Volcanoes, list/Pinatubo

Location and Characteristics
The Mount Pinatubo volcano is located on the island of Luzon,Philippines and is one volcano of a chain of compsite volcanoes called the Luzon volcanic arc. This arc goes along the west coast of the island because of the subduction of Manila Trench which is off to the west.It is located on a convergent boundary where two tectonic plates collide (Tectonic plates are pieces of the Earth's lithosphere that are on the Earth's asthenosphere.).

What Happens at This Type of Boundary
One of these plates is made of oceanic crust while the other is made of continental crust. The oceanic plate gets subducted(one tectonic plate moving under the other) and magma forms after processes within the Earth. This magma evenutually rises to the Earth's surface and causes a volcanic eruption of explosive nature because of the silica-rich magma.

Characteristics
Mt.Pinatubo is a composite volcano because of its broad base and because the slopes of the volcano get steeper the closer you get to the summit. Composite volcanoes are made with a combination of pyroclastic material and lava. Composite volcanoes are also referred to as stratovolcanoes.

The Reawakening
Mt.Pinatubo had been dormant for about 500 years. And then a major earthquake with a magnitude of 7.8(quite similar to the San Francisco earthquake of 1906)rocked the area about 60 miles(100 kilometers)northeast of the Pinatubo volcano. This is what people think awoke the volcano from its long sleep. Apart from landslides,several minor local earthquakes and steam emissions, the Pinatubo volcano seemed undisturbed.However,scientists were wrong because in March and April 1991,magma started rising from more than 20 miles(32 kilometers) below Pinatubo leading to small earthquakes and steam explosions which led to the blasting of 3 craters. In April,May,and June 1991 many minor earthquakes occurred and noxious sulphur dioxide was released.

The Eruption
On the days of June 7-11 of 1991 the magma began to ooze out of the central vent but no real danger was presented due to lost pressure. On June 12th the first explosive eruption occurred and then the big one erupted on June 15th. The volcano exploded woth massive power,releasing 5 cubic kilometers of volcanic material and releasing ash clouds that rose 22 miles(35 kilometers)into the air. Fine ash travelled to the Indian Ocean and satellites tracked the ash cloud a handful of times around the globe. Very hot ash,gas,and pumice bits and fragments(pyroclastic flows)blanketed deep valleys with deposits as deep as 660 feet(200 meters)thick.The eruption blasted out a great deal of magma and rock from below the volcano,causing the summit to collapse and form a caldera(depression)a whopping 1.6 miles(2.5 kilometers)across. Typhoon Yunya was nearby when the volcano erupted so the ash from the eruption and the water from the typhoon mixed together to form tephra with a high water content. Ash and water also mixed on the ground forming lahars(mud flows).

Damage/Effects and a Lasting Impact
The eruption affected many people and their lives, but much more would've been affected if scientists hadn't been able to predict the volcano's eruption.If it weren't for the Philippines Institute of Volcanology and Seismology(PHILVOLCS) and the United States Geological Survey(USGS),many more lives would've been affected.Commmercial planes and other aircraft were warned of the rising ash cloud and most were able to avoid it.The tephra that rained down caused the roofs of many homes near the eruption site to collapse when it crushed them with massive weight and force. Lahars(destructive mud flows)also posed a threat to lives and property. The lahars completely submerged houses with a mud-like substance.The ash clouds also affected two nearby U.S. military bases-the Clark Air Base and the Subic Bay Naval Station. About 20million tons of sulphur dioxide was blasted into the Earth's stratosphere. This cloud spreaded around the world and temporarily caused global temperatures to drop half a degree Celsius even a few years after the eruption. The death toll was 300-800 people. Native people who lived on the slopes of Pinatubo called the Aeta Highlanders(20,000 people)were forced to move and many of them today still live in resettlement camps awaiting the day they can return to their home. The other 200,000(about)people who were evacuated prior and to the big eruption have returned but they still face threats of lahars burying their homes.

Other Eruptions/Eruption History
There have been seven other eruptions.The first ever eruption was around 7460 B.C. and it was an explosive eruption that had pyroclastic flows. The most recent eruption was in 1993 and there was some damage to property,there were evacuations,and there were some deaths. It was not as effective as the 1991 eruption,though.

References/Bibliography
http://www.georesources.co.uk/pinatubo.htm

http://www.volcano.si.edu/world/volcano.cfm?vnum=0703-083&volpage=erupt

http://vulcan.wr.usgs.gov/Volcanoes/Philippines/Pinatubo/Publications/Wolfe/wolfe_report.html

Berry,Kathleen M.,et al.Earth Science