Volcano
Causes of volcanic eruptions
Key things you should know aboutvolcanoes:
A volcano is a cone-shaped mountain or hill,formed by eruptions of lava and ash.
When magma reaches the Earth's surface it iscalled lava. When the lava cools, it formsrock.
Volcanic eruptions can happen at destructive andconstructive boundaries, but not atconservative boundaries.
Some volcanoes happen underwater, along thesea bed or ocean floor.
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How volcanoes form
•Magma rises through cracks orweaknesses in the Earth's crust.
•Pressure builds up inside the Earth.
•When this pressure is released, magmaexplodes to the surface causing avolcanic eruption.
•The lava from the eruption cools to formnew crust.
•Over time, after several eruptions, therock builds up and a volcano forms.
What happens inside a volcano
•A volcano has several important features:
•The magma chamber is a collection of magmainside the Earth, below the volcano.
•The main vent is the main outlet for the magmato escape.
•Secondary vents are smaller outlets throughwhich magma escapes.
•The crater is created after an eruption blows thetop off the volcano.
•An eruption occurs when pressure in the magmachamber forces magma up the main vent, towardsthe crater at the top of the volcano. Some magmawill also be forced out of the secondary vent at theside of the volcano.
Image shows a diagram of volcano. The main vent leads from the magma chamber (at the heart of the volcano), to the crater at the top. Smaller secondary vents lead from the magma chamber to the side of the volcano.
Different types of volcano
•One way to describe volcanoes is in terms ofactivity:
•Some volcanoes are still active and eruptfrequently.
•Some volcanoes are dormant (sleeping).
•Some volcanoes are extinct (never likely to eruptagain).
•The other way of describing volcanoes is by theirshape or type. There are cone volcanoes, shieldvolcanoes and composite volcanoes.
•Cone volcanoes:
•These are usually found at destructiveboundaries.
•Cone volcanoes are tall and steep-sided.
•Cone volcanoes are formed by eruptions of thick,viscous (sticky) lava.
•The thick lava moves relatively slowly andhardens quickly to form new rock - this explainsthe formation of a cone shape.
•Eruptions tend to be violent
image: illustration of a side view of a cone volcano. It is shaped like an upside down V and has a tall, narrow cone with steep slopes. The lava in the main vent is thick.
Shield volcanoes:
•These are usually found atconstructive boundaries.
•They are low, with gentlysloping sides.
•Shield volcanoes are formedby eruptions of thin, runny lava.
•Eruptions here tend to befrequent but relatively gentle.
image: illustration of a side view of a shield volcano. It is shaped like an upturned shield. It has a low, wide cone with gentle slopes. The lava in the main vent is fluid.
Composite volcanoes:
•These volcanoes are composed(made up) of alternating layers oflava and ash (other volcanoes justconsist of lava).
•The eruptions from thesevolcanoes may be a pyroclasticflow rather than a lava flow. Apyroclastic flow is a mixture of hotsteam, ash, rock and dust.
•A pyroclastic flow can roll down thesides of a volcano at very highspeeds and with temperatures ofover 400° C.
image: illustration of a side view of a composite volcano. The cone is higher and steeper than the shield, but less steep than a cone volcano. It is made up of alternating layers of ash and lava.
Effects of volcanic eruptions
   Volcanic eruptions can have a devastating effecton people and the environment. However, unlikeearthquakes, volcanoes can also have a positiveimpact on an area. These positive impacts canhelp to explain why people choose to live nearvolcanoes.
Graphical table shows the pros and cons of volcanoes. Positive impacts: The dramatic scenery created by volcanic eruptions attracts tourists. This brings income to an area. The lava and ash deposited during a volcanic eruption breaks down to provide valuable nutrients for the soil. This creates very fertile soil, which is good for agriculture. The high level of heat and activity inside Earth, close to a volcano, can provide opportunities for generating energy. This is called geothermal energy. Negative impacts: Many lives can be lost as a result of a volcanic eruption. If the ash and mud from a volcanic eruption mix with rain water or water from melting snow, fast moving mudflows are created. These flows are called lahars. Lava flows and lahars can destroy settlements and clear areas of woodland or agriculture. Human and natural landscapes can be destroyed and changed forever.
Case study: Chances Peak,
Montserrat, 1995-97
                                          
Volcanic ash covers the town of Plymouth, Montserrat
Montserrat is a small island in the Caribbean. There is a volcanicarea located in the south of the island, called Soufriere Hills.
The volcanic peak in this area is called Chances Peak, which hadbeen dormant for over 300 years. Then in 1995, the volcanobegan to give off warning signs of an eruption (smallearthquakes and eruptions of dust and ash). Once Chances Peakhad woken up it then remained active for a period of 5 years.The most intense eruptions occurred in 1997.
During this time, Montserrat was devastated by pyroclasticflows. The small population of the island (11,000 people) wasevacuated in 1995 to neighbouring islands. The evacueesbecame refugees.
Despite the evacuations, 19 people were killed by the eruptions.This is because a small group of people chose to stay behind onthe island and watch over their crops.
Volcanic eruptions and lahars have destroyed large areas ofMontserrat. The capital, Plymouth, has been covered in layers ofash and mud. Homes and buildings have been destroyed.
The graphic shows the progress of the eruption and its impact onthe island.
                                                         
Volcanic activity has calmed down in recent years and peoplehave begun to return to the island.
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image: map of Montserrat showing the location of the volcano and the 5 pyroclastic flows that occurred through 1997 and 1998. 17 settlements were destroyed.
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