24 Characteristics of Igneous Rocks
Magma – beginning of the rock cycle
Magma is molten rock inside the earth. It is the source of all igneous rocks, coming from under the Earth under the pressure from surrounding rocks and because molten rocks have lesser density. Because the earth was largely molten at its origin, magma may be considered the beginning of the rock cycle. Igneous rocks contain information about how they originate. 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.
The study of igneous rocks enables us to understand the igneous part of geologic history. For example, at the end of the Permian period, beginning 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. At that time, a huge amount of basalt erupted onto the earth. 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. Those scientists are studying the information contained in the basalts of that age to further test their hypotheses.
Igneous rocks contain three essential sources of information: their minerals, their overall chemical composition, and their igneous texture. Igneous rock names are based on specific combinations of these features. Igneous rocks also contain isotopic information that is used in determining absolute ages and in further characterizing the origin of the magma. Special equipment and expertise is required to conduct isotopic and precise chemical analyses. 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.
All igneous rocks, other than pure volcanic glass, contain minerals. The minerals provide details on the chemical composition of the rock, and on the conditions in which the magma originated, cooled, and solidified. 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.
Most magmas are predominantly silicate liquids, composed largely of silica tetrahedra that have not yet bonded together to become silicate minerals. 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. Amount of silica, together with temperature, amount of volatiles (water + carbon dioxide) regulates magma’s property called viscosity: a measure of the ease of flow. For example, low viscosity means that magma is fluid or semi-fluid; high viscosity means that magma is thick and resists to flow.
Magma forms due to the heat transfer from the earth interior, decompression due to the change of the depth while travelling up and availability of volatile components, such as gases (e.g. carbon dioxide) and water. Once magma has formed inside the earth, its composition may be modified. Minerals can grow from the magma and separate from it, changing the chemistry of the remaining liquid. Or, one body of magma can mix with another that has a different composition.
Magmas come in a range of compositions, from rich in silica and poor in iron and magnesium (felsic, from feldspar & Si) to moderate in silica and high in iron and magnesium (mafic, from Mg and Fe). Felsic igneous rocks, as a whole rock, tend to have light colors or shades: white, pink, light brown, light gray. Mafic igneous rocks, on the whole, tend to be dark colored, commonly black or dark gray. Most mafic magma originates by melting of rocks in the mantle that are extremely rich in iron and magnesium. Felsic magma usually originates in the crust or by the shedding of mafic minerals as magma rises through the crust.
The igneous texture tells us how the magma cooled and solidified. Magma can solidify into igneous rock in several different ways, each way resulting in a different igneous texture. Magma may stay within the earth, far below ground level, and crystallize into plutonic igneous rock (also known as intrusive igneous rock). Or, magma may flow out onto surface of the earth as a lava flow. 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. Lava flows and pyroclastic material are volcanic igneous rock (also known as extrusive igneous rock). The igneous texture of a rock describes whether the rock has mineral crystals or is glassy, the size of the mineral grains, and the rock’s porosity (empty spaces).
