Caldera
The largest and most explosive volcanic eruptions eject tens to hundreds of
cubic kilometers of magma onto the Earth's
surface. When such a large volume of magma is removed from beneath a volcano,
the ground subsides or collapses into
the emptied space, to form a huge depression called a caldera.
Some calderas are more than 25 kilometers in diameter and several kilometers deep. -- Excerpt from: Brantley, 1994, Volcanoes of the United States:
USGS General Interest Publication
Cinder Cones and Scoria 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. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication
Composite Volcanoes and Stratovolcanoes
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,
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.
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. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication
Composite volcanoes tend to erupt
explosively and pose considerable danger to nearby life and property. -- Excerpt from: Tilling, Topinka, and Swanson, 1990,
Eruptions of Mount St. Helens: Past, Present, and Future: USGS General
Interest Publication
"Continental" Volcanoes
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 granite layer, magmas are commonly modified or changed in
composition and erupt on the surface to form volcanoes constructed of nonbasaltic rocks. -- Excerpt from:
Tilling, 1985, Volcanoes:
USGS General Interest Publication
"Island-Arc" Volcanoes
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
nonbasaltic rocks. -- Excerpt from:
Tilling, 1985, Volcanoes:
USGS General Interest Publication
Lava Plateaus
In some shield-volcano 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. -- Excerpt from: Tilling, 1985, Volcanoes:
USGS General Interest Publication
Lava Domes
Lava (usually dacite or rhyolite)
that is too sticky to flow far from its vent forms steep-sided mounds called
lava domes. -- Excerpt from: Wright and Pierson, 1992, USGS Circular 1973
Maars and Tuff Cones
Also called "tuff cones", maars are shallow, flat-floored craters
that scientists interpret have formed above diatremes
(a general term for a volcanic vent or pipe formed by the explosive energy of gas-charged magmas)
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 rocks and
rocks torn from the walls of the diatreme. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication
"Oceanic" Volcanoes
In a typical "oceanic" environment,
volcanoes are alined 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. -- Excerpt from:
Tilling, 1985, Volcanoes:
USGS General Interest Publication
Shield Volcanoes
Shield volcanoes 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 a
a warrior's shield. They are built up slowly by the accretion of
thousands of flows of highly fluid basaltic (from basalt, a
hard, dense dark volcanic rock) 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. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication
Submarine Volcanoes, Ridges, and Vents
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, deepwater eruptions may
not disturb the ocean surface. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication
Supervolcanoes
The term supervolcano has no specifically defined scientific meaning. It was used by the
producers of a British TV program in 2000 to refer to volcanoes that have generated Earth's
largest volcanic eruptions. As such, a supervolcano would be one that has produced an
exceedingly large, catastrophic explosive eruption and a giant caldera. Because Yellowstone has
produced three such very large caldera-forming explosive eruptions in the past 2.1 million
years, the producers considered it to be a supervolcano.
Because there is no well-defined minimum size for a "supervolcano", there is no exact number of
such volcanoes. Examples of volcanoes that produced exceedingly voluminous pyroclastic
eruptions and formed large calderas in the past 2 million years would include Yellowstone, Long
Valley in eastern California, Toba in Indonesia, and Taupo in New Zealand. Other
"supervolcanoes" would likely include the large caldera volcanoes of Japan, Indonesia, Alaska
(e.g., Aniakchak, Emmons, Fisher), and other areas. -- Excerpt from USGS Yellowstone Volcano Observatory Website, 2002
Tuyas
A "Tuya" is a volcano formed under a glacier.
Because volcanic activity in western
Canada was contemporaneous with the ebb and flow of
Cordilleran glaciations,
many of the volcanoes display ice contact features. Mount Garibaldi
itself is a supraglacial volcano which erupted onto a regional ice sheet.
Others, such as Hoodoo Mountain, were contained within basins thawed in
the ice and assumed the flat-topped form of tuyas. Still others, such as the
subglacial mounds of the Clearwater Field, were erupted under glacial ice
to form piles of pillow lava and hyaloclastite. -- Excerpt from: J.G. Souther, 1990, IN:
Wood and Kienle, 1990, Volcanoes of North America: United States and Canada:
Cambridge University Press, 354p.
Volcanic Fields and Lava Fields, Mafic Volcanoes, Monogenetic
Volcanoes
Mafic volcanoes typically erupt for brief time intervals (weeks to perhaps centuries), but some can grow almost as
large as composite volcanoes. Subsequent eruptions in the region typically issue from new vents and, over tens to
hundreds of thousands of years, build broad fields of many volcanoes. Prominent mafic volcanoes in the Three
Sisters region include North Sister, Mount Bachelor, Belknap Crater, Black Butte, and Mount Washington.
Hundreds more mafic volcanoes form the High Cascades of central Oregon between the neighboring composite
volcanoes of Mount Jefferson, 60 kilometers (40 miles) north of Three Sisters, Newberry volcano, a similar
distance southeast, and Crater Lake, 120 kilometers (75 miles) south. -- Excerpt from: Scott, et.al., 2001, Volcano Hazards in the Three Sisters Region, Oregon: USGS Open-File Report 99-437
Composite volcanoes
erupt episodically over tens
to hundreds of thousand of years and can display a
wide range of eruption styles.
Monogenetic volcanoes typically erupt for only
brief time intervals -- weeks to perhaps
centuries -- and generally display a narrower
range in eruptive behavior. Most monogenetic
volcanoes are basaltic in composition but... a few are of andesite
and dacite composition -- that is, with a relatively
higher silica content. -- Excerpt from:
Walder, et.al., 1999,
Volcano Hazards in the Mount Jefferson Region, Oregon:
USGS Open-File Report 99-24
Monogenetic volcanic fields
are collections of cinder cones, and/or
Maar vents and associated lava flows and
pyroclastic deposits.
Sometimes a stratovolcano is at the center of the field. -- Excerpt from:
Wood and Kienle, 1990, Volcanoes of North America: United States and Canada:
Cambridge University Press, 354p.
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BibleClass123.com Volcano Types, #2
