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as dormant volcanoes; and only those with no historic record are regarded as extinct volcanoes.
Volcanoes are unpredictable because, like the stress that builds up and suddenly causes an earthquake, stress builds up inside the earth with no sign of change occurring at the surface. How much pressure is needed to cause renewed activity is not easy to guess; neither is the rate at which the pressure is building. An eruption is a sudden release of pressure from within or from below the crust. Once the pressure has been released, the eruption stops.
Magma
Whatever the kind of material that is erupted from a volcano, it has a common source – a chamber deep underground, usually in the lower part of the crust. It is called a magma chamber.
Magma is a mixture of molten materials and liquefied gases. It can be made up of a variety of materials, which is partly responsible for the different types and frequency of eruptions.
The most important control is the proportion of silica to other parts (mainly iron and magnesium compounds) of the magma. If the proportion of silica (which will form quartz when it cools) is above about 60 per cent, the magma will be very sticky and will almost certainly erupt explosively. If the magma contains between about 45
A magma chamber has its source deep within the crust, often in the upper mantle. A large plume of molten rock begins to rise, melting its way upwards over perhaps millions of years. When it is close to the surface, the upper part of the crust begins to break up, and this allows magma to flow to the surface as a volcano. A magma chamber may feed a number of volcanoes. When it eventually cools, a magma chamber solidifies into granite and is called a batholith. Batholiths are found at the cores of many mountain systems.
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