Mafic and Silicic Magmas
Although compositions cover a wide spectrum, most magmas contain 40 to 75 wt% SiO₂ (Box a). Magmas richest in SiO₂ also tend to be rich in Al₂O3. They may contain appreciable amounts of FeO and Fe2O3, but are usually deficient in MgO. We term such magmas silicic (Si rich), sialic (Si and Al rich), or felsic. Light colored minerals dominate felsic rocks, so many geologists use the term felsic to refer to any light-colored igneous rock, even if the chemical composition is unknown. At the other end of the spectrum, magmas with < 50 wt% SiO₂ are usually rich in MgO and contain more FeO and Fe₂O3 than silicic magmas. Thus, we call them mafic (Mg and Fe rich) and in some extreme cases, ultramafic. They are usually dark in color. The term intermediate describes rocks with compositions between mafic and silicic.
Melting of rocks occurs at many places in the Earth, and magma compositions reflect their sources. Mid-ocean ridge and ocean hot-spot magmas are mostly mafic; subduction zone magmas are generally silicic to intermediate. Continental rifts produce a variety of magma types. Besides distinctions between, mafic, intermediate, and silicic rocks, petrologists often classify igneous rocks based on the alkali (K₂O + Na₂O) and alkaline earth (CaO) contents; alkalic rocks are those with high (K₂O + Na₂O): CaO ratios. Some rare and unusual magma types produce igneous rocks rich in nonsilicate minerals such as carbonates or phosphates, but we will not consider them here.
Rocks of different compositions have different melting temperatures because some elements combine to promote melting. Silicon and oxygen, in particular, promote melting because they form very stable molten polymers (long chains of Si and O). Silicic minerals, and SiO2-rich rocks, therefore, melt at lower temperatures than mafic minerals and SiO2-poor rocks. Conversely, magmas of different compositions crystallize at different temperatures. Temperatures measured in flowing lavas generally range from 900 to 1,100 °C (1,650 to 2,010 °F) with higher temperatures corresponding to basaltic (mafic) lavas and lower temperatures to andesitic rhyolitic (intermediate to silicic) lavas. Eventually, as a magma cools, it crystallizes; the first crystals form at the liquidus temperature. With further cooling of up to 200 °C (390 °F), the magma completely solidifies. The last drop of melt crystallizes at the solidus temperature. Some magmas crystallize at temperatures well above 1,000 °C (1,830 °F), but granites and other silicic magmas may crystallize at temperatures as low as 700 °C (1,290 °F).
Volatiles
Magmas also contain volatiles (gas, liquid, or vapour). H2O and CO2 are the most common, but compounds of sulfur, chlorine, and several other elements may also be present. Consequently, although igneous minerals crystallize at high temperatures, they may contain H2O, CO2 or other gaseous components. Volatiles may separate from a melt to form bubbles, most commonly in cooling lava, creating vesicles as the magma solidifies. Water is especially important in the crystallization process. A small amount will appreciably lower melting and crystallization temperatures, change magma viscosity, and produce large amounts of vapour or steam. Steam is often responsible for explosions such as those that took place at Mount Saint Helens on May 18, 1980.
Slowly cooling magmas do not crystallize all at once. After partial crystallization, the remnant melt may contain water and dissolved incompatible elements that did not enter any of the minerais already formed. When the remnant melt finally crystallizes, pegmatites containing minerals rich in incompatible elements such as potassium (K), rubidium (Rb), lithium (Li), beryllium (Be), boron (B), or rare earth elements (REEs) may result. Pegmatites often contain large euhedral crystals because the water acts as a flux and promotes crystal growth and Many spectacular and valuable mineral specimens come from pegmatites.