Types of volcanic
eruptions facts.
During
a volcanic eruption, lava, tephra (ash, lapilli, volcanic bombs and
blocks), and various gases are expelled from a volcanic vent or fissure.
Several types of volcanic eruptions have been distinguished by
volcanologists. These are often named after famous volcanoes where that
type of behavior has been observed. Some volcanoes may exhibit only one
characteristic type of eruption during a period of activity, while
others may display an entire sequence of types all in one eruptive
series.
There
are three different metatypes of eruptions. The most well-observed are
magmatic eruptions, which involve the decompression of gas within magma
that propels it forward. Phreatomagmatic eruptions are another type of
volcanic eruption, driven by a the compression of gas within magma, the
direct opposite of the process powering magmatic activity. The last
eruptive metatype is the Phreatic eruption, which is driven by the
superheating of steam via contact with magma; these eruptive types often
exhibit no magmatic release, instead causing the granulation of
existing rock.
.
Within
these wide-defining eruptive types are several subtypes. The weakest
are Hawaiian and submarine, then Strombolian, followed by Vulcanian and
Surtseyan. The stronger eruptive types are Pelean eruptions, followed by
Plinian eruptions; the strongest eruptions are called "Ultra Plinian."
Subglacial and Phreatic eruptions are defined by their eruptive
mechanism, and vary in strength. An important measure of erruptive
strength is Volcanic Explosivity Index (VEI), a magnitudic scale ranging
from 0 to 8 that often correlates to eruptive types.
Eruption mechanisms
Diagram showing the scale of VEI correlation with total ejecta volume.
Volcanic eruptions arise through three main mechanisms:
Gas release under decompression causing magmatic eruptions.
Thermal contraction from chilling on contact with water causing phreatomagmatic eruptions.
Ejection of entrained particles during steam eruptions causing phreatic eruptions.
There
are two types of eruptions in terms of activity, explosive eruptions
and effusive eruptions. Explosive eruptions are characterized by
gas-driven explosions that propels magma and tephra. Effusive eruptions,
meanwhile, are characterized by the outpouring of lava without
significant explosive eruption.
Volcanic
eruptions vary widely in strength. On the one extreme there are
effusive Hawaiian eruptions, which are characterized by lava fountains
and fluid lava flows, which are typically not very dangerous. On the
other extreme, Plinian eruptions are large, violent, and highly
dangerous explosive events. Volcanoes are not bound to one eruptive
style, and frequently display many different types, both passive and
explosive, even the span of a single eruptive cycle.Volcanoes do not
always erupt vertically from a single crater near their peak, either.
Some volcanoes exhibit lateral and fissure eruptions. Notably, many
Hawaiian eruptions start from rift zones, and some of the strongest
Surtseyan eruptions develop along fracture zones.
Volcano explositivity index
The
Volcanic Explosivity Index (commonly shortened VEI) is a scale, from 0
to 8, for measuring the strength of eruptions. It is used by the
Smithsonian Institution's Global Volcanism Program in assessing the
impact of historic and prehistoric lava flows. It operates in a way
similar to the Richter scale for earthquakes, in that each interval in
value represents a tendfold increasing in magnitude (it is logarithmic).
The vast majority of volcanic eruptions are of VEIs between 0 and 2.
Volcanic eruptions by VEI index
| VEI | Plume height | Eruptive volume * | Eruption type | Frequency | Example |
|---|---|---|---|---|---|
| 0 | <100 m (330 ft) | 1,000 m3 (35,300 cu ft) | Hawaiian | Continuous | Kilauea |
| 1 | 100–1,000 m (300–3,300 ft) | 10,000 m3 (353,000 cu ft) | Hawaiian/Strombolian | Months | Stromboli |
| 2 | 1–5 km (1–3 mi) | 1,000,000 m3 (35,300,000 cu ft) † | Strombolian/Vulcanian | Months | Galeras (1992) |
| 3 | 3–15 km (2–9 mi) | 10,000,000 m3 (353,000,000 cu ft) | Vulcanian | Yearly | Nevado del Ruiz (1985) |
| 4 | 10–25 km (6–16 mi) | 100,000,000 m3 (3.53×109 cu ft) | Vulcanian/Peléan | Few years | Galunggung (1982) |
| 5 | >25 km (16 mi) | 1 km3 (0.24 cu mi) | Plinian | 5–10 years | Mount St. Helens (1980) |
| 6 | >25 km (16 mi) | 10 km3 (2 cu mi) | Plinian/Ultra Plinian | 1,000 years | Krakatoa (1883) |
| 7 | >25 km (16 mi) | 100 km3 (20 cu mi) | Ultra Plinian | 10,000 years | Tambora (1815) |
| 8 | >25 km (16 mi) | 1,000 km3 (200 cu mi) | Ultra Plinian | 100,000 years | Lake Toba (74 ka) |
| * This is the minimum eruptive volume neccessary for the eruption to be considered within the category. | |||||
| ** Values are a rough estimate. Exceptions occur. |
| † There is a discontinuity between the 2nd and 3rd VEI level; instead of increasing by a magnitude of 10, the value increases by a magnitude of 100 (from 10,000 to 1,000,000). |
Magmatic eruptions
Magmatic
eruptions produce juvenile clasts during explosive decompression from
gas release. They range in intensity from the relatively small lava
fountains on Hawaii to catastrophic Ultra Plinian eruption columns more
than 30 km (19 mi) high, bigger than the AD 79 eruption that buried
Pompeii.
Hawaiian eruption
Diagram
of a Hawaiian eruption. (key: 1. Ash plume 2. Lava fountain 3. Crater
4. Lava lake 5. Fumaroles 6. Lava flow 7. Layers of lava and ash 8.
Stratum 9. Sill 10. Magma conduit 11. Magma chamber 12. Dike)
Hawaiian
eruptions are a type of volcanic eruption, named after the Hawaiian
volcanoes with which this eruptive type is hallmark. Hawaiian eruptions
are the calmest types of volcanic events, characterized by the effusive
eruption eruption of very fluid basalt-type lavas with low gasous
content. The volume of ejected material from Hawaiian eruptions is less
than half of that found in other eruptive types. Steady production of
small amounts of lava builds up the large, broad form of a shield
volcano. Eruptions are not centralized at the main summit as with other
volcanic types, and often occur at vents around the summit and from
fissure vents radiating out of the center.
Hawaiian
eruptions often begin as a line of vent eruptions along a fissure vent,
a so-called "curtain of fire." These die down as the lava beings to
concentrate at a few of the vents. Central-vent eruptions, meanwhile,
often take the form of large lava fountains (both continuous and
sporatic), which can reach heights of hundreds of meters or more. The
particles from lava fountains usually cool in the air before hitting the
ground, resulting in the accumulation of cindery scoria fragments;
however, when the air is especially thick with clasts, they cannot cool
off fast enough due to the surrounding heat, and hit the ground still
hot, the accumulation of which forms splatter cones. If eruptive rates
are high enough, they may even form splatter-fed lava flows. Hawaiian
eruptions are often extremely long lived; Pu'u O'o, a cinder cone of
Kilauea, has been erupting continuously since 1983. Another Hawaiian
volcanic feature is the formation of active lava lakes, self-maintaining
pools of raw lava with a thin crust of semi-cooled rock; there are
currently only 5 such lakes in the world, and the one at Kīlauea's
Kupaianaha vent is one of them.
Ropey pahoehoe lava from Kilauea, Hawaiʻi.
Flows
from Hawaiian eruptions are basaltic, and can be divided into two types
by their structural characteristics. Pahoehoe lava is a relatively
smooth lava flow that can be billowy or ropey. They can move as one
sheet, by the advancement of "toes," or as a snaking lava column. A'a
lava flows are denser and more viscous then pahoehoe, but tend to move
slower. Flows can measure 2 to 20 m (7 to 66 ft) thick. A'a flows are so
thick that the outside layers cools into a rubble-like mass, insulating
the still-hot interior and preventing it from cooling. A'a lava moves
in a peculiar way—the front of the flow steepens due to pressure from
behind until it breaks off, after which the general mass behind it moves
forward. Pahoehoe lava can sometimes become A'a lava due to increasing
viscosity or increasing rate of shear, but A'a lava never turns into
pahoehoe flow.
Hawaiian
eruptions are responsible for several unique volcanological objects.
Small volcanic particles are carried and formed by the wind, chilling
quickly into teardrop-shaped glassy fragments known as Pele's tears
(after Pele, the Hawaiian volcano deity). During especially high winds
these chunks may even take the form of long drawn out rods, known as
Pele's hair. Sometimes basant aerates into reticulite, the lowest
density rock type on earth.
Although
Hawaiian eruptions are named after the volcanoes of Hawaii, they are
not neccessarily restricted to them; the largest lava fountain ever
recorded formed on the island of Izu Ōshima (on Mount Mihara) in 1986, a
1,600 m (5,249 ft) gusher that was more than twice as high as the
mountain itself (which stands at 764 m (2,507 ft)).
Volcanoes known to have Hawaiian activity include:
Pu'u
O'o, a parasitic cinder cone located on Kilauea on the island of
Hawaiʻi which has been erupting continuously since 1983. The eruptions
began with a 6 km (4 mi)-long fissure-based "curtain of fire" on January
3. These gave way to centralized eruptions on the site of Kilauea's
east rift, eventually building up the still active cone.
For a list of all of the volcanoes of Hawaii, see List of volcanoes in the Hawaiian - Emperor seamount chain.
Mount Etna, Italy.
Mount Mihara in 1986 (see above paragraph)
Strombolian eruption
Diagram
of a Strombolian eruption. (key: 1. Ash plume 2. Lapilli 3. Volcanic
ash rain 4. Lava fountain 5. Volcanic bomb 6. Lava flow 7. Layers of
lava and ash 8. Stratum 9. Sill 10. Magma conduit 11. Magma chamber 12.
Dike)
Strombolian
eruptions are a type of volcanic eruption, named after the volcano
Stromboli, which has been erupting continuously for centuries.
Strombolian eruptions are driven by the bursting of gas bubbles within
the magma. These gas bubbles within the magma accumulate and coalesce
into large bubbles, called gas slugs. These grow large enough to rise
through the lava column.Upon reaching the surface, the difference in air
pressure causes the bubble to burst with a loud pop,throwing magma in
the air in a way similar to a soap bubble. Because of the high gas
pressures associated with the lavas, continued activity is generally in
the form of episodic explosive eruptions accompanied by the distinctive
loud blasts. During eruptions, these blasts occur as often as every few
minutes.
The
term "Strombolian" has been used indiscriminately to describe a wide
variety of volcanic eruptions, varying from small volcanic blasts to
large eruptive columns. In reality, true Strombolian eruptions are
characterized by short-lived and explosive eruptions of lavas with
intermediate viscosity, often ejected high into the air. Columns can
measure hundreds of meters in height. The lavas formed by Strombolian
eruptions are a form of relatively viscous basaltic lava, and its end
product is mostly scoria. The relative passivity of Strombolian
eruptions, and its non-damaging nature to its source vent allow
Strombolian eruptions to continue unabated for thousands of years, and
also makes it one of the least dangerous eruptive types.
Strombolian
eruptions eject volcanic bombs and lapilli fragments that travel in
parabolic paths before landing around their source vent. The steady
accumulation of small fragments builds cinder cones composed completely
of basaltic pyroclasts. This form of accumulation tends to result in
well-ordered rings of tephra.
Strombolian
eruptions are similar to Hawaiian eruptions, but there are differences.
Strombolian eruptions are noisier, produce no sustained eruptive
columns, do not produce some volcanic products associated with Hawaiian
volcanism (specifically Pele's tears and Pele's hair), and produce fewer
molten lava flows (although the eruptive material does tend to form
small rivulets).
Volcanoes known to have Strombolian activity include:
Parícutin,
Mexico, which erupted from a fissure in a cornfield in 1943. Two years
into its life, pyroclastic activity began to wane, and the outpouring of
lava from its base became its primary mode of activity. Eruptions
ceased in 1952, and the final height was 424 m (1,391 ft). This was the
first time that scientists are able to observe the complete life cycle
of a volcano.
Mount Etna, Italy, which has displayed Strombolian activity in recent eruptions, for example in 1981, 1999, 2002-2003, and 2009.
Mount
Erebus in Antarctica, the southernmost active volcano in the world,
having been observed erupting since 1972. Eruptive activity at Erebrus
consists of frequent Strombolian activity.
Stromboli
itself. The namesake of the mild explosive activity that it possesses
has been active throughout historical time; essentially continuous
Strombolian eruptions, occasionally accompanied by lava flows, have been
recorded at Stromboli for more than a millennium.
Vulcanian eruption
Diagram
of a Vulcanian eruption. (key: 1. Ash plume 2. Lapilli 3. Lava fountain
4. Volcanic ash rain 5. Volcanic bomb 6. Lava flow 7. Layers of lava
and ash 8. Stratum 9. Sill 10. Magma conduit 11. Magma chamber 12. Dike)
Vulcanian
eruptions are a type of volcanic eruption, named after the volcano
Vulcano, which also gives its name to the word Volcano. It was named so
following Giuseppe Mercalli's observations of its 1888-1890 eruptions.
In Vulcanian eruptions, highly viscous magma within the volcano make it
difficult for vesiculate gases to escape. Similar to Strombolian
eruptions, this leads to the buildup of high gas pressure, eventually
popping the cap holding the magma down and resulting in an explosive
eruption. However, unlike Strombolian eruptions, ejected lava fragments
are not aerodynamical; this is due to the higher viscosity of Vulcanian
magma and the greater incorporation of crystalline material broken off
from the former cap. They are also more explosive than their Strombolian
counterparts, with eruptive columns often reaching between 5 and 10 km
(3 and 6 mi) high. Lastly, Vulcanian deposits are andesitic to dacitic
rather than basaltic.
Initial
Vulcanian activity is characterized by a series of short-lived
explosions, lasting a few minutes to a few hours and typified by the
ejection of volcanic bombs and blocks. These eruptions wear down the
lava dome holding the magma down, and it disintegrates, leading to much
more quiet and continuous eruptions. Thus an early sign of future
Vulcanian activity is lava dome growth, and its collapse generates an
outpouring of pyroclastic material down the volcano's slope.
Deposits
near the source vent consist of large volcanic blocks and bombs, with
so-called "bread-crust bombs" being especially common. These deeply
cracked volcanic chunks form when the exterior of ejected lava cools
quickly into a glassy or fine-grained shell, but the inside continues to
cool and vesiculate. The center of the fragment expands, cracking the
exterior. However the bulk of Vulcanian deposits are fine grained ash.
The ash is only moderately dispersed, and its abundance indicates a high
degree of fragmentation, the result of high gas contents within the
magma. In some cases these have been found to be the result of
interaction with meteoric water, suggesting that Vulcanian eruptions are
partially hydrovolcanic.
Volcanoes that have exhibited Vulcanian activity include:
Sakurajima, Japan has been the site of Vulcanian activity near-continuously since 1955.
Tavurvur, Papua New Guinea, one of several volcanoes in the Rabaul Caldera.
Irazú Volcano in Costa Rica exhibited Vulcanian activity in its 1965 eruption.
Peléan eruption
Diagram
of Peléan eruption. (key: 1. Ash plume 2. Volcanic ash rain 3. Lava
dome 4. Volcanic bomb 5. Pyroclastic flow 6. Layers of lava and ash 7.
Stratum 8. Magma conduit 9. Magma chamber 10. Dike)
Peléan
eruptions (or nuée ardente) are a type of volcanic eruption, named
after the volcano Mount Pelée in Martinique, the site of a massive
Peléan eruption in 1902 that is one of the worst natural disasters in
history. In Peléan eruptions, a large amount of gas, dust, ash, and lava
fragmets are blown out the volcano's central crater, driven by the
collapse of rhyolite, dacite, and andesite lava dome collapses that
often create large eruptive columns. An early sign of a coming eruption
is the growth of a so-called Peléan or lava spine, a bulge in the
volcano's summit preempting its total collapse. The material collapses
upon itself, forming a fast-moving pyroclastic flow (known as a
block-and-ash flow) that moves down the side of the mountain at
tremendous speeds, often over 150 km (93 mi) per hour. These massive
landslides make Peléan eruptions one of the most dangerous in the world,
capable of tearing through populated areas and causing massive loss of
life. The 1902 eruption of Mount Pelée caused tremendous destruction,
killing more than 30,000 people and competely destroying the town of St.
Pierre, the worst volcanic event in the 20th century.
Peléan
eruptions are characterized most prominently by the incandescent
pyroclastic flows that they drive. The mechanics of a Peléan eruption
are very similar to that of a Vulcanian eruption, except that in Peléan
eruptions the volcano's structure is able to withstand more pressure,
hence the eruption occurs as one large explosion rather than several
smaller ones.
Volcanoes known to have Peléan activity include:
Mount
Pelée, Martinique. The 1902 eruption of Mount Pelée completely
devastated the island, destroying the town of St. Pierre and leaving
only 3 survivors. The eruption was directly preceded by lava dome
growth.
Mayon
Volcano, the Philippines most active volcano. It has been the site of
many different types of eruptions, Peléan included. Approximarly 40
ravines radiate from the summit and provide pathways for frequent
pyroclastic flows and mudslides to the lowlands below. Mayon's most
violent eruption occurred in 1814 and was responsible for over 1200
deaths.
The
1951 Peléan eruption of Mount Lamington. Prior to this eruption the
peak had not even been recognized as a volcano. Over 3,000 people were
killed, and it has become a benchmark for studying large Peléan
eruptions.
 |  |  |
Plinian eruption
Diagram
of a Plinian eruption. (key: 1. Ash plume 2. Magma conduit 3. Volcanic
ash rain 4. Layers of lava and ash 5. Stratum 6. Magma chamber)
Plinian
eruptions (or Vesuvian) are a type of volcanic eruption, named for the
historical AD 79 eruption of Mount Vesuvius that buried the Roman towns
of Pompeii and Herculaneum, and specifically for its chronicler Pliny
the Younger. The process powering Plinian eruptions starts in the magma
chamber, where dissolved volatile gases are stored in the magma. The
gases vesiculate and accumulate as they rise through the magma conduit.
These bubbles agglutinate and once they reach a certain size (about 75%
of the total volume of the magma conduit) they explode. The narrow
confines of the conduit force the gases and associated magma up, forming
an eruptive column. Eruption velocity is controlled by the gas contents
of the column, and low-strength surface rocks commonly crack under the
pressure of the eruption, forming a flared outgoing structure that
pushes the gases even faster.
These
massive eruptive columns are the distinctive feature of a Plinian
eruption, and reach up 2 to 45 km (1 to 28 mi) into the atmosphere. The
densest part of the plume, directly above the volcano, is driven
internally by gas expansion. As it reaches higher into the air the plume
expands and becomes less dense, convection and thermal expansion of
volcanic ash drive it even further up into the stratosphere. At the top
of the plume, powerful prevailing winds drive the plume in a direction
away from the volcano.
These
highly explosive eruptions are associated with volatile-rich dacitic to
rhyolitic lavas, and occur most typically at stratovolcanoes. Eruptions
can last anywhere from hours to days, with longer eruptions being
associated with more felsic volcanoes. Although they are associated with
felsic magma, Plinian eruptions can just as well occur at basaltic
volcanoes, given that the magma chamber differentiates and has a
structure rich in silicon dioxide.
Plinian
eruptions are similar to both Vulcanian and Strombolian eruptions,
except that rather than creating discrete explosive events, Plinian
eruptions form sustained eruptive columns. They are also similar to
Hawaiian lava fountains in that both eruptive types produce sustained
eruption columns maintained by the growth of bubbles that move up at
about the same speed as the magma surrounding them.
Regions
affected by Plinian eruptions are subjected to heavy pumice airfall
affecting an area 0.5 to 50 km3 (0 to 12 cu mi) in size. The material in
the ash plume eventually finds its way back to the ground, covering the
landscape in a thick layer of many cubic kilometers of ash.
However
the most dangerous eruptive feature are the pyroclastic flows generated
by material collapse, which move down the side of the mountain at
extreme speeds of up to 700 km (435 mi) per hour and with the ability to
extend the reach of the eruption hundreds of kilometers. The ejection
of hot material from the volcano's summit melts snowbanks and ice
deposits on the volcano, which mixes with tephra to form lahars, fast
moving mudslides with the consistency of wet concrete that move at the
speed of a river rapid.
Major Plinian eruptive events include:
The
historical AD 79 eruption of Mount Vesuvius buried the Roman towns of
Pompeii and Herculaneum under a layer of ash and tephra. It is the model
Plinian eruption. Mount Vesuvius has erupted multiple times since then,
for example in 1822.
The
1980 eruption of Mount St. Helens in Washington, which ripped apart the
volcano's summit, was a Plinian eruption of Volcanic Explosivity Index
(VEI) 5.
The
strongest types of erupions, with a VEI of 8, are so-called
"Ulta-Plinian" eruptions, such as the most recent one at Lake Toba 74
thousand years ago, which put out 2800 times the material erupted by
Mount St. Helens in 1980.
Hekla
in Iceland, an example of basaltic Pilian volcanism being its 1947-48
eruption. The past 800 years have been a pattern of violent initial
eruptions of pumice followed by prolonged extrusion of basaltic lava
from the lower part of the volcano.
Pinatubo
in the Philippines on 15 June 1991, which produced 5 km3 (1 cu mi) of
dacitic magma, a 40 km (25 mi) high eruption column, and released 17
megatons of sulfur dioxide.
Phreatomagmatic eruption
Phreatomagmatic
eruptions are eruptions that arise from interactions between water and
magma. They are driven from thermal contraction (as opposed to magmatic
eruptions, which are driven by thermal expansion) of magma when it comes
in contact with water. This temperature difference between the two
causes violent water-lava interactions that make up the eruption. The
products of phreatomagmatic eruptions are believed to be more regular in
shape and finer grained than the products of magmatic eruptions because
of the differences in eruptive mechanisms.
There
is debate about the exact nature of Phreatomagmatic eruptions, and some
scientists believe that fuel-coolant reactions may be more critical to
the explosive nature than thermal contraction. Fuel coolant reactions
may fragment the volcanic material by propagating stress waves, widening
cracks and increasing surface area that ultimetly lead to rapid cooling
and explosive contraction-driven eruptions.
Surtseyan eruption
Diagram
of a Surtseyan eruption. (key: 1. Water vapor cloud 2. Compressed ash
3. Crater 4. Water 5. Layers of lava and ash 6. Stratum 7. Magma conduit
8. Magma chamber 9. Dike)
A
Surtseyan eruption (or hydrovolcanic) is a type of volcanic eruption
caused by shallow-water interactions between water and lava, named so
after its most famous example, the eruption and formation of the island
of Surtsey off the coast of Iceland in 1963. Surtseyan eruptions are the
"wet" equivalent of ground-based Strombolian eruptions, but because of
where they are taking place they are much more explosive. This is
because as water is heated by lava, it flashes in steam and expands
violently, fragmenting the magma it is in contact with into fine-grained
ash. Surtseyan eruptions are the hallmark of shallow-water volcanic
oceanic islands, however they are specifically confined to them.
Surtseyan eruptions can happen on land as well, and are caused by rising
magma that comes into contact with an aquifer (water-bearing rock
formation) at shallow levels under the volcano. The products of
Surtseyan eruptions are generally oxidized palagonite basalts (though
andesitic eruptions do occur, albeit rarely), and like Strombolian
eruptions Surtseyan eruptions are generally continuous or otherwise
rhythmic.
A
distinct defining feature of a Surtseyan eruption is the formation of a
pyroclastic surge (or base surge), a ground hugging radial cloud that
develops along with the eruption column. Base surges are caused by the
gravitational collapse of a vaperous eruptive column, one that is denser
overall then a regular volcanic column. The densest part of the cloud
is nearest to the vent, resulting a wedge shape. Associated with these
laterally moving rings are dune-shaped depositions of rock left behind
by the lateral movement. These are occasionally disrupted by bomb sags,
rock that was flung out by the explosive eruption and followed a
ballistic path to the ground. Accumulations of wet, spherical ash known
as accretionary lapilli is another common surge indicator.
Over
time Surtseyan eruptions tend to form maars, broad low-relief volcanic
craters dug into the ground, and tuff rings, circular structures built
of rapidly quenched lava. These structures are associated with a single
vent eruption, however if eruptions arise along fracture zones a rift
zone may be dug out; these eruptions tend to be more violent then the
ones forming a tuff ring or maars, an example being the 1886 eruption of
Mount Tarawera. Littoral cones are another hydrovolcanic feature,
generated by the explosive deposition of basaltic tephra (although they
are not truly volcanic vents). They form when lava accumulates within
cracks in lava, superheats and explodes in a steam explosion, breaking
the rock apart and depositing it on the volcano's flank. Consecutive
explosions of this type eventually generate the cone.
Volcanoes known to have Surtseyan activity include:
Surtsey,
Iceland. The volcano built itself up from depth and emerged above the
Atlantic Ocean off the coast of Iceland in 1963. Initial hydrovolcanics
were highly explosive, but as the volcano grew out rising lava started
to interact less with the water and more with the air, until finally
Surtseyan activity waned and became more Strombolian in character.
Ukinrek Maars in Alaska, 1977, and Capelinhos in the Azores, 1957, both examples of above-water Surtseyan activity.
Mount Tarawera in New Zealand erupted along a rift zone in 1886, killing 150 people.
Ferdinandea,
a seamount in the Mediterranean Sea, breached sea level in July 1831
and was the source of a dispute over sovereignty between Italy, France,
and Great Britain. The volcano did not build tuff cones strongly enough
to withstand erosion, and disappeared back below the waves soon after it
appeared.
The
underwater volcano Hunga Tonga in Tonga breached sea level in 2009.
Both of its vents exhibited Surtseyan activity for much of the time. It
was also the site of an earlier eruption in May 1988.
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Submarine eruption
Diagram
of a Submarine eruption. (key: 1. Water vapor cloud 2. Water 3. Stratum
4. Lava flow 5. Magma conduit 6. Magma chamber 7. Dike 8. Pillow lava)
Submarine
eruptions are a type of volcanic eruption that occurs underwater. An
estimated 75% of the total volcanic eruptive volume is generated by
submarine eruptions near mid ocean ridges alone, however because of the
problems associated with detecting deep sea volcanics, they remained
virtually unknown until advances in the 1990s made it possible to
observe them.
Submarine
eruptions are generated by seamounts (underwater volcanoes), and are
driven by one of two processes. Volcanoes near plate boundaries and
mid-ocean ridges are built by the decompression melting of mantle rock
that floats up to the crustal surface. Eruptions near subducting zones,
meanwhile, are driven by subducting plates that adds volatiles to the
rising plate, raising its melting point. Each process generates
different rock; mid-ocean ridge volcanics are primarily basaltic,
whereas subduction flows are mostly calc-alkaline, and more explosive
and viscous.
Spreading
rates along mid-ocean ridges vary widely, from 2 cm (0.8 in) per year
at the Mid-Atlantic Ridge, to up to 16 cm (6 in) along the East Pacific
Rise. Higher spreading rates are a probably cause for higher levels of
volcanism. The technology for studying seamount eruptions did not exist
until advancements in hydrophone technology made it possible to "listen"
to acoustic waves, known as T-waves, released by submarine earthquakes
associated with submarine volcanic eruptions. The reason for this is
that land-based seismometers cannot detect sea-based earthquakes below a
magnitude of 4, but acoustic waves travel well in water and long
periods of time. A system in the North Pacific, maintained by the United
States Navy and originally intended for the detection of submarines,
has detected an event on average every 2 to 3 years.
The
most common underwater flow is pillow lava, a circular lava flow named
after its unusual shape. Less common are glassy, marginal sheet flows,
indicative of larger-scale flows. Volcaniclastic sedimentary rocks are
common in shallow-water environments. As plate movement starts to carry
the volcanoes away from their eruptive source, eruption rates start to
die down, and water erosion grinds the volcano down. The final stages of
eruption caps the seamount in alkalic flows. There are about 100,000
deepwater volcanoes in the world, although most are beyond the active
stage of their life. Some exemplery seamounts are Loihi Seamount, Bowie
Seamount, Cross Seamount, and Denson Seamount.
Subglacial eruption
A
diagram of a Subglacial eruption. (key: 1. Water vapor cloud 2. Crater
lake 3. Ice 4. Layers of lava and ash 5. Stratum 6. Pillow lava 7. Magma
conduit 8. Magma chamber 9. Dike)
Subglacial
eruptions are a type of volcanic eruption characterized by interactions
between lava and ice, often under a glacier. The nature of
glaciovolcanism dictates that it occurs at areas of high latitude and
high altitude. It has been suggested that subglacial volcanoes that are
not actively erupting often dump heat into the ice covering them,
producing meltwater. This meltwater mix means that subglacial eruptions
often generate dangerous jökulhlaups (floods) and lahars.
The
study of glaciovolcanism is still a relatively new field. Early
accounts described the unusual flat-topped steep-sided volcanoes (called
tuyas) in Iceland that were suggested to have formed from eruptions
below ice. The first English-language paper on the subject was published
in 1947 by William Henry Mathews, describing the Tuya Butte field in
northwest British Columbia. The eruptive process that builds these
structures, originally inferred in the paper, begins with volcanic
growth below the glacier. At first the eruptions resemble those that
occur in the deep sea, forming piles of pillow lava at the base of the
volcanic structure. Some of the lava shatters when it comes in contact
with the cold ice, forming a glassy breccia called hyaloclastite. After a
while the ice finally melts into a lake, and the more explosive
eruptions of Surtseyan activity begins, building up flanks made up of
mostly hyaloclastite. Eventually the lake boils off from continued
volcanism, and the lava flows become more effusive and thicken as the
lava cools much more slowly, often forming columnar jointing.
Well-preserved tuyas show all of these stages, for example
Hjorleifshofdi in Iceland.
Products
of volcano-ice interactions stand as various structures, whose shape is
dependent on complex eruptive and environmental interactions. Glacial
volcanism is a good indicator of past ice distribution, making it an
important climatic marker. Since they are imbedded in ice, as ice
retracts worldwide there are concerns that tuyas and other structures
may destabalize, resulting in mass landslides. Evidence of
volcanic-glacial interactions are evident in Iceland and parts of
British Columbia, and it's even possible that they play a role in
deglaciation.
Glaciovolcanic
products have been identified in Iceland, British Columbia, Hawaii and
Alaska, the Cascade Range, South America and even on the planet
Mars.Volcanoes known to have subglacial activity include:
Mauna
Kea in tropical Hawaii. There is evidence of past subglacial eruptive
activity on the volcano in the form of a subglacial deposit on its
summit. The eruptions originated about 10,000 years ago, during the last
ice age, when the summit of Mauna Kea was covered in ice.
In
2008, the British Antarctic Survey reported a volcanic eruption under
the Antarctica ice sheet 2,200 years ago. It is believed to be that this
was the biggest eruption in Antarctica in the last 10,000 years.
Volcanic ash deposits from the volcano were identified through an
airborne radar survey, buried under later snowfalls in the Hudson
Mountains, close to Pine Island Glacier.
Iceland,
well known for both glaciers and volcanoes, is often a site of
subglacial eruptions. An example an eruption under the Vatnajökull ice
cap in 1996, which occurred under an estimated 2,500 ft (762 m) of ice.
As
part of the search for life on Mars, scientists have suggested that
there may be subglacial volcanoes on the red planet. Several potential
sites of such volcanism have been reviewed, and compared extensively
with similar features in Iceland:
"Viable
microbial communities have been found living in deep (2800 m)
geothermal groundwater at 349 K and pressures over 300 bar..Furthermore,
microbes have been postulated to exist in basaltic rocks in rinds of
altered volcanic glass. All of these conditions could exist in polar
regions of Mars today where subglacial volcanism has occurred."
—Jack Farmer, Arizona State University
Phreatic eruption
Diagram
of a Phreatic eruption. (key: 1. Water vapor cloud 2. Volcanic bomb 3.
Magma conduit 4. Layers of lava and ash 5. Stratum 6. Water table 7.
Explosion 8. Magma chamber)
Phreatic
eruptions (or steam-blast eruptions) are a type of eruption driven by
the expansion of steam. When cold ground or surface water coming into
contact with hot rock or magma it superheats and explodes, fracturing
the surrounding rock and thrusting out a mixture of steam, water , ash,
volcanic bombs, and volcanic blocks. The distinguishing feature of
phreatic explosions is that they only blast out fragments of
pre-existing solid rock from the volcanic conduit; no new magma is
erupted.Because they are driven by the cracking of rock stata under
pressure, Phreatic activity does not always result in an eruption; if
the rock face is strong enough to withstand the explosive force,
outright eruptions may not occur, although cracks in the rock will
probably develop and weaken it, furthering future eruptions.
Often
a precursor of future volcanic activity, Phreatic eruptions are
generally weak, although there have been exceptions. Some Phreatic
events may be triggered by earthquake activity, another volcanic
precursor, and they may also travel along dike lines. Phreatic eruptions
form base surges, lahars, avalanches, and volcanic block "rain." They
may also release deadly toxic gas able to sufficate anyone in range of
the eruption.
Volcaoes known the exhibit Phreatic activity include:
Mount St. Helens, which exhibited Phreatic activity just prior to its catastrophic 1980 eruption (which was itself Plinian.
Taal Volcano, Philippines, 1965.
La Soufrière of Guadeloupe (Lesser Antilles), 1975-1976 activity.
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