Apr 23 2014
Magma is a hot mixture of molten or semi-molten rock and volatiles that is found below the Earth’s surface. It is also composed of gas bubbles, dissolved gas, and suspended crystals. It is a complex, high-temperature fluid substance with temperatures ranging between 700 and 1300°C. However, rare carbonatite melts may have a temperature of 600°C, and komatiite may be as hot as 1600°C.
Magma usually gets collected in magma chambers which either cause volcanic eruption or convert into a pluton. It has an ability to intrude into adjacent rocks to forms igneous sills or dikes, extrude onto the surface in the form of lava, or explode as tephra forming pyroclastic rock.
Formation of Magma
The Earth’s mantle has very high temperatures, thanks to heat generated by radioactivity, the planet’s formation, and other physical means. This is due to the high pressure created within the mantle. The high pressure, in turn, raises the melting point of the solid rock in the mantle. As a result, magma forms by three different means:
- Heat transfer – The rising magma transfers heat to the cold rocks surrounding the mantle before solidification. If the surrounding rocks are already hot enough to melt, it takes up more heat to form rhyolitic magma
- Decompression melting – This process involves the upward movement of the mantle to a low-pressure region. The reduction of overlying pressure melts the rocks, resulting in the formation of magma
- Flux melting – The presence of water or other volatile agents, like sulfur and carbon dioxide gases, in the rock body increases the melting effect. This accounts for the formation of volcanoes near subduction zones, where carbonaceous matter, sediment, and water are released to form magma
Cooling of Magma
Volcanic eruption and crystallization within the mantle, to form pluton, are the two known processes through which magma ceases to exist. In both processes, magma tends to cool and form igneous rocks. When magma starts to cool, it forms solid mineral phases, some of which settle down in the magma chamber, resulting in cumulates associated with the formation of mafic layered intrusions.
Magma that cools within the magma chamber generally forms plutonic rock bodies, such as granite, diorite, and gabbro, based on the composition of the magma. If the magma erupts, volcanic rocks such as rhyolite, andesite, and basalt are formed.
Sources and Further Reading