The Global Distribution of Volcanoes

Features of a Volcano

Physical Parts

  • Magma chamber – large underground pool of magma.
  • Lava – magma, once it reaches the surface.
  • Crater – bowl-shaped basin in the top of the volcano.
  • Vent – central tube which magma travels through.
  • Secondary cones – eruptions from other vents may build up secondary cones on the flanks.

An Eruption

  • Ash, Steam and Gas – material thrown out by the volcano.
  • Volcanic Bombs – larger material thrown out by the force of eruption.
  • Lava Flow – Most traditionally associated with volcanoes, but probably one of the least dangerous hazards to humans. Lava flows only travel up to a couple of km/h so it is possible to move out of their way. However, they can bury and incinerate any land or property that they travel over.
  • Pyroclastic Flows – These are giant clouds of ash and gas. They are extremely dangerous because they can travel up to 500 km/h, reach distances of 30km and can be over 700 degrees centigrade in temperature. They will burn, knock over or bury anything in their path.
  • Lahars – These are a secondary hazard and normally occur on snow-covered volcanoes. Hot ash and gas melt the snow and then mix. They then travel down the volcano as a fast-moving mudflow which can drown or bury anything in their path.
  • Ash Clouds – Not as fast-moving as a pyroclastic flow, but ash clouds can still be very disruptive. The weight of falling ash can collapse buildings and destroy crops. They can reduce sunlight by blocking out the sun and even cause problems for air travel like the recent Iceland volcano.
  • Lava or Volcanic Bombs (tephra) – When volcanoes erupt they often throw out semi molten pieces of rocks. As long as humans are a safe distance they don’t really pose any problems. However, because of their heat they can start fires.
  • Poisonous Gases – When volcanoes erupt they can release poisonous gases like carbon monoxide and sulphur dioxide. These can kill humans or animals if they are too close, but they can also contribute to the greenhouse effect.

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Types of Volcanoes

Shield Volcanoes

Shield volcanoes are very gently sloped and made from layers of lava

Composite Volcanoes

Composite volcanoes are steep and made from layers of ash and lava

Cinder Volcanoes

Cinder volcano are almost a mix of both but made from mainly ash

Predicting a Volcanic Eruption

Volcanoes are a lot easier to predict than earthquakes because they normally give some warning signs. Scientists (volcanologists) will look for some of the following changes to try to predict a likely volcano:

  • Change in the shape or size of volcano.
  • Change in the temperature of a volcano.
  • Change in the amount and type of gases being released.
  • Earthquake activity.
  • Changes in plant and animal life.
  • Changes in local hydrology e.g. temperature and chemical composition of nearby rivers.

Why Live Near a Volcano

Even though volcanoes are extremely hazardous places, many people still choose to live on, or near them. Some of the reasons why people do this include:

  • Ornamental – Their beauty, places like Mount St. Helen’s are beautiful to look at and enjoy.
  • Geological – Minerals, it is possible to mine minerals like sulphur from volcanoes geothermal potential (cheap and clean renewable energy) e.g. Iceland and El Salvador.
  • Tourism – tourists like to view and walk up volcanoes e.g. Santa Ana volcano or Pacaya volcano. There is often hot springs near volcanoes which tourists and locals can enjoy e.g. Mt. Arenal in Costa Rica or the hundreds of them in Japan.
  • Farming – Land around volcanoes is very fertile because of all the minerals, therefore many people choose to farm the land.
  • Poverty – people simply can’t afford to live anywhere else apart from the marginal land on volcanoes, shortage of space and high population density. San Salvador is slowly growing up El Boqueron because of the shortage of space.
  • Complacency – because the volcano has not erupted for a long time.
  • Evacuation – Confidence that they will be given an adequate warning to evacuate.
  • Place of Living – Families have always lived in the area and don’t want to leave.

Case Study: Mount St. Helen’s, Washington (MEDC/HIC) – News Article

Destruction strikes in northwest USA, Washington at 8:32 am yesterday flooding the land with pyroclastic flow (a flow of molten ash and debris in the form of a moving cloud that can reach speeds of 700 km/h) and engulfing the sky in an ash cloud. Because of this the after math shows that 57 people were killed (of which 9 were reporters)and 250 homes, 47 bridges, 24 km of railways, and 298 km of highway were destroyed. Mount St. Helen’s last erupted a long time ago thus why the pressure in the magma chamber built up over time, then because of an earthquake measuring 5.1 on the Richter scale causing the eruption that reduced the elevation of the mountain’s summit from 2,950 m to 2,550 m replacing it with a 1 mile (1.6 km) wide horseshoe-shaped crater. The debris avalanche was up to 0.7 cubic miles (2.9 km3) in volume.

Mount Pinatubo – Philippines (MEDC) Case Study

Mount Pinatubo resides in the Philippines on the island of Luzon, near the three points of the Philippine provinces of Zambales, Tarlac, and Pampanga. The volcano last erupted in 1991 and has an elevation of 1,486m.

Mount Pinatubo is an infrequently active strata volcano in the Cabusilan Mountains on the island of Luzon, near the three points of the Philippine provinces of Zambales, Tarlac, and Pampanga. Before the volcanic activities of 1991, its eruptive history was unknown to most people. It was heavily eroded, inconspicuous and obscured from view. It was covered with dense forest which supported a population of several thousand indigenous people, the Aetna’s, who fled to the mountains during the Spanish conquest of the Philippines.

The volcano’s Politian / Ultra-Politian eruption on 15 June 1991 produced the second largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta in the Alaska Peninsula. Complicating the eruption was the arrival of Typhoon Yunya (Diding), bringing a lethal mix of ash and rain to areas surrounding the volcano. Successful predictions at the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives, but the surrounding areas were severely damaged by pyroclastic flows, ash deposits, and subsequently, by the lahars caused by rainwater re-mobilizing earlier volcanic deposits causing extensive destruction to infrastructure and changing the river systems months to years after the eruption.

The effects of the eruption were felt worldwide. It ejected roughly 10,000,000,000 tonnes (1.1×1010 short tons) or 10 km3 (2.4 cu mi) of magma, and 20,000,000 tonnes (22,000,000 short tons) SO, bringing vast quantities of minerals and metals to the surface environment. It injected more particulate into the stratosphere than any eruption since Krakatoa did in 1883. Over the following months, the aerosols formed a global layer of sulphuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F) in the years 1991-93, and ozone depletion temporarily increased substantially.

Case Study: Mount Krakatoa, Indonesia (LEDC/LIC)

Mount Krakatoa in Indonesia is a volcanic island situated in the Sunda Strait between the islands of Java and Sumatra in Indonesia. The name is also used for the surrounding island group comprising the remnants of a much larger island of three volcanic peaks which was obliterated in a cataclysmic 1883 eruption, unleashing huge tsunamis (killing more than 36,000 people) and destroying over two-thirds of the island. The explosion is considered to be the loudest sound ever heard in modern history, with reports of it being heard up to 3,000 miles (4,800 km) from its point of origin. The shock waves from the explosion were recorded on barographs around the globe.
In 1927 a new island, Anak Krakatau, or “Child of Krakatoa”, emerged from the caldera formed in 1883 and is the current location of eruptive activity.

Indonesia has over 130 active volcanoes, the most of any nation. They make up the axis of the Indonesian island arc system, which was produced by northeast-ward subduction of the Indo-Australian plate. A majority of these volcanoes lie along Indonesia’s two largest islands, Java and Sumatra. These two islands are separated by the Sunda Strait, which is located at a bend in the axis of the island arc. Krakatoa is directly above the subduction zone of the Eurasian Plate and the Indo-Australian Plate where the plate boundaries make a sharp change of direction, possibly resulting in an unusually weak crust in the region.


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