Magma is molten rock within the Earth's crust

Magma is rock within the earth that is completely melted. This happens between 800 and 1300° C, depending on the pressure and the rock's composition.

When magma emerges onto the Earth's surface it becomes lava.

Magma production

Decompression melting occurs when the temperature of the rock remains the same but the pressure is reduced. This occurs due to it moving towards the surface eg in a mantle plume.

Flux melting occurs when rock is close to melting point and water is added. Water is a flux that promotes melting by reducing the melting point of the rock.

Heat-induced melting occurs due to an increase in temperature. It is the rarest form of melting in the mantle and usually only occurs in mantle plumes or hotspots.

Magma composition

Mafic magmas are rich in iron, magnesium and calcium. They tend to be composed of around 50% silica.

Felsic magmas are composed around 75% silica. They are lower in iron, magnesium and calcium than mafic magma, but contain more sodium and potassium. Felsic magmas also tend to contain more volatiles such as water, carbon dioxide and sulfur dioxide.

Source: Earle, S (2015) Physical Geology#Chapter 3 Intrusive Igneous rocks

See: 1.1c magma is formed at three plate tectonic settings for places where magma is formed