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

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earthquake about 120 miles (193 km) east of Rabaul in 1983,

earthquake activity diminished in 1984 and 1985. Emissions

of carbon dioxide from a crater on Tavurvur killed birds in

1981, and fresh activity from fumaroles destroyed plants at

a site slightly more than one mile northwest of Tavurvur in

1983.

In view of the recent and historic unrest at Rabaul, there

is a chance that new eruptions will occur there in the near

future. Calderas apparently were formed by eruptions there

some 3,500 years and 1,400 years ago, and eruptions of

comparable magnitude were considered possibilities for the

years ahead. With such events in mind, scientists and public

officials prepared an emergency plan that would respond in

several stages, up to and including evacuation of the local

population, to a threatened eruption at Rabaul. On Sep-

tember 19, 1994, both Vulcan and Tavurvur began to erupt

simultaneously and without warning. The eruption was pow-

erful (VEI = 4) producing heavy amounts of ash (up to 30

inches [75 cm] deep) and forcing the evacuation of 50,000

people. Because of the plan, only one person was killed. The

eruption ended in 1995.

See also radioactivity.

Rahoum See Ambrim.

Rainier, Mount Washington, United States One of the

most famous volcanoes on Earth, Mount Rainier stands near

Seattle and is part of the Cascade Mountains, a volcanic

range associated with the Cascadia Subduction Zone off

the Pacific coast of northwest United States. Mount Rainier

is a stratovolcano and has been dormant since the mid-

1800s, when a minor eruption reportedly took place.

The most recent major eruption of Mount Rainier is

thought to have occurred approximately 2,000 years ago and

laid down a thick layer of tephra east of the mountain. This

eruption also is believed to have produced a lava cone at

the summit of the volcano, as well as large mudflows. One

mudflow, estimated to have occurred almost 3,000 years ago,

traveled down the South Puyallup River and Tahoma Creek

valleys as a mass of mud hundreds of feet thick; this same

flow is thought to have exposed the core of the volcano. In

any future eruptions, mudflows may recur on a large scale,

through melting of glacial ice on the peak, and may present

a threat to nearby communities. The potential for devastat-

ing avalanches from Mount Rainier is considerable because

internal heat is believed to have turned the mountain core into

soft material that might fail easily during a powerful earth-

quake (a distinct possibility in the seismically active Pacific

Northwest), releasing overlying material to fall downslope

The sleeping giant Mount Rainier overlooks an encroaching population

of western Washington. It poses one of the biggest volcanic threats in the

United States. (Courtesy of the CVO-USGS)

212 radioactivity

in vast amounts. Such an avalanche, in turn, might allow

an explosion comparable to the one that destroyed the area

around Mount Saint Helens during the eruptions of 1980.

The White River valley is believed to be especially vulnerable

to major avalanches from Mount Rainier, should conditions

ever permit them to occur. rockfalls said to have exceeded

10 million cubic yards (7.9 million m

) occurred in the White

River Valley in 1963.

Mount Rainier is noted for its steam caves, a network

of ice tunnels and chambers about 1.5 miles (2.4 km) long

altogether. The steam caves are produced by heat from the

volcano acting on firn, a moderately dense form of ice that

results when snow accumulating on the volcano is com-

pressed to a density not quite that of the crystalline ice found

in glaciers. Heat from the volcano melts spaces between the

ice and the rock of the mountain. Warm air circulating in

these spaces enlarges them. The result is a network of pas-

sageways within and beneath the ice. The steam caves rep-

resent a delicate equilibrium between fumarole activity

and the accumulation of snow and ice on the volcano. The

caves were discovered in 1870 when climbers took shelter in

them, although there is evidence that Native Americans knew

of the caves’ existence long before European-descended set-

tlers arrived in the Northwest. Most of the tunnel network

exists in the East Crater atop Mount Rainier, although the

steam caves extend also to the adjacent West Crater. Similar

systems of steam caves are found at nearby Mount Baker and

also at Wrangell in Alaska.

Ranau caldera, Sumatra, Indonesia Located on the

Semangko Fault Zone, the Ranau caldera is occupied in part

by Lake Ranau. The caldera has undergone some unrest but

no major eruptions during the late 19th and early 20th centu-

ries. On two occasions, in 1887 and 1888, large numbers of

dead fish were discovered on Lake Ranau and on its beaches.

The lake water was discolored and had an odor of sulfur.

The deaths of the fish were attributed to volcanic activity

in the lake. A powerful earthquake in 1895 reportedly pro-

duced seiches in Lake Ranau; dead fish were noted again on

the surface of the lake in December 1903. This fish kill was

accompanied by discoloration of the lake waters, as in 1887

and 1888, and a smell of sulfur was detected near the shore.

Rasshua volcano, Kuril Islands, Russia The Rasshua vol-

cano has been active on an intermittent basis since the early

19th century. There appears to have been some unrest at the

site in 1810, but details are unavailable. In 1846, there was

a moderate to large eruption (VEI = 3). Increased steaming

activity at Rasshua in 1946 preceded a similar increase in

activity at nearby Sarichev volcano, which erupted five days

later. fumaroles at Rasshua became considerably more

active in 1957, and small explosions were reported. Young

lava flows extend down the eastern, northwestern, and

southern flanks of the volcano.

Raung See Ijen.

Rayleigh waves These earthquake waves occur only along

Earth’s surface (i.e., surface waves). They make the ground

surface move like the waves on the ocean. This motion is

technically referred to as retrograde, which means that they

orbit in an elliptical pattern in the direction opposite to that

of vibrations from the earthquake epicenter. Such motion is

highly destructive but fortunately only in the area of the epi-

center. Energy of the waves decreases rapidly away from the

source. Throw a pebble in a lake, and watch the rings form.

They model Rayleigh waves. Rayleigh waves, also known as

ground roll, are named after John William Strutt, Lord Ray-

leigh (1842–1919), British mathematician and physicist.

Raymond Fault California, United States The Raymond

Fault is about five miles (8 km) long and is located several

miles southeast of Pasadena. The fault has exhibited surface

rupture within historical times and is suspected of being the

source of the Los Angeles earthquake of 1855.

Reaumur, Mount See Long Island.

recurrence interval A recurrence interval is the average

amount of time between earthquakes of a given magnitude

for a given area. There are different recurrence intervals

Crushing of the base of a column in an overpass during the 1971 San

Fernando earthquake. The damage done to the column is characteristic of

that done by a Rayleigh wave. The motion of the ground is like an ocean

wave. The return upward motion causes the crushing because it is held in

place by the overpass. (Courtesy of the USGS)

recurrence interval 213

for various-size earthquakes in an area and different recur-

rence intervals for the same-size earthquakes between areas.

A recurrence interval for a given city might be 50 years for

a magnitude 7.0 earthquake. Recurrence intervals can be

among the best tools for earthquake prediction because

the tectonic stress buildup for a fault is consistent and the

strength of the rocks does not vary. As long as the historical

records are old enough and complete, a recurrence interval

can be determined. The problem is that they are only accu-

rate to 15 or 20 years. This kind of uncertainty is usually not

acceptable to most people.

Redoubt volcano, Alaska, United States The stratovol-

cano Redoubt, located some 110 miles (173 km) southwest

of Anchorage, is one of the easternmost in the Aleutian

Islands. It underwent major eruptions several times in the

20th century including four moderate to large ones (VEI = 3)

since 1966. It has erupted at least 30 times in the last 10,000

years. Major explosive eruptions also have occurred in this

century at nearby Spurr and Augustine volcanoes. Redoubt’s

eruption of December 14–19, 1989, produced columns of

ash that rose to altitudes exceeding 40,000 feet (12,192 m)

and had serious effects on airline travel in the vicinity of the

volcano. The ashfalls from this series of explosive incidents

extended over the heavily populated Cook Inlet area and

inland to the vicinity of Fairbanks. A period of lava dome

building activity followed these ashfalls and produced more

than a dozen such domes over four months. Each of the

domes was destroyed by explosive eruptions that produced

ash clouds, although the clouds did less to disrupt air travel

than the clouds from the early phase of the volcano’s erup-

tion. More than 20 significant falls of tephra were observed

at Redoubt between December 14, 1989, and the end of

April 1990. A fall of tephra on December 15 included coarse-

grained deposits rich in pumice, but subsequent tephra falls

were generally characterized by ash-size material. Damage

and loss of revenue from ash and debris flows totaled some

$160 million, making this eruption second only to Mount

Saint Helens in cost in U.S. history.

Recent eruptions of Redoubt have allowed few direct

observations because of poor weather conditions and short

periods of sunlight in the winter. Studies of these erup-

tions have yielded considerable information, however, about

lightning in the clouds surrounding the volcano. Ordinar-

ily, lightning is rare in the vicinity of Redoubt, but lightning

discharges have been observed during most of the eruptions

of Redoubt that released ash. A commercial lightning detec-

tion system deployed by the Alaska Volcano Observatory

detected lightning in 11 of 12 eruptions and located the light-

ning strikes in nine of those eruptions, following the activa-

tion of the system early in 1990. Most of the strikes between

clouds and the ground occurred close to the mountain, and

the number of cloud-to-ground discharges was found to be

related to the amount of ash released. Lightning discharges in

the early stages of the eruption had a negative charge, whereas

discharges later in the eruption were positive. This pattern

indicates that coarse particles of ash, expelled early in the

eruption, carry a negative charge, and finer particles expelled

in later stages of the eruption carry a positive charge. “Intra-

cloud” strikes of lightning, within the volcanic cloud itself,

tend to be more numerous than cloud-to-ground strikes and

to occur even at great distances from the volcano. Intracloud

discharges during the February 15, 1990, eruption of Redoubt

occurred up to about 80 miles (128 km) from the mountain.

Intracloud lightning may prove useful in developing sys-

tems capable of tracking airborne volcanic clouds. Satellites

have been used to track ejecta from Redoubt in more than

20 eruptions since late 1989. Environmental observation

satellites in geostationary and polar orbits have monitored

emissions of ash and volcanic debris. Data provided by the

advanced very high resolution radiometer (AVHRR) on two

U.S. weather satellites in polar orbit were used to detect a

large mudflow along the Drift River near Redoubt as the

event occurred during the February 15, 1990, eruption, and

to provide warnings to Anchorage. Studies of AVHRR images

of eruptions of Redoubt and Augustine indicate that volca-

nic clouds can be detected under various weather conditions,

whether during day or night or over water or land. Multiple

infrared channels are useful here, but there is as yet no single

algorithm capable of detecting all eruption clouds.

See also seismology.

rift A breach or “crack” between two masses of crustal

rock that once were joined together. The process by which

continental crust is pulled apart and a divergent margin

is created. Rifting includes normal faulting (horsts and gra-

bens as well) and resulting earthquakes. It can lead to basalt

volcanism (flood basalts) and rhyolitic volcanism in

some cases. Eventually, it will create an ocean basin with a

mid-ocean ridge.

A dip-slip fault in which the rock on top of the fault plane (hanging wall)

moves up relative to the rock under the fault (footwall). Movement is

shown by offset of the rock layers across the fault. (Courtesy of the

USGS)

rift 215

rift valley A long, linear crack in Earth’s crust that occurs

where magma rising from below reaches the surface as

flood basalts and rhyolitic cones. They are also areas of

intense seismic activity related to movement on normal faults.

These faults form scarps along the valley walls. Drainage

into the basin leads to thick sedimentary deposits that com-

monly contain petroleum deposits. The Great Rift Valley

of Africa is a good example of a rift valley and is the early

phase in the development of a divergent margin.

right-lateral fault A type of strike-slip fault in which

the rock on the opposite side of the fault from the observer

moves to the right. They are also called dextral strike-slip

faults. The San Andreas Fault is a right-lateral strike-slip

fault.

Rincón de la Vieja volcano, Costa Rica Rincón de la

Vieja, “Nook where the Old One lives,” is the biggest vol-

cano in the northern part of Costa Rica and appears to be

located inside nested calderas. The volcano is made up of

several volcanic centers arrayed along a ridge. It has erupted

at least 16 times since 1851, most of which are phreatic.

Explosive eruptions were recorded between 1860 and 1863,

but details are unavailable. Explosive activity was observed

again at the volcano in April 1922, together with evidence of

a very powerful recent eruption. A strong eruption occurred

on June 4, 1922, preceded by sulfur dioxide emissions

and loud noises from fumaroles. In 1963, the volcano was

reported to be steaming and giving off sulfur gases. Vapor

emissions from the volcano in 1965 preceded an eruption

of ash in 1966. On a single day (February 23, 1968), more

than 20 eruptions occurred in a half hour. These eruptions

required that villages in the area be evacuated. Steam blasts

from the volcano occurred in 1983 that affected locations

more than a mile from the crater. Another eruption took

place in 1984, and more eruptive activity is thought to have

occurred in the autumn of 1985. Minor explosions were

recorded in 1986–87, 1992, 1995, and 1998. The 1995 erup-

tion forced the evacuation of 300 families.

ring dikes Circular dikes around a volcano. If magma is

removed from the magma chamber beneath a volcano, it can

undergo caldera collapse. The volcano and the area around

it collapse because they are no longer being held up by the liq-

uid. A series of concentric faults and cracks develop around

the collapsing volcano. As they do, magma will squeeze up

along the cracks and faults forming ring dikes.

“Ring of Fire” This is the popular name for a narrow

belt of intense earthquake and volcanic activity that follows

approximately the borders of the Pacific Ocean basin and

includes most of the active volcanoes on Earth. The Ring of

Fire extends from Tierra del Fuego, at the southern extremity

of South America, northward along the Andes Mountains

of South America, through Central America, along the

Cascade Mountains in the Pacific Northwestern United

States and southwestern Canada, down the Aleutian

Islands in Alaska, westward to volcano-filled Kamchatka

Peninsula, and southward to Japan and eastern China, the

Philippine Islands, Indonesia, New Guinea, and New

Map of the world showing the “Ring of Fire” that encircles the Pacific Ocean basin as well as the distribution of volcanoes in the ring. Other major

volcanic areas of the world are also shown.

216 rift valley

Zealand. Altogether, the Ring of Fire measures some 30,000

miles (48,280 km) in length and encloses an area of approxi-

mately 70 million square miles (181,299,167 km

Because the Ring of Fire contains some of the largest cities

in the world, notably Los Angeles and Tokyo, it has been the

site of numerous fatalities from earthquakes and volcanic erup-

tions over the centuries, and the potential for loss of life in

future earthquakes and eruptions is great despite all precautions

that may be taken. Adding to the potential for damage from

earthquakes and eruptions around the Ring of Fire is the danger

of tsunamis, which can carry destruction all the way across the

Pacific Ocean and even farther. The following selected listings

of major earthquakes and volcanic eruptions along the Ring

of Fire for the past several centuries provide some idea of the

destructive potential of seismic and volcanic activity along it:

• China, 1556. Although little historical information is avail-

able about this earthquake, it is thought to have affected

three provinces and killed more than 800,000 people.

• Japan, 1596. An earthquake offshore reportedly generated

a tsunami that destroyed the island of Uryu-Jima complete-

ly and caused more than 4,000 deaths.

• Philippine Islands, 1616. An eruption of the volcano

Mayon buried numerous villages near the mountain under

ash, although casualty figures are unavailable. A later

eruption in 1766 is said to have killed some 2,000 people

and produced large flows of hot mud, possibly from lakes

that spilled out of the caldera of the volcano. An addi-

tional 2,000 people were reported killed in an 1814 erup-

tion of Mayon. The volcano has erupted frequently since

then, but casualty figures have been comparatively low.

• Japan, 1737. A tsunami said to have been more than 200

feet (61 m) high struck the north shores of Japan and also

Russia’s Kamchatka Peninsula.

• Chile, 1757. A major earthquake occurred at Concepción,

allegedly accompanied by a tsunami. Some 5,000 people

perished, and 10,000 were injured.

• Mexico, 1759. The emergence of Jorullo volcano is

thought to have killed some 200 people. Jorullo’s activity

continued for approximately 40 years.

• Java, Indonesia, 1772. Powerful earthquakes at Papanda-

jan generated a huge depression, approximately six miles

(10 km) wide and 15 miles (24 km) long. An entire town

was destroyed, and some 2,000 people were killed.

• Japan, 1793. The volcano Unzen exploded, killed some

50,000 people, and deposited pumice on the sea in layers

thick enough to support a person.

• Venezuela, 1812. An earthquake at Caracas killed about

10,000 people there and thousands more in nearby com-

munities.

• Indonesia, 1815. The eruption of Tambora is estimated to

have cast out more than 30 cubic miles of solid material in

a single week and killed 92,000 people both directly and

by resulting disease and famine.

• Java, Indonesia, 1822. The volcano Galung Gung erupt-

ed, killing some 4,000 people.

• Chile, 1822. An earthquake at Valparaiso killed some

10,000 people and uplifted the shoreline several feet,

exposing shipwrecks on the ocean floor.

• Chile, 1835. One of the most calamitous earthquakes in

history destroyed Concepción and Santiago on February

20 and was accompanied by a tsunami that caused wide-

spread destruction along the shore.

• New Zealand, 1855. A very strong earthquake in southeast

New Zealand uplifted the Rimutaka mountain range by

several feet.

• Japan, 1857. An earthquake and subsequent fire destroyed

Tokyo and killed an estimated 100,000 people.

• Ecuador, 1877. The most violent recorded eruption of the

volcano Cotopaxi killed some 1,000 people and destroyed

much of the mountain’s summit.

• Indonesia, 1883. The eruption of the volcanic island

Krakatoa in the Sunda Strait generated a huge (120-

foot [37-m]-high) tsunami that killed 36,000 people along

shorelines near the volcano. The tsunami circled the globe

several times before diminishing completely. Ash ejected

into the upper atmosphere from this eruption made sunsets

redder in the following year.

• Alaska, 1891. A series of extremely powerful earthquakes

near Yakutat Bay raised a nearby mountain range almost

50 feet (15 m) and affected an area of some 200,000

square miles (517,998 km

• Guatemala, 1902. An earthquake and subsequent fire

destroyed Guatemala City and killed more than 12,000

people.

• Formosa (Taiwan), 1906. An earthquake reportedly

destroyed more than 6,000 buildings and took some 1,300

lives.

• California, United States, 1906. The San Francisco earth-

quake of this year is perhaps the most famous seismic event

in United States history.

• Chile, 1906. An earthquake struck Valparaiso and killed

approximately 1,500 people.

• Java, Indonesia, 1919. Kelud volcano erupted and gener-

ated a vast flow of hot water and mud that is thought to

have killed more than 5,000 people.

• China, 1920. Gansu Province experienced a powerful

earthquake that reportedly killed about 200,000 people.

• Japan, 1923. The “great Kanto earthquake” destroyed

much of Tokyo, killing approximately 143,000 people and

leaving a half-million more homeless.

• Japan, 1927. An earthquake almost as powerful as the

Kanto earthquake struck the Tango Peninsula on Japan’s

western shore, killing some 3,000 people and destroying

approximately 14,000 buildings.

• Netherlands East Indies (now Indonesia), 1928. The vol-

cano Rokotinda erupted and killed more than 200 persons

with landslides and falling rocks.

• Java, Indonesia, 1931. Merapi volcano’s eruption lasted

three weeks and took more than 1,000 lives.

• California, United States, 1933. The Long Beach earth-

quake killed more than 100 people and caused extensive

damage.

• Chile, 1939. An earthquake destroyed Concepción, killing

some 50,000 people and leaving perhaps 750,000 more

without shelter.

• Japan, 1946. An undersea earthquake generated large tsuna-

mis in the Inland Sea that obliterated some 50 communities

“Ring of Fire” 217

along the shore. About 2,000 people are thought to have

died in the tsunami, and possibly 500,000 were left home-

less after the waves had passed.

• Ecuador, 1949. An earthquake in central Ecuador killed

some 6,000 people and injured perhaps 20,000 others.

• Japan, 1952. The eruption of a submarine volcano at

Myozin-Syo in the Bonin Islands destroyed a ship, leaving

no survivors, when the vessel passed over a vent during the

eruption.

• Hawaii, United States, 1960. A tsunami that accompa-

nied an earthquake in Chile caused extensive destruction

in Hawaii. Some 60 people died in Hilo City. More than

400 other people were killed when the wave reached Japan

and the Philippine Islands, in addition to the 2,000 people

killed in Chile.

• Alaska, United States, 1964. The Good Friday earth-

quake of 1964, with its accompanying tsunami, killed

more than 100 people and carried destruction down the

Pacific coast to Crescent City, California.

• Chile, 1968. Almost 500 people died in an earthquake that

struck central Chile. The earthquake itself was responsible

for only a few of these deaths. Most resulted from the fail-

ure of dams that released their impounded water and bur-

ied communities in mud.

• Peru, 1970. A huge earthquake set off a devastating ava-

lanche where house-size blocks traveled at speeds up to

270 miles (435 km) per hour. A total of 66,000 people

were killed in all.

• California, United States, 1971. The San Fernando earth-

quake was small by some standards (fewer than 100 persons

killed) but caused heavy damage to property and became

one of the most intensively studied earthquakes in history.

• China, 1976. The Tangshan earthquake by official reports

caused 250,000 fatalities. However, it is suspected that

more than 655,000 actually perished.

• Guatemala, 1976. A devastating earthquake on the

Motagua Fault killed more than 23,000 people.

• Mexico, 1985. The devastating Mexico City earthquake

demonstrated that city’s vulnerability to seismic events.

Much of the city is built on unconsolidated sediment,

which reacted to passing seismic waves in such a way that

numerous buildings collapsed. The earthquake originated

not in the Mexico City area but rather along the Pacific

coast of Mexico.

• California, United States, 1989. The Loma Prieta earth-

quake that hit San Francisco damaged the Bay Bridge

between San Francisco and Oakland and killed motorists

whose vehicles were crushed under a collapsing segment of

highway in the East Bay region.

• Philippines, 1991. The eruption of the long sleeping giant,

Mount Pinatubo killed more than 3,000 people and forced

the closure of Clark Air Force Base. It had a VEI of 5.

• California, United States, 1994. The North Ridge earth-

quake devastated the Los Angeles area, killing only 60 peo-

ple but causing more than $20 billion in property damage.

• Japan, 1995. The Kobe earthquake killed more than 5,500

people and reduced much of the city to rubble.

• Taiwan, 1999. The Tai-Ching earthquake killed some 2,400

people and caused $14 billion in property damage.

Rio Grande Rift New Mexico, United States The Rio

Grande Rift began its activity about 13 million years ago. It

borders the Colorado Plateau and extends through the San

Luis Valley between the Sangre de Cristo Mountains and the

San Juan Mountains. The northern part of the rift lies near

Taos and Albuquerque, New Mexico. The lower portion of

the rift is characterized by flat territory, such as the Plains of

Saint Augustin. Signs of volcanic activity are abundant along

the Rio Grande Rift. An example is the Valles caldera in the

Jemez Mountains near Los Alamos. This is one of the larg-

est calderas known and is believed to have formed through

the eruption and eventual collapse of a volcano whose out-

put of tephra filled in large portions of the rift in the vicin-

ity of Los Alamos about 1.12 million years ago. Cinder cones

in the Valles Caldera area indicate that volcanism was active

here only a few thousand years ago. Numerous lava flows

are visible along the rift, notably the Malpais lava beds near

Carriozo. This lava flow may be seen at Valley of Fires State

Park. Molten rock may still underlie the rift; there is evidence

of a pool of magma between 10 and 15 miles (16 and 24 km)

deep beneath the rift near Socorro. The Rio Grande Rift has

been investigated as a possible source of geothermal energy.

Riviera earthquake, France and Italy An earthquake took

the Riviera at the French-Italian border by surprise at 6 a.m.

on February 23, 1887. The seismic waves were felt strongly

in the cities of San Remo and Genoa, Italy; Monte Carlo,

Monaco; and Nice, France. Earthquakes are not common in

this part of the Mediterranean basin, so preparation was

poor. In addition, the quake struck on the morning of Ash

Wednesday in towns that were crowded with tourists there

to celebrate Mardi Gras. The earthquake was estimated at a

6–6.2 on the Richter scale and an X on the modified Mer-

calli scale. This earthquake marked the first use of the Cec-

chi instrument, an early seismograph. The sea level was said

to have dropped three feet (1 m) after the first shock and

then rose six feet (2 m), suggesting that a tsunami or seiche

accompanied the event. In all, about 2,000 people lost their

lives. Equally striking was the mass exodus of people—espe-

cially tourists—immediately after the event. Some 50,000

people choked the rail systems, boats, and other modes of

transportation in a mad attempt to escape.

Roccamonfina volcano, Italy The stratovolcano Roc-

camonfina is located some 30 miles (48 km) north of Vesu-

vius on the western shore of the Italian Peninsula. The

volcano is about 11 miles (17.6 km) in diameter and has two

calderas. The main caldera is approximately four miles (6.4

km) in diameter and occupies the summit of the mountain,

whereas a second, smaller caldera is located on the north side

of the volcano. Roccamonfina has many parasitic cones.

The volcano has generally been quiet within historical times

but has had one recorded period of unrest, around 270 b.c.,

when a flame reportedly rose up and burned for several days.

What exactly happened in this case is uncertain. It may or

may not have been an actual eruption.

rock avalanche A general term for rock masses moving

downhill at high speed. They include rock and debris falls and

218 Rio Grande Rift

debris flows. Rocks and debris can break loose as the result

of an earthquake among other causes and race downhill. In

rugged terrains and high elevation, these masses can attain

speeds of well over 200 miles (322 km) per hour. They will

devastate anything in their way and only stop at the bottom

of a valley. The most spectacular example of a rock or debris

flow avalanche was in Nevados Huascarán in 1970.

rockfall In contrast to a rockslide, which stays in contact

with the surface throughout its movement, a rockfall is a mass

of rocks that breaks loose from the bedrock and falls through

the air to the slope below. Most of the travel is a free fall, but it

may include bouncing as well. Rockfalls typically break away

from the bedrock along steeply inclined joints. Undercutting of

slopes by rivers, wind, or human activities also hastens rock-

falls. Toppling is a particular type of rockfall that involves the

rotation of the rock mass away from the exposure around a

fixed fulcrum. Whatever the mechanism, repeated rockfalls or

rockslides produce a pile of rock rubble at the foot of a cliff or

slope. This rubble is called talus. Earthquakes in mountain-

ous regions produce numerous rockfalls.

rockslide A mass of solid rock that slides quickly down

an inclined discontinuity. The most common discontinuities

are bedding and fractures. In sedimentary rocks, bedding is

usually the primary discontinuity. With bedding, a rigid layer

such as limestone or sandstone typically breaks free and slides

down an underlying, softer layer such as shale. In crystalline

rocks (igneous and metamorphic), the most common discon-

tinuities are joints and faults. The rock masses may break

loose along steeply inclined joints through frost-wedging pro-

cesses, but they slide along shallower joints and faults. Rock-

slides may be generated by a number of processes, but they

are especially common during earthquakes. The surface

waves shake loose otherwise stable rock masses, and they

slide into the valleys below with disastrous results.

Rodinia There was another supercontinent that preceded

Pangaea, and it has been named Rodinia. It was constructed

during the middle to late Proterozoic and was finally

assembled about 1 billion years ago. The continents were in

completely different positions and have been shuffled around

during later plate tectonic interactions. For example, the

Block diagram of a cliff showing a rockfall with talus slope at the base

Rodinia 219

east coast of the United States appears to have faced onto

South America. The building of this continent included signif-

icant earthquake and volcanic activity along active belts. The

Blue Ridge, New Jersey–Hudson Highlands, Adirondacks,

Green Mountains, and Long Mountains of eastern North

America contain such a mobile belt. However, evidence for

the building of this supercontinent is abundant worldwide.

Rodinia broke apart during the late Proterozoic and into the

Cambrian.

Rostaq earthquake, Afghanistan On February 4, 1998, an

earthquake of magnitude 5.9 occurred in the Afghanistan-

Tajikstan border region. At least 2,323 deaths were reported

as were 818 injuries. At least 8,094 houses were destroyed,

and 6,725 livestock were killed.

Rotorua caldera, North Island, New Zealand The Rotorua

caldera is located in the Taupo volcanic zone and is noted for

its variable hydrothermal activity. In 1886, there appar-

ently was a failure for several weeks in a hot-water supply

to baths at Rotorua, although this change may or may not

have been connected with activity at the caldera. In 1932 a

strong hydrothermal explosion took place in Lake Rotorua.

Ruapehu volcano, North Island, New Zealand Adjacent

to the volcanoes Ngauruhoe and Tongariro, Ruapehu has a

crater lake at its summit whose eruptions cause occasional

mudflows. It has erupted about 50 times since 1861. Most

of the eruptions are phreatic. A mudflow in 1953 destroyed

a railroad bridge and brought about the deaths of more than

150 people in a train wreck. According to studies of the vol-

Block diagram of tilted rock layers and a rockslide or debris slide along the layers

220 Rostaq

cano that span the last 75,000 years, Ruapehu has been in

a phase of low-emission-volume, low-magnitude eruptions. It

has been in this phase for the past 1,800 years. It erupted in

1997 and again in 1999. The 1997 eruption was the largest

in 400 years. It included explosions, a six-mile (10-km)-high

eruptive column, and a lahar.

Ruby Seamount Mariana Islands It is a submarine vol-

cano located about 25 miles (40 km) northwest of Saipan,

Guam. The last major eruption was in 1966, but there was a

minor eruption in 1995 as well. tsunami alerts were posted

during this eruption.

run-up The run-up height is the elevation of the water level

in feet or meters caused by the arrival of a tsunami that is

above sea level at the expected tidal elevation. The run-up

height is the added height of sea level caused by a tsunami. In

the vast majority of tsunamis, run-up heights are on the order

of centimeters. There are historical accounts of tsunamis that

claim run-ups of greater than 200 feet (60 m). These would

be extremely rare if they ever existed.

rupture Slippage along a fault plane that produces break-

age or rupturing of rock. After stress exceeds the strength of

the fault rock, rupture occurs, producing an earthquake. Sur-

face rupture does not necessarily coincide with the epicenter

of an earthquake. It can be related to slumping and shifting

of fault blocks in the subsurface.