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

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eruptions do tend to follow minor but notable changes, such

as small earthquakes and the formation of fissures in the

caldera. Eruptions were recorded in 1708 and 1800 and every

several years thereafter up to the late 1980s. Most eruptions

have been explosive with associated lava flows. Piton de la

Fournaise is thought to be fed by basaltic magma. Geolo-

gists believe that the magma is stored in a central reservoir

near the surface before erupting through the summit and

through rifts on the volcano’s flanks. Apparently, inflation

occurs at the summit for some weeks or months before an

eruption. Reinflation, however, does not appear to start soon

after each new eruption. Piton de la Fournaise is presumed to

be a hot spot volcano like Hawaii. A volcanic observatory

has been set up there.

plagioclase Probably the most common igneous mineral,

plagioclase is a type of feldspar, a silicate with a frame-

work structure. At high temperature, plagioclase is calcium

rich. It is common in basalt and gabbro. At low temper-

atures, plagioclase is sodium-rich. it occurs in granite and

rhyolite. In intermediate rocks, the plagioclase is half-

calcium and half-sodium. It is most common in intermediate

rocks.

plastic deformation See ductile.

plateau basalt See flood basalts.

plate tectonics One of the foundations of modern geology,

the theory of plate tectonics has been described as “the glue

that holds geology together.” It describes the interactions

among several large, and a number of smaller, rigid plates of

Earth’s lithosphere. The lithosphere is composed of Earth’s

crust, which forms the top layer adhered to a bottom layer

of rigid mantle rock. The interactions among the plates

are complex, but for the purpose of understanding earth-

quakes and volcanic eruptions, plate tectonic theory may be

described as follows. Lithospheric plates float, so to speak,

atop the denser rocks of the asthenosphere (soft upper

mantle) by a process known as isostasy. These plates are not

stable but instead move around the planet. Thermally induced

convection current in the mantle drive this motion or con-

tinental drift.

The plates interact with each other along their margins.

There are three different types of margins: convergent

where plates are pushed together, divergent where plates are

pulled apart, and transform where plates slide laterally past

each other. There are three different kinds of convergent mar-

gins, depending upon the types of crust involved. They are

ocean-ocean, ocean-continent, and continent-continent colli-

sions. The ocean-ocean and ocean-continent collisions form

subduction zones in which the downgoing ocean crust is

consumed in the mantle. In ocean-continent collisions, also

called Andean margins after the type example, the ocean crust

is subducted and melted to form a magmatic arc. Magmatic

arcs are seismically and volcanically active and very danger-

ous. Ocean-ocean collisions form island arcs, where ocean

crust is subducted beneath other ocean crust in a deep ocean

trench. Examples such as the Aleutian Islands are equally

Ashfall deposits buried this straw hut at the base of Mount Pinatubo after

the 1991 eruption. Note the odd landscape in the background resulting

from erosion caused by the typhoon the following day. (Courtesy of the

CVO-USGS)

The mudflow generated by the typhoon that struck the Philippines the day

after the eruption of Mount Pinatubo was far more disastrous than the

eruption itself. (Courtesy of the USGS)

202 plagioclase

Diagram of plate boundary types, magmatic arcs above subduction zones (convergent boundary), and a mid-ocean ridge comprising a divergent boundary.

Fracture zones form transform boundaries. A hot spot is also shown.

Tectonic map of the Earth showing the major tectonic plates. There are three different types of boundaries: the dark lines with triangles are convergent

boundaries (subduction zones), the double lines are mid-ocean ridges, and the single lines are transform faults. (Courtesy of the USGS)

plate tectonics 203

active and dangerous. Continent-continent collisions are

more like car accidents: One plate plows under the other, but

most of the deformation is taken up in crushing and elevating

the rock. This scenario is called a Himalaya margin after the

type example. There is intense seismic activity at the margins

but no volcanic activity.

divergent boundaries form more in stages. On con-

tinental crust, the margin begins with uplift followed by

normal faulting and graben formation. Seismic activity

is common in this phase. Next, extensive volcanic activity

occurs including flood basalts and possible rhyolite vol-

canoes as in the Basin and Range Province. The crust is

then pulled apart, and ocean crust forms between, making

a narrow ocean basin like the Red Sea. Finally, a full ocean

basin with a mid-ocean ridge forms including extensive

seismicity and volcanism.

Transform margins are large strike-slip faults that

separate lithospheric plates. Transform margins must con-

nect other types of plate margins. Therefore, there are

divergent-divergent, divergent-convergent, and convergent-

convergent transform margins. About 99% of transform

margins are the divergent-divergent type. They form small

offsets all along every mid-ocean ridge of various sizes and

are almost exclusively on ocean crust. A major exception to

this rule is the San Andreas Fault of California, which is

on continental crust. Like the San Andreas Fault, these other

transform faults are also highly seismically active. Regard-

less of the type of transform margin, they are all similar in

their seismic activity and general lack of volcanic activity.

Larger transform faults are called fracture zones. Other

examples of transform faults include the South Alpine Fault

in New Zealand and the Motagua Fault in Guatemala,

both of which have produced devastating earthquakes.

New ocean crust is formed along mid-ocean ridges,

where molten rock rising from the asthenosphere solidifies.

The newly formed rock moves outward to either side of the

ridge in a pattern much like that of objects on a moving con-

veyer belt. Volcanic activity is therefore commonplace along

the ridges and may build up whole islands, of which Iceland

is a spectacular example. As much ocean crust is destroyed in

subduction zones as is produced at the mid-ocean ridges to

preserve the size of Earth. As the subducted plate descends

into the asthenosphere, the plate melts, and its lighter compo-

nents rise back toward the surface as molten rock, or magma.

This magma finds its way to the surface through openings in

the crust and produces volcanoes. (Not all volcanic activity

is linked to subduction zones; hot spots, or points where

ascending plumes of molten rock are thought to approach

Earth’s surface, may be responsible for volcanic activity, as in

the case of the Hawaiian Islands.)

The source of energy for the movement of crustal plates

has been a subject of debate since plate tectonics was devel-

oped from the earlier theory of continental drift. In recent

years, it has been widely believed that much of the energy

behind the motions of crustal plates is derived from convec-

tion cells, or circulation patterns produced within the mantle

by convective activity.

See also convection current; Earth, internal

structure of; Wegener, Alfred.

Plinian eruption A type of volcanic eruption character-

ized by incredible vertical explosions that shoot gas, ash, and

ejecta many miles into the atmosphere. These eruptions are

the most violent of the Vesuvian events. They are commonly

the final phase in an eruptive cycle after the pipe has been

cleared by earlier eruptive types. They have VEIs of 3–8.

plug A volcanic neck.

pluton A body of intrusive, typically coarse-grained igneous

rock. Plutons intrude forcibly or passively into the preexist-

ing country rock of an area and cool underground. Through

slow cooling, the minerals grow large and form a crystal-

line, phaneritic texture. Tabular dikes and sills are less circu-

lar than stocks and batholiths. Laccoliths and lopoliths are

subvolcanic intrusions. They are typically forcible intrusions in

contrast to tabular intrusions which are typically passive.

Production of new ocean crust at a mid-ocean ridge during a reversing

magnetic field of the Earth. The stripes on the ocean floor show normal

and reversed magnetic polarity over the past 5 million years.

204 Plinian eruption

plutonism The formation of plutons and related processes.

Poás volcano, Costa Rica Located northeast of the com-

munities of Naranjo, Grecia, and San Pedro, Poás volcano has

arisen within a pair of nested calderas and is suspected of

having a lake rich in sulfur, underlain by a layer of molten

sulfur, in its main crater. Molten sulfur has sometimes been

expelled in eruptions of Poás. Most of its eruptions, which

have been recorded on numerous occasions in the 19th and

20th centuries, have been small and either magmatic or phre-

atic in character. It has erupted at least 39 times since 1828,

but the activity has been nearly continuous. Notable eruptions

in 1828, 1834, and 1880 deposited ash on nearby communi-

ties. Other significant eruptions occurred in 1910 and 1953.

The 1910 event began with the eruption of a geyser, and a

plume rose to an altitude of more than four miles (6.4 km).

Mud covered the whole top portion of the volcano. Numer-

ous strong earthquakes occurred later that year, including one

especially powerful earthquake that caused widespread dam-

age in nearby communities. A 1953 eruption eliminated a

crater lake and dropped ash 30 miles (48 km) or more away

from the volcano. An eruption column rose about two miles

(3 km) over the volcano in 1976, and geysers erupted in 1979,

1980, and 1988. The crater lake gradually drained in 1989

and was replaced by a six-foot (1.8-m)-deep pool of liquid

sulfur. This was the first observation of a sulfur lake on Earth,

although liquid sulfur is common on Io, Jupiter’s nearest

moon. Summit fumarole activity continued through 1995.

Pompeii and Herculaneum volcanoes, Italy Two of the

most famous cities in the history of volcanology, the Roman

communities of Pompeii and Herculaneum were destroyed

in the eruption of Vesuvius in a.d. 79. The destruction of

the two cities has provided a popular theme for authors,

notably Edward Bulwer-Lytton in his novel The Last Days

of Pompeii. Bulwer-Lytton was apparently influenced by the

historian Dion Cassius, who reported that the fallout from

Vesuvius buried the people of Pompeii as they sat in the the-

ater. This melodramatic scenario appears to have little basis

in fact, and it is thought that Dion Cassius was merely draw-

ing on traditional (and exaggerated) accounts of the catastro-

phe. Other of Dion Cassius’s statements sound improbable,

to say the least, especially his report that a great number of

giants appeared in the vicinity of the volcano during the erup-

tion. An eyewitness account of the event was made by Pliny

the Younger (see Appendix B).

Excavations at Pompeii have revealed much about life in

that coastal city some 2,000 years ago. One visitor, quoted in

Charles Morris’s study of volcanoes, The Volcano’s Deadly

Work, commented on “the weirdness of the scene” at Pom-

peii and the remarkable preservation of details of life there:

We have before us the narrow lanes, paved with

tufa, in which Roman wagon wheels have worn deep

ruts. We cross streets on stepping-stones which san-

daled feet long ago polished. We see the wine shops

with empty jars, counters stained with liquor, stone

mills where the wheat was ground, and the very

ovens in which bread was baked more than eigh-

teen centuries ago. “Welcome” is offered us at one

silent, broken doorway; at another we are warned

to “Beware of the dog!” The painted figures, some

of them so artistic and rich in colors that pictures

of them are disbelieved, the mosaic pavements, the

empty fountains . . . the marble pillars and the small

gardens are there just as the owners left them. Some

of the walls are scribbled over by the small boys of

Pompeii in strange characters which mock modern

erudition. In places we read the advertisements of

gladiatorial shows, never to come off, the names of

candidates for legislative office who were never to

sit. There is nothing like this elsewhere.

The streets of Pompeii must have had a charm

unapproached by those of any city now in existence.

The stores, indeed, were wretched little dens. Two

or three of them commonly occupied the front of

a house on either side of the entrance, the ostium;

but when the door lay open, as was usually the

case, a passerby could look into the atrium, prettily

decorated and hung with rich stuffs. The sunshine

entered through an aperture in the roof.

As the life of the Pompeiians was all outdoors,

their pretty homes stood open always. There was

indeed a curtain betwixt the atrium and the pen-

style, but it was drawn only when the master gave

a banquet. Thus a wayfarer in the street could see,

beyond the hall described and its busy servants,

the white columns of the peristyle, with creepers

trained around them, flowers all around, and jets of

water playing through pipes which are still in place.

In many cases the garden itself could be observed

between the pillars of the further gallery, and rich

paintings on the wall beyond that.

But how far removed those little palaces of Pom-

peii were from our notion of well-being is scarcely to

be understood by one who has not seen them. It is a

Map of the Mount Vesuvius (Naples) area of Italy showing the relation of

the volcano to the two destroyed cities of Pompeii and Herculaneum.

Pompeii and Herculaneum 205

question strange in all points of view where the fam-

ily slept in the houses, nearly all of which had no sec-

ond story. In the most graceful villas the three to five

sleeping chambers round the atrium and four round

the peristyle were rather ornamental cupboards than

aught else. One did not differ from another, and if

these were devoted to the household the slaves,

male and female, must have slept on the floor out-

side. The master, his family and his guest used these

small, dark rooms which were apparently without

such common luxuries as we expect in the humblest

home. All their furniture could hardly have been

more than a bed and a footstool . . . The kitchen of

each villa certainly was not furnished with such inge-

nuity, expense or thought as the stories of Roman

gourmandising would have led us to expect. In [one]

house . . . the cook seems to have been employed in

frying eggs at the moment when increasing danger

put him to flight. His range, four partitions of brick,

was very small; a knife, a strainer, a pan lay by the

fire just as they fell from the slave’s hand. Pompeii

was buried by falling tephra, whereas Herculaneum

was covered by mudflows.

Popocatépetl volcano, Mexico The word Popocatépetl

means “smoking mountain” and was given to the volcano by

the Aztec. The stratovolcano stands south of Mexico City

and has undergone explosive eruptions, most of them small,

on more than 36 occasions in recorded history, 15 since the

arrival of the Spanish in 1519. There was a major eruption in

1720. Popocatépetl was one of the first volcanoes into which

European explorers actually made a descent, to collect sulfur

for gunpowder. The most recent eruption was from 1995 to

1999 and forced the evacuation of 75,000 people. It started to

erupt again in the spring of 2000. Some 20 million people live

close enough to the volcano to be threatened by eruptions.

porphyritic A textural term for volcanic rocks meaning that

there are two grain-size populations. Large grains (usually one-

eighth to one-half inch [2 mm–1 cm]) of a mineral form perfect

crystals floating in a fine-grained matrix. This texture reflects

a two-stage history. The large crystals were formed while the

magma was cooling in the magma chamber. Because they

were surrounded by pure liquid, they could grow without

bumping into other grains. That is why they form crystals. The

minerals are varieties that form at higher temperatures than

the rest of the rock. Otherwise, they would also be crystallizing

as well. Later, the crystals and liquid are erupted from a vol-

cano during an active phase. The liquid cools quickly during

the eruption producing very small mineral grains and/or glass

to fill between the crystals. In basalt, the large crystals could

be olivine or plagioclase and the matrix is pyroxene and

plagioclase. In andesites and dacites, the large crystals are

plagioclase and possibly hornblende, pyroxene, or biotite.

The fine minerals are quartz and plagioclase. In rhyolite,

the large grains are k-feldspar and possibly quartz or biotite.

The matrix is quartz, K-feldspar, and plagioclase. These rela-

tions are determined by the crystallization sequence as shown

by Bowens Reaction Series.

Port Royal earthquake, Jamaica The earthquake that

hit Port Royal on June 7, 1692, struck about 11:43 a.m. (In

1959, divers brought up the remains of a watch that appar-

ently stopped at that moment.) The earthquake caused the

waterfront built on sandy soil to fall into the sea. A tavern,

a warehouse, and other buildings slid seaward and crumbled

into the waters. Approximately half the town was wiped out.

Precambrian All time from the beginning of Earth 4.6

billion years ago until the beginning of the Paleozoic Age

about 535 million years ago. The Precambrian can be sub-

divided into the early Archean and the later Proterozoic.

The division between the two was 2.5 billion years ago. Bil-

lion years is written Ga in scientific terminology and it means

“giga annum,” where annum means “years.”

precursor Any number of changes in the geological con-

ditions that foreshadows the coming of an earthquake on a

fault. These changes can include several small earthquakes,

local uplift or subsidence along a fault, changes in water levels

in wells, and increased radon emissions, among others. Most

times it is difficult to recognize and properly evaluate these

precursors otherwise earthquakes would not be as deadly.

pressure ridge Common topographic ridges resulting both

in earthquakes and volcanoes. The ridges are formed through

lateral transverse shortening of the land surface. In volcanic

flows, the crust on the top of the flow can fold into a ridge as

the result of pressure generated from flowing subsurface lava.

primary effects The direct hazards from a volcanic erup-

tion. These include lava flows,

NUÉE ARDENTEs, lahars,

explosions, ash fallout, and any other direct effects. In con-

trast, secondary effects might include flooding from dammed

rivers from lava flows, polluted water, ruined crops and fam-

ine, disease-resulting pollution, interrupted transportation

routes and services, and any other hazard that is not directly

caused by the eruption.

Proterozoic The later part of Precambrian.

pseudotachylite Rock within a fault that is melted by

movement during an earthquake. It is then hardened in the

fault. In shallow rocks near the surface, there is a lot of fric-

tion on a fault during an earthquake. The friction can heat up

the rock and melt it. The newly formed liquid fills in the fault

plane or cracks around the fault. The solidified melt is pseudo-

tachylite. Tachylite is volcanic glass and pseudo means “false.”

Puerto Rico Puerto Rico experiences numerous earth-

quakes, but highly destructive ones are rare. However, the

Caribbean Sea, in which Puerto Rico is located, is an area of

high-intensity seismic and volcanic activity.

On October 11, 1918, an earthquake of Richter mag-

nitude 7.5, with Mercalli intensities in the range of VIII–IX,

caused property damage estimated at about $4 million and

killed more than 100 people. The earthquake was centered in

the northwestern Mona Passage and was accompanied by a

tsunami that caused widespread destruction and loss of life.

206 Popocatépetl

The sea withdrew before the tsunami’s arrival, then returned,

and reached heights possibly greater than 19 feet (5.8 m). A

church in Aguada was destroyed, and all brick buildings were

ruined in Anasco. Structures of wood or reinforced concrete

reportedly came through the earthquake with little damage

where building materials were sound. At Aguadilla, the earth-

quake caused serious damage to buildings constructed on

alluvium. The collapse of a factory at Mayagüez killed several

people. Damage to railways, bridges, pipelines, and chimneys

was widespread, and two cable links in the Mona Passage

were broken. Strong aftershocks followed the October 11

earthquake on October 24 and November 12.

Puget Sound Washington, United States An inlet of the

Pacific Ocean between the Olympic Peninsula to the west

and the mainland to the east, Puget Sound is an area of

intense seismic activity.

See also Cascadia Subduction Zone; Cascade Moun-

tains; Washington.

pumice An extremely lightweight, “foamy” volcanic rock,

pumice forms when magma bearing a high concentration of

dissolved gases solidifies as glass just as the gases are bub-

bling out of solution. It forms on top of a magma or lava

pool just as foams such as whipped cream would. Pumice is

cast out in large quantities during the eruptions of some vol-

canoes and is so lightweight that it may float on water. In

eruptions of certain volcanoes on islands or near seacoasts,

pumice may accumulate on the water and form great float-

ing “rafts” capable of supporting a man’s weight. Such rafts

of pumice may impede navigation. They are also the vehicle

by which certain species of plants and animals migrate from

island to island across the Pacific Ocean.

Puracé volcano, Colombia The dacitic shield volcano

capped by an andesitic stratovolcano Puracé has undergone

explosive eruptions on more than two dozen occasions since

1827, 12 of which were in the 20th century. Two of these

eruptions caused fatalities.

Puu Oo volcanic cone, Hawaii, United States This cone,

whose name means “hill of the Oo Bird,” was produced by

an eruption of Kilauea that began in 1983 and came to be

known as the Puu Oo eruption. Puu Oo underwent several

episodes of eruption in the 1980s and reached a height of

more than 750 feet (229 m). Although episodes of eruption

became generally shorter, the extrusion rate of lava increased,

and fire fountains rose higher. Some fire fountains rose

approximately 1,200 feet (366 m) into the air, approaching

the height of the Empire State Building in New York City.

P-waves See seismology.

pyroclastic Made up of fragmented pieces of rock that

were produced in an explosive volcanic eruption. The ejecta

are transported, deposited, and lithified just as a clastic rock.

The pyro part of the word means “fire,” so it is a clastic rock

produced by fire. An ignimbrite, for example, is composed of

pyroclastic material.

pyroclastic flow More commonly known as an ash flow,

a pyroclastic flow is made up of clastic material included and

carried along in a flow of ash. The pyroclastic flow behaves

as a fluid, hence its name. An ignimbrite is an example of

a pyroclastic flow. Such a flow may extend outward many

This pyroclastic flow of hot ash and debris is moving down the slopes of

Mount Saint Helens. In the second photo, a geologist studies the larger

material deposited by the flow. (Courtesy of the CVO-USGS)

pyroclastic flow 207

miles from the volcano that produced it. A NUÉE ARDENTE is

a fast-moving cloud of gas released in a volcanic eruption and

containing pyroclastic material.

pyrometamorphism Rock that has undergone extreme

metamorphism in lava or magma. As magma intrudes into

preexisting rock, pieces of the cool rock are broken off into

the extremely hot liquid. The broken rock is immediately

cooked at very high temperatures. The minerals and tex-

ture change accordingly. The same process happens when

extremely hot lava runs over the ground surface. The soil

beneath is immediately cooked and new minerals and tex-

tures are produced. Pyro means “fire.”

pyroxene A common igneous mineral in mafic, ultra-

mafic, and, less commonly, intermediate rocks. It includes

clinopyroxene and orthopyroxene. Pyroxene is a ferromag-

nesian mineral. It is a silicate with a chain structure.

208 pyrometamorphism

quake See earthquake.

quartz The most common mineral in Earth’s crust, quartz

is found in many felsic and intermediate igneous rocks.

It is especially common in granite and pegmatite, where it

can make up 25% or more of the rock. It can also be com-

mon in rhyolite, but quartz takes on two different struc-

tures at high temperatures that are called cristobalite and

tridymite. Because the grain size is so small in rhyolite,

commonly the quartz grains cannot be seen. Quartz is nearly

pure silicon dioxide (SiO

) and has a framework type sili-

cate structure. Impurities give quartz colors forming smokey

quartz (gray), rose quartz (pink), amethyst (purple), or any of

the hydrated (water-bearing) family such as agates and opal.

Quartz can also be a replacement mineral in any volcanic

rock and commonly fills amygdules and cavities.