Key facts

• A volcano is formed by eruptions of lava and ash.
• Volcanoes are usually cone shaped mountains or hills.
• When magma reaches the Earth's surface it is called lava. When the lava cools, it forms rock.
• Volcanic eruptions can happen at destructive and constructive boundaries, but not at conservative boundaries.
• Some volcanoes happen underwater, along the seabed or ocean floor.

Magma rises through cracks or weaknesses in the Earth's crust.

1. Pressure builds up inside the Earth.
2. When this pressure is released, eg as a result of plate movement, magma explodes to the surface causing a volcanic eruption.
3. The lava from the eruption cools to form new crust.
4. Over time, after several eruptions, the rock builds up and a volcano forms.

• The magma chamber is a collection of magma inside the Earth, below the volcano.
• The main vent is the main outlet for the magma to escape.
• Secondary vents are smaller outlets through which magma escapes.
• The crater is created after an eruption blows the top off the volcano.

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.

• still active and erupt frequently
• dormant (temporarily inactive but not fully extinct)
• extinct (never likely to erupt again)

Shield volcanoes

• Shield volcanoes are usually found at constructive or tensional boundaries.
• They are low, with gently sloping sides.
• They are formed by eruptions of thin, runny lava.
• Eruptions tend to be frequent but relatively gentle.

Composite volcanoes

• Composite volcanoes are made up of alternating layers of lava and ash (other volcanoes just consist of lava). They are usually found at destructive or compressional boundaries.
• The eruptions from these volcanoes may be a pyroclastic flow rather than a lava flow. A pyroclastic flow is a mixture of hot steam, ash, rock and dust.
• A pyroclastic flow can roll down the sides of a volcano at very high speeds and with temperatures of over 400°C.

A supervolcano is a volcano on a massive scale. It is different from a volcano because:

• it erupts at least 1,000 km³ of material (a large volcano erupts around 1 km³)
• it forms a depression, called a caldera (a volcano forms a cone shape)
• a supervolcano often has a ridge of higher land around it
• a supervolcano erupts less frequently - eruptions are hundreds of thousands of years apart

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 acaldera. 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.

Every year millions of visitors come to see the related features, such as geysersand 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.

PositiveNegative 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 calledlahars. 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.

It's not possible to prevent earthquakes and volcanic eruptions. However, careful management of these hazards can minimise the damage that they cause. Prediction is the most important aspect of this, as this gives people time to evacuate the area and make preparations for the event.

Predicting and preparing for volcanoes

Unfortunately volcanic eruptions and earthquakes cannot be prevented.

Managing hazards such as earthquakes and volcanoes can be done by:prediction and preparation.

Predicting eruptions

As a volcano becomes active, it gives off a number of warning signs. These warning signs are picked up by volcanologists (experts who study volcanoes) and the volcano is monitored.

Warning signsMonitoring techniques Hundreds of small earthquakes are caused as magma rises up through cracks in the Earth's crust. Seismometers are used to detect earthquakes. Temperatures around the volcano rise as activity increases. Thermal imaging techniques and satellite cameras can be used to detect heat around a volcano. When a volcano is close to erupting it starts to release gases. The higher the sulfur content of these gases, the closer the volcano is to erupting. Gas samples may be taken and chemical sensors used to measure sulfur levels.

A detailed plan is needed for dealing with a possible eruption. Everyone who could be affected needs to know the plan and what they should do if it needs to be put into action. Planning for a volcanic eruption includes:

• creating an exclusion zone around the volcano
• being ready and able to evacuate residents
• having an emergency supply of basic provisions, such as food
• funds need to be available to deal with the emergency and a goodcommunication system needs to be in place

Earthquakes are not as easy to predict as volcanic eruptions. However, there are still some ways of monitoring the chances of an earthquake:

• Laser beams can be used to detect plate movement.
• A seismometer is used to pick up the vibrations in the Earth's crust. An increase in vibrations may indicate a possible earthquake.
• Radon gas escapes from cracks in the Earth's crust. Levels of radon gas can be monitored - a sudden increase may suggest an earthquake.

Many of the prediction techniques used to monitor earthquakes are not 100 per cent reliable. Planning and preparing for an earthquake is therefore very important. People living in earthquake zones need to know what they should do in the event of a quake. Training people may involve holding earthquake drills and educating people via TV or radio. 

People may put together emergency kitsand store them in their homes. An emergency kit may include first-aid items, blankets and tinned food. Earthquake-proof buildings have been constructed in many major cities, eg theTransamerica Pyramid in San Francisco. Buildings such as this are designed to absorb the energy of an earthquake and to withstand the movement of the Earth. Roads and bridges can also be designed to withstand the power of earthquakes.

A tsunami is a huge wave, usually caused by volcanic or earthquake activity under the ocean, which can eventually crash onto the shoreline. The effects on a community can be devastating.

What causes a tsunami?

When an earthquake, volcano or landslide happens on the ocean floor, water is displaced. This water forms the start of the tsunami.

When the waves reach shallower water:

• their height can increase by several metres
• the shallow water slows the wave
• the waves get closer together

It is hard to see that a tsunami is approaching. The most obvious sign is the coastal water retreats just before the waves reach the shore. This is actually the trough of the wave following behind.