This basics page focuses on igneous rocks and gives you the background needed to understand the terms used in the igneous rock classification table. The igneous texture describes whether the rock has mineral crystals or is glassy, the size of the mineral grains, and the rock’s porosity (empty spaces). The igneous texture of a rock is not how it feels in your hand, not whether it is rough or smooth.
Lava flows and pyroclastic material are volcanic igneous rock (also known as extrusive igneous rock). Another way that igneous rock forms is by magma erupting explosively into the air and falling to earth in pieces known as pyroclastic material, also called tephra. Or, magma may flow out onto surface of the earth as a lava flow. Magma may stay within the earth, far below ground level, and crystallize into plutonic igneous rock (also known as intrusive igneous rock). Magma can solidify into igneous rock in several different ways, each way resulting in a different igneous texture. The igneous texture tells us how the magma cooled and solidified.
Felsic magma usually originates in the crust or by the shedding of mafic minerals as magma rises through the crust. Most mafic magma originates by melting of rocks in the mantle that are extremely rich in iron and magnesium. Mafic igneous rocks, on the whole, tend to be dark colored, commonly black or dark gray. Felsic igneous rocks, as a whole rock, tend to have light colors or shades: white, pink, light brown, light gray. Magmas come in a range of compositions, from rich in silica and poor and iron and magnesium (felsic) to moderate in silica and high in iron and magnesium (mafic). Or, one body of magma can mix with another that has a different composition. Minerals can grow from the magma and separate from it, changing the chemistry of the remaining liquid. Once magma has formed inside the earth, its composition may be modified. The chemical composition of an igneous rock tells us about the origin of the magma, beginning with which type of rock melted within the earth to form the magma in the first place, and how deep in the earth the melting occurred. Most magmas are predominantly silicate liquids, composed largely of silica tetrahedra that have not yet bonded together to become silicate minerals. Geologists conduct chemical analyses of minerals to determine the temperatures and pressures at which they formed and to identify the dissolved gases and chemical elements that were present in the magma. The minerals provide details on the chemical composition of the rock, and on the conditions in which the magma originated, cooled, and solidified. Fortunately, with some basic training and practice anyone can learn to identify the minerals, composition and texture of an igneous rock name the rock and interpret key information about its origins.Īll igneous rocks, other than pure volcanic glass, contain minerals. Special equipment and expertise is required to conduct isotopic and precise chemical analyses.
Igneous rocks also contain isotopic information that is used in determining absoloute ages and in further characterizing the origin of the magma. Igneous rock names are based on specific combinations of these features. Igneous rocks contain three essential sources of information: their minerals, their overall chemical composition, and their igneous texture. Those scientists are studying the information contained in the basalts of that age to further test their hypotheses. Many geologists think that the gases and particles released into the atmosphere by those eruptions may have been a major factor in the end of Triassic mass extinction. At the end of the Triassic, a huge amount of basalt erupted onto the earth. For example, at the end of the Triassic period, 245 million years ago, the greatest mass extinction ever known took place, wiping out more life forms on earth than the mass extinction that led to the demise of dinosaurs 65 million years ago at the end of the Cretaceous. The study of igneous rocks enables us to understand the igneous part of geologic history. By carefully analyzing igneous rocks and interpreting the information they contain, we can deduce processes that take place within the earth and we can understand volcanic processes that take place on the earth’s surface. Igneous rocks contain information about how they originate. Because the earth was largely molten at its origin, magma may be considered the beginning of the rock cycle.
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