Geography: Vulcanicity

Most volcanic activity is associated with plate tectonic processes and is mainly located along plate margins.

• along oceanic ridges where plates are moving apart - Mid Atlantic ridge

• rift valleys. African rift valley has a number of volcanoes along it including Mt Kenya

• on or near subduction zones. 'ring of fire' that surrounds the pacific ocean is assoiciated with subduction

• over hot spots such as the one in the middle of the Pacific ocean

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Destructive

Type of magma - Acidic (rhyolite)

Characteristics - Slow flowing viscous

Type of eruption - Potentially explosive: lava shatters into pieces

Materials erupted - Lava bombs, ash, dust

Frequency - Infrequent: long dormancy period

Volcano - Composite and ash-cinder

Constructive

Type of magma - Basaltic (basalt)

Characteristics - Runny

Type of eruption - Little violence: gases easily escape

Materials erupted - Mainly lava

Frequency - Regular and can be continuous

Volcano - Lava Plateau and shiel

Magma is intruded into the crust, where it later solidifies into a range of features.

Sill - sheet-like intrusive igneous rock formed when magma is injected into sedimentary bedding surfaces. Usually form from low viscosity magma. (High force waterfall, River Tees)

Dyke - a vertical intrusion with horizontal cooling cracks. Cut across the bedding planes of rock. (Isouthern isle of arran, scotland)

Batholith - large body of igneous rock formed beneath the Earth’s surface by the intrusion and solidification of magma. It is commonly composed of coarse-grained rocks (e.g., granite ) with a surface exposure of 100 square km or larger, (Hambledown Tor, Dartmoor)

Note: the area surrounding the Batholith is altered by the heat and pressure of the intrusion to form metamorphic aureole (i.e limestone can be turned into marble)

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Lava plateau - generally flat and featureless. Basaltic in nature and therefore travel great distances. (Antrim laval plateau, Northern Ireland - covers an area of about 1650 Km2)

Basic/shield volcano - formed from free flowing lava. gentle sides and cover a great area. (Mauna Loa, Hawaii - 4km above sea level)

Acid/Dome volcano - steep-sided convex cones, consiting of viscous lava (Rhyolite). (Puy, south-central France)

Ash and cinder volcano - formed from ash and cinder and volcanic bombs ejected from the crater. sides are steep and symmetrical. (Paricutin, Mexico -1,345 feet above ground)

Composite cones - consists of layers of ash and lava, usually Andesitic. (Mt Etna, Sicily - 10,925 feet)

Calderas - build-up of gases, which eventually cause so much oressure to build up, that an extremem expolsion takes place, removing the summit. crater left can be flooded by the sea or become a lake. (Krakatoa, Indonesia - indo-australian plate subduction)

Geyser - Surface water works its way down to an average depth of around 2,000 metres (6,600 ft) where it meets up with hot rocks. The resultant boiling of the pressurized water results in the geyser effect of hot water and steam spraying out of the geyser's surface vent. (Old Faithful, Yellowstone National Park, Wyoming, USA)

Hot springs - A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. (Hanmar Springs, NZ)

Boiling mud - If a Hot spring mixes with surface depsosits, boiling mud is formed. (Many can be found in Iceland)

Solfatara - Small volcanic areas without cones, produced by gases (Sulphurous) escaping to the surface. (Bay of Naples, Italy)