Volcano Types
http://www.geology.sdsu.edu/how_volcanoes_work/Volcano_types.html
1. Scoria Cone –
2. Shield Volcano –
3. Strato Volcano –
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VOLCANO TYPES
GENERIC FEATURES
A volcanic vent is an opening exposed on the earth's surface where volcanic material is
emitted. All volcanoes contain a central vent underlying the summit crater of the volcano.
The volcano's cone-shaped structure, or edifice, is built by the more-or-less symmetrical
accumulation of lava and/or pyroclastic material around this central vent system. The
central vent is connected at depth to a magma chamber, which is the main storage area
for the eruptive material. Because volcano flanks are inherently unstable, they often
contain fractures that descend downward toward the central vent, or toward a shallow-level
magma chamber. Such fractures may occasionally tap the magma source and act as conduits for flank eruptions along
the sides of the volcanic edifice. These eruptions can generate cone-shaped accumulations of volcanic material, called
parasitic cones. Fractures can also act as conduits for escaping volcanic gases, which are released at the surface through
vent openings called fumaroles.
Summit Crater
Parasitic Cones
Fumarole
MAIN VOLCANO TYPES
Although every volcano has a unique eruptive history, most can be grouped into three main types based largely on their
eruptive patterns and their general forms. The form and composition of the three main volcano types are summarized
here:
VOLCANO
TYPE
VOLCANO
SHAPE
SCORIA CONE
Straight sides with steep slopes;
large summit crater
SHIELD VOLCANO
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Very gentle slopes; convex
upward
COMPOSITION
ERUPTION
TYPE
Basalt tephra;
occasionally andesitic
Strombolian
Basalt lava flows
Hawaiian
STRATOVOLCANO
Gentle lower slopes, but steep
upper slopes; concave upward;
small summit crater
Highly variable;
alternating basaltic to
rhyolitic lavas and tephra
with an overall andesite
composition
Plinian
SUBORDINATE VOLCANO TYPES -- Lava and tephra can erupt from vents other than these three main volcano types.
A fissure eruption, for example, can generate huge volumes of basalt lava; however, this type of eruption is not associated
with the construction of a volcanic edifice around a single central vent system. Although point-source eruptions can
generate such features as spatter cones and hornitos, these volcanic edifices are typically small, localized, and/or
associated with rootless eruptions (i.e., eruptions above the surface of an active lavaflow, unconnected to an overlying
magma chamber) . Vent types related to hydrovolcanic processes generate unique volcanic structures, discussed
separately under hydrovolcanic eruptions.
WHEN IS A VOLCANO CONSIDERED ACTIVE, DORMANT, OR EXTINCT?
Classifying a volcano as active, dormant, or extinct is a subjective and inexact exercise. A volcano is generally considered
active if it has erupted in historic time. This definition, however, is rather ambiguous, because recorded history varies from
thousands of years in Europe and the Middle East, to only a few hundred years in other regions of the world, like the
Pacific Northwest of the United States. Scientists generally consider a volcano active if it is currently erupting, or exhibiting
unrest through earthquakes, uplift, and/or new gas emissions. The Smithsonian Institution's catalog of active volcanoes,
recognizes 539 volcanoes with historic eruptions. In addition, there are 529 volcanoes that have not erupted in historic
times, but which exhibit clear evidence of eruption in the past 10,000 years. These latter volcanoes are probably best
considered "dormant," since they have the potential to erupt again.
Whether or not inactive volcanoes are considered truly extinct, or just dormant, depends partly on the average repose
interval between eruptions. As noted in eruptive variability, explosive eruptions like those at Toba and Yellowstone have
repose intervals of hundreds of thousands of years, whereas non-explosive eruptions have very short repose intervals.
Thus, the Yellowstone region, which has not experienced an eruption for 70,000 years, can not be considered extinct. In
fact, many scientists consider Yellowstone to be active because of high uplift rates, frequent earthquakes, and a very
active geothermal system. Many inactive scoria cones, on the other hand, may be viewed as extinct shortly after they
erupt, because such volcanoes are typically monogenetic and only erupt once.
Scoria cones, also known as cinder cones, are the most common type of volcano. They are also the smallest type, with
heights generally less than 300 meters. They can occur as discrete volcanoes on basaltic lava fields, or as parasitic cones
generated by flank eruptions on shield volcanoes and stratovolcanoes.
Scoria cones are composed almost wholly of ejected basaltic tephra. (Ash, lapilli, bombs or blocks)The tephra is most
commonly of lapilli size, although bomb-size fragments and lava spatter may also be present. The tephra fragments
typically contain abundant gas bubbles (vesicles), giving the lapilli and bombs a cindery (or scoriaceous) appearance.
The rapid eruption of expanding gases results in the obliteration and fragmentation of magma and rock. The greater the
explosivity, the greater the amount of fragmentation. Individual eruptive fragments are called pyroclasts ("fire
fragments"). Tephra (Greek, for ash) is a generic term for any airborne pyroclastic accumulation. Whereas tephra is
unconsolidated, a pyroclastic rock is produced from the consolidation of pyroclastic accumulations into a coherent rock
type.
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CLASSIFICATION OF PYROCLASTS - Tephra is classified on the basis of pyroclast size:
ASH -- Very fine-grained fragments (<
2 mm), generally dominated by broken
glass shards, but with variable amounts
of broken crystal and lithic (rock)
fragments. Courtesy of USGS.
LAPILLI -- Pea- to walnut-size
pyroclasts (2 to 64 mm). They often
look like cinders. In water-rich
eruptions, the accretion of wet ash may
form rounded spheres known as
accretionary lapilli (left). Courtesy of
USGS.
BLOCKS AND BOMBS -- Fragments
>64 mm. Bombs are ejected as
incandescent lava fragments which
were semi-molten when airborne, thus
inheriting streamlined, aerodynamic
shapes. Blocks (not shown) are ejected
as solid fragments with angular shapes.
Courtesy of J.P. Lockwood, USGS.
The tephra accumulates as scoria-fall deposits which build up around the vent to form the volcanic edifice (cone-shaped
structure) is built by the more-or-less symmetrical accumulation of lava. The edifice has very steep slopes, up to 35
degrees, although older eroded scoria cones typically have gentler slopes, from 15 to 20 degrees. Unlike the other two
main volcano types, scoria cones have straight sides and very large summit craters, with respect to their relatively small
edifices. They are often symmetric, although many are asymmetric due to (1) the build up of tephra on the downwind flank
of the edifice, (2) elongation of the volcano above an eruptive fissure, or (3) partial rafting of an outer wall of the volcano
due to basalt lava oozing outward from beneath the volcano edifice. Where scoria cones have been breached, they
typically reveal red-oxidized interiors.
Scoria cone on Mauna Kea
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Sunset Crater scoria cone
La Poruna scoria cone
Scoria cones are generated by Strombolian eruptions, (true strombolian activity is characterized by short-lived, explosive
outbursts of pasty lava ejected a few tens or hundreds of meters into the air) which produce eruptive columns of basalt
tephra. Many scoria cones are monogenetic in that they only erupt once, in contrast to shield volcanoes and
stratovolcanoes. An exception is the Cerro Negro volcano in Nicaragua, which is the Earth's most historically active
scoria cone. It is one of several parasitic cones on the northwest flank of Las Pilas volcano. Cerro Negro has erupted
more than twenty times since it was born in 1850. Its most recent eruptions were in 1992 and 1995.
Shield volcanoes are broad, low-profile features with basal diameters that vary from a few kilometers to over 100
kilometers (e.g., the Mauna Loa volcano, Hawaii). Their heights are typically about 1/20th of their widths. The lower slopes
are often gentle (2-3 degrees), but the middle slopes become steeper (~10 degrees) and then flatten at the summit. This
gives shield volcanoes a flank morphology that is convex in an upward direction. Their overall broad shapes result from
the extrusion of very fluid (low viscosity) basalt lava that spreads outward from the summit area, in contrast to the vertical
accumulation of airfall tephra around scoria-cone vents, and the build-up of viscous lava and tephra around
stratovolcanoes. Cross-sections through shield volcanoes reveal numerous thin flow units of pahoehoe basalt which has a
higher fluidity (lower viscosity) is a product of their lower SiO2 (silica) contents, typically < 1 m thick. Pyroclastic deposits
are minor (< 1%) and of limited dispersal, generally from flank eruptions associated with parasitic scoria cones, or from
rare, localized hydrovolcanic eruptions.
Very thin pahoehoe flow units
(< 0.5 m thick) on the flank of a
shield volcano in western
Saudi Arabia. Photo by Vic
Camp.
The Mauna Loa volcano on the Big Island of Hawaii, seen here in both oblique and satellite views, is the world's largest
shield volcano:
Mauna Loa from the southeast
Mauna Loa satellite view
Shield volcanoes are generated by Hawaiian eruptions. However, there is some variability in their eruptive style, which
translates into variations in shield morphology and size. The almost perfect symmetry and small volume (~15 km3) of
Icelandic shields, for example, stands in marked contrast to the elongation and huge volume (thousands of km3) of
Hawaiian shields. These variations are largely attributed to the monogenetic, small-volume, centralized summit
eruptions, typical of icelandic shields, and the polygenetic, large-volume, linear fissure eruptions, typical of most hawaiian
shields. Still different are the symmetrical Galapagos shields, shown below, which have steep middle slopes (>10
degrees) and flat tops occupied by large and very deep calderas.
Calderas are some of the most spectacular features on Earth. They are large volcanic craters that form by two different
methods: 1) an explosive volcanic eruption; or, 2) collapse of surface rock into an empty magma chamber. These shield
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types appear to be generated by ring-fracture eruptions, which delineate the sides of the caldera and mark the site of
caldera collapse. Geology.com
Stratovolcanoes, also known as composite cones, are the most picturesque and the most deadly of the volcano types.
Their lower slopes are gentle, but they rise steeply near the summit to produce an overall morphology that is concave in
an upward direction. The summit area typically contains a surprisingly small summit crater. This classic stratovolcano
shape is exemplified by many well-known stratovolcanoes, such as Mt. Fuji in Japan, Mt. Mayon in the Philippines, and
Mt. Agua in Guatemala.
Mt. Mayon
Mt. Agua
In detail, however, stratovolcano shapes are more variable than these classic examples, primarily because of wide
variations in eruptive style and composition. Some may contain several eruptive centers, a caldera, or perhaps an
amphitheater as the result of a lateral blast (e.g., Mt. St. Helens).
Typically, as shown in the image to the left, stratovolcanoes have a layered or stratified
appearance with alternating lava flows, airfall tephra, pyroclastic flows, volcanic
mudflows (lahars), and/or debris flows. The compositional spectrum of these rock types
may vary from basalt to rhyolite in a single volcano; however, the overall average
composition of stratovolcanoes is andesitic. Many oceanic stratovolcanoes tend to be
more mafic than their continental counterparts. The variability of stratovolcanoes is
evident when examining the eruptive history of individual volcanoes. Mt. Fuji and Mt.
Etna, for example, are dominanted by basaltic lava flows, whereas Mt. Rainier is
dominated by andesitic lava, Mt. St. Helens by andesitic-to-dacitic pyroclastic material,
and Mt. Lassen by dacitic lava domes.
A pyroclastic flow is a fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the
flank of a volcanic edifice. These awesome features are heavier-than-air emulsions that move much like a snow
avalanche, except that they are fiercely hot, contain toxic gases, and move at phenomenal, hurricane-force speeds, often
over 100 km/hour. They are the most deadly of all volcanic phenomena.
Only ten elements make up the bulk of most magmas: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg),
titanium (Ti) calcium (Ca), sodium (Na), potassium (K), and phosphorous (P). Because oxygen and silicon are by far the
two most abundant elements in magma, it is convenient to describe the different magma types in terms of their silica
content (SiO2). The magma types vary from mafic magmas, which have relatively low silica and high Fe and Mg
contents, to felsic magmas, which have relatively high silica and low Fe and Mg contents. Mafic magma will cool and
crystallize to produce the volcanic rock basalt, whereas felsic magma will crystallize to produce dacite and rhyolite.
Intermediate-composition magmas will crystallize to produce the rock andesite. Because the mafic rocks are enriched in
Fe and Mg, they tend to be darker colored than the felsic rock types.
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SiO2 CONTENT
MAGMA TYPE
VOLCANIC ROCK
~50%
Mafic
Basalt
~60%
Intermediate
Andesite
~65%
Felsic (low Si)
Dacite
~70%
Felsic (high Si)
Rhyolite
Stratovolcanoes typically form at convergent plate margins, where one plate descends beneath an adjacent plate at the
site of a subduction zone. Examples of subduction-related stratovolcanoes can be found in many places in the world, but
they are particularly abundant along the rim of the Pacific Ocean, a region known as Ring of Fire. In the Americas, the
Ring of Fire includes stratovolcanoes forming the Aleutian islands in Alaska, the crest of the Cascade Mountains in the
Pacific Northwest, and the high peaks of the Andes Mounains in South America. A satellite view of three stratovolcanoes
from the Andes is shown here:
Three Andean stratovolcanoes in
northern Ecaudor
The eruptive history of most stratovolcanoes is delineated by highly explosive Plinian eruptions. These dangerous
eruptions are often associated with deadly pyroclastic flows composed of hot volcanic fragments and toxic gases that
advance down slopes at hurricane-force speeds. Like shield volcanoes, stratovolcanoes are polygenetic; however, they
differ from shield volcanoes in that they erupt infrequently, with typical repose intervals of hundreds of years between
eruptions. Most active stratovolcanoes worldwide appear to be < 100,000 years old, although some, like Mt. Rainier, may
be more than 1 million years old.
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