How does magma form?

Magma can be described as melted rock deep within the Earth, usually originating from the melting of the upper mantle or crust. The magma will generally contain silica tetrahedra as well as many metal cations such as calcium and magnesium which are all randomly organised within the melt and can freely move past one another. But how does the magma form? Well, magma is formed by the partial melting of the mantle and crust and this can occur in three ways. The first way is called heat-transfer melting. Rising magma or rock will bring heat with it, and so can melt the surrounding mantle or crustal rock. For example, magmas generated in the mantle tend to be around 1200 degrees Celsius, whereas the more silicate minerals such as quartz and orthoclase feldspar (common in continental crustal rocks) begin to partially melt at around 650-850 degrees Celsius. Therefore, the crustal rock will begin to partially melt due to the introduction of heat from rising magma. Another method of melting is by the introduction of volatiles. Volatiles, such as water and carbon dioxide are characterised by their low melting points and ability to evaporate easily. Water is commonly brought down on the subducting plate at convergent plate margins in the form of hydrous minerals (such as amphiboles and serpentine which is a form of water-altered olivine). However, as the subducting plate continues deeper into the upper mantle, it heats up and the hydrous minerals become less stable and break down, releasing water into mantle. Once within the mantle, the water can help break the chemical bonds between the molecules within the dry mantle rock and lowers its melting temperature, and so begins melting the mantle. The final method of melting rock is known as decompression melting. During decompression melting, rock from within the mantle is brought to the surface adiabatically (no exchange of heat or energy with its surroundings) and so the lithostatic pressure decreases. This means that the parcel of rising rock crosses the solidus, and so at this point the thermal vibration of the molecules is no longer counteracted by the lithostatic pressure and the rock begins to partially melt.