Page 43 - Curriculum Visions Dynamic Book

 Basaltic material oozing out from a Hawaiian volcano. Basalt is very low in silicon and so will not produce violent eruptions.
magma: the molten rock that forms a balloon- shaped chamber in the rock below a volcano. It is fed by rock moving upwards from below the crust.
viscous: slow moving, syrupy. A liquid that has a low viscosity is said to be mobile.
Chemistry of magma chambers
The minerals that pour over the Earth’s surface or that are sent high into the sky as ash, are formed at great depth in pockets of molten rock known as magma chambers.
Magma chambers do not have a uniform chemical composition. This is because they receive new materials from below and, from time to time, they send newly formed minerals to the surface.
Scientists have found that the type of minerals created depend on the length of time the magma sits in the magma chamber before it is emptied and refreshed from below.
The longer the waiting period (the longer the volcano is dormant), the more the minerals become dominated by silica as opposed to the other mineral-forming elements. What happens is a kind of separating process, where the silica rises to the top of the chamber, and the other heavier elements are pushed below.
An eruption of rock rich in silica (say between 65 and 75% silica) does not flow easily. It is called a viscous lava. As a result it quickly solidifies as it reaches the air. The pressure of the rising magma behind, causes this syrupy material to be blown to pieces, leading to an explosion of great violence and the ejection of huge volumes of fragments of lava, which cool to form ash as they fall through the air. On the other hand, if the magma has been refreshed relatively recently, the magma
has not had time to separate out and it remains less rich in silica (say 45–55%). As a result the lava is runny and able to flow from the volcano easily, with little build up of pressure.
It is rich in dark iron and manganese minerals such as olivine and hornblende. An eruption of this lava is not as violent and therefore less liable to lead to disaster.
Scientists can look at the materials surrounding a volcano and find out how old they are. They can also examine the minerals they contain. From this information, it is possible to work out how often the volcano erupts on average, and what kind of material it ejects on each occasion. Using this chemical information, and the length of time since the last eruption, the degree of violence of the next eruption can be predicted.
 The violent eruption of Mt St Helens in 1980 was due to the high silica content of the magma in the chamber below the volcano.
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