Determining whether an igneous rock is phaneritic or aphanitic is just the first stage in giving it a name. The second stage is determining its chemical com
Fountaining and lava flow from Pu'u O eruption of Kilauea, Hawaii, January 31, 1984 (J.D. Griggs, USGS)
ponents. The composition of magma is controlled by the most abundant elements in the Earth, including silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), hydrogen (H), and oxygen (O). Oxygen is the most abundant ion in the crust of the Earth, so petrologists usually express compositional variations of magmas in terms of oxides. For most magmas the largest constituent is represented by the combination of one silicon atom with two oxygen atoms, forming silicon dioxide, more commonly called silica. Three very narrow compositional variations in the silica content of magma are common. The first type has about 50 percent silica (SiO2), the second 60 percent, and the third 70 percent. These volcanic rock types are called basalt, andesite, and rhyolite, and the corresponding plutonic rocks gabbro, diorite, and granite. The table "Classification of Igneous Rocks" discusses the different kinds of igneous rocks. some of the variation in the nature of different types of volcanic eruptions can be understood by examining what causes magmas to have such a wide range in composition.
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