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Distribution of Volcanic Activity

Volcanoes are vents on the earth's surface through which magma is able to reach the surface as lava. Where magma reaches the surface in an eruption, this is known as extrusive activity. In total, there are around 500-600 active volcanoes in the world. The distribution of the world's volcanic activity is not random. The occurence of volcanic activity is clearly associated with the presence of boundaries of the earth's crustal plates.

Volcanism and Plate Margin Processes (including magma formation)

Destructive Boundaries

Example: Pacific Ring of Fire (Main area) - e.g. Mount St Helens (Washington State)

• ~80% of volcanoes are found at destructive plate margins
• this is where two plates converge and one plate is subducted under the other
• magma is formed by re-melted oceanic crust during subduction and partial melting of the asthenosphere;
• magma rises up (less dense) and may also melt some of the continental crust, giving rise to andesitic magma
• reaches the surface in an explosive eruption;

Constructive Boundaries

Example: Iceland (North American and Eurasian plates diverging - at the Mid-Atlantic Ocean Ridge)

• ~15% of volcanoes are found at constructive plate boundaries
• this is where two plates diverge (move apart) resulting in the upwelling of basaltic magma from the asthenosphere;
• upwelling of the magma causes some surface doming and the divergence causes some release of pressure - allowing molten magam to rise along cracks in the crustal rocks;
• some magma reaches the surface creating sub-marine volcanoes - which may eventually grow to above sea-level;

INTRAPLATE VOLCANISMS

Example: Hawaiian Islands (middle of Pacific Plate)

• the exception to the correlation of volcanism to the existence of plate boundaries can be accounted for by hot spot activity (see previous notes on hot spot activity)
• hot spots are plumes of hot semi-molten rock, rising from inside the mantle
• magma erupts at the surface (may be at vents, or as fissure eurptions, creating continental flood basalts (layers of runny basaltic lava covering the surface)

VOLCANIC STRUCTURE

The shape of a volcanic structure will vary according to the nature of the eruption and the magma type (see characteristics and formation of extrusive landforms). However there are number of main characteristics of volcanic cones which can often be identified - i.e. main vent, magma chamber, secondary cones etc. (see diagram below).

If the main volcanic vent becomes blocked by solidified magma, the continual rise of magma results in the build up of pressure and magma is forced through weaknesses in the rock, creating secondary vents and erupting on the flank of the volcano as a secondary (or 'parasitic') cone.

See also the following links / resources...

Excellent

A list of earth's active volcanoes (from MTU)

Volcano Index on Volcano World

Recent earthquakes and active volcanoes (interactive volcano map - scroll to bottom of page - click on red triangles for update on recent activity)