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

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gabbro A mafic rock that is characterized by coarse

grains. Gabbro is composed of plagioclase and pyroxene

and less commonly olivine and hornblende. Gabbro is the

plutonic equivalent of basalt. dikes and sills of gabbro are

common in areas of basalt volcanism. They make up a large

proportion of oceanic crust. There are also some large gab-

bro bodies on continental crust as well. The Palisades Sill

in New York–New Jersey is composed of diabase, which is a

textural variety of gabbro. stocks and batholiths (circular

plutons) of gabbro are rare but can yield significant quanti-

ties of platinum and chromium.

Gadamsa See Asawa.

Gademota See Asawa.

Galápagos Islands Ecuador The Galápagos Islands are

located off the west coast of South America and are the

summits of volcanoes along a hot spot. The islands are per-

haps best known for their giant tortoises and for remarkable

variations in the bodily forms of animal populations from

one island to another. The most active of the volcanoes in

the Galápagos Islands is Fernandina, a major eruption of

Flooded caldera rim forming an arcuate island in the Galápagos, Ecuador (Courtesy of NOAA)

which took place in 1968 and again in 1995. Other volca-

noes include Alcedo, Cerro Azul, Darwin, Ecuador, Sierra

Negra, and Wolf, all on Isabela Island.

Galung Gung volcano, Java Not quite 7,000 feet (2,134

m) high, Galung Gung is adjacent to the Plain of Ten Thou-

sand Hills, which actually number several thousand less. They

are thought to have been formed by a lahar that accompa-

nied one of Galung Gung’s eruptions. Galung Gung erupted

twice in 1822 (VEI = 5) and destroyed no fewer than 114

villages and killed more than 4,000 people. Since then it has

erupted four times, most recently in 1984. The 1982 eruption

(VEI = 4) killed about 68 people and caused $15 million in

property damage.

Gamalama volcano, Halmahera, Indonesia Also known

as Peak of Ternate, Gamalama stratovolcano forms a

seven-mile (11-kilometer)-wide island. It has had at least 70

eruptions since 1538. Eruptions from 1771 to 1775 caused

fatalities including a

NUÉE ARDENTE that killed more than

1,300 people in 1775. Other eruptions that resulted in fatali-

ties occurred in 1838, 1871, and 1962. The most recent erup-

tion was in 1993. Most of the eruptions are explosive with

VEIs of 2–3.

Gansu earthquake, China An earthquake estimated at

magnitude 8.6 on the Richter scale struck the Chinese

province of Gansu (formerly Kansu) on December 16, 1920,

killing approximately 180,000 people. An additional 20,000

deaths during the following months were attributed to lack of

shelter during bitter winter weather. Natural dams formed by

landslides had to be destroyed to prevent flooding.

Garibaldi, Mount British Columbia, Canada A volcanic

mountain at the northern end of the Cascade Range.

Gaua Island Vanuatu Located in the northern portion of

the Vanuatu archipelago, Gaua Island is a stratovolcano

with a caldera occupied by a lake, Steaming Hill Lake, and

by a postcaldera cone, Mount Gharat. From 1963 to 1982,

Mount Gharat underwent at least 13 eruptions. Most erup-

tions were small (VEI = 2) and lasted only one to two days.

The 1963 and 1973–74 eruptions lasted several months.

Geger Halang caldera, Java, Indonesia The Geger Halang

caldera is located in central Java near Juningan and Telaga

and has a history of eruptive activity dating back to the late

17th century. How the caldera originated is uncertain. Pos-

sibly the caldera collapsed, but landslide activity has also

Small resurgent dome volcanoes within a larger caldera scatter across the landscape of the Galápagos Islands, Ecuador, in the Pacific Ocean. (Courtesy

of NOAA)

94 Galung Gung

been put forward to account for the calderas formation.

Before the collapse, a stratovolcano is thought to have

occupied the site of the caldera. The stratovolcano Cereme

is situated on the caldera’s northern rim. It has erupted six

times between 1698 and 1951. An explosive eruption in

1698 reportedly caused many deaths. Other eruptions are

recorded in 1772, 1775, and 1805. Earthquakes and sub-

sidence in 1876 at a site several kilometers away from the

caldera may have been unrelated to volcanism at Geger

Halang. Emissions of sulfur gas increased in 1917 as did

fumarolic activity in 1924. A series of eruptions in 1937

and 1938 was accompanied by a large number of strong

earthquakes. An eruption in 1951 consisted of a single det-

onation followed by an emission of thick smoke. tectonic

earthquakes occurred in 1973 but evidently were unrelated

to volcanism at the caldera.

Gemini Seamount volcano, eastern New Hebrides A

submarine volcano that has been erupting regularly since

1996. It was reported to have produced explosions every

three to nine minutes. These eruptions produce steam and

ash.

geodesy Geodesy is the science of determining the size and

shape of the Earth, both on the large and local scale, and the

precise locating of points on the surface. Geodedic surveys are

conducted at all scales on a regular basis, now with the aid of

satellite telemetry. These surveys can help to identify areas that

are bulging or subsiding, which may be precursors to earth-

quakes and volcanic eruptions. They can also determine uplift

and subsidence as the result of seismic and volcanic events.

geology Literally translated, geology means “science of the

Earth,” but it is more than that. It is a composite science that

encompasses all other sciences—biology, physics, and chem-

istry as well as many aspects of engineering—and even has

subdisciplines of each with composite names (geochemistry,

geophysics, etc.). It is the study of the chemical and engi-

neering properties of the materials of the Earth—rocks, min-

erals, soils, fluids (water, oil, etc.), and gases. It is the study

of the internal and external processes that affect the planet,

including plate tectonics, deformation, surface process

(geomorphology), magma generation and movement, and

even environmental processes. It is also the study of the his-

tory of the planet, the internal and external changes that have

Static electricity generated during an eruption produces lightning. (Courtesy of the USGS)

geology 95

taken place through time, and the history of life through fos-

sils (paleontology). In the 19th century, geology was by far

the most influential of the sciences, and most scientists were

geologists. In the 20th (and 21st) century, humans relied

on the expertise of geologists to provide virtually all energy

needs. It is geologists who find all of the oil, natural gas, and

nuclear energy. Now geologists have added environmental

analysis and cleanup to their repertoire. Considering that

when a particle, gas, or fluid settles to the ground or pene-

trates the surface, it becomes the realm of the geologist, virtu-

ally all of the Earth is covered by this science.

geomorphology The study of surface landforms and their

evolution. These forms are grouped by the type of environ-

ment in which they occur. Deserts form dunes and ripples,

mesas and buttes, and related features. Beaches form dunes,

spits, and strands of sand. Glacial environments include

outwash plains, gravel piles and other features. Rivers have

gravel bars and sand. The landforms and surface deposits are

in the realm of geomorphology.

geophysical monitoring Monitoring of volcanoes using

geophysical techniques to predict eruptions. Swarms of

microearthquakes or small earthquakes record the move-

ment of magma upward in the magma chambers prior to

eruption. By monitoring seismic activity, geologists can pre-

dict eruptions and recommend evacuation.

geophysicist Scientist who studies the physical properties

of Earth. It is the geophysicist who monitors seismic activ-

ity on faults. They determine the source and magnitude of

earthquakes as well as the resulting damage. Their work is

used to construct seismic risk maps. Eventually, their studies

may lead to reliable earthquake prediction. Geophysicists also

do the geophysical and topographic monitoring of volcanoes

to help predict eruptions.

geophysics Science of the physical properties of Earth

including seismology, the study of earthquakes.

Georgia United States Although the state of Georgia is not

itself highly susceptible to earthquakes, it has been affected

strongly on occasion by powerful earthquakes in neighbor-

ing states, such as the New Madrid earthquake and the

Charleston, South Carolina, earthquake. An earthquake

in northern Georgia on November 1, 1875, affected an area

of about 150 by 200 miles (241 by 322 km). Portions of the

state, especially along its coast, are vulnerable to damage

from lique-faction in the event of future strong earthquakes.

geothermal energy Geothermal energy may be defined

as energy that is naturally given off by Earth. Sources of

enhanced geothermal energy may include geysers, hot

springs, and volcanoes. Although facilities for exploiting

geothermal energy have been built at numerous locations in

several countries. The potential for utilizing this source of

energy appears to be unlimited. Considering that at just a few

feet underground, the temperature remains a constant 55°F.

There is great potential for supplemental cooling in the sum-

mer and heating in the winter. Obviously, other systems must

be utilized as well, but it is much easier to warm a house from

55°F than from 25°F. On the other hand, in some areas, the

geothermal energy can provide all or nearly all of the energy

needs of a community. In these areas, heat from within Earth

is concentrated in formations at or near the surface where the

heat is stored and can be tapped conveniently.

The geology of geothermal energy resources is complex,

but commercially exploitable geothermal resources tend to be

concentrated along certain areas. Of particular interest are

areas of crustal spreading, where new crust solidifies from

molten rock rising to the surface, and along plate boundar-

ies, where converging plates give rise to conditions favoring

increased heat flow to the surface. In either case, magma

rising from below brings large amounts of heat toward the

surface. Heat from the magma converts underground water,

whether natural or introduced, into geothermal systems,

which may be dominated either by liquid or by vapor. The

former tend to be high-pressured, whereas the latter are com-

paratively low-pressured. Geothermal systems have been used

for centuries on a small scale for heating, but application of

geothermal energy to other uses began only in the late 18th

and early 19th centuries, starting in Italy.

Development of geothermal energy resources in New

Zealand began shortly after World War II, and the first

such facility there was finished in 1958, at the Wairakei

fields. Wairakei derives its energy from a huge volume of hot

rock believed to be supplied with water by rainwater seep-

ing down from the surface. Development is thought to have

touched only part of New Zealand’s geothermal energy

resources. Geothermal heating facilities are widespread in

Iceland, which lies along the Mid-Atlantic Ridge and

is the site of intense and frequent volcanic activity. In this

arrangement, hot water from underground is distributed from

central locations to users and achieves good results with less

pollution that other heating systems based on combustion

of fossil fuels. Geothermal heating in Iceland also is used in

greenhouses and in various industrial processes, such as dry-

ing seaweed and washing wool. The preeminent geothermal

power facility in the United States is located at the geysers

in northern California near San Francisco and is oper-

ated by Pacific Gas and Electric Company. Other sources of

geothermal energy exist at numerous locations in the United

States, and some are used for heating, but other factors, such

as distance from major population centers, restrict the avail-

ability of these heat sources for generation of power or other

large-scale commercial applications. Japan and Mexico also

have, or have plans to develop, geothermal power facilities.

Generating electricity using geothermal energy causes

various problems such as mineral-rich water corroding metals

and building up on turbines. In some locations, earthquakes

are associated with high heat flow from Earth’s interior.

Sometimes the earthquakes are only minor, but some may be

much more powerful. Earthquakes in geothermal areas are

thought to be connected to motion on faults along which

geothermal fluids flow. Volcanic activity also is associated

with areas suitable for geothermal power production because

volcanoes themselves constitute areas of high heat flow.