Volcanic Eruptions of Mafic Magma

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Mafic volcanoes have many features in common, even though their shapes vary:

the viscosity of the magma is low because the silica content is low, consequently the basalt lavas are fluid and can be fast flowing

the gas content of the magma is high but the low viscosity allows the gas to escape so that eruptions are quiet andnon-explosive

eruptions are frequent, sometimes daily, but every few years

activitiy is described as effusive and classified as Hawaiian

they occur at divergent plate margins or hot spots.

Shield Volcano

A shield volcano is chatacterised by gentle slopes of less than 10 degrees. They are composed almost entirely of thin basalt lava flows built up from a central vent and fissure eruptions on the flanks of the volcano. The low viscosity of the magma allows the lava to travel down the sides of the volcano on a gentle slop, but as it cools and its viscosity increases, its thickness builds up on the lower flanks, forming a somewhat steeper lower slope. Most shield volcanoes have a rougly circular or oval shape in map view. Very little pyroclastic material is found within a shield volcano.

Fissure Eruptions

Iceland has a number of fissures parallel to the axial rift of the mid-ocean ridge and these can erupt huge quantites of fluid lava that spreads out over a large area. Lava erupted from the 30km long Lki fissure in 1783-84, and covered an area of 560km2. The lavas from these fissure eruptions build up to form plateaus. Vast quantities of volcanic gases, particularly SO2, are released, which are believed to affect the climate. The eastern United States recorded the lowest ever winter average temperature in 1783-84, about 4.8 degrees below average. Europe alo experienced an abnormally severe winter.

Columnar Jointing

Where lava flows are more than 3m thick, the inside of the flow will cool steadily and slowly, perhaps over weeks instead of the hours or days for the outside of the lava flow. Hexagonal columns form in the centre of these thick lava flows as it contracts during cooling. The cooling originates at equally spaced centres and proceeds in all directions from these centres. Contraction causes tnsion cracks to start to form half-way between each cooling centre.

Submarine Eruptions

When lava is erupted underwater the outer skin cools very rapidly while the inside is still molten. Pressure of move lava behind causes the front to break through to form a new pillow shape. Pillow lavas are recongised by their characteristic rounded shape. Each pillow has a rounded top and a sagging bottom, so pillow lava can be used as a way-up structure to identify the youngest rocks. Vesicles may be present towards the outer upper edge of the pillows and the spaces betewen pillows may be infilled with fine glassy materials.

Volcanic Explosivity Index

The VEI was propsed in 1982 as a way to describe the relative size or magnitude of explosive volcanic eruptions. It is a 0 to 8 index of increasing explosivity., Each increase in number represents an increase by a factor of ten.

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