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. Higher SiO 2 content magmas have higher viscosity than lower SiO 2 content magmas Lower Temperature magmas have higher viscosity than higher temperature magmas. When magmas reach the surface of the Earth they erupt from a vent. They may erupt explosively or non-explosively. Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earth's surface.