Volcanoes and volcanic eruptions
Volcanoes form when magma reaches the Earth's surface, causing eruptions of lava and ash. They occur at destructive (compressional) and constructive (tensional) plate boundaries.
The immediate effects of volcanic eruptions can be devastating, but they may be beneficial in the long term.
An eruption occurs when pressure in the magma chamber forces magma up the main vent, towards the crater at the top of the volcano. Some magma will also be forced out of the secondary vent at the side of the volcano.
Volcanoes can be described in terms of activity and can be:
Volcanoes can also be described by their shape or type - shield or composite.
A supervolcano is a volcano on a massive scale. It is different from a volcano because:
Magma cannot escape to the surface and collects under the lower crust.
An 'uplifted bulge' begins to form under the lower crust as the magma chamber enlarges.
Cracks appear on the surface. Gas and ash erupt from the magma chamber through these cracks.
The magma chamber collapses and a depression is formed. This is called a caldera.
Yellowstone is one example of a supervolcano. Three huge eruptions have happened in the last 3 million years. the last eruption was 630,000 years ago, and was 1,000 times bigger than the Mount St Helens eruption in 1980.
The large volume of material from the last Yellowstone eruption caused the ground to collapse, creating a depression called a caldera [caldera: A cauldron / depression shaped tectonic feature usually formed from the collapse of a magma chamber. ]. The caldera is 55 km by 80 km wide. The next eruption is predicted to have catastrophic worldwide effects.
The supervolcano at Yellowstone is formed because of a volcanic hotspot [hotspot: An area of the Earth's crust where an unusually high amount of heat flow is causing volcanic activity. ].
Every year millions of visitors come to see the related features, such as geysers [geyser: A spring of water which has been heated geothermally which erupts intermittently. ] and hot springs. Old Faithful is one example of a geyser.
Volcanic eruptions can have a devastating effect on people and the environment.
However, unlike earthquakes, volcanoes can also have a positive impact on an area. These positive impacts can help to explain why people choose to live near volcanoes.
|The dramatic scenery created by volcanic eruptions attracts tourists. This brings income to an area.||Many lives can be lost as a result of a volcanic eruption.|
|The lava and ash deposited during an eruption breaks down to provide valuable nutrients for the soil. This creates very fertile soil which is good for agriculture||If the ash and mud from a volcanic eruption mix with rain water or melting snow, fast moving mudflows are created. These flows are called lahars [lahar: A destructive volcanic landslide or mudflow, consisting of a mixture of volcanic debris, mud, rock and water. ].|
|The high level of heat and activity inside the Earth, close to a volcano, can provide opportunities for generating geothermal energy.||Lava flows and lahars can destroy settlements and clear areas of woodland or agriculture.|
|Human and natural landscapes can be destroyed and changed forever.|
Montserrat is a small island in the Caribbean. There is a volcanic area located in the south of the island on Soufriere Hills called Chances Peak. Before 1995 it had been dormant [dormant: A volcano is classed as dormant when it is temporarily inactive but not fully extinct. ] for over 300 years. In 1995 the volcano began to give off warning signs of an eruption (small earthquakes and eruptions of dust and ash). Once Chances Peak had woken up it then remained active for five years. The most intense eruptions occurred in 1997.
During this time, Montserrat was devastated by pyroclastic flows [pyroclastic flow: A very hot mixture of volcanic debris that flows downhill at high speeds. ]. The small population of the island (11,000 people) was evacuated [evacuated: When people have been removed or sent away from an area, usually for their own safety. ] in 1995 to the north of Montserrat as well as to neighbouring islands and the UK.
Despite the evacuations, 19 people were killed by the eruptions as a small group of people chose to stay behind to watch over their crops.
Volcanic eruptions and lahars [lahar: A destructive volcanic landslide or mudflow, consisting of a mixture of volcanic debris, mud, rock and water. ] have destroyed large areas of Montserrat. The capital, Plymouth, has been covered in layers of ash and mud. Many homes and buildings have been destroyed, including the only hospital, the airport and many roads.
The graphic shows the progress of the eruption and its impact on the island.
Volcanic activity has calmed down in recent years and people have begun to return to the island.
You might be asked to consider the values and attitudes or opinions of people involved in the eruption, such as refugees or aid workers for example.
Mount St Helens is on the plate boundary between the Juan de Fuca plate and North American plate. When it erupted it permanently changed the surrounding landscape.
|Primary effects||Secondary effects|
Nuée ardente [nuee ardente: A French phrase, literally meaning 'glowing cloud', which describes the cloud of volcanic debris formed by the collapse of a volcanic dome. ] (hot ash and gas) destroyed forests and logging camps.
63 people were killed, mainly by poisonous gases.
Lahars [lahar: A destructive volcanic landslide or mudflow, consisting of a mixture of volcanic debris, mud, rock and water. ] (mudflows of ash and water) covered an extensive area surrounding the volcano.
Ash blocked rivers destroying popular fishing sites and causing flooding. This in turn destroyed crops and livestock.
Flooding destroyed communications such as road and railway bridges.
Sediment carried downstream ruined barge transport on the Columbia River.