Volcanism: The Origin of Magma
||Magma is molten rock from inside the Earth. It contains many gaseous constituents, which largely escape at the Earth's surface. The basic substance of most magma is silicia (SiO2). The more silicia is contained in the magma, the more viscous the magma becomes and the more viscous the magma is the more explosive the eruptions of the volcanoes.
The magma from the Earth's mantle can rise directly via "hot spots" and makes volcanoes arise such as those on Hawaii. This magma also reaches the surface at divergent continental boundaries, where it becomes basalt rock. The volcanoes that produce basalt are often Hawaiian in their eruption type.
At convergent plate boundaries, the so-called subuction zones, the oceanic Earth's crust descends into the Earth's mantle and is partially fused. The material of the fused crust changes the composition of the magma.
Water that is transported with the oceanic crust into the Earth's interior reduces the melting temperature of the rocks and increases the gas pressure in the magma. This is another reason for the high explosivity of the volcanoes in the vicinity of convergent plate boundaries. The lava from these volcanoes leads to the production of rocks of andesite, dacite and rhyolite. These types of lava can be formed from basaltic magma if it matures for a long time in a magma chamber.
2 processes are responsible for the rising of magma from greater depths. The first part of the rise occurs because the molten magma has a lower density than the surrounding plastic rock of the Earth's mantle. If the hydrostatic pressure is balanced, the magma gathers and forms a magma chamber. Due to cooling and crystallisation, the gas pressure inside the magma chamber increases until the pressure is so great that the gases escape explosively. The magma rises through the volcanic pipe and the volcano erupts.
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