Volcanoes Of The Philippines: Stratovolcano

A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers (strata) of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions. The lava that flows from stratovolcanoes typically cools and hardens before spreading far due to high viscosity. The magma forming this lava is often felsic, having high-to-intermediate levels of silica (as in rhyolite, dacite, or andesite), with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows are uncommon and have traveled as far as 15 km (9.3 mi).

Diagram of a Stratovolcano

Stratovolcanoes are sometimes called "composite volcanoes" because of their composite layered structure built up from sequential outpourings of eruptive materials. They are among the most common types of volcanoes, in contrast to the less common shield volcanoes. Two famous stratovolcanoes are Krakatoa, best known for its catastrophic eruption in 1883 and Vesuvius, famous for its destruction of the towns Pompeii and Herculaneum in AD79.
Stratovolcanoes are common in subduction zones, forming chains along plate tectonic boundaries where oceanic crust is drawn under continental crust (Continental Arc Volcanism, e.g. Cascade Range, central Andes) or another oceanic plate (Island arc Volcanism, e.g. Japan, Aleutian Islands). The magma that forms stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust, is released into mantle rock of the asthenosphere above the sinking oceanic slab.

Mount Fuji, Japan

The release of water from hydrated minerals is termed "dewatering," and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths. The water freed from the rock lowers the melting point of the overlying mantle rock, which then undergoes partial melting and rises due to its lighter density relative to the surrounding mantle rock, and pools temporarily at the base of the lithosphere. The magma then rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition (see Classification of igneous rock).

Mount Mayon, Philippines

When the magma nears the top surface, it pools in a magma chamber under or within the volcano. There, the relatively low pressure allows water and other volatiles (mainly CO₂, SO₂, Cl₂, and H₂O) dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened, releasing CO₂. Once a critical volume of magma and gas accumulates, the obstacle (mass blockage) of the volcanic cone is overcome, leading to a sudden explosive eruption.
In recorded history, explosive eruptions at subduction zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations. Subduction-zone stratovolcanoes, like Mount St. Helens and Mount Pinatubo, typically erupt with explosive force: the magma is too stiff to allow easy escape of volcanic gases. As a consequence the tremendous internal pressures of the trapped volcanic gases remain in the pasty magma.

Mount Pinatubo, Philippines

Following the breaching of the magma chamber, the magma degasses explosively. Such an explosive process can be likened to shaking a bottle of carbonated water vigorously, and then quickly removing the cap. The shaking action nucleates the dissolution of CO₂ from the liquid as bubbles, increasing the internal volume. The gases and water gush out with speed and force.
Two Decade Volcanoes erupted in 1991 provide examples of stratovolcano hazards. On June 15, Mount Pinatubo spewed ash 40 kilometres (25 mi) into the air and produced huge pyroclastic flows and mudflows that devastated a large area around the volcano. Pinatubo, located 90 km (56 mi) from Manila, had been dormant for 600 years before the 1991 eruption, which ranks as one of the largest eruptions in the 20th Century. Also in 1991, Japan's Unzen Volcano, located on the island of Kyushu about 40 km (25 mi) east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapse of this erupting dome generated ash flows that swept down the mountain's slopes at speeds as high as 200 km/h (120 mph). Unzen is one of more than 75 active volcanoes in Japan; an eruption in 1792 killed more than 15,000 people — the worst volcanic disaster in the country's history.

Mount Tavurvur, Papua New Guinea

The 79 CE Plinian eruption of Mount Vesuvius, a stratovolcano looming adjacent to Naples, completely covered the cities of Pompeii and Herculaneum with pyroclastic surge deposits. The death toll ranged between 10,000 and 25,000. Mount Vesuvius is recognized as one of the most dangerous volcanoes, jointly because of its potential for powerful explosive eruptions and the high population density of the area (around 3 million people) around its perimeter.

Anak Krakatua volcano, Indonesia

List of Stratovolcano

     Chile
*Llaima
*Irruputuncu on the border of Bolivia and Chile
*Ojos del Salado, highest Volcano in the
         World.
*Ojos del Salado is in the border
        between Argentina and Chile
*Villarrica
*Cerro Arenales
*Calbuco
*Callaqui
*Mount Hudson
*Copahue
*Lascar Volcano
*Nevados de Chillán

Mount Villarica, a westernmost of three large
stratovolcano in Chile

Italy

*Mount Etna in Sicily - recently erupted;
April, 2010
*Mount Vesuvius
*Stromboli
*Vulcano

El Salvador

*Chaparrastique
*Ilamatepec - recently erupted; October, 2005

Guatemala

*Pacaya
*Acatenango

Mount Arenal in Costa Rica

*Volcan de Fuego
*Santa María

Indonesia

*Semeru in Java
*Mount Agung in Bali
*Mount Batur in Bali
*Galunggung
*Krakatoa
*Anak Krakatau
*Mount Merapi in Central Java
*Mount Tambora
*Ambang
*Mount Bromo, East Java

Mount Etna in Italy

*Mount Sinabung, Sumatra

United States

*Lassen Peak in California
*Mount Hood in Oregon
*Mount Baker in Washington
*Mount Rainier in Washington
*Mount St. Helens in Washington
*Cleveland volcano in Alaska
*Mount Redoubt in Alaska
*Mount Shishaldin in Alaska
*Mount Akutan in Alaska
*Augustine Volcano, Cook Inlet, Alaska
*Mount Katmai, Katmai National Park and
Preserve, Alaska
*Mount Pavlof, Alaska

Mount St. Helens in Skamania County, Washington USA

*Korovin Volcano,
Alaska

Japan

*Mount Fuji
*Mount Mihara on Izu Oshima
*Sakurajima
*Mount Unzen
*Mount Adatara

New Zealand

*Mount Taranaki/Egmont in Egmont National
Park in North Island
*Mount Ngauruhoe in North Island

Mount Krakatua, is a active stratovolcano in
Volcanic Island,
Indonesia

*Mount Ruapehu in Tongariro National Park
*White Island in Bay Of Plenty

Iceland

*Eyjafjallajokull

Papua New Guinea

*Tavurvur - recently erupted September 2006
*Ulawun

Philippines

*Mayon Volcano
*Taal Volcano
*Mount Pinatubo

Mount Llaima, is one of the largest and most
active stratovolcano in Chile

Ecuador

*Pichincha Volcano
*Tungurahua
*Sangay
*Reventador

Mexico

*Popocatépetl

Sakurajima is a former island that connected
to Mainland, but it form as an active
stratovolano in Japan

*Colima
volcano

Mount Taranaki, is an active but quiescent stratovolcano
in New Zealand. It sits upon the remains of three older
volcanic complexes which lie to the northwest.

*Anatahan in Northern Mariana
Islands
*Ol Doinyo Lengai in Tanzania
*Cumbre Vieja, Tenerife, Canary
Islands

Stratovolcano