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

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there is flood basalt volcanism from fissure eruptions.

These lava flows form the most voluminous volcanic depos-

its on the continental crust. Examples include the Columbia

Plateau of Washington, the Deccan traps of India, the

Piranha basin of South America, the Watchung basalts of

New Jersey, and the Karoo deposits of Africa. Some areas

also produce rhyolite volcanoes. Unlike the basalts that

simply spill out of cracks in the ground, the rhyolites form

well-developed cones. These volcanoes are highly explosive.

The Basin and Range Province of the western United

States, the Great Rift Valley system of Africa and the Rio

Grande Rift of Texas form good examples of these stages

of rifting. The next two stages involve the development of an

ocean basin. First is a restricted linear basin that contains a

mid-ocean ridge and is very active. Later, the basin becomes

wide and forms a full ocean basin. Volcanism is purely basal-

tic of the tholeiitic variety in these stages. Normal faults

continue to be active in both stages, but transform faults

dominate as the earthquake producers in the full ocean basin.

New oceanic crust is created in the mid-ocean ridge in both

stages. The youngest part of the ocean basin is therefore at

the mid-ocean ridge and becomes progressively older away

from and toward the continents. The new crust is largely lay-

ers of pillow lavas cross cut by feeder dikes to the under-

water volcanoes. The Red Sea is a good example of the early

stage of basin development, and the Atlantic Ocean is an

example of the later stages.

Dominica Lesser Antilles Dominica is situated in the

northern portion of the Lesser Antilles island arc and

has an unnamed caldera with a history of unrest dating

back to the late 17th century. The caldera is estimated to

be about six miles (10 km) wide. Several vents are present,

including Grande Soufrière Hills, Morne Macaque (Mico-

trin), Morne Trois Pitons, the Valley of Desolation, and

Watt Mountain. There is disagreement as to whether these

vents belong to a single volcano. Other vents on the island

include Foundland and Morne Anglais. Only one eruption

has occurred here within historical times, a minor steam

explosion in 1880.

There has been considerable earthquake activity at

Dominica over the past several centuries, however, starting

with the report of a moderately strong earthquake in 1673.

Emissions of gas and numerous strong earthquakes were

reported in 1765, and more strong quakes occurred in 1816

and 1838. In 1843 a very strong tectonic earthquake took

place near Dominica and was felt both there and on nearby

islands. Lesser earthquakes occurred at intervals of sev-

eral years through 1849. A moderately strong earthquake

in 1879 was accompanied by a minor phreatic eruption

and a black cloud rising from the area of solfataras. It is

uncertain whether the earthquake was linked with the erup-

tive activity. Seismic swarms have been recorded at Dominica

in 1967, 1971, 1974, and 1976. A moderately strong earth-

quake occurred approximately 100 miles (160 km) north of

Dominica in 1976 and was viewed as the result of magma

movements.

dormant volcano Literally, a sleeping volcano. This vol-

cano was active in the past and may well be active in the

future but under the current conditions is inactive. Volcanoes

may stay inactive for thousands of years and suddenly spring

back to life. Dormancy is really not measured in geological

terms but in historical terms. If the volcano has been active

in historical times but is currently inactive, it is termed dor-

mant. If the volcano has not been active in historical times, it

is deemed inactive. The problem, however, is that there really

is no distinction between active but not currently erupting

and dormant.

ductile Deformation of rock that does not produce earth-

quakes. Ductile deformation of rock means that instead of

breaking, it bends or flows. Ductile deformation depends

upon composition and other factors. Clay is ductile at surface

conditions. granite, on the other hand, cracks and frac-

tures under surface conditions. At depth, under high temper-

ature conditions, however, even granite is ductile. Because it

can’t rupture, no earthquakes can occur. This transition from

brittle rupturing rock to ductile deformation occurs at about

six to nine miles (10 to 15 km) depth for common continen-

tal crust. It is somewhat deeper for oceanic crust. This

transition means that there are few earthquakes deeper than

six to nine miles (10 to 15 km).

Duguna See Asawa.

Dukono volcanic complex, Halmahera, Indonesia The

Dukono volcanic complex is one of the most active volcanoes

in Indonesia. Its first recorded eruption was in 1550, and it

was later active in 1719, 1868, and 1901. Dukono has been

in nearly continuous eruption since 1933. The eruptions are

moderately explosive (VEI = 3) and produced lava flows

and lahars. Other volcanoes in the complex include Ibu (one

eruption in 1911), Todoko-ranu, and Jailolo. The only other

active volcano is Gamkonora. It erupted 12 times between

1564 and 1987. The 1673 eruption was very strong (VEI = 5)

and produced a large tsunami that devastated several nearby

coastal villages.

dust See volcanic dust.

72 Dominica

Earth, internal structure of Earthquakes and volcanoes

are manifestations of processes at work deep within Earth.

These processes in turn are evidence of Earth’s internal struc-

ture, models of which have undergone considerable evolution

in the 20th century. On a large scale, the internal features of

Earth may be categorized as follows:

1. Core. This is the innermost layer of Earth, a dense,

approximately spherical mass of very hot rock that

accounts for roughly one-third of the planet’s mass and

one-sixth of its volume. The core is divided into a solid

inner core and a liquid outer layer.

2. Mantle. The mantle, the thick layer of rock surrounding the

core, contains about two-thirds of Earth’s mass and more

than four-fifths of its volume. An outer layer of the mantle

is called the asthenosphere and is involved closely with

driving plate tectonics. There is also a division within

the upper and lower mantle based on mineral structure.

3. Crust. The crust, or surface layer, of Earth is very thin

compared to the mantle and core (five to 80 miles [8 to

129 km] thick) and accounts for only a tiny fraction of

Earth’s total mass and volume. The crust floats on the

asthenosphere by a process called isostasy, in which the

relatively lightweight rocks of the crust are supported by

the denser rocks below. The crust is divided into a number

of individual, rigid plates that interact with one another

through various processes, generating earthquakes and

volcanic activity. The crust contains many faults that

produce numerous earthquakes.

earthflow An earthflow is a type of mass movement. It is

a viscous flow of saturated, fine-grained materials that moves

downslope at speeds ranging from barely perceptible up to

about 10 miles per hour (about 0.1 m/sec). Materials suscep-

tible to earthflow include clay, fine sand and silt, and fine-

grained pyroclastic material (primarily ash). The velocity

and distance of the earthflow is controlled by water content,

with faster and longer movements through higher water con-

tents. Some earthflows may continue to move for years. Earth-

flows normally begin when water content increases either

through rain, melting (normal heating or volcanic erup-

tion), or liquefaction during an earthquake. Shaking from

an earthquake may also initiate flow in a saturated soil. The

flows tend to bulge in the middle as they move, which in turn

channels more fluid to the middle while the edges dry out.

Earthflows will stop moving when the water content drops.

earthquake See seismology.

earthquake hazard An earthquake hazard is considered

to be any of the damaging effects and processes of an earth-

quake that may affect the normal activities of people. Exam-

ples of earthquake hazards include surface faulting, ground

shaking, landslides, liquefaction, slumping, fissures, ava-

lanches, tsunamis, and seiches, among others.

earthquake light This phenomenon consists of a pecu-

liar glow that sometimes is reportedly seen in the sky during

earthquakes. What causes earthquake light is uncertain, but it

has been suggested that methane escaping from underground

during earthquakes is ignited somehow and burns near the

surface, giving off light.

earthquake risk Earthquake risk is the probable damage

to buildings, roads, services, and other infrastructure and the

number of people that are expected to be killed or injured

during an earthquake of a particular size in a particular loca-

tion. Earthquake risk is a probabilistic model for a specific

seismic event. It varies considerably with each area depending

upon population and emergency readiness.

earthquake swarm Many earthquakes preceding a volca-

nic eruption. As magma move upward in the crust, it pushes

rock out of the way. It forces open cracks in the rock. Each

break results in a small earthquake. These earthquakes are

typically of magnitudes of 4 or less, but some larger earth-

quakes are possible. There can be hundreds to thousands of

earthquakes in such swarms. volcanologists use this kind

of seismic activity to predict eruptions. The correlation is not

foolproof. Many earthquake swarms do not foretell a coming

eruption but rather just movement of magma.

East Pacific Rise A mid-ocean ridge 6,000 miles (9,700

km) long, the East Pacific Rise is the site of remarkable geo-

logical formations and ecological communities on the seabed.

An expedition to the East Pacific Rise in 1980 discovered that

superheated (more than 700°F), mineral-laden water flowing

from springs, or “hydrothermal vents,” on the seabed had

generated peculiar “chimneys” formed of minerals deposited

when the hot water met the cold ambient waters of the deep

ocean. Around the vents lived giant worms and large clams

and crabs in an ecosystem supported by bacteria feeding on

hydrogen sulfide, oxygen and carbon dioxide in the water

from the vents.

Eboshi-dake See Daisetsu-Tokachi.

economic effects of earthquakes and volcanoes The

economic effects of earthquakes and volcanic eruptions

can be tremendous. After witnessing the effects of a power-

ful earthquake in Chile, the 19th-century British naturalist

Charles Darwin wrote:

Earthquakes alone are sufficient to destroy the pros-

perity of any country. If beneath England the now

inert subterranean forces should exert those powers,

Diagram of Earth with a cutaway to show its internal structure. The center of Earth has an inner core (solid) surrounded by an outer core (liquid). The

mantle encases the core and is divided into a lower and an upper mantle based on the physical character of the rocks and minerals. The composition

is the same—peridotite. The crust is very thin in comparison with the rest of the planet so that the blowup inset is required to illustrate the difference

between ocean crust and continental crust.

74 East Pacific Rise

which most assuredly in former geological ages they

have exerted, how completely would the entire condi-

tion of the country be changed! What would become

of the lofty houses, thickly packed cities, great manu-

factories, the beautiful public and private edifices?

If the new period of disturbance were first to com-

mence by some great earthquake in the dead of the

night, how terrific would be the carnage! England

would at once be bankrupt; all papers, records, and

accounts would from that moment be lost. Govern-

ment being unable to collect the taxes, and failing to

maintain its authority, the hand of violence and rap-

ine would remain uncontrolled. In every large town

famine would go forth, pestilence and death follow-

ing in its train.

A single earthquake or volcanic eruption can cause prop-

erty damage in the hundreds of millions of dollars, and a

very powerful shock or eruption occurring in or near a heav-

ily populated area might cause damage into the billions of

dollars. Of particular concern in our time is the anticipated

earthquake in the Tokyo, Japan, metropolitan area. Tokyo

has undergone major earthquakes at intervals of approxi-

mately 75 years during the last few centuries, and the most

recent major earthquake there occurred in 1928. The cost

of rebuilding Tokyo after the next “superquake” has been

estimated at $1 trillion. In these days of an increasingly inte-

grated global economy, a highly destructive earthquake in

Tokyo might have drastic economic effects in many other

countries as well, including the United States.

Ecuador Ecuador is located on the western shore of

South America, where the westward-moving continen-

tal crustal plate encounters oceanic crust and generates

earthquakes where the two plates collide. Ecuador has a his-

tory of highly destructive seismic activity, notably the great

earthquake of August 5, 1949. This earthquake, of magni-

tude 7.5 on the Richter scale, was centered 25 miles (40

km) deep and affected an area of 1,500 square miles (3,885

) along the eastern side of the Andes, particularly

Ecuador’s highland plateau. The earthquake killed more

than 6,000, injured some 20,000, and left approximately

100,000 people homeless. More than 50 cities and towns

experienced severe damage, and total damage was estimated

at more than $60 million.

Ecuador has some very active volcanoes as well includ-

ing Altar, Antisana, Cayambe, Chimborazo, Cotopaxi,

Cusin, Guagua Pichincha, Ilinzia, Imbabura, Mojanda,

Ninahuilca, Pasochoa, Reventador, Ruminahui, Sangay,

Sumaco, and Tungurahua. Chimborazo is the largest of

the volcanoes, standing 20,700 feet (6,309 m). Sangay is

probably the most active of the volcanoes, erupting nearly

continuously since 1934. Guagua Pichincha is probably the

most dangerous volcano because it overlooks Quito, the

capital city of Ecuador. The city has been devastated sev-

eral times by the larger of its 25 historical eruptions. The

worst eruption was in 1660 when a foot of ash blanketed

the city.

Edo earthquake and tsunamis, Japan A submarine earth-

quake struck an area south of the Boso Peninsula just east

of Tokyo Bay on December 30, 1703. The estimated Rich-

ter magnitude for this event was 8.0. The quake destroyed

the city of Edo, one of the more prosperous settlements in the

area. Bathymetric studies along with drill coring in this area

indicated that the seafloor was uplifted about 17 feet (5.1

m) in several places. As a result, some previously outlying

reefs became dry land. Uplift also occurred along the coast

at various locations, producing tsunamis that struck the

entire coast. There was extensive damage in nine provinces.

The official reported death toll for this disaster was 5,233

people, but there are some data that indicate a death toll as

high as 37,000.

Egon volcano, Indonesia Egon is among a group of vol-

canoes in the Flores Islands. The other volcanoes include

Lewotobi, Ilimuda, and Lereboleng. Egon erupted in 1907

and likely in 1888–89. Lebwotobi had at least 19 eruptions

between 1675 and 1991. Most eruptions are moderate in size

with a VEI of 2–3. The largest eruptions were in 1869 and

1907.

Egypt Egypt’s position on the northern and northeastern

edge of the African plate make it subject to earthquakes,

though not directly associated with volcanoes. The Sinai

border is along the Red Sea, which is an active divergent

margin capable of sustaining active faults. It also faces the

Eastern Mediterranean Sea, which is highly active by virtue

of the north-dipping Hellenic Arc subduction zone and

impingement of Turkey, which is being forced westward

through extrusion tectonics. These complex interactions

yield complex faulting and earthquake activity.

Most of the historic activity is within the Nile valley and

Nile Delta, where soft sediments amplify the waves, causing

great destruction. Notable historic earthquakes include the

a.d. 365 Alexandria event that destroyed the historic light-

house and caused tens of thousands of deaths; the 1303 Alex-

andria earthquake that claimed 10,000 lives, and the 1754

Cairo earthquake that claimed 40,000 lives. In all, there

were 58 historical earthquakes between 2200 b.c. and a.d.

1900, with modified Mercalli intensities between V and

IX, 22 of which are well documented. Instrumental seismicity

(recorded on a seismograph) was from 1900 to the present.

Four distinct zones of activity were apparent from these data:

Northern Red Sea–Gulf of Suez–Alexandria trending north-

west-southeast, Gulf of Aqaba–Levant Fault trending north

northeast–south southwest, Eastern Mediterranean–Cairo–

Faiyum trending northeast-southwest, and Egypt–Mediter-

ranean Coast tending east-west. Most of the faults in these

zones experienced primarily strike-slip motion. The most

recent powerful earthquake was in Cairo in 1995 with a 7.0

magnitude.

ejecta Solid volcanic fragmental material is ejecta regardless

of composition or internal textural relations. The classification

is based purely on the size of the fragments. Fine ash or volca-

nic dust is less than 0.2 inch (0.5 cm); coarse ash is between 0–

.08 inch (0.01–0.2 cm). lapilli are 0.8–2.5 inches (0.2–6.4 cm)

ejecta 75

and along with ash is lithified into a tuff. bombs are 2.5–10

inches (6.4–25 cm), and blocks are larger than 10 inches (25

cm). Rocks of these ejecta are called agglomerate.

elastic rebound theory This is the basic accepted theory for

how earthquakes are generated. The idea is that faults will

remain locked with no slippage while tectonic forces slowly

build up stress within the rocks. When the stress exceeds

the strength of the rocks or locks on the fault, the fault slips,

releasing or converting all of the stored stress energy in the

form of heat and seismic waves in an earthquake.

elastic strain Elastic strain means that a material will

strain as a direct function of the stress applied and then

return to its original shape after the stress is released. This

deformation is referred to as recoverable because it is not per-

manent. Springs and rubber bands exhibit elastic behavior:

They stretch out as a function of how hard they are pulled

and then return to their original shape when they are let go.

Seismic waves cause elastic strain in rocks. The rocks deform

as the wave passes through but return to their original shape

afterward. The amount that rocks deform in this process is

minuscule, especially in comparison with springs and rubber

bands. Even if you could slow the wave down and watch it

pass through a small piece of rock, you still wouldn’t be able

to see the rock change its shape. Both the change in and the

return to shape are essentially instantaneous. For some sub-

stances, the return to original shape is not immediate. Cello-

phane behaves in this manner. It crushes immediately, but it

will slowly uncrush after it is let go. This kind of slow recov-

ery is referred to as anelastic.

Eldfell See Heimaey.

Eldgja See Katla.

embryonic volcano A volcanic pipe that is filled with

volcanic breccia but for which there is no surface expres-

sion. These features are considered to have formed through

phreatic explosions. They are precursors to full volcanoes if

activity continues in the area.

Emmons Lake caldera, Aleutian Islands, Alaska, United

States The Emmons Lake caldera is located near Pavlof

volcano in an area of frequent seismic and volcanic activity.

Emperor Seamounts northern Pacific Ocean A chain

of submerged volcanic mountains in the northern Pacific

Ocean, the Emperor Seamounts are part of a string of vol-

canic seamounts and islands reaching almost from Hawaii

to Russia’s Kamchatka Peninsula. The Emperor Seamounts

and the Hawaiian Islands are both part of this same chain,

which formed as the Pacific crustal plate moved over a

stationary hot spot of volcanic activity. A sharp bend in the

chain marks the division between the Emperor Seamounts

and the Hawaiian Islands and indicates that the Pacific

crustal plate changed its direction of motion abruptly, from

north to northwest.

See also Iceland.

Penguins with Mount Erebus in the background. Mount Erebus is the most active volcano in Antarctica. (Courtesy of the USGS)

eruption 77

eruption cycle The sequence of events that happens dur-

ing an eruption episode. It depends on the volcano as to what

the cycle will include and in what order. Typically, an erup-

tion cycle begins with an earthquake swarm as the magma

moves into place. fumarole activity commonly follows

next as the groundwater is heated to the boiling point. The

first eruption commonly includes gas emissions and a fair

amount of ash. Explosions begin and bombs are hurled from

the volcano. lava flows from the main vent in the middle

stages of the eruption cycle. Later, lava may flow from other

vents. Some volcanoes pour lava out continuously through

the eruption. In this case, the volume tends to start out slow

and increase in the middle of the eruption and then decrease

again at the end. In others, eruptions are catastrophic with a

huge amount of lava and ejecta being emitted at once sepa-

rated at once, punctuating otherwise relatively quiet periods.

In subduction-related stratovolcanoes and rhyolite vol-

canoes, the middle of the cycle commonly involves an explo-

sion and loss of a significant portion of the volcanic cone.

Later, it may be rebuilt in a resurgent cone and significant

ash and ejecta emissions. In shield volcanoes as in hot spots

or divergent boundaries, there are no explosions and loss

of the cones. They produce far less ash than the subduction

related volcanoes.

Erzincan earthquake, Turkey On December 26, 1939, the

right-lateral strike-slip North Anatolian Fault produced

another of its catastrophic earthquakes—this one in the city

of Erzincan. The magnitude of the earthquake was 7.9 on

the Richter scale. It produced surface ruptures, landslides,

rockfalls, and liquefaction. It also produced significant

tsunamis on the Black Sea near Fasta Bay. There are reports

that these waves achieved heights of 50–60 feet (17–20 m)

locally. Strong aftershocks continued for several day

after the main shock, and minor aftershocks continued for

months.

Some 33,000 people lost their lives in this disaster, and

many tens of thousands were injured. Over 110,000 homes

were heavily damaged. A large winter storm exacerbated this

situation dropping snow on the ruins and bringing plummet-

ing temperatures and strong winds. Many people died from

exposure, and rescue efforts were hampered.

The Erzincan earthquake, although not the biggest, may

have been the most significant in the history of Turkey. As a

result, the first seismic design codes for buildings were estab-

lished in 1940, and the first earthquake zonation (hazard)

map for Turkey was produce in 1942.

escarpment Also known simply as a scarp, an escarpment

is a steep cliff located at the edge of a flat or nearly flat area.

An escarpment typically forms where there is vertical motion

along a dip-slip fault.

Etna, Mount Sicily, Italy One of the most famous volca-

noes, Etna stands some 10,750 feet (3,500 m) tall and has a

circumference greater than 90 miles (150 km). The volcano

has a gentle slope and resembles a shield volcano, such as

those of the Hawaiian Islands, more than a steeply sloping

volcano, such as Fuji or Vesuvius. Numerous lava flows

have occurred from the flanks of the volcano. Charles Mor-

ris, in his 1902 survey of volcanoes, The Volcano’s Deadly

Work, described the activity of Etna:

There is a great similarity in the character of the

eruptions of Etna. Earthquakes presage the out-

burst, loud explosions follow, rifts and bocche del

fuoco [“mouths of fire,” or small lava vents] open

in the sides of the mountain; smoke, sand, ashes

and scoriae are discharged, the action localizes itself

in one or more craters, cinders are thrown up and

accumulate around the crater and cone, ultimately

lava rises and frequently breaks down one side of

the cone where resistance is least; then the eruption

is at an end.

Etna has erupted at least 190 times from 1226 b.c. to the

present. Most of these eruptions are moderate (VEI = 1–2).

Virgil, Pindar, Seneca, Pythagoras, and Thucydides all men-

The eruption column above the 1980 eruption of Mount Saint Helens

(Courtesy of the USGS)

78 eruption cycle

tioned the volcano in their writings. Perhaps the most famous

eruption of Etna occurred in 1669, when powerful explo-

sions destroyed part of the volcano’s summit and lava from

a fissure on the volcano’s flank flowed some 10 miles to the

sea and damaged the town of Catania. The 1669 eruption is

notable for an attempt to divert a lava flow. About 50 towns-

people from Catania reportedly dug a passage that drained

away lava and apparently reduced the immediate threat to

Catania, but when the flow changed direction and menaced

the village of Paterno, inhabitants of that community drove

away the Catanians and ended their attempt to divert the

lava. This effort, though ultimately unsuccessful, showed that

engineering could alter the direction of lava flows.

An eruption in 1755 was accompanied not only by emis-

sions of lava but also by great flows of water, which resulted

when lava came in contact with snow and ice on the summit.

The water flowed down the flanks of the mountain, carry-

ing along with it great quantities of unconsolidated volcanic

material. The volume of water involved in this eruption was

estimated at approximately 16 million cubic feet (453,070

). It formed a channel two miles (3 km) wide and 34 feet

(10 m) deep in places and flowed at the rate of approximately

40 miles (64 km) per hour. Another great eruption occurred

in 1819. Although few deaths are attributed to Etna his-

torically, in 1843, 36 people were killed in a phreatic explo-

sion. An extremely violent eruption that lasted more than

nine months occurred in 1852. This eruption is thought to

have produced flows of lava more than 2 billion cubic feet

(56,633,694 m

) in volume over an area of some three square

miles (7.8 km

) More than three centuries after the attempt

to divert a lava flow during the eruption of 1669, volcanolo-

gists in Italy asked the United States military to help with

another such experiment during the eruption of 1992. The

attempt was called Operation Volcano Buster and involved

using explosives to blast a hole in a lava tunnel 6,000 feet

(1,830 m) up the flank of Mount Etna. Helicopters then

dropped large blocks of concrete into the hole in the hope of

stopping the lava flow. The experiment, however, was con-

sidered only a partial success at best.

Etna is still quite active. The 1991–93 eruption was the

largest in the last 300 years. In the 1995–96 eruption, explo-

sions and fire fountains were common. That eruption never

really ended; it has just increased and decreased in intensity

ever since. As of April 2000, air traffic routes are still being

diverted around the erupting volcano.

Eurasian crustal plate One of the major crustal plates

that are believed to occupy the surface of the earth, the Eur-

asian plate generally underlies the continents of Europe and

Asia, although portions of Asia also are found in the Ara-

bian crustal plate and Indo-Australian crustal plate.

The borders of the Eurasian plate are noted for intense earth-

quake and volcanic activity. On its western edge, the mid-

ocean ridge in the middle of the North Atlantic Ocean is

the site of volcanism, notably in areas such as Iceland. The

eastern edge of the Eurasian plate follows part of the trace of

the “Ring of Fire,” the belt of seismic activity and volcanism

that surrounds much of the Pacific Ocean basin. Volcanic

activity is especially pronounced along this border of the Eur-

asian plate in regions such as Japan and the Kamchatka Pen-

insula of Russia. The southeast edge of the Eurasian plate

displays abundant evidence of volcanic activity, both ancient

and recent. This is the region of the highly destructive volca-

noes of the Indonesian archipelago, such as Krakatoa, the

1883 eruption of which has become virtually a synonym for

a natural catastrophe. Along the southern border of the Eur-

asian plate, earthquakes are frequent and powerful where the

Eurasian plate adjoins the Indo-Australian and Arabian plates.

The ongoing collision between the Indian and Eurasian plates

is believed to have generated the Himalaya Mountains, among

other prominent features of southern Eurasia.

See also Indonesia; Iran; Philippine Islands; plate

tectonics.

explosive eruption An eruption that produces an energetic

ejection of pyroclastic material. andesitic, dacitic, and

rhyolitic volcanoes produce the most pyroclastic material

and the most explosive eruptions. There are two reasons for

the explosivity. These lavas and magmas are highly viscous

and sticky. They tend to clog up the volcanic pipe. Clogged-

up vents cause the pressure to increase in the volcano. The

other factor is that these types of magmas carry a lot of water

and other gasses. These components are referred to as vola-

tiles. Under pressure, these volatiles are held in the magma

as liquid. When the pressure is released, they can come out of

the magma. Because it is so hot, they immediately turn to gas.

Gas occupies 22.4 times as much space as the same amount

of liquid at room temperature. In other words, boil one gal-

lon of water and it will produce 22.4 gallons of steam. At

volcanic temperatures, the increase in volume is even greater.

This radical increase in volume is instantaneous and causes

the volcano to explode. As an analogy, shake a bottle of soda.

Nothing happens until you remove the cap and then it foams

over or explodes. When you remove the cap, you release the

pressure and the carbon dioxide, which was formerly held in

the liquid, instantaneously turns to gas, producing the foam.

extinct volcano A volcano that is inactive and for which

there is no chance of renewed activity. The transition from

dormant to inactive and extinct is just a matter of time.

If the volcano has not erupted in historic times and is not

expected to erupt in the future, then it is considered extinct.

The distinction between inactive and extinct is unclear.

extrusion tectonics Otherwise known as escape tecton-

ics, extrusion tectonics involve the lateral movement of

landmasses out of the way of an ongoing continental colli-

sion. If a person were to place a piece of clay or putty in his

or her hand and then smash the other fist into it. The clay

would squirt out of the sides as the fist made impact. con-

tinents do the same thing. As a continent moves toward

another through subduction, the oceanic crust eventu-

ally becomes exhausted, and the two continents impact. The

subducting continent, here termed the rigid indenter, will

attempt to drive beneath the other, and the margins of each

continent will crumple with massive amounts of deforma-

tion, both faulting and folding. As the collision proceeds, the

overriding continent develops a series of crossing strike-slip

extrusion tectonics 79

faults, one direction dextral and the other direction sinis-

tral, behind the collision zone. This zone of crossing strike-

slip faults is known as a syntaxis. If one side of the overriding

plate faces onto an ocean basin, this is called a “free face.”

the strike-slip faults that accommodate movement of land in

the direction of the free face will become very active, facili-

tating large-scale bulk movement of continent. The strike-slip

faults in the other direction will be less active. Once the first

Map showing the classic area for escape tectonics, the Himalayan orogeny. The Indochina block was forced out of the way of the impinging Indian plate

along the Red River Fault for about 25 million years and accommodating over 600 miles (1,000 km) of left-lateral strike-slip movement. When the Red River

Fault became inactive 20 million years ago, left-lateral movement shifted to the Altyn Tagh Fault which also underwent many hundreds of kilometers of

offset. A new fault connecting to Lake Baikal has initiated more recently. The three block diagrams show an analog model of escape tectonics. A wooden

block (rigid indenter) models India by being driven into a block of clay that models Asia. The left side of the clay is fixed, but the right side (free face) is

able to undergo tectonic escape. The sequence models the Himalayan collision.

80 extrusion tectonics

piece of continent escapes the collision, the old strike-slip

fault becomes inactive, and a new strike-slip fault begins to

extrude a second, deeper piece out of the way.

The best example of extrusion tectonics can be found

in the Himalayan orogeny. India is the rigid indenter, Asia

the overriding plate, and the Pacific Ocean the free face.

Upon impact of India with Asia, the huge Red River Fault

began sinistral strike-slip movement that continued for about

25 million years. It moved Indochina to the east and com-

pletely out of the way of the collision and rotated it from an

east-west orientation to a north-south orientation where it

presently lies. The Red River Fault became inactive, and the

Altyn Tagh Fault to the north then began sinistral strike-slip

movement and pushed central China, the second piece, east-

ward and out of the way. At present, the Altyn Tagh Fault is

becoming less active, and a new fault system that connects to

Lake Baikal and Kamchatka, far to the north is initiating.

Another example of escape tectonics is in the Zagros

orogeny. The Arabian crustal plate (rigid indenter) sepa-

rated from Africa and drove northward until it collided

with the margin of southern Asia. This collision produced the

Zagros Mountains of Iran and Iraq. As the two continents

pushed together, Turkey extruded westward and into the

eastern Mediterranean Sea (free face), into which it now

juts. Extrusion of Turkey westward is accommodated on the

North Anatolian Fault, which undergoes dextral strike-slip

movement and regularly produces some of the world’s dead-

liest earthquakes.

extrusive rock igneous rock that flows, or has flowed,

from a fissure or vent on Earth’s surface.