Products of Volcanoes

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Volcanic gases

All atmospheric gases, except photosynthetic oxygen, were released by volcanic eruption. These gases are dissolved in the magma while it is under pressre at depth, then as the pressure decreases near the surface the gases are released by an eruption. The main gas is water vapour in the form of steam, which accounts for 70% of all gases released. The other main gases are CO2 (12%), N (7%) and SO2 (7%), with trace amounts of H, CO, S, Ar, Cl and F. These gases can combine with hydrogen and water to produce numerous toxic compounds such as HCl, H, H2SO4 and H2S. Rain can be very acid close to a volcano. Fumaroles are often a sign of gas escaping and activity below the surface.

Lava Flows

Lava flows from recent eruptions can be very distinctive but ancient lavas are more difficult to recognise as they usually lack the surface features of rubbly blocks and flow patterns.

Mafic Flows

These low visocity flows of basalt are thin and widespread over many kilometres because they are fluid. They cool to form:

aa lava flows with a rough blocky jagged surface

pahoehoe flows with a smooth or ropey texture

The pahoehoe flows are less visoucs and the aa flows and as the lava cools it comes even more viscous so that a pahoehoe flow may become an aa. Erupting lavas have temperatures in the range of 1000 to 1200 degrees, but most do not flow very fast. Lava in a lava tube may move more rapidly but most flows are at walking pace with scoria falling down from the top to the front of a flow. It they cool underwater, pillow lavas may form.

Intermediate and Silicic Flows

Viscous flows of andesite are irregular and tend to be limited to the area around the cone. Silicic flows of rhyolite are rare as they are very viscous so they tend to be thick but small in area. Very sticky lava can form lava domes within the crater and often shows flow banding.

Pyroclastic Material

Magma can be forcefully or explosive ejected into the atmosphere as particles called pyroclasts:

ash is the smallest particles, less than 2mm, and forms the pyroclastic rock tuff when consolidated

lapilli are particles are between 2mm and 64mm and forms a lapilli tuff

bombs and blocks are the coarsest particles, over 64mm, and form the pyroclastic rock agglomerate close to the crater of the volcano

nuee ardentes are the gaseous magma that forms the fast moving pyroclastic flows that can flow several kilometres and cool to form the rock ignimbrite

Pumice has already been described as a silicic igneous rock. It is so light tht is can travel a long distance from the volcano like the pyroclasts. When Krakatoa erupted, pumice fell on ships several kilometres from the volcano and floated on the sea looking like snow.

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