A stratovolcano, also known as a composite volcano, is a typically Volcanic cone volcano built up by many alternating layers (Stratum) of hardened lava and tephra.
The term composite volcano is used because strata are usually mixed and uneven instead of neat layers.
The existence of stratovolcanoes on other bodies of the Solar System has not been conclusively demonstrated.
Distribution
Stratovolcanoes are common at
subduction zones, forming chains and clusters along
Plate tectonics boundaries where an
oceanic crust plate is drawn under a continental crust plate (continental arc volcanism, e.g.
Cascade Range,
Andes, Campania) or another
oceanic crust plate (
island arc volcanism, e.g.
Japan,
Philippines,
Aleutian Islands).
Stratovolcanoes also occur in some other geological settings, for example as a result of intraplate volcanism on oceanic islands far from plate boundaries. Examples are Teide in the Canary Islands,
Formation
Subduction volcanoes form when
hydrous are pulled down into the mantle on the slab. These hydrous minerals, such as
chlorite and serpentine, release their water into the mantle which decreases its
melting point by . The release of water from
minerals is termed "
dewatering", and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths.
This allows the mantle to partially melt and generate
magma. This is called
flux melting. The magma then rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition. When the magma nears the top surface, it pools in a
magma chamber within the crust below the stratovolcano.
The processes that trigger the final eruption remain a question for further research. Possible mechanisms include:
-
Magma differentiation, in which the lightest, most silica-rich magma and volatiles such as water, , and sulfur dioxide accumulate in the uppermost part of the magma chamber. This can dramatically increase pressures.
-
Fractional crystallization of the magma. When anhydrous such as feldspar crystallize out of the magma, this concentrates volatiles in the remaining liquid, which can lead to a second boiling that causes a gas phase (carbon dioxide or water) to separate from the liquid magma and raise magma chamber pressures.
-
Injection of fresh magma into the magma chamber, which mixes and heats the cooler magma already present. This could force volatiles out of solution and lower the density of the cooler magma, both of which increase pressure. There is considerable evidence for magma mixing just before many eruptions, including magnesium-rich olivine crystals in freshly erupted silicic lava that show no reaction rim. This is possible only if the lava erupted immediately after mixing since olivine rapidly reacts with silicic magma to form a rim of pyroxene.
-
Progressive melting of the surrounding country rock.
These internal triggers may be modified by external triggers such as sector collapse, , or interactions with groundwater. Some of these triggers operate only under limited conditions. For example, sector collapse (where part of the flank of a volcano collapses in a massive landslide) can only trigger the eruption of a very shallow magma chamber. Magma differentiation and thermal expansion also are ineffective as triggers for eruptions from deep magma chambers.
Hazards
In
recorded history, explosive eruptions at
subduction zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations.
stratovolcanoes, such as Mount St. Helens,
Mount Etna and
Mount Pinatubo, typically erupt with explosive force because the
magma is too
Viscosity to allow easy escape of
.
As a consequence, the tremendous internal pressures of the trapped volcanic gases remain and intermingle in the pasty
magma. Following the breaching of the
Volcanic vent and the opening of the crater, the
magma degasses explosively. The magma and gases blast out with high speed and full force.
Since 1600 Common Era, nearly 300,000 people have been killed by volcanic eruptions. Most deaths were caused by and , deadly hazards that often accompany explosive eruptions of subduction-zone stratovolcanoes.
Two Decade Volcanoes that erupted in 1991 provide examples of stratovolcano hazards. On 15 June, Mount Pinatubo erupted and caused an Volcanic ash to shoot 40 km (25 mi) into the air. It produced large pyroclastic surges and lahar floods that caused a lot of damage to the surrounding area.
The eruption of Mount Vesuvius in 79 AD is the most famous example of a hazardous stratovolcano eruption. Pyroclastic surges completely smothered the nearby ancient cities of Pompeii and Herculaneum with thick deposits of ash and pumice ranging from 6–7 meters deep. Pompeii had 10,000–20,000 inhabitants at the time of eruption.
Ash
In addition to potentially affecting the climate,
volcanic ash clouds from explosive eruptions pose a serious hazard to
aviation.
clouds consist of silt- or sand-sized pieces of rock, mineral,
volcanic glass. Volcanic ash grains are jagged, abrasive, and don't dissolve in water.
For example, during the 1982 eruption of
Galunggung in
Java, British Airways Flight 9 flew into the ash cloud, causing it to sustain temporary engine failure and structural damage.
Although no crashes have happened due to ash, more than 60, mostly commercial aircraft, have been damaged. Some of these incidents resulted in emergency landings.
are a threat to health when inhaled and are also a threat to property. A square yard of a 4-inch thick volcanic ash layer can weigh 120–200 pounds and can get twice as heavy when wet. Wet ash also poses a risk to electronics due to its conductive nature.
Dense clouds of hot volcanic ash can be expelled due to the collapse of an
Eruption column, or laterally due to the partial collapse of a
volcanic edifice or
lava dome during explosive eruptions. These clouds are known as pyroclastic surges and in addition to volcanic ash, they contain hot
lava,
pumice,
Volcanic rock, and volcanic gas. Pyroclastic surges flow at speeds over 50 mph and are at temperatures between 200 °C – 700 °C. These surges can cause major damage to property and people in their path.
Lava
Lava from stratovolcanoes are generally not a significant threat to humans or animals because the highly
Viscosity lava moves slowly enough for everyone to evacuate. Most deaths attributed to lava are due to related causes such as
and
from
toxic gas.
can bury homes and farms in thick
volcanic rock which greatly reduces property value.
However, not all stratovolcanoes erupt viscous and sticky lava.
Nyiragongo, near
Lake Kivu in
central Africa, is very dangerous because its
magma has an unusually low
Silicate, making it much less viscous than other stratovolcanoes. Low
viscosity lava can generate massive
, while lava of thicker viscosity can solidify within the vent, creating a
volcanic plug. Volcanic plugs can trap
volcanic gas and create pressure in the magma chamber, resulting in violent eruptions.
Lava is typically between 700 and 1,200 °C (1,300–2,200 °F).
Volcanic bombs
are masses of unconsolidated rock and lava that are ejected during an eruption. Volcanic bombs are classified as larger than 64mm (2.5 inches). Anything from 2 to 64mm is classified as
lapilli.
When erupted, volcanic bombs are still molten and partially cool and solidify on their descent. They can form ribbon or oval shapes that can also flatten on impact with the ground.
Volcanic bombs are associated with Strombolian and Vulcanian eruptions and
basaltic lava. Ejection velocities ranging from 200 to 400 m/s have been recorded causing volcanic bombs to be destructive.
Lahar
(from a Javanese term for volcanic mudflows) are a mixture of volcanic debris and water. Lahars can result from heavy rainfall during or before the eruption or interaction with ice and snow. Meltwater mixes with volcanic debris causing a fast moving mudflow. Lahars are typically about 60% sediment and 40% water.
Depending on the abundance of volcanic debris the lahar can be fluid or thick like concrete.
have the strength and speed to flatten structures and cause great bodily harm, gaining speeds up to dozens of kilometers per hour.
In the
Armero tragedy of Nevado del Ruiz in
Colombia, pyroclastic surges melted snow and ice atop the 5,321 m (17,457 ft) high Andean volcano. The ensuing lahar killed 25,000 people and flooded the city of
Armero and nearby settlements.
Volcanic gas
As a volcano forms, several different
Volcanic gas mix with
magma in the volcanic chamber. During an eruption the gases are then released into the
atmosphere which can lead to toxic human exposure. The most abundant of these
Volcanic gas is
Water (
water) followed by
Carbon dioxide (
carbon dioxide),
Sulfur dioxide (
sulfur dioxide),
Hydrogen sulfide (
hydrogen sulfide), and HF (hydrogen fluoride).
If at concentrations of more than 3% in the air, when breathed in
Carbon dioxide can cause dizziness and difficulty breathing. At more than 15% concentration
Carbon dioxide causes death.
Carbon dioxide can settle into depressions in the land, leading to deadly, odorless pockets of gas.
classified as a respiratory, skin, and eye irritant if come into contact with. It is known for its pungent egg smell and role in ozone depletion and has the potential to cause acid rain downwind of an eruption.
has an even stronger odor than
Sulfur dioxide as well as being even more toxic. Exposure for less than an hour at concentrations of over 500 ppm causes death.
HF and similar species can coat
ash particles and once deposited can poison soil and water.
Gases are also emitted during volcanic degassing, which is a passive release of
gas during periods of dormancy.
Eruptions that affected global climate
As per the above examples, while eruptions like
Mount Unzen have caused deaths and local damage, the impact of the June 1991 eruption of
Mount Pinatubo was seen globally.
The eruptive columns reached heights of and dumped 17 megatons of SO
2 into the lower
stratosphere.
The
that formed from the
sulfur dioxide (SO
2),
carbon dioxide (CO
2), and other
dispersed around the world. The
Sulfur dioxide in this cloud combined with water (both of volcanic and atmospheric origin) and formed
sulfuric acid, blocking a portion of the sunlight from reaching the
troposphere.
This caused the global temperature to decrease by about from 1992 to 1993. These
caused the
ozone layer to reach the lowest concentrations recorded at that time.
An eruption the size of Mount Pinatubo affected the weather for a few years; with warmer winters and cooler summers observed.
A similar phenomenon occurred in the April 1815, the eruption of Mount Tambora on Sumbawa island in Indonesia. This eruption is recognized as the most powerful eruption in recorded history.
List
See also
