Magmas are less dense than surrounding rocks, and will therefore move upward. If magma makes it to the surface it will erupt and later crystallize to form an extrusive or volcanic rock . If it crystallizes before it reaches the surface it will form an igneous rock at depth called a plutonic or intrusive igneous rock . Fast cooling on the surface results in many small crystals or quenching to a glass. Slow cooling at depth in the earth results in fewer much larger crystals, gives rise to phaneritic texture .
Porhpyritic texture develops when slow cooling is followed by rapid cooling.
Types of Magma Chemical composition of magma is controlled by the abundance of elements in the Earth. Since oxygen is so abundant, chemical analyses are usually given in terms of oxides. SiO 2 is the most abundant oxide. Gas gives magmas their explosive character, because the gas expands as pressure is reduced. Mostly H 2 O with some CO 2 Minor amounts of Sulfur, Cl , and F Rhyolitic or granitic magmas usually have higher gas contents than basaltic or gabbroic magmas.