Igneous Rock: What is Magma?
Magma is molten rock that flows within the earth. It may be completely liquid or, more commonly, a fluid mixture of liquid, solid crystals, and dissolved gases. When magma reaches the Earth's surface, we call it lava, or molten rock that flows ground.
Magma forms when underground temperature become high enough to break the bonds in some minerals, causing the minerals to melt. The rock then changes from a crystalline solid to a fluid mix containing freely moving ions and atoms as well as some still-solid crystalline fragments. Different minerals melt out of the rock at different temperatures as the heat gradually increases, with the minerals having the highest melting points remaining the longest as still-solid fragments in the magma. At the same time, the composition of the magma changes as each newly molten mineral enters and enriches it.
When heat dissipates from magma, its bonds no longer break and new bonds start to form. Firstly, some of the free atoms and ions in the liquid bond to form tiny crystals. Additional ions and atoms bond at prescribed sites in the crystal structures. The crystals grow until they touch the edges of adjacent crystals. As cooling progresses, different minerals crystallize from the magma, again changing the magma's composition. If cooling continues long enough, the entire body of magma will become solidified as igneous rock.