The rock that results from cooling magma depends on three things:
- The composition of the magma
- The rate at which it was cooled
- The volatile content
The composition, e.g. basaltic (generally referred to as mafic from the high content of Magnesium and Iron) or granitic (generally referred to as felsic due to the high content of feldspar) is the primary factor. Basaltic magmas are low in silica (silicon bonded with oxygen) and high in magnesium and iron. Grantic rocks high in silica, high in aluminum, sodium, potassium and (often) volatiles.
The rate at which magma cools is a second primary factor in determining what a rock looks like (its texture). If magma cools slowly, and this happens most often deep in the earth, the crystals that form have time to grow large and are often well formed. When magma cools fast, as it does at or near the Earth’s surface, the crytals that start to from have little time to grow large and the rock that forms tend to be very fine grained.
The final factor, volatile content, only really becomes important during the later stages of crystallization when the retention of volatiles in magma can allow crystals to grow large, even with relatively rapid cooling.
For all these reasons, mafic magma can give give rise to basalt if it cools fast (small crystals hard to see with the naked eye), diabase (USA)/dolerite (UK) if cooled more slowly (small crystals but just visible with the naked eye) or gabbro (large crystals easily visible with the naked eye)
…. crazy that americans have different names than the rest of the world for the same rocks, but go figure Aluminium (UK)/Aluminum (USA) 🙂
In the same way felsic magma can form rhyolite if cooled fast (as with basalt), microgranite if cooled more slowly (as with diabase/dolerite) or granite if cooled really slowly (as with gabbro). In some cases, volatiles are retained until the final stages of crystallization, and rocks formed with very large crystals called pegmatites can form.
So, just to confuse students, the same magma can give rise to at least three basic textures with different names..
Basalt-Dolerite/Diabase-Gabbro
Rhyolite-Microgranite-Granite-Pegmatite.