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

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Lituya Bay Alaska, United States Located on the south-

eastern coast of Alaska, Lituya Bay is famous for the

destruction caused by a wave that resulted from a landslide

associated with a powerful earthquake in 1958. A rockslide

along the shore of the bay set off a wave that destroyed tim-

ber as high as 1,720 feet (524 m). At various other locations

along the shore of the bay, the wave swept away forest at

elevations of 600 (183 m), 160 (49 m), 130 (40 m), and 90

feet (27.4 m). An island in the bay was denuded of timber up

to an elevation of 80 feet (24.4 m). Three boats were in the

bay at the time of the incident. One was destroyed, the wave

carried the second across a spit at the mouth of the bay and

into the ocean, and the third boat survived the wave in the

shelter of the island mentioned earlier. Return waves in the

bay formed a small seiche.

Liwa earthquake, southern Sumatra, Indonesia On Febru-

ary 15, 1994, an earthquake of magnitude 6.6 occurred. At

least 207 people were killed, and 2,000 were injured. Between

direct damage, mudslides, and landslides, more than 6,000

homes and buildings and 75,000 people were left homeless.

Damage is estimated at $169 million.

local magnitude The recorded magnitude of an earth-

quake less than 62 miles (100 km) from epicenter. Because

seismic waves move away from the earthquake in a spheri-

cal front, they quickly lose their destructive power. It is

like the rings that form on a lake when a rock is thrown in.

The height of the wave is high when the ring is small near

the source. As the ring grows larger, the height of the wave

becomes progressively smaller very quickly. The reason is

that all of the energy must be spread over a rapidly increasing

circle. At any place, energy reflected in amplitude of the wave

must be small. Seismic waves do the same thing. The local

magnitude really monitors the area around an earthquake

where the greatest destruction occurs because that is where

the waves have the greatest magnitude.

locked fault A fault may be locked if the frictional strength

exceeds the applied stress for a long period of time. Typically,

something prevents an active fault from slipping in a seg-

ment, and there are no earthquakes for a much longer than

usual period of time. An asperity, an intrusion or other rock

body, or intersecting faults are the most common culprits. In

some cases, a bulge or elevated area may develop over the

locked segment. This is the suspected case in the Palmdale

bulge along the San Andreas Fault in California. The prob-

lem is that when the stress finally overcomes the obstruction

to movement, the resulting earthquake is much stronger than

usual.

loess Loess is windblown dust from deserts or other arid

regions that accumulates on downwind slopes in non-arid

areas. This fine dust is eroded by wind and carried in suspen-

sion until it encounters an area of lower wind velocity. Large

hills and mountains can create enough of a wind shadow to

allow deposition locally. Otherwise, the dust remains aloft

and can circulate in the atmosphere for a long period of time.

Where it accumulates as loess, typically on slopes, it can be

quite thick but not particularly stable as the result of the fine

size. China has particularly thick loess deposits. It is depos-

its that have failed in strong earthquakes, destroying both the

communities that inhabited them as well as those in the val-

leys below. Some of the largest death tolls from earthquakes

occurred in the Gansu province from this process.

Loihi Seamount Hawaiian island chain, United States The

Loihi Seamount is located southeast of the island of Hawaii

and is some 3,000 feet (914 m) below sea level. It is the

youngest volcano in the Hawaiian chain. The seamount

appears to have undergone recent volcanic activity and has

two craters on its summit. Earthquake activity under Loihi

in the 1970s may have been linked with eruptions. It is esti-

mated that Loihi will reach the surface of the Pacific Ocean

in a few tens of thousands of years.

Lokon volcano, Sulawesi (Celebes), Indonesia An andes-

ite stratovolcano with at least 21 historic eruptions. The

most recent eruption was in 1992. Most eruptions are small

to moderate (VEI = 2) and involve pyroclastic emissions.

Loma Prieta earthquake, California Almost equal in fame

to the 1906 catastrophe was the less powerful but nonetheless

highly destructive earthquake that struck the San Francisco

Bay area on October 17, 1989. This earthquake was the larg-

est in northern California since the San Francisco earth-

quake 83 years earlier. In terms of damage, the earthquake is

thought to have been the single most expensive natural disas-

ter in U.S. history up to that time. Estimates of damage are

as high as $10 billion. Known as the Loma Prieta earthquake

after a mountain near its epicenter, the 1989 disturbance was

centered just south of San Jose, in the vicinity of Gilroy, Wat-

sonville, and Santa Cruz, and was caused by movement along

the San Andreas Fault at a depth of more than 11 miles

The bare area on the right side of the photo was once as forested as

the left side of the photo before the immense 1957 seiche stripped away

the trees 1,700 feet (524 m) up the hillslope facing onto Lituya Bay,

Alaska. (Courtesy of the USGS)

152 Lituya Bay

(18 km). The rock along the western side of the fault moved

northward several feet. The earthquake occurred at 5:04 p.m.,

measured 7.1 on the Richter scale, lasted some 15 seconds,

and is thought to have been the most powerful earthquake in

California since the Kern County earthquake in 1952.

Although media coverage concentrated on destruction

in San Francisco, where structures built on unconsolidated

material suffered serious damage, destruction from the Loma

Prieta earthquake was greatest within a radius of about 20

miles (32 km) from the epicenter. Buildings collapsed in

Hollister, Gilroy, and Santa Cruz, and major damage was

reported in Watsonville as well. A portion of Highway 101

collapsed over a slough near Watsonville. The earthquake

reportedly shifted hundreds of frame homes in Santa Cruz

County from their foundations and blocked highway access

to the community of Santa Cruz, which was cut off by rock-

falls and damaged bridges. Collapsing walls of aged build-

ings allegedly killed three people in Santa Cruz. Damage was

light at the University of California at Santa Cruz but rela-

tively heavy at nearby Stanford University, where destruction

was estimated at $165 million. The ground cracked in many

locations near the epicenter in the Santa Cruz Mountains, but

no signs of surface rupture along the fault itself were found,

although one estimate put the length of the underground

rupture from this earthquake at about 30 miles (48 km).

One crack near Highway 17 measured more than 600 yards

(550 m) long and several feet wide. Sixty-seven people were

reported killed by the Loma Prieta earthquake.

Loss of human life was concentrated in western Oakland,

on the east side of the bay, where a portion of Interstate 880,

some 1.5 miles (2.4 km) long and called the Cypress Viaduct,

collapsed and crushed dozens of vehicles and their occu-

pants as the upper level of the two-level highway fell on the

lower level. Survivors on the lower level were few; although

one man was found alive 89 hours after the collapse, he died

soon afterward. Another collapse occurred on the Bay Bridge

between San Francisco and Oakland, where a 50-foot (15-m)-

long segment of the bridge’s upper deck fell onto the lower.

Heavy damage was also reported to the Embarcadero Free-

way in downtown San Francisco, and damage including land-

slides and harm to bridges forced the closing of Highways 1

and 101 south of San Francisco.

The earthquake interrupted the third game of the World

Series at Candlestick Park. Fires from a ruptured gas main

consumed portions of the Marina district, but the fires were

brought under control within hours. The Marina district also

exhibited widespread damage from liquefaction because the

district was built on fill laid down early in the century. Other

Aerial photograph of the collapsed Cypress viaduct of route I-880 in Oakland, California, as a result of the 1989 Loma Prieta earthquake. This collapse

caused most of the casualties suffered during this event. (Courtesy of the USGS)

Loma Prieta 153

areas of heavy damage included portions of the Mission dis-

trict and the area south of Market Street. Significant damage

also occurred in the Mission, Haight, Sunset, and Tenderloin

neighborhoods.

Lomo Poisentepe See Apoyo.

Long Beach See California.

Long Island caldera, Papua New Guinea The Long Island

volcanic complex is located on the northern coast of Papua

New Guinea and includes two stratovolcanoes, Cerisy

Peak and Mount Reaumur, on opposite sides of Lake Wis-

dom, a caldera lake some six miles (9.5 km) wide. Motmot

Island, a small island within the lake, was built by volcanic

activity within historical times. Powerful seismic and volca-

nic events appear to have occurred at the caldera some 300

years ago, but records were not kept on a scientific basis. An

eruption in 1660 is believed to have been as powerful as the

Krakatoa eruption of 1883. Eruptions occurred in 1933,

1938, 1943, 1953, and 1955 from a vent or vents under

the lake and built up Motmot Island. Discolored water in

the lake, accompanied other volcanic activity in 1968, 1974,

1976, and 1993.

long-period wave A seismic wave with a period of six sec-

onds or greater. The period of a wave is the time it takes for

an entire wave to pass a point. If you were standing in the

ocean, it would be the time it takes between the wave crests

that hit you. Long-period waves mean a slow rise and fall as

they pass. These waves tend to be less damaging.

Long Valley caldera, California, United States This cal-

dera near Mammoth Lakes appears to have undergone a

series of eruptions 500 to 600 years ago that produced rhyo-

lite domes in and near the caldera. It is perhaps the largest

modern rhyolitic centers in North America. A much earlier

eruption (about 3.6 million years ago) is thought to have

been so powerful that it sent a pyroclastic flow rushing up

the flank of the Sierra Nevada and over the mountains. The

output from this eruption is believed to have flowed down

Owens Valley for some 40 miles (64 km) and to have covered

almost 600 square miles (1,554 km

) of land in the present

states of California and Nevada with thick Ash layers. A

famous deposit called the Bishop Tuff appears to date from

this eruption. The Ashfall from this eruption has been

traced into what is now Nebraska. The eruption is thought

to have forced some 150 cubic miles (615 km

) of magma to

the surface in the form of Plinian eruptions. This volume of

ash makes the Long Valley eruption among the largest docu-

mented.

Eruptions in the area continued at Mono-Inyo Craters

volcanic field. The most recent eruption was at Mono Lake in

1720 and 1850. Earthquake activity and uplift in the caldera

during the 1980s provided evidence that magma was moving

underground at that time. In 1994, dying forests were found

to be attributed to carbon dioxide concentrations of 30% to

96% within the soil.

Los Alamos New Mexico, United States Los Alamos is

built on a volcanic plateau made of ashflow deposits laid

down in an eruption that is believed to have resulted in the

collapse of the volcano summit and the formation of a cal-

dera, now called Valle Grande.

Los Angeles earthquakes, California, United States The

Los Angeles area has been the location of numerous earth-

quakes, some of which in historical times have been powerful

and destructive. One of the first earthquakes experienced by

European-descended Americans in this part of California

occurred during a visit by Captain Gaspar de Portola, gover-

nor of Mexico, in July 1769. Another series of earthquakes

in 1812 caused such widespread damage to Spanish missions

in southern California that 1812 came to be known as “el

año de los temblores,” or “the year of the earthquakes.”

Other notable earthquakes in southern California history

include the Fort Tejon earthquake of 1857; the San Jacinto

earthquake of 1899; the Imperial Valley earthquakes of

1915 and 1979; the San Jacinto earthquake of 1918; the

Santa Barbara earthquake of 1925; the Long Beach 1933

earthquake; the Terminal Island earthquakes of 1949, 1951,

1955, and 1961; the Kern County earthquake of 1952; the

San Fernando earthquake of 1971; the Santa Barbara

earthquake of 1978; and North Ridge earthquake of 1994.

It is widely assumed that an earthquake of unprecedented

destructive potential is possible in the greater Los Angeles

area, although it is difficult to predict when and where it may

occur. Projected casualty figures from the Federal Emergency

Management Agency (FEMA), in the case of a postulated

earthquake of magnitude 8.3 on the San Andreas Fault, put

deaths at 3,000 to 12,500 and hospitalized injured at 12,000

to 50,000, with 52,000 long-term homeless. The same agency

projected figures for another postulated earthquake, of mag-

nitude 7.5 on the Newport-Inglewood Fault Zone, at 4,400

to 21,000 killed and 17,600 to 84,000 hospitalized injured,

with 192,000 long-term homeless.

The Marina district of San Francisco, California, suffered much worse

destruction than the rest of the city during the 1989 Loma Prieta

earthquake. The destruction shown here at Beach and Divisadero Streets

is a good example. (Courtesy of the USGS)

154 Lomo Poisentepe

The greater Los Angeles area is highly susceptible to

earthquakes because of a complex and unstable geology

generated by the collision between northward-moving Baja

California and the deeply rooted Sierra Nevada range. This

collision has produced Southern California’s Transverse

Ranges and a network of active faults, some of which have

the potential to generate highly destructive earthquakes.

Prominent fault zones in the Los Angeles area include the

following:

• The Elsinore Fault Zone extends almost 120 miles (192

km) northwest from the Mexican border to the northern

boundary of the Santa Ana Mountains.

• The Newport-Inglewood Fault Zone is actually a collection

of small faults along a line extending past Newport Beach

and Santa Monica all the way into Baja California. The

powerful Long Beach Earthquake of 1933 occurred along

this fault zone.

• The Palos Verdes Hills Fault lies about five miles (8 km)

west of Long Beach.

• The San Andreas Fault Zone is the most famous fault

affecting the Los Angeles area and extends approximately

along a line connecting Palmdale, San Bernardino, and

Banning and then along the southern boundary of the Little

San Bernardino Mountains. The San Andreas Fault Zone

was the site of the Fort Tejon earthquake of 1857.

• The San Pedro Basin Fault Zone extends approximately

from Point Dune down the San Pedro Channel to the east

of Catalina Island.

• The small but nonetheless dangerous San Fernando Fault,

about 20 miles (32 km) south of the San Andreas Fault,

was responsible for the destructive San Fernando earth-

quake of 1971.

Louisiana earthquakes, United States Destructive earth-

quakes originating within Louisiana itself appear to be

infrequent, although the state occupies the lower end of the

seismically active Mississippi Valley. A strong earthquake on

October 19, 1930, damaged buildings at Napoleonville, and

another earthquake on November 19, 1958, caused alarm in

Baton Rouge.

Love wave Named after the British physicist A. E. H. Love

(1853–1940), Love waves are earthquake waves that are

restricted to shallow depths near ground surface. The motion

is horizontal and perpendicular to the direction of travel,

with no vertical motion involved. They are very destructive

waves that cause structures to fall over sideways as the result

of their side-to-side movement.

low velocity zone At 37–155 miles (60–250 km) depth

beneath Earth’s surface, seismic velocities decrease abruptly.

At both shallower and deeper levels, velocities increase con-

sistently. The reason that the velocity drops at this depth is

because the rocks behave in a ductile manner there. Above

it rocks are brittle. Considering that seismic waves are

elastic, ductile deformation is not conducive to high speeds.

The response of the gum-like material is much slower than

the rubber bandlike response of the brittle rocks. In terms

of plate tectonics, the low velocity zone corresponds to the

asthenosphere. Lithospheric plates float on this layer.

Luzon earthquake, Philippines At 7:26 a.m. on July 16,

1990, one of the most infamous earthquakes in the history

of the Philippines struck the city of Luzon. The main shock

had a magnitude of 7.8 and was followed by numerous

aftershocks for many months. The death toll was 1,621,

with more than 3,100 injuries reported. It could have been

far more devastating if it had occurred later in the day. land-

slides, liquefaction, and abrupt subsidence caused exten-

sive damage to buildings, roads, and utility services. Total

damage was estimated at $2 billion.

Luzon 155

maar Also known as a tuff cone, a maar is a shallow, flat-

floored crater that is believed to have formed above diatremes

as the result of a violent explosion of gas or steam. Maars

are typically 200–6,500 feet (60–1,950 m) across and 30–650

feet (9–195 m) deep. The craters are commonly filled with

water, forming lakes. The rims of maars are low and consist

of a mixture of fragments of volcanic rock and rocks from

the walls of the diatreme.

Macdonald Seamount Tubuai Islands and Seamounts,

South Pacific Ocean This seamount reaches to within about

100 feet (30 m) of the surface. Its activity was discovered in

1967 when undersea hydrophones picked up noise from it.

Bubbles of gas and fragments of basalt were seen issuing

from the seamount in 1987.

mafic A compositional term for magma and lava, meaning

that they are rich in iron and magnesium. The word derives

from ma for magnesium and f for Fe (iron). Mafic rocks

include basalt for volcanic rocks and gabbro for plutonic

rocks. These rocks are composed of pyroxene and calcium-

rich plagioclase but commonly include olivine as well.

They are very dark in color. Rocks of this type are formed at

very high temperatures relative to other igneous rocks.

magma Molten rock underground is generally known as

magma, as distinct from lava, which is molten rock that has

emerged onto the surface from a volcanic vent. Magma dif-

fers greatly in composition and behavior. Some magmas are

high in dissolved gases and, when they reach Earth’s surface

and the high subsurface pressures on them are removed,

they form the frothy rocks pumice and scoria as dissolved

gases bubble out of solution as the rock hardens. Such gas-

rich magma tends to produce explosive eruptions because of

its high gas content. Alternatively, magma that is relatively

low in dissolved gases may emerge from vents as streams or

floods of highly fluid lava that may flow across the land for

many miles before solidifying and halting. Rocks produced

by such eruptions include basalt. Great flows of basalt from

eruptions of this kind may be found in the Pacific Northwest

of the United States, especially the Columbia Plateau and

Snake River Plain. When magma solidifies underground, it

may form plutons such as dikes (vertical walls of igneous

rock created when magma enters a vertical crack in subter-

ranean rock and solidifies there) or sills (shelves of igne-

ous rock that form in similar fashion but along strata of

country rock). Masses of magma that solidify at depth are

known generally as plutons. When magma in the chimney of

a volcano solidifies and remains standing after the external

layers of the volcano have been eroded away by wind and

water, the resulting columnar structure is known as a vol-

canic neck. Where magma reservoirs near the surface heat

groundwater, the result may be an area suitable for geo-

thermal energy production. Movement of magma under-

ground is thought to be responsible for minor earthquakes

that may signal the imminent eruption of a volcano.

magma chamber The store of magma beneath a volcano

that provides the molten rock for lava and ejecta during an

eruption. Magma is ever-present in these large chambers up to a

kilometer beneath the surface. It can remain there for long peri-

ods of time without an eruption taking place. However, if it is

charged with additional magma or volatiles (especially water)

from deeper or even laterally within the earth or it is squeezed

by local stress, the magma may ascend to the volcano. This

process produces an eruption. Pulses of magma may feed the

chamber periodically and drive the eruptions. Movement of

the magma in the chamber is accompanied by an earthquake

swarm as the magma opens fractures. If all the magma is

drained from the chamber, the volcano may undergo caldera

collapse in which massive subsidence of the complex occurs. If

the magma crystallizes underground, it forms a pluton.

magmatic arc In a convergent boundary between oce-

anic crust and continental crust, an igneous belt devel-

ops over a subduction zone. This igneous belt is analogous

157

to an island arc on ocean crust, except that a magmatic arc

is on continental crust. These arcs exhibit extensive volca-

nic activity like island arcs but much more plutonic activity.

The prime example of a magmatic arc is the Andes Moun-

tains of South America. The volcanic rocks are andesites

but dominantly dacites and even rhyodacites in some cases.

The plutonic rocks are primarily granodiorites but gran-

ites are not uncommon. These extensive plutons coalesce to

form huge batholiths exemplified by the Sierra Nevada

of California.

magnitude A measure of the energy released in an earth-

quake.

See also Earth, internal structure of.

mare On Earth’s Moon, a plain of igneous rock pro-

duced by lava flows following a major meteorite impact is

known as a mare (plural, maria), from the Latin word for sea.

Viewed from Earth, the lunar maria resemble seas, although

the effective lack of atmosphere on the moon prevents liquid

water from existing there. The lunar maria are thought to be

relatively recent additions to the lunar surface because they

exhibit few impact structures (craters), a sign that their sur-

face is younger than other, more heavily cratered areas. None-

theless, these seas of basalt are more than 3 billion years old.

Earth may have had a similar history in its early development.

Mars Although little is known about the geology of Mars

compared to that of Earth, volcanism is known to have

played a large part in shaping the surface of the red planet.

Unmanned probes to Mars have revealed the existence of

large volcanoes, such as Olympus Mons and the volcanoes

along nearby Tharsis Ridge in the northern hemisphere.

Olympus Mons is the largest volcano in the solar system to

the best of our knowledge. Although volcanic mountains

are evident on its surface, Mars appears to lack the pattern

of shifting and colliding crustal plates that characterizes tec-

tonic and volcanic activity on Earth.

It has been suggested that material expelled through the

volcanoes of Tharsis Ridge came from magma underlying

Valles Marinensis, a great valley thought to have been formed

by subsidence as molten material underground was removed

through eruptions. Volcanism evidently has given the north-

ern hemisphere a geomorphology differing from that of

the southern hemisphere in ways besides the mere existence

of volcanic mountains. Impact cratering is much more visible

in the southern hemisphere, whereas in the northern hemi-

sphere, tephra from eruptions apparently has covered many

impact craters and other such features. The Martian volca-

noes affect the planet’s meteorology on a small but interest-

ing scale; clouds form in the lee of Olympus Mons and are

thought to have been observed through telescopes on Earth

at various times before probes reached Mars. Mars, like the

moon, appears to be much quieter than Earth from a seismic

viewpoint. Evidence of erosive activity on the Martian sur-

face indicates that Mars once possessed substantial amounts

of liquid water and a denser atmosphere than at present.

Although the Martian atmosphere is only a fraction as dense

as that of Earth, occasional dust storms on Mars are capable

of transporting large quantities of sediment aerially. These

storms are capable of hiding even the giant Martian volca-

noes from view.

marsh gas See methane.

The 1972 Managua earthquake caused the collapse and complete

destruction of this three-story, reinforced concrete Customs House office

building. (Courtesy of the USGS)

Tyrrhena Patera is an example of a Martian volcano with large valleys

on the sides. The valleys are interpreted to have formed as the result of

water rather than lava flow. There must have been more water on Mars at

one time. (Courtesy of NASA)

marsh gas 159

Marum, Mount See Ambrim.

Maryland United States The state of Maryland is not espe-

cially noted for earthquake activity, but the state has experi-

enced substantial earthquakes on occasion. The eastern portion

of the state has a considerable potential for damage from liq-

uefaction in any future major earthquakes because much of

the urbanized area of eastern Maryland is built atop moist,

unconsolidated soil that might be expected to lose coherence

in a strong earthquake. Maryland sometimes has felt shocks

that originated in adjacent Virginia. On April 24, 1758,

Annapolis experienced an earthquake (also felt in Pennsylva-

nia) that lasted about a half-minute and followed noises from

underground. Shocks in Harford County, Maryland, on March

11–12, 1883, made clocks stop. On January 2, 1885, Frederick

County was shaken by an earthquake that also was felt in Vir-

ginia and knocked small objects off of shelves and furniture;

this earthquake was reportedly silent except for rattling noises

from windows. There is a likely impact structure in Chesa-

peake Bay that may localize some of the activity.

Masaya volcanic complex, Nicaragua The Masaya com-

plex, a caldera with three small stratovolcanoes in its

center (Masaya, Nindiri, and Santiago), has been reported

active since the 16th century. It is the most active volcano

in the region with at least 19 eruptive periods since 1524.

The most recent eruption was in 1999. Although most

Nicaraguan volcanoes have the steep sides characteristic of

pyroclastic cones, Masaya has a wide, low configuration

(shield volcano), having been built up from basaltic lava

flows without a great amount of explosive eruptions. A lava

lake like that of Kilauea in Hawaii formed in Masaya in the

15th century and again from 1965–1979. Masaya’s activity

has also included lava flows and explosions. It also has a

history of sulfur dioxide emissions.

Masaya, however, is most famous for a huge eruption in

4,550 b.c. It had a VEI of 5 and was one of the largest erup-

tions in the last 10,000 years.

Mascara earthquake, northern Algeria On September 18,

1994, an earthquake of magnitude 5.9 occurred. At least

159 people were killed, 289 were injured, and more than

10,000 were left homeless.

Mashu¯ See Kutcharo.

Massachusetts United States The state of Massachusetts

is located in New England, one of the most seismically active

regions of the United States, and has a history of strong

earthquakes dating back to colonial times. Some of the most

powerful earthquakes in United States history have occurred

in Massachusetts. Eastern Massachusetts in particular has a

history of strong earthquakes, often accompanied by remark-

able acoustical effects, such as rumbling noises. In addition

to earthquakes that originate within Massachusetts, the state

sometimes undergoes earthquakes originating in other areas

of New England and also along the Saint Lawrence Valley

between the United States and Canada. Portions of Massa-

chusetts, especially in the east, appear highly susceptible to

damage in any future major earthquake because parts of the

large urban area in and around Boston are built on moist,

unconsolidated soil that would be vulnerable to liquefac-

tion as earthquake waves passed through it.

Possibly the greatest earthquake in the history of Mas-

sachusetts occurred on November 18, 1755. This earthquake

is thought to have had an epicenter east of Cape Ann and

a Mercalli intensity of approximately VIII. The earthquake

was felt over an area of some 300,000 square miles (776,996

), from Nova Scotia to Chesapeake Bay, and some 200

miles (322 km) at sea, where a ship in deep water experienced

a shock that made it feel as if the vessel had run aground.

(Vessels in harbors along the shore had much the same

experience.) Starting with a sound like thunder, the earth-

quake made it difficult to keep one’s footing so that people

caught in the earthquake had to hold on to nearby objects

to keep from being thrown down. In Boston, chimneys fell,

and the ground motion reportedly resembled that of waves

at sea. Treetops swayed vigorously, weather vanes fell from

buildings, and stone walls were knocked over. In parts of

Massachusetts, cracks formed in the earth, and sand moved

upward through the cracks to the surface. One curious effect

of this earthquake was that it appears to have killed fish in

great numbers along the seacoast. Another powerful shock

occurred on November 22, 1755, and still more shocks barely

a month later on December 19.

mass wasting Mass wasting encompasses all downhill

movement of mass, including soil and rock, in response

to gravity. These movements are generally classified based

upon composition and velocity. If the dominant material is

rock, the movement may either be a rockfall or a rock-

slide, depending upon the mode of movement. Otherwise,

mass wasting of debris and soil is generally divided by type of

movement, whether fluid-saturated (fluid-type flow) or fluid-

bearing (coherent or debris-type movement). The categories

of mass movement in each of these types are subdivided by

velocity. The slowest coherent movement is creep, transition-

ing to slump, debris slide, debris avalanche, and stur-

zstrom with increasing velocity. Water-saturated flows are

solifluction at its slowest, transitioning to earthflow and

mudflow with increasing velocity.

LANDSLIDE is the general

term for several of these mass movements.

Matsushiro caldera, Japan The Matsushiro caldera

is located on the island of Honshu¯ and has not experienced

any eruptions within historical times, but earthquake activ-

ity is frequent in the vicinity of the caldera and has received

extensive study. There are various opinions on what may be

happening under the caldera. One view is that intrusion of

magma may be involved. Other processes, including hydro-

thermal activity, have been suggested to account for unrest

at Matsushiro.

Mauna Kea volcano, Hawaii, United States Mauna Kea is

located on the island of Hawaii and is the tallest mountain

on Earth, more than 30,000 feet (9,144 m) from its base on

the seafloor to the summit. Unlike Mauna Loa and Kilauea,

also on Hawaii, Mauna Kea is not active.

160 Marum, Mount

Mauna Loa volcano, Hawaii, United States Mauna Loa,

a basaltic shield volcano located on the island of Hawaii,

is one of the most active volcanoes on Earth but also one of

the least dangerous because its lava is low in dissolved gases.

Therefore, it is not given to producing violent, explosive

eruptions. lava flows from Mauna Loa occur every several

years on the average, tend to last only a few days, and have

amounted to more than 3 billion cubic yards (2.4 billion m

)

in the past century. Lava emerges from the summit crater

(called Mokuaweoweo), from rift zones that extend from

that crater, and from two pit craters, South Pit and North

Bay, adjacent to the main caldera that was produced by

subsidence. One of five shield volcanoes that make up the

island of Hawaii, the largest island in the Hawaiian chain,

Mauna Loa has erupted on numerous occasions since records

of eruptions began to be kept in the early 19th century. An

impressive caldera on the summit of Mauna Loa is approxi-

mately two to three miles (3.2 to 4.8 km) in diameter and

formed several hundred years ago when collapse accompa-

nied the withdrawal of magma from an underground cham-

ber. The caldera walls stand about 600 feet (183 m) high in

some places.

In the late 1800s and the first half of the 20th century,

Mauna Loa was very active. The volcano fell quiet in 1950

but became active again in 1975. An eruption in 1984 was

associated with precursory earthquakes. Relatively little

is known about activity at Mauna Loa compared with the

nearby Kilauea volcano because Mauna Loa was quiet

from 1950 to 1975 when methods for studying volcanic

activity were advancing. Earthquake activity and other evi-

dence usually provide abundant warning that an eruption is

forthcoming, and geologists have had considerable success

in predicting the volcanic eruptions. Eruptions of Mauna

Loa are notable for “curtains of fire,” fountainlike erup-

tions of extremely fluid lava that may last a day or so before

subsiding.

On occasion, lava flows from Mauna Loa have threat-

ened the nearby city of Hilo. One flow in 1881 stopped only

at the edge of town, and another lava flow approaching Hilo

in 1935 made history because of an attempt to divert the lava

by aerial bombardment. The U.S. Army Air Corps, on the

advice of the director of the Hawaiian Volcano Observatory,

bombed the lava flow at selected points in hopes of changing

its direction. The theory behind this tactic was that bombing

might release gas and molten rock beneath the “crust” of the

lava flow and thus stop its advance. The bombing appeared

to succeed, for the lava flow halted soon after the bombard-

ment. There is disagreement, however, about whether the

bombing actually stopped the lava or whether the lava flow

was about to cease anyway.

Mayon volcano, Philippine Islands The beautiful strato-

volcano Mayon has a long history of destructive eruptions.

It has erupted 47 times since 1616. Of these, 18 produced

NUÉE ARDENTEs and 12 resulted in fatalities. The 1616 erup-

tion was extremely violent and is said to have destroyed doz-

ens of villages near the volcano. In 1766, another eruption,

characterized by flows of hot mud down the volcano flanks,

killed some 2,000 people. The 1814 eruption of Mayon

caused at least another 2,000 deaths. Major eruptions took

place in 1886, 1888, and 1897. In the 1897 eruption, approx-

imately 400 people are believed to have died. Eruptions in

1914 and 1928 were less costly in terms of lives lost. The pre-

vious eruption was in 1993. It killed 68 people and prompted

the evacuation of 60,000 others. The latest eruption lasted

from 1999 to March 2000 and resulted in an evacuation of

more than 10,000 but no deaths.

Mayon is steeped in legend. Magayon was the jealous

uncle of a beautiful princess. As she escaped with a lover,

Magayon chased her. The gods sent a landslide that buried

Magayon. His anger now wells up as eruptions from Mayon.

Chart showing the types of mass movements and their conditions of

velocity and water content

Mayon 161