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Title: Volcanoes
Author: Tilling, Robert I.
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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[Illustration: The eruption of Cerro Negro Volcano, near Leon,
Nicaragua, during November 1968.]

       U. S. Department of the Interior / U. S. Geological Survey


                          by Robert I. Tilling

[Illustration: Cover and Title Page: Lava fountains and flows, Mauna
Loa, Hawaii, July 6, 1975.]

Volcanoes destroy and volcanoes create. The catastrophic eruption of
Mount St. Helens on May 18, 1980, made clear the awesome destructive
power of a volcano. Yet, over a time span longer than human memory and
record, volcanoes have played a key role in forming and modifying the
planet upon which we live. More than 80 percent of the Earth’s
surface—above and below sea level—is of volcanic origin. Gaseous
emissions from volcanic vents over hundreds of millions of years formed
the Earth’s earliest oceans and atmosphere, which supplied the
ingredients vital to evolve and sustain life. Over geologic eons,
countless volcanic eruptions have produced mountains, plateaus, and
plains, which subsequent erosion and weathering have sculpted into
majestic landscapes and formed fertile soils.

Ironically, these volcanic soils and inviting terranes have attracted,
and continue to attract, people to live on the flanks of volcanoes.
Thus, as population density increases in regions of active or
potentially active volcanoes, mankind must become increasingly aware of
the hazards and learn not to “crowd” the volcanoes. People living in the
shadow of volcanoes must live in harmony with them and expect, and
should plan for, periodic violent unleashings of their pent-up energy.

This booklet presents a generalized summary of the nature, workings,
products, and hazards of the common types of volcanoes around the world,
along with a brief introduction to the techniques of volcano monitoring
and research.

[Illustration: ]

On August 24, A.D. 79, Vesuvius Volcano suddenly exploded and destroyed
the Roman cities of Pompeii and Herculaneum. Although Vesuvius had shown
stirrings of life when a succession of earthquakes in A.D. 63 caused
some damage, it had been literally quiet for hundreds of years and was
considered “extinct.” Its surface and crater were green and covered with
vegetation, so the eruption was totally unexpected. Yet in a few hours,
hot volcanic ash and dust buried the two cities so thoroughly that their
ruins were not uncovered for nearly 1,700 years, when the discovery of
an outer wall in 1748 started a period of modern archeology. Vesuvius
has continued its activity intermittently ever since A.D. 79 with
numerous minor eruptions and several major eruptions occurring in 1631,
1794, 1872, 1906 and in 1944 in the midst of the Italian campaign of
World War II.

[Illustration: ]

In the United States on March 27, 1980, Mount St. Helens Volcano in the
Cascade Range, southwestern Washington, reawakened after more than a
century of dormancy and provided a dramatic and tragic reminder that
there are active volcanoes in the “lower 48” States as well as in Hawaii
and Alaska. The catastrophic eruption of Mount St. Helens on May 18,
1980, and related mudflows and flooding caused significant loss of life
(57 dead or missing) and property damage (over $1.2 billion). Mount St.
Helens is expected to remain intermittently active for months or years,
possibly even decades.

The word “volcano” comes from the little island of Vulcano in the
Mediterranean Sea off Sicily. Centuries ago, the people living in this
area believed that Vulcano was the chimney of the forge of Vulcan—the
blacksmith of the Roman gods. They thought that the hot lava fragments
and clouds of dust erupting from Vulcano came from Vulcan’s forge as he
beat out thunderbolts for Jupiter, king of the gods, and weapons for
Mars, the god of war. In Polynesia the people attributed eruptive
activity to the beautiful but wrathful Pele, Goddess of Volcanoes,
whenever she was angry or spiteful. Today we know that volcanic
eruptions are not supernatural but can be studied and interpreted by

                        The Nature of Volcanoes

Volcanoes are mountains, but they are very different from other
mountains; they are not formed by folding and crumpling or by uplift and
erosion. Instead, volcanoes are built by the accumulation of their own
eruptive products—lava, bombs (crusted over lava blobs), ashflows, and
tephra (airborne ash and dust). A volcano is most commonly a conical
hill or mountain built around a vent that connects with reservoirs of
molten rock below the surface of the Earth. The term volcano also refers
to the opening or vent through which the molten rock and associated
gases are expelled.

[Illustration: Fountaining lava and volcanic debris during the 1959
Kilauea Iki eruption of Kilauea Volcano, Hawaii.]

Driven by buoyancy and gas pressure the molten rock, which is lighter
than the surrounding solid rock, forces its way upward and may
ultimately break through zones of weaknesses in the Earth’s crust. If
so, an eruption begins, and the molten rock may pour from the vent as
non-explosive lava flows, or it may shoot violently into the air as
dense clouds of lava fragments. Larger fragments fall back around the
vent, and accumulations of fall-back fragments may move downslope as ash
flows under the force of gravity. Some of the finer ejected materials
may be carried by the wind only to fall to the ground many miles away.
The finest ash particles may be injected miles into the atmosphere and
carried many times around the world by stratospheric winds before
settling out.

Molten rock below the surface of the Earth that rises in volcanic vents
is known as _magma_, but after it erupts from a volcano it is called
_lava_. Originating many tens of miles beneath the ground, the ascending
magma commonly contains some crystals, fragments of surrounding
(unmelted) rocks, and dissolved gases, but it is primarily a liquid
composed principally of oxygen, silicon, aluminum, iron, magnesium,
calcium, sodium, potassium, titanium, and manganese. Magmas also contain
many other chemical elements in trace quantities. Upon cooling, the
liquid magma may precipitate crystals of various minerals until
solidification is complete to form an _igneous_ or _magmatic rock_.

[Illustration: An idealized diagram of a volcano in an oceanic
environment (left) and in a continental environment (right).]

The diagram below shows that heat concentrated in the Earth’s upper
_mantle_ raises temperatures sufficiently to melt the rock locally by
fusing the materials with the lowest melting temperatures, resulting in
small, isolated blobs of magma. These blobs then collect, rise through
conduits and fractures, and some ultimately may re-collect in larger
pockets or reservoirs (“holding tanks”) a few miles beneath the Earth’s
surface. Mounting pressure within the reservoir may drive the magma
further upward through structurally weak zones to erupt as lava at the
surface. In a continental environment, magmas are generated in the
Earth’s crust as well as at varying depths in the upper mantle. The
variety of molten rocks in the crust, plus the possibility of mixing
with molten materials from the underlying mantle, leads to the
production of magmas with widely different chemical compositions.

If magmas cool rapidly, as might be expected near or on the Earth’s
surface, they solidify to form igneous rocks that are finely crystalline
or glassy with few crystals. Accordingly, lavas, which of course are
very rapidly cooled, form volcanic rocks typically characterized by a
small percentage of crystals or fragments set in a matrix of _glass_
(quenched or super-cooled magma) or finer grained crystalline materials.
If magmas never breach the surface to erupt and remain deep underground,
they cool much more slowly and thus allow ample time to sustain crystal
precipitation and growth, resulting in the formation of coarser grained,
nearly completely crystalline, igneous rocks. Subsequent to final
crystallization and solidification, such rocks can be exhumed by erosion
many thousands or millions of years later and be exposed as large bodies
of so-called _granitic_ rocks, as, for example, those spectacularly
displayed in Yosemite National Park and other parts of the majestic
Sierra Nevada mountains of California.

Lava is red hot when it pours or blasts out of a vent but soon changes
to dark red, gray, black, or some other color as it cools and
solidifies. Very hot, gas-rich lava containing abundant iron and
magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava
high in silicon, sodium, and potassium flows sluggishly, like thick
honey in some cases or in others like pasty, blocky masses.

[Illustration: Two Polynesian terms are used to identify the surface
character of Hawaiian lava flows. Aa, a basalt with a rough, blocky
appearance, much like furnace slag, is shown at the top. Pahoehoe, a
more fluid variety with a smooth, satiny and sometimes glassy
appearance, is shown at the bottom.]

All magmas contain dissolved gases, and as they rise to the surface to
erupt, the confining pressures are reduced and the dissolved gases are
liberated either quietly or explosively. If the lava is a thin fluid
(not viscous), the gases may escape easily. But if the lava is thick and
pasty (highly viscous), the gases will not move freely but will build up
tremendous pressure, and ultimately escape with explosive violence.
Gases in lava may be compared with the gas in a bottle of a carbonated
soft drink. If you put your thumb over the top of the bottle and shake
it vigorously, the gas separates from the drink and forms bubbles. When
you remove your thumb abruptly, there is a miniature explosion of gas
and liquid. The gases in lava behave in somewhat the same way. Their
sudden expansion causes the terrible explosions that throw out great
masses of solid rock as well as lava, dust, and ashes.

The violent separation of gas from lava may produce rock froth called
_pumice_. Some of this froth is so light—because of the many gas
bubbles—that it floats on water. In many eruptions, the froth is
shattered explosively into small fragments that are hurled high into the
air in the form of volcanic cinders (red or black), volcanic ash
(commonly tan or gray), and volcanic dust.

[Illustration: During the 1959 eruption of Kilauea Iki, fountaining lava
and volcanic debris completely blocked several of the roads in the
Hawaii Volcanoes National Park.]

                      Principal Types of Volcanoes

Geologists generally group volcanoes into four main kinds—cinder cones,
composite volcanoes, shield volcanoes, and lava domes.

                              Cinder cones

Cinder cones are the simplest type of volcano. They are built from
particles and blobs of congealed lava ejected from a single vent. As the
gas-charged lava is blown violently into the air, it breaks into small
fragments that solidify and fall as _cinders_ around the vent to form a
circular or oval cone. Most cinder cones have a bowl-shaped _crater_ at
the summit and rarely rise more than a thousand feet or so above their
surroundings. Cinder cones are numerous in western North America as well
as throughout other volcanic terrains of the world.

[Illustration: Schematic representation of the internal structure of a
typical cinder cone.]

In 1943 a cinder cone started growing on a farm near the village of
Parícutin in Mexico. Explosive eruptions caused by gas rapidly expanding
and escaping from molten lava formed cinders that fell back around the
vent, building up the cone to a height of 1,200 feet. The last explosive
eruption left a funnel-shaped crater at the top of the cone. After the
excess gases had largely dissipated, the molten rock quietly poured out
on the surrounding surface of the cone and moved downslope as lava
flows. This order of events—eruption, formation of cone and crater, lava
flow—is a common sequence in the formation of cinder cones.

[Illustration: Parícutin Volcano, Mexico, is a cinder cone rising
approximately 1,200 feet above the surrounding plain.]

During 9 years of activity, Parícutin built a prominent cone, covered
about 100 square miles with ashes, and destroyed the town of San Juan.
Geologists from many parts of the world studied Parícutin during its
lifetime and learned a great deal about volcanism, its products, and the
modification of a volcanic landform by erosion.

                          Composite volcanoes

Some of the Earth’s grandest mountains are _composite_
volcanoes—sometimes called _stratovolcanoes_. They are typically
steep-sided, symmetrical cones of large dimension built of alternating
layers of lava flows, volcanic ash, cinders, blocks, and bombs and may
rise as much as 8,000 feet above their bases. Some of the most
conspicuous and beautiful mountains in the world are composite
volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador,
Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens
and Mount Rainier in Washington.

Most composite volcanoes have a crater at the summit which contains a
central vent or a clustered group of vents. Lavas either flow through
breaks in the crater wall or issue from fissures on the flanks of the
cone. Lava, solidified within the fissures, forms dikes that act as ribs
which greatly strengthen the cone.

[Illustration: Schematic representation of the internal structure of a
typical composite volcano.]

The essential feature of a composite volcano is a conduit system through
which magma from a reservoir deep in the Earth’s crust rises to the
surface. The volcano is built up by the accumulation of material erupted
through the conduit and increases in size as lava, cinders, ash, etc.,
are added to its slopes.

When a composite volcano becomes dormant, erosion begins to destroy the
cone. As the cone is stripped away, the hardened magma filling the
conduit (the volcanic plug) and fissures (the dikes) becomes exposed,
and it too is slowly reduced by erosion. Finally, all that remains is
the plug and dike complex projecting above the land surface—a telltale
remnant of the vanished volcano.

[Illustration: Shishaldin Volcano, an imposing composite cone, towers
9,372 feet above sea level in the Aleutian Islands, Alaska.]

An interesting variation of a composite volcano can be seen at Crater
Lake in Oregon. From what geologists can interpret of its past, a high
volcano—called Mount Mazama—probably similar in appearance to
present-day Mount Rainier was once located at this spot. Following a
series of tremendous explosions about 6,800 years ago, the volcano lost
its top. Enormous volumes of volcanic ash and dust were expelled and
swept down the slopes as ash flows and avalanches. These large-volume
explosions rapidly drained the lava beneath the mountain and weakened
the upper part. The top then collapsed to form a large depression, which
later filled with water and is now completely occupied by beautiful
Crater Lake. A last gasp of eruptions produced a small cinder cone,
which rises above the water surface as Wizard Island near the rim of the
lake. Depressions such as Crater Lake, formed by collapse of volcanoes,
are known as _calderas_. They are usually large, steep-walled,
basin-shaped depressions formed by the collapse of a large area over,
and around, a volcanic vent or vents. Calderas range in form and size
from roughly circular depressions 1 to 15 miles in diameter to huge
elongated depressions as much as 60 miles long.

[Illustration: Crater Lake, Oregon; Wizard Island, a cinder cone, rises
above the lake surface.]

                 The Evolution of a Composite Volcano:

[Illustration: a. Magma, rising upward through a conduit, erupts at the
Earth’s surface to form a volcanic cone. Lava flows spread over the
surrounding area.]

[Illustration: b. As volcanic activity continues, perhaps over spans of
hundreds of years, the cone is built to a great height and lava flows
form an extensive plateau around its base. During this period, streams
enlarge and deepen their valleys.]

[Illustration: c. When volcanic activity ceases, erosion starts to
destroy the cone. After thousands of years, the great cone is stripped
away to expose the hardened “volcanic plug” in the conduit. During this
period of inactivity, streams broaden their valleys and dissect the lava
plateau to form isolated lava-capped mesas.]

[Illustration: d. Continued erosion removes all traces of the cone and
the land is worn down to a surface of low relief. All that remains is a
projecting plug or “volcanic neck,” a small lava-capped mesa, and
vestiges of the once lofty volcano and its surrounding lava plateau.]

                            Shield volcanoes

Shield volcanoes, the third type of volcano, are built almost entirely
of fluid lava flows. Flow after flow pours out in all directions from a
central _summit_ vent, or group of vents, building a broad, gently
sloping cone of flat, domical shape, with a profile much like that of a
warrior’s shield. They are built up slowly by the accretion of thousands
of highly fluid lava flows called basalt lava that spread widely over
great distances, and then cool as thin, gently dipping sheets. Lavas
also commonly erupt from vents along fractures (rift zones) that develop
on the flanks of the cone. Some of the largest volcanoes in the world
are shield volcanoes. In northern California and Oregon, many shield
volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000
feet. The Hawaiian Islands are composed of linear chains of these
volcanoes including Kilauea and Mauna Loa on the island of Hawaii—two of
the world’s most active volcanoes. The floor of the ocean is more than
15,000 feet deep at the bases of the islands. As Mauna Loa, the largest
of the shield volcanoes (and also the world’s largest active volcano),
projects 13,677 feet above sea level, its top is over 28,000 feet above
the deep ocean floor.

In some eruptions, basaltic lava pours out quietly from long fissures
instead of central vents and floods the surrounding countryside with
lava flow upon lava flow, forming broad plateaus. Lava plateaus of this
type can be seen in Iceland, southeastern Washington, eastern Oregon,
and southern Idaho. Along the Snake River in Idaho, and the Columbia
River in Washington and Oregon, these lava flows are beautifully exposed
and measure more than a mile in total thickness.

[Illustration: Mauna Loa Volcano, Hawaii, a giant among the active
volcanoes of the world; snow-capped Mauna Kea Volcano in the distance.]

[Illustration: The internal structure of a typical shield volcano.]

                               Lava domes

[Illustration: A sketch of the havoc wrought in St. Pierre Harbor on
Martinique during the eruption of Mont Pelée in 1902.]

Volcanic or lava domes are formed by relatively small, bulbous masses of
lava too viscous to flow any great distance; consequently, on extrusion,
the lava piles over and around its vent. A dome grows largely by
expansion from within. As it grows its outer surface cools and hardens,
then shatters, spilling loose fragments down its sides. Some domes form
craggy knobs or spines over the volcanic vent, whereas others form
short, steep-sided lava flows known as “coulees.” Volcanic domes
commonly occur within the craters or on the flanks of large composite
volcanoes. The nearly circular Novarupta Dome that formed during the
1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and
200 feet high. The internal structure of this dome—defined by layering
of lava fanning upward and outward from the center—indicates that it
grew largely by expansion from within. Mont Pelée in Martinique, Lesser
Antilles, and Lassen Peak and Mono domes in California are examples of
lava domes. An extremely destructive eruption accompanied the growth of
a dome at Mont Pelée in 1902. The coastal town of St. Pierre, about 4
miles downslope to the south, was demolished and nearly 30,000
inhabitants were killed by an incandescent, high-velocity ash flow and
associated hot gases and volcanic dust. Only two men survived; one
because he was in a poorly ventilated, dungeon-like jail cell and the
other who somehow made his way safely through the burning city.

[Illustration: The Novarupta Dome formed during the 1912 eruption of
Katmai Volcano, Alaska.]

[Illustration: Schematic representation of the internal structure of a
typical volcanic dome.]

                       Other Volcanic Structures

                             Plugs (necks)

Congealed magma, along with fragmental volcanic and wallrock materials,
can be preserved in the feeding conduits of a volcano upon cessation of
activity. These preserved rocks form crudely cylindrical masses, from
which project radiating dikes; they may be visualized as the fossil
remains of the innards of a volcano (the so-called “volcanic plumbing
system”) and are referred to as volcanic _plugs_ or _necks_. The igneous
material in a plug may have a range of composition similar to that of
associated lavas or ash, but may also include fragments and blocks of
denser, coarser grained rocks—higher in iron and magnesium, lower in
silicon—thought to be samples of the Earth’s deep crust or upper mantle
plucked and transported by the ascending magma. Many plugs and necks are
largely or wholly composed of fragmental volcanic material and of
fragments of wallrock, which can be of any type. Plugs that bear a
particularly strong imprint of explosive eruption of highly gas-charged
magma are called _diatremes_ or _tuff-breccia_.

Volcanic plugs are believed to overlie a body of magma which could be
either still largely liquid or completely solid depending on the state
of activity of the volcano. Plugs are known, or postulated, to be
commonly funnel shaped and to taper downward into bodies increasingly
elliptical in plan or elongated to dike-like forms. Typically, volcanic
plugs and necks tend to be more resistant to erosion than their
enclosing rock formations. Thus, after the volcano becomes inactive and
deeply eroded, the exhumed plug may stand up in bold relief as an
irregular, columnar structure. One of the best known and most
spectacular diatremes in the United States is Ship Rock in New Mexico,
which towers some 1,700 feet above the more deeply eroded surrounding
plain. Volcanic plugs, including diatremes, are found elsewhere in the
western United States and also in Germany, South Africa, Tanzania, and

[Illustration: Ship Rock, San Juan County, New Mexico.]


Also called “tuff cones,” _maars_ are shallow, flat-floored craters that
scientists interpret have formed above diatremes as a result of a
violent expansion of magmatic gas or steam; deep erosion of a maar
presumably would expose a diatreme. Maars range in size from 200 to
6,500 feet across and from 30 to 650 feet deep, and most are commonly
filled with water to form natural lakes. Most maars have low rims
composed of a mixture of loose fragments of volcanic rock and rocks torn
from the walls of the diatreme.

Maars occur in the western United States, in the Eifel region of
Germany, and in other geologically young volcanic regions of the world.
An excellent example of a maar is Zuni Salt Lake in New Mexico, a
shallow saline lake that occupies a flat-floored crater about 6,500 feet
across and 400 feet deep. Its low rim is composed of loose pieces of
basaltic lava and wallrocks (sandstone, shale, limestone) of the
underlying diatreme, as well as random chunks of ancient crystalline
rocks blasted upward from great depths.

[Illustration: Zuni Salt Lake Maar, Catron County, New Mexico.]

                          Nonvolcanic craters

Some well-exposed, nearly circular areas of intensely deformed
sedimentary rocks, in which a central vent-like feature is surrounded by
a ring-shaped depression, resemble volcanic structures in gross form. As
no clear evidence of volcanic origin could be found in or near these
structures, scientists initially described them as “cryptovolcanic,” a
term now rarely used. Recent studies have shown that not all craters are
of volcanic origin. Impact craters, formed by collisions with the Earth
of large meteorites, asteroids, or comets, share with volcanoes the
imprints of violent origin, as evidenced by severe disruption, and even
local melting, of rock. Fragments of meteorites or chemically detectable
traces of extraterrestrial materials and indications of strong forces
acting from above, rather than from below, distinguish impact from
volcanic features.

Other possible explanations for these nonvolcanic craters include
subsurface salt-dome intrusion (and subsequent dissolution and
collapse); collapse caused by subsurface limestone dissolution and/or
ground-water withdrawal; and collapse related to melting of glacial ice.
An impressive example of an impact structure is Meteor Crater, Ariz.,
which is visited by thousands of tourists each year. This impact crater,
4,000 feet in diameter and 600 feet deep, was formed in the geologic
past (probably 30,000-50,000 years before present) by a meteorite
striking the Earth at a speed of many thousands of miles per hour.

In addition to Meteor Crater, very fresh, morphologically distinct,
impact craters are found at three sites near Odessa, Tex., as well as 10
or 12 other locations in the world. Of the more deeply eroded, less
obvious, postulated impact structures, there are about ten
well-established sites in the United States and perhaps 80 or 90
elsewhere in the world.

[Illustration: Meteor Crater, Arizona.]

[Illustration: Mount St. Helens, about noon, May 18, 1980.]

                      Types of Volcanic Eruptions

During an episode of activity, a volcano commonly displays a distinctive
pattern of behavior. Some mild eruptions merely discharge steam and
other gases, whereas other eruptions quietly extrude quantities of lava.
The most spectacular eruptions consist of violent explosions that blast
great clouds of gas-laden debris into the atmosphere.

The type of volcanic eruption is often labeled with the name of a
well-known volcano where characteristic behavior is similar—hence the
use of such terms as “Strombolian,” “Vulcanian,” “Vesuvian,” “Peléan,”
“Hawaiian,” and others. Some volcanoes may exhibit only one
characteristic type of eruption during an interval of activity—others
may display an entire sequence of types.

In a Strombolian-type eruption observed during the 1965 activity of
Irazú Volcano in Costa Rica, huge clots of molten lava burst from the
summit crater to form luminous arcs through the sky. Collecting on the
flanks of the cone, lava clots combined to stream down the slopes in
fiery rivulets.

[Illustration: Irazú Volcano, Costa Rica, 1965.]

In contrast, the eruptive activity of Parícutin Volcano in 1947
demonstrated a “Vulcanian”-type eruption, in which a dense cloud of
ash-laden gas explodes from the crater and rises high above the peak.
Steaming ash forms a whitish cloud near the upper level of the cone.

In a “Vesuvian” eruption, as typified by the eruption of Mount Vesuvius
in Italy in A.D. 79, great quantities of ash-laden gas are violently
discharged to form a cauliflower-shaped cloud high above the volcano.

[Illustration: Parícutin Volcano, Mexico, 1947.]

[Illustration: Mount Vesuvius Volcano, Italy, 1944.]

In a “Peléan” or “Nuée Ardente” (glowing cloud) eruption, such as
occurred on the Mayon Volcano in the Philippines in 1968, a large
quantity of gas, dust, ash, and incandescent lava fragments are blown
out of a central crater, fall back, and form tongue-like, glowing
avalanches that move downslope at velocities as great as 100 miles per
hour. Such eruptive activity can cause great destruction and loss of
life if it occurs in populated areas, as demonstrated by the devastation
of St. Pierre during the 1902 eruption of Mont Pelée on Martinique,
Lesser Antilles.

“Hawaiian” eruptions may occur along fissures or fractures that serve as
linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii
in 1950; or they may occur at a central vent such as during the 1959
eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In
fissure-type eruptions, molten, incandescent lava spurts from a fissure
on the volcano’s rift zone and feeds lava streams that flow downslope.
In central-vent eruptions, a fountain of fiery lava spurts to a height
of several hundred feet or more. Such lava may collect in old pit
craters to form lava lakes, or form cones, or feed radiating flows.

[Illustration: Mauna Loa Volcano, Hawaii, 1950.]

“Phreatic” (or steam-blast) eruptions are driven by explosive expanding
steam resulting from cold ground or surface water coming into contact
with hot rock or magma. The distinguishing feature of phreatic
explosions is that they only blast out fragments of preexisting solid
rock from the volcanic conduit; no new magma is erupted. Phreatic
activity is generally weak, but can be quite violent in some cases, such
as the 1965 eruption of Taal Volcano, Philippines, and the 1975-76
activity at La Soufrière, Guadeloupe (Lesser Antilles).

The most powerful eruptions are called “plinian” and involve the
explosive ejection of relatively viscous lava. Large plinian
eruptions—such as during 18 May 1980 at Mount St. Helens or, more
recently, during 15 June 1991 at Pinatubo in the Philippines—can send
ash and volcanic gas tens of miles into the air. The resulting ash
fallout can affect large areas hundreds of miles downwind. Fast-moving
deadly pyroclastic flows (“nuées ardentes”) are also commonly associated
with plinian eruptions.

[Illustration: Kilauea Volcano, Hawaii, 1959.]

[Illustration: Taal Volcano, Philippines, 1965.]

                          Submarine Volcanoes

Submarine volcanoes and volcanic vents are common features on certain
zones of the ocean floor. Some are active at the present time and, in
shallow water, disclose their presence by blasting steam and rock-debris
high above the surface of the sea. Many others lie at such great depths
that the tremendous weight of the water above them results in high,
confining pressure and prevents the formation and explosive release of
steam and gases. Even very large, deep-water eruptions may not disturb
the ocean surface.

[Illustration: Schematic representation of a typical submarine eruption
in the open ocean.]

The unlimited supply of water surrounding submarine volcanoes can cause
them to behave differently from volcanoes on land. Violent, steam-blast
eruptions take place when sea water pours into active shallow submarine
vents. Lava, erupting onto a shallow sea floor or flowing into the sea
from land, may cool so rapidly that it shatters into sand and rubble.
The result is the production of huge amounts of fragmental volcanic
debris. The famous “black sand” beaches of Hawaii were created virtually
instantaneously by the violent interaction between hot lava and sea
water. On the other hand, recent observations made from deep-diving
submersibles have shown that some submarine eruptions produce flows and
other volcanic structures remarkably similar to those formed on land.
Recent studies have revealed the presence of spectacular,
high-temperature hydrothermal plumes and vents (called “smokers”) along
some parts of the mid-oceanic volcanic rift systems. However, to date,
no direct observation has been made of a deep submarine eruption in

During an explosive submarine eruption in the shallow open ocean,
enormous piles of debris are built up around the active volcanic vent.
Ocean currents rework the debris in shallow water, while other debris
slumps from the upper part of the cone and flows into deep water along
the sea floor. Fine debris and ash in the eruptive plume are scattered
over a wide area in airborne clouds. Coarse debris in the same eruptive
plume rains into the sea and settles on the flanks of the cone. Pumice
from the eruption floats on the water and drifts with the ocean currents
over a large area.

[Illustration: Submarine eruption of Myojin-sho Volcano, Izu Islands,
Japan on September 23, 1952.]

                  Geysers, Fumaroles, and Hot Springs

Geysers, fumaroles (also called _solfataras_), and hot springs are
generally found in regions of young volcanic activity. Surface water
percolates downward through the rocks below the Earth’s surface to
high-temperature regions surrounding a magma reservoir, either active or
recently solidified but still hot. There the water is heated, becomes
less dense, and rises back to the surface along fissures and cracks.
Sometimes these features are called “dying volcanoes” because they seem
to represent the last stage of volcanic activity as the magma, at depth,
cools and hardens.

Erupting geysers provide spectacular displays of underground energy
suddenly unleashed, but their mechanisms are not completely understood.
Large amounts of hot water are presumed to fill underground cavities.
The water, upon further heating, is violently ejected when a portion of
it suddenly flashes into steam. This cycle can be repeated with
remarkable regularity, as for example, at Old Faithful Geyser in
Yellowstone National Park, which erupts on an average of about once
every 65 minutes.

[Illustration: Old Faithful Geyser, Yellowstone National Park, Wyoming.]

Fumaroles, which emit mixtures of steam and other gases, are fed by
conduits that pass through the water table before reaching the surface
of the ground. Hydrogen sulfide (HࠢS), one of the typical gases issuing
from fumaroles, readily oxidizes to sulfuric acid and native sulfur.
This accounts for the intense chemical activity and brightly colored
rocks in many thermal areas.

Hot springs occur in many thermal areas where the surface of the Earth
intersects the water table. The temperature and rate of discharge of hot
springs depend on factors such as the rate at which water circulates
through the system of underground channelways, the amount of heat
supplied at depth, and the extent of dilution of the heated water by
cool ground water near the surface.

[Illustration: Black Growler steam vents (fumaroles), Norris Basin,
Yellowstone National Park, Wyoming.]

[Illustration: Mammoth Hot Springs, Yellowstone National Park, Wyoming.]

                          Volcano Environments

There are more than 500 active volcanoes (those that have erupted at
least once within recorded history) in the world—50 of which are in the
United States (Hawaii, Alaska, Washington, Oregon, and
California)—although many more are hidden under the seas. Most active
volcanoes are strung like beads along, or near, the margins of the
continents, and more than half encircle the Pacific Ocean as a “Ring of

[Illustration: The distribution of some of the Earth’s 500 active

Many volcanoes are in and around the Mediterranean Sea. Mount Etna in
Sicily is the largest and highest of these mountains. Italy’s Vesuvius
is the only active volcano on the European mainland. Near the island of
Vulcano, the volcano Stromboli has been in a state of nearly continuous,
mild eruption since early Roman times. At night, sailors in the
Mediterranean can see the glow from the fiery molten material that is
hurled into the air. Very appropriately, Stromboli has been called “the
lighthouse of the Mediterranean.”

Some volcanoes crown island areas lying near the continents, and others
form chains of islands in the deep ocean basins. Volcanoes tend to
cluster along narrow mountainous belts where folding and fracturing of
the rocks provide channelways to the surface for the escape of magma.
Significantly, major earthquakes also occur along these belts,
indicating that volcanism and seismic activity are often closely
related, responding to the same dynamic Earth forces.

In a typical “island-arc” environment, volcanoes lie along the crest of
an arcuate, crustal ridge bounded on its convex side by a deep oceanic
trench. The granite or granitelike layer of the continental crust
extends beneath the ridge to the vicinity of the trench. Basaltic
magmas, generated in the mantle beneath the ridge, rise along fractures
through the granitic layer. These magmas commonly will be modified or
changed in composition during passage through the granitic layer and
erupt on the surface to form volcanoes built largely of non-basaltic

[Illustration: Mount Sinabung, Sumatra (By J. Baylor Roberts (c)
National Geographic Society).]

[Illustration: Island-arc environment]

In a typical “oceanic” environment, volcanoes are aligned along the
crest of a broad ridge that marks an active fracture system in the
oceanic crust. Basaltic magmas, generated in the upper mantle beneath
the ridge, rise along fractures through the basaltic layer. Because the
granitic crustal layer is absent, the magmas are not appreciably
modified or changed in composition and they erupt on the surface to form
basaltic volcanoes.

[Illustration: Mauna Kea Volcano, Hawaii.]

[Illustration: Oceanic environment]

In the typical “continental” environment, volcanoes are located in
unstable, mountainous belts that have thick roots of granite or
granitelike rock. Magmas, generated near the base of the mountain root,
rise slowly or intermittently along fractures in the crust. During
passage through the granitic layer, magmas are commonly modified or
changed in composition and erupt on the surface to form volcanoes
constructed of nonbasaltic rocks.

[Illustration: Mount Adams, Washington.]

[Illustration: Continental environment]

                         Plate-Tectonics Theory

[Illustration: In the Pacific Northwest, the Juan de Fuca Plate plunges
beneath the North American Plate, locally melting at depth; the magma
rises to feed and form the Cascade volcanoes.]

According to the now generally accepted “plate-tectonics” theory,
scientists believe that the Earth’s surface is broken into a number of
shifting slabs or plates, which average about 50 miles in thickness.
These plates move relative to one another above a hotter, deeper, more
mobile zone at average rates as great as a few inches per year. Most of
the world’s active volcanoes are located along or near the boundaries
between shifting plates and are called “plate-boundary” volcanoes.
However, some active volcanoes are not associated with plate boundaries,
and many of these so-called “intra-plate” volcanoes form roughly linear
chains in the interior of some oceanic plates. The Hawaiian Islands
provide perhaps the best example of an “intra-plate” volcanic chain,
developed by the northwest-moving Pacific plate passing over an inferred
“hot spot” that initiates the magma-generation and volcano-formation
process. The peripheral areas of the Pacific Ocean Basin, containing the
boundaries of several plates, are dotted by many active volcanoes that
form the so-called “Ring of Fire.” The “Ring” provides excellent
examples of “plate-boundary” volcanoes, including Mount St. Helens.

The accompanying figure shows the boundaries of lithosphere plates that
are presently active. The double lines indicate zones of spreading from
which plates are moving apart. The lines with barbs show zones of
underthrusting (subduction), where one plate is sliding beneath another.
The barbs on the lines indicate the overriding plate. The single line
defines a strike-slip fault along which plates are sliding horizontally
past one another. The stippled areas indicate a part of a continent,
exclusive of that along a plate boundary, which is undergoing active
extensional, compressional, or strike-slip faulting.

[Illustration: Major tectonic plates of the Earth.]

                       Extraterrestrial Volcanism

[Illustration: Mariner 9 imagery of Olympus Mons Volcano on Mars
compared to the eight principal Hawaiian Islands at the same scale
(Mariner 9 Image Mosaic, NASA/JPL).]

Volcanoes and volcanism are not restricted to the planet Earth. Manned
and unmanned planetary explorations, beginning in the late 1960’s, have
furnished graphic evidence of past volcanism and its products on the
Moon, Mars, Venus and other planetary bodies. Many pounds of volcanic
rocks were collected by astronauts during the various Apollo lunar
landing missions. Only a small fraction of these samples have been
subjected to exhaustive study by scientists. The bulk of the material is
stored under controlled-environment conditions at NASA’s Lunar Receiving
Laboratory in Houston, Tex., for future study by scientists.

From the 1976-1979 Viking mission, scientists have been able to study
the volcanoes on Mars, and their studies are very revealing when
compared with those of volcanoes on Earth. For example, Martian and
Hawaiian volcanoes closely resemble each other in form. Both are shield
volcanoes, have gently sloping flanks, large multiple collapse pits at
their centers, and appear to be built of fluid lavas that have left
numerous flow features on their flanks. The most obvious difference
between the two is size. The Martian shields are enormous. They can grow
to over 17 miles in height and more than 350 miles across, in contrast
to a maximum height of about 6 miles and width of 74 miles for the
Hawaiian shields.

Voyager-2 spacecraft images taken of Io, a moon of Jupiter, captured
volcanoes in the actual process of eruption. The volcanic plumes shown
on the image rise some 60 to 100 miles above the surface of the moon.
Thus, active volcanism is taking place, at present, on at least one
planetary body in addition to our Earth.

[Illustration: Spacecraft image, made in July 1979, shows volcanic plume
rising some 60 to 100 miles above the surface of Io, a moon of Jupiter
(Voyager 2 photo, NASA).]

                    Volcano Monitoring and Research

It has been said that the science of “volcanology” originated with the
accurate descriptions of the eruption of Vesuvius in A.D. 79 contained
in two letters from Pliny the Younger to the Roman historian Tacitus.
Pliny’s letters also described the death of his uncle, Pliny the Elder,
who was killed in the eruption. Actually, however, it was not until the
19th century that serious scientific inquiry into volcanic phenomena
flourished as part of the general revolution in the physical and life
sciences, including the new science of “geology.” In 1847, an
observatory was established on the flanks of Vesuvius, upslope from the
site of Herculaneum, for the more or less continuous recording of the
activity of the volcano that destroyed the city in A.D. 79. Still,
through the first decade of the 20th century, the study of volcanoes by
and large continued to be of an expeditionary nature, generally
undertaken after the eruption had begun or the activity had ceased.

[Illustration: The U.S. Geological Survey’s Hawaiian Volcano
Observatory, on the crater rim of Kilauea Volcano.]

Perhaps “modern” volcanology began in 1912, when Thomas A. Jaggar, Head
of the Geology Department of the Massachusetts Institute of Technology,
founded the Hawaiian Volcano Observatory (HVO), located on the rim of
Kilauea’s caldera. Initially supported by an association of Honolulu
businessmen, HVO began to conduct systematic and continuous monitoring
of seismic activity preceding, accompanying, and following eruptions, as
well as a wide variety of other geological, geophysical, and geochemical
observations and investigations. Between 1919 and 1948, HVO was
administered by various Federal agencies (National Weather Service, U.S.
Geological Survey, and National Park Service), and since 1948 it has
been operated continuously by the Geological Survey as part of its
Volcano Hazards Program. The more than 75 years of comprehensive
investigations by HVO and other scientists in Hawaii have added
substantially to our understanding of the eruptive mechanisms of Kilauea
and Mauna Loa, two of the world’s most active volcanoes. Moreover, the
Hawaiian Volcano Observatory pioneered and refined most of the commonly
used volcano-monitoring techniques presently employed by other
observatories monitoring active volcanoes elsewhere, principally in
Indonesia, Italy, Japan, Latin America, New Zealand, Lesser Antilles
(Caribbean), Philippines, and Kamchatka (U.S.S.R.).

What does “volcano monitoring” actually involve? Basically, it is the
keeping of a detailed “diary” of the changes—visible and invisible—in a
volcano and its surroundings. Between eruptions, visible changes of
importance to the scientists would include marked increase or decrease
of steaming from known vents; emergence of new steaming areas;
development of new ground cracks or widening of old ones; unusual or
inexplicable withering of plant life; changes in the color of mineral
deposits encrusting fumaroles; and any other directly observable, and
often measurable, feature that might reflect a change in the state of
the volcano. Of course, the “diary” keeping during eruptive activity
presents additional tasks. Wherever and whenever they can do so safely,
scientists document, in words and on film, the course of the eruption in
detail; make temperature measurements of lava and gas; collect the
eruptive products and gases for subsequent laboratory analysis; measure
the heights of lava fountains or ash plumes; gage the flow rate of ash
ejection or lava flows; and carry out other necessary observations and
measurements to fully document and characterize the eruption. For each
eruption, such documentation and data collection and analysis provide
another building block in constructing a model of the characteristic
behavior of a given volcano or type of eruption.

Volcano monitoring also involves the recording and analysis of volcanic
phenomena not visible to the human eye, but measurable by precise and
sophisticated instruments. These phenomena include ground movements,
earthquakes (particularly those too small to be felt by people),
variations in gas compositions, and deviations in local electrical and
magnetic fields that respond to pressure and stresses caused by the
subterranean magma movements.

Some common methods used to study invisible, volcano-related phenomena
are based on:

1. Measurement of changes in the shape of the volcano—volcanoes
gradually swell or “inflate” in building up to an eruption because of
the influx of magma into the volcano’s reservoir or “plumbing system”;
with the onset of eruption, pressure is immediately relieved and the
volcano rapidly shrinks or “deflates.” A wide variety of instruments,
including precise spirit-levels, electronic “tiltmeters,” and
electronic-laser beam instruments, can measure changes in the slope or
“tilt” of the volcano or in vertical and horizontal distances with a
precision of only a few parts in a million.

2. Precise determination of the location and magnitude of earthquakes by
a well-designed seismic network—as the volcano inflates by the rise of
magma, the enclosing rocks are deformed to the breaking point to
accommodate magma movement. When the rock ultimately fails to permit
continued magma ascent, earthquakes result. By carefully mapping out the
variations with time in the locations and depths of earthquake foci,
scientists in effect can track the subsurface movement of magma,
horizontally and vertically.

[Illustration: Scientist, wearing asbestos gloves and gas mask, samples
volcanic gases from active vent.]

3. Measurement of changes in volcanic-gas composition and in magnetic
field—the rise of magma high into the volcanic edifice may allow some of
the associated gases to escape along fractures, thereby causing the
composition of the gases (measured at the surface) to differ from that
usually measured when the volcano is quiescent and the magma is too deep
to allow gas to escape. Changes in the Earth’s magnetic field have been
noted preceding and accompanying some eruptions, and such changes are
believed to reflect temperature effects and/or the content of magnetic
minerals in the magma.

Recording historic eruptions and modern volcano-monitoring in themselves
are insufficient to fully determine the characteristic behavior of a
volcano, because a time record of such information, though perhaps long
in human terms, is much too short in geologic terms to permit reliable
predictions of possible future behavior. A comprehensive investigation
of any volcano must also include the careful, systematic mapping of the
nature, volume, and distribution of the products of prehistoric
eruptions, as well as the determination of their ages by modern isotopic
and other dating methods. Research on the volcano’s geologic past
extends the data base for refined estimates of the recurrence intervals
of active versus dormant periods in the history of the volcano. With
such information in hand, scientists can construct so-called “volcanic
hazards” maps that delineate the zones of greatest risk around the
volcano and that designate which zones are particularly susceptible to
certain types of volcanic hazards (lava flows, ash fall, toxic gases,
mudflows and associated flooding, etc.).

A strikingly successful example of volcano research and volcanic-hazard
assessment was the 1978 publication (Bulletin 1383-C) by two Geological
Survey scientists, Dwight Crandell and Donal Mullineaux, who concluded
that Mount St. Helens was the Cascade volcano most frequently active in
the past 4,500 years and the one most likely to reawaken to erupt, “...
perhaps before the end of this century.” Their prediction came true when
Mount St. Helens rumbled back to life in March of 1980. Intermittent
explosions of ash and steam and periodic formation of short-lived lava
domes continued throughout the decade. Analysis of the volcano’s past
behavior indicates that this kind of eruptive activity may continue for
years or decades, but another catastrophic eruption like that of May 18,
1980, is unlikely to occur soon.

On 18 May 1982, the U.S. Geological Survey (USGS) formally dedicated the
David A. Johnston Cascades Volcano Observatory (CVO) in Vancouver,
Washington, in memory of the Survey volcanologist killed two years
earlier. This facility—a sister observatory to the Hawaiian Volcano
Observatory—facilitates the increased monitoring and research on not
only Mount St. Helens but also the other volcanoes of the Cascade Range
of the Pacific Northwest. More recently, in cooperation with the State
of Alaska, the USGS established the Alaska Volcano Observatory in March
1988. The work being done at these volcano observatories provides
important comparisons and contrasts between the behavior of the
generally non-explosive Hawaiian shield volcanoes and that of the
generally explosive composite volcanoes of the Cascade and Alaskan
Peninsula-Aleutian chains.

                          Volcanoes and People

Volcanoes both harass and help mankind. As dramatically demonstrated by
the catastrophic eruption of Mount St. Helens on May 1980 and of
Pinatubo in June 1991, volcanoes can wreak havoc and devastation in the
short term. The types of volcanic and associated hazards are not
described in this booklet but treated in several of the publications
listed in _Suggested Reading_. However, it should be emphasized that the
short-term hazards posed by volcanoes are balanced by benefits of
volcanism and related processes over geologic time. Volcanic materials
ultimately break down to form some of the most fertile soils on Earth,
cultivation of which fostered and sustained civilizations. People use
volcanic products as construction materials, as abrasive and cleaning
agents, and as raw materials for many chemical and industrial uses. The
internal heat associated with some young volcanic systems has been
harnessed to produce geothermal energy. For example, the electrical
energy generated from The Geysers geothermal field in northern
California can meet the present power consumption of the city of San

The challenge to scientists involved with volcano research is to
mitigate the short-term adverse impacts of eruptions, so that society
may continue to enjoy the long-term benefits of volcanism. They must
continue to improve the capability for predicting eruptions and to
provide decision makers and the general public with the best possible
information on high-risk volcanoes for sound decisions on land-use
planning and public safety. Geo-scientists still do not fully understand
how volcanoes really work, but considerable advances have been made in
recent decades. An improved understanding of volcanic phenomena provides
important clues to the Earth’s past, present, and possibly its future.

                           Suggested Reading

  Decker, Robert, and Decker, Barbara, 1989, Volcanoes: W.H. Freeman and
  Company, New York, 285 p. (Revised edition).

  Editors, 1982, Volcano: in the series Planet Earth, Alexandria,
  Virginia, Time-Life Books, 176 p.

  Harris, S.L., 1988, Fire mountains of the West: The Cascade and Mono
  Lake Volcanoes: Missoula, Montana, Mountain Press Publishing Company,
  379 p.

  Heliker, Christina, 1990, Volcanic and seismic hazards on the Island
  of Hawaii: Reston, Virginia, U.S. Geological Survey, 48 p.

  Macdonald, G.A., 1972, Volcanoes: Englewood Cliffs, New Jersey,
  Prentice-Hall, Inc., 510 p.

  Simkin, Tom, Tilling, R.I., Taggart, J.N., Jones, W.J., and Spall,
  Henry, compilers, 1989, This dynamic planet: World Map of volcanoes,
  earthquakes, and plate tectonics: U.S. Geological Survey, Reston,
  Virginia, prepared in cooperation with the Smithsonian Institution,
  Washington, D.C. (scale 1:30,000,000 at equator).

  Tilling, R.I., Heliker, Christina, and Wright, T.L., 1989, Eruptions
  of Hawaiian volcanoes: Past, present, and future: Reston, Virginia,
  U.S. Geological Survey, 54 p.

  Tilling, R.I., Topinka, Lyn, and Swanson, D.A., 1990, Eruptions of
  Mount St. Helens: Past, present, and future: Reston, Virginia, U.S.
  Geological Survey, 56 p. (Revised edition).

  Tilling, R.I., 1991, Monitoring active volcanoes: Reston, Virginia,
  U.S. Geological Survey, 13 p. (Revised edition).

  Wood, C.A., and Kienle, Jurgen, 1990, Volcanoes of North America:
  United States and Canada: New York, Cambridge University Press, 354 p.

[Illustration: The port city of St. Pierre on the island of Martinique;
Mont Pelée is in the background. In 1902, this city was entirely
destroyed by pyroclastic flows; about 30,000 people died.]

[Illustration: ]

As the Nation’s principal conservation agency, the Department of the
Interior has responsibility for most of our nationally owned public
lands and natural and cultural resources. This includes fostering sound
use of our land and water resources; protecting our fish, wildlife, and
biological diversity; preserving the environmental and cultural values
of our national parks and historical places; and providing for the
enjoyment of life through outdoor recreation The Department assesses our
energy and mineral resources and works to ensure that their development
is in the best interests of all our people by encouraging stewardship
and citizen participation in their care. The Department also has a major
responsibility for American Indian reservation communities and for
people who live in island territories under U S administration

  U.S. Geological Survey
  Information Services
  P.O. Box 25286
  Denver, CO 80225

            For sale by the U.S. Government Printing Office
Superintendent of Documents, Mail Stop: SSOP, Washington, DC 20402-9328

                           U.S. GOVERNMENT PRINTING OFFICE: 1996 421-281

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