCTION

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other tsunamis are described for comparison. Finally, there are many tables to

place the magnitude of these recent disasters into historical context.

The third edition of Encyclopedia of Earthquakes and Volcanoes provides a

unique single source on historical earthquakes and volcanic eruptions for students

or laypeople. Many of the sources included are obscure and difficult to obtain.

With the new “Preface Essay on Plate Tectonics,” this edition may serve as a stand-

alone reference for introductory courses on earthquakes and volcanoes.

xvi Introduction

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ENTRIES A–Z

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aa A Hawaiian word (pronounced AH-ah), aa is a particu-

lar kind of lava flow with an irregular, jagged surface. Aa

is very stiff and blocky because much of its mass is hardened

lava. It flows slowly, with lava rubble tumbling down the

advancing slope. It typically occurs far from the volcanic

vent at the leading edge of the flow.