A volcano is a vent at the surface of the Earth, through which magma and other volcanic materials are ejected.
They are proof that the Earth is active and that the plates are in constant motion.
They are a example of a natural hazard.
Their spread relates closely to plate margins.
The area around the Pacific Ocean is especially prone to volcanoes is known as the Pacific Ring of Fire.
Composite (Cone) Volcanoes
Steep slopes and a narrow base.
Often secondary cones, layers of thick ash and lava.
Infrequent, but violent eruptions.
Usually found at destructive plate boundaries.
They are made of alternating layers of lava and ash.
The eruptions may be a pyroclastic flow, which 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.
Thick magma means that it doesn't travel very far before solidifying, giving the volcano its cone shape.
Shield Volcanoes
Wide base and gentle slopes.
Low, rounded peak.
Layers of runny lava, with little ash - which is why there's a wide base, it can travel far before solidifying.
Eruptions frequent and non-violent.
Volcanic Activity
Most volcanic activity is associated with plate tectonics and largely occurs along plate boundaries. It occurs:
Along oceanic ridges where plates are moving apart, e.g. Mid-Atlantic Ridge and Iceland.
Rift valleys, e.g. Africa Rift Valley with volcanoes such as Mount Kenya and Mount Kilimanjaro.
On or near subduction zones; the Pacific Ring of Fire has the most violent activity.
Over hot spots, e.g. Hawaiian islands in the Pacific.
These are linked to the different kinds of plate margin, emissions and lava.
Cross Section of a Volcano
Volcanic Material
Pyroclastic Material - Magma can be forcefully or explosively ejected into the atmosphere as particles called pyroclasts. The term 'pyroclastic' describes all the fragmental material formed by explosive eruptions including bomb, blocks, lapilli and ash.
Ash - Consists of rock, mineral and volcanic glass fragments. Smallest particles are less than 2mm. It is hard, so abrasive, and doesn't dissolve in water.
Lapilli - Small pieces of material, similar to pumice ranging in size from 2-64mm across.
Bombs and Blocks - Volcanic bombs are lava fragments that were ejected while viscous and are larger than 64mm in diameter. A volcanic block is a solid rock fragment greater than 64mm in diameter that was ejected from a volcano during an explosive eruption. Blocks commonly consist of solidified pieces of old lava flows that were part of a volcano's cone.
Hazards of Volcanoes
Lava flows burn and bury crops.
Submarine, coastal or island eruptions can trigger tsunamis.
Ash fall ruins crops and machinery, disrupts transport, pollutes the air and causes breathing difficulties.
Dust emissions endanger air transport and seed torrential rainstorms resulting in dangerous wet ash and mud lahars.
Violent eruptions of pyroclastic flows destroy life and property.
Volcanic melting of snow creates dangerous lahars.
Volcanic dust absorbs solar energy, and so lowers atmosphere temperatures.
Benefits of Volcanoes
Lava flows can create land e.g. Hawaii.
Hot rocks allow the development of geothermal power.
Igneous rocks contain valuable mineral deposits, e.g. gold, silver, copper and diamonds.
Volcanic sulphur is an important raw material in the pharmaceutical and agrochemical industry.
Igneous rocks create excellent building materials such as granite.
Extinct volcanoes have made excellent defensive sites, e.g. Edinburgh Castle.
Lava and ashes weather quickly into fertile soils.
Popular tourist attractions generate huge revenues. Geothermal tourism is also a big business.