Volcanoes are formed along two types of plate boundary: destructive and constructive boundaries. The basic shape of a volcano is similar throughout the world, however there are many factors which influence how the volcano is built.
Volcanoes occur where molten rock (magma) is allowed to escape to the surfaceo f the earth. This usually occurs at plate boundaries through cracks in thecrust called vents.
Once it has reached the surface, the magma becomes known as lava. The composition of the lava determines the shape of the final volcano.
Volcanoes also throw out ash, steam, dust, pumice, and gases, which can be poisonous. However it is the lava that mainly helps to shape the volcano.There are three main volcanic cones: acid lava cones, composite cones and basic lava cones.
Acid cone volcanoes are steep sided due to the fact that the lava is thick and acidic, meaning that it doesn't flow far before solidifying, for example Mt. Pelee.
Shield cone volcanoes are wide-based, with gentle slopes. Their lava is runny and thin, which means that it can travel a long way before cooling and solidifying. Often these eruptions are non-violent and can last for years, such as the one at Kilaueain Hawaii.
Composite Cone volcanoes are steep-sided,and made of alternate layers of ash and lava. Often the lava cools to create a plug in the vent, meaning that a huge explosion is needed to remove it.The best example is Mt. St. Helens.
Mt. St. Helen's, May 1980 (The U.S.A.)
- Erupted on the morning of May 18th 1980, after being inactive for over 120 years.
- Caused by the subduction of the oceanic Juan de Fuca plate under the continental North American plate, on the West Coast of the United States. The volcano forms part of the Rockies Range of mountains.
- After minor earthquakes and a small eruption during March, scientists carefully monitored the mountain. By the beginning of May a bulge had developed on the North side of the mountain. This activity forced the Local Authorities to create a "Red Zone" around the volcano, from where any residents were evacuated, and no one was allowed to re-enter. This saved a lot of people from the disaster.
- At approximately 8.30 a.m. on the 18th May, the volcano erupted a small amount of ash and stem. Then, a couple of minutes later, something occurred that nobody could have predicted. An earthquake caused the bulge to collapse down the side of the mountain. This released the full power of the explosive gases behind as the mountain literally blew its own side off.
- The material from the massive landslide sped down the mountain, filling Spirit Lake and then racing on as a mud flow (it had mixed with the lake water) down the Toutle River and Kalama River.
- The explosions from the side of the mountain sent a speeding cloud of gas, steam and dust across the ground to the North of the volcano, flattening everything in its path. The trees of the forests around the mountain were toppled as if they were matchsticks, with the majority of them facing in the same direction.
- These explosions also instantly melted all of the snow on the mountain, adding to the mudflows that were racing down the mountain and into the rivers. These flows took all with them, including huge trees and even bridges.
- During the remainder of the day huge gas and ash eruptions occurred, spreading a massive cloud of ash that took 7 days to entirely circle the world. The town of Yakima, 120 miles away, was so badly affected by the dust and ash that residents could only go outside if they wore facemasks.
- The mountain managed to reduce its height by nearly 400 metres and blew an amphitheatre shape hole in its side, 3km long and 500 metres wide.
- 61 people were killed by the eruption, mainly by the poisonous gases, but also by being caught up in the massively swollen rivers.
- No wildlife, either plants or trees, survived within the 25km blast zone. However vegetation has already made great strides to re-colonise the area.
Mt. Pinatubo, June 1991 (The Philippines)
- Mt. Pinatubo had not erupted for over 600 years. Its slopes had become fertile, well-cultivated paddy fields. People did not expect it to erupt.
- On 12th June the mountain erupted. Measurements and predictions by scientists had meant that over 200,000 people had been evacuated by the time that the mountain erupted.
- The eruption sent a huge cloud of gas and ash up into the atmosphere. Torrential rain then caused much of the ash to be deposited back on the ground as mud.
- An area of over 600km in radius had ash falls from the volcano, with nearly 50cm falling near the mountain itself.
- Most terrifying of all were the lahar's that was produced. These are huge, speeding mudslides, formed by the ash and the torrential rain that swept down covering entire villages in a think layer of mud, often up to 10 feet deep. They destroyed over 200,000 homes and covered 50,000 hectares of farmland.
- Although a relatively small number of people were killed (350), the effects of the eruption were devastating. Diseases such as malaria and cholera spread quickly in the refugee camps set up to help the evacuee's. Over the next few years, heavy rains caused ash and dust from the eruption to create more devastating lahars.