Magma Formation
Introduction
Magma is a molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites.
Formation of Magma
Magma formation involves a complex series of processes that occur within the Earth's crust and upper mantle. The process begins with the melting of existing rocks under extreme temperature and pressure conditions. This melting can occur in several ways, including decompression melting, flux melting, and heat-transfer melting.
Decompression Melting
Decompression melting occurs when the Earth's mantle is forced upwards, reducing the pressure and causing the rock to melt. This often happens at mid-ocean ridges, where tectonic plates are diverging, or moving apart. The mantle rock rises to fill the gap, and as it moves closer to the surface, the pressure decreases, leading to partial melting.
Flux Melting
Flux melting happens when water and other volatile materials mix with mantle rocks. The water lowers the melting point of the rock, causing it to melt into magma. This process is common at subduction zones, where one tectonic plate is forced under another, carrying water into the mantle.
Heat-Transfer Melting
Heat-transfer melting is the process where hot magma rises from the deep mantle into the cooler upper levels. The heat from the magma causes the surrounding rock to melt, creating more magma. This process often occurs in mantle plumes, where heat from the core drives the upward movement of magma.
Composition of Magma
The composition of magma can vary widely, depending on the types of rocks that were melted to form it and any subsequent processes that may have altered its composition. In general, magma is composed of a mixture of molten or semi-molten rock, along with dissolved gases and sometimes suspended crystals.
Silicate Magma
Most magma is composed of silicate minerals. These are minerals that contain silicon and oxygen, along with a variety of other elements. The specific composition of the magma will determine its behavior and the type of igneous rock it will form when it cools.
Basaltic Magma
Basaltic magma is high in iron, magnesium, and calcium, and low in potassium and sodium. This type of magma is formed by the melting of the Earth's mantle and is common at mid-ocean ridges and hot spots.
Andesitic Magma
Andesitic magma is intermediate in composition, being less mafic (rich in magnesium and iron) than basaltic magma but more mafic than rhyolitic magma. It is associated with subduction zones, where the crust is being forced into the mantle.
Rhyolitic Magma
Rhyolitic magma is high in potassium and sodium, and low in iron, magnesium, and calcium. This type of magma is formed by the melting of continental crust and is common in continental volcanic arcs.
Magma Movement and Eruption
Magma is less dense than the surrounding rock, which causes it to rise towards the surface. As it rises, it can encounter obstacles that cause it to pool in magma chambers. If the pressure in the magma chamber becomes too great, the magma can erupt from a volcano.
Magma Chambers
Magma chambers are large pools of magma that form in the Earth's crust. They can be tens to hundreds of kilometers in size, and they serve as reservoirs for magma before it erupts.
Volcanic Eruptions
When magma reaches the surface, it is called lava, and it can erupt from volcanoes in several ways. The type of eruption can be influenced by the composition of the magma, the amount of dissolved gases in the magma, and the efficiency of gas escape from the magma.