Volcanoes

Volcanoes are simply vents at the earth's surface through which lava and other volcanic products are erupted. Although many volcanoes are cone-shaped, different types of volcano exist according to their location and the products they are made up of.

The distribution of volcanoes
Volcanoes occur in narrow, linear belts and are mostly found along destructive boundaries with large numbers found around the Pacific Ring of Fire (the area marking the boundary of the Pacific plate). They are also found at constructive plate margins such as the Mid-Atlantic Ridge. An exception to the general distribution of volcanoes is those found in the middle of the Pacific Plate (the Hawiian islands) which are formed due to hotspot activity.

As well as describing the distribution of volcanoes you will need to be able to describe and explain their occurence at plate boundaries:

Volcanoes at Destructive Boundaries:
1. Plates move together due to convection currents
2. Heavier oceanic plate is subduced
3. Friction between the plates and heat from the interior causes the subducting plate to met
4. Melting of the plate creates molten magma
5. This magma is less dense than its surrounding and therefore rises and is erupted at the surface through a weakness in the crust creating a volcano.
6. Volcanoes at Destructive boundaries tend to be quite explosive due to the build up of pressure and gases.

Volcanoes at Constructive Boundaries:
1. Plates move away from each other due to convection currents
2. This creates a weakness / 'gap' in the crust
3. Magma is a able to rise to plug the gap forming lava flows and submarine volcanoes
4. As magma continues to build up above the surface of the ocean, volcanic islands (such as Surtsey) may form.
5. Eruptions at constructive boundaries tend to be gentle with little pressure build up

Volcanoes at Hotspots:
As mentioned in the last post, volcanoes may also be created at Hotspots - for example the Hawaiian Islands (with volcanoes such as Kilauea). See this animation as a reminder of how hotspot activity can result in the formation of volcanoes.

Structure of a Volcano:

Some Volcanic eruptions are more explosive than others due to the type of magma. At destructive margins andesitic magma gives rise to acid lava which is thick and sticky. As gases can't escape easily pressure builds up resulting in violent explosions.

In contrast at constructive margins, basaltic lava gives rise to basic lava which is thin and runny and from which gases escape easily. These eruptions are therefore more gentle in nature and less explosive.

Follow up Links:
Some great images and information on individual volcanoes at Volcano World
Animation - eruption of a stratovolcano (Savage Earth)
Volcanoes Online - a detailed source of information on types of volcanoes

Key Term Check:
Volcano - a vent on the earth's surface through which lava erupts
Hotspot - a thermal plume of magma which rises underneath a tectonic plate
Magma Chamber - an underground chamber storing molten rock
Secondary Crater - a small crater often cut into the side of a large cone (may form due to a blockage in the main cone)
Crater - a large opening at the top of a volcano from which gases, lava and ash etc. escape
Vent - pipe taking magma towards the main crater
Pyroclastic Flow - a cloud of burning ash, gases and other volcanic material which can travel downslope at great speeds.
Volcanic Bombs - large, hot boulders ejected during an eruption
Magma - molten rock under the ground
Lava - molten rock which reaches the ground surface

The photographs in this post are courtesy of the USGS