Formation Of Igneous Rocks
The magmas that form igneous rock are hot, chemical soups containing a complex mixture of many different elements. As they cool, many different minerals could form. Indeed, two magmas with identical composition could form quite distinct sets of minerals, depending on the conditions of crystallization.
As a magma cools, the first crystals to form will be of minerals that become solid at relatively high temperatures (usually olivine and a type of feldspar known as anorthite). The composition of these early-formed mineral crystals will be different from the initial composition of the magma. Consequently, as these growing crystals take certain elements out of the magma in certain proportions, the composition of the remaining liquid changes. This process is known as magmatic differentiation. Sometimes, the early-formed crystals are separated from the rest of the magma, either by settling to the floor of the magma chamber, or by compression that expels the liquid, leaving the crystals behind.
As the magma cools to temperatures below the point where other minerals begin to crystallize (such as pyroxene and another type of feldspar known as bytownite), their crystals will start to form as well. However, early-formed minerals often cannot coexist in magma with the later-formed mineral crystals. If the early-formed minerals are not separated from the magma, they will react with or dissolve back into the magma over time. This process repeats through several cycles as the temperature of the magma continues to cool to the point where the remaining minerals become solid. The final mix of minerals formed from a cooling magma depends on three factors: the initial composition of the magma, the degree to which already-formed crystals separate from the magma, and the speed of cooling.
INTRUSIONS
When magma intrudes a region of the crust and cools, the resulting mass of igneous rock is called an intrusion. Geologists describe intrusions by their size, their shape, and whether they are concordant, meaning they run parallel to the structure of neighboring rocks, or discordant, meaning they cut across the structure of neighboring rocks. An example of a concordant intrusion is a horizontal bed formed when magma flows between horizontal beds of neighboring rock. A discordant intrusion would form when magma flows into cracks in neighboring rock, and the cracks lie at an angle to the neighboring beds of rock.
A batholith is an intrusion with a cross-sectional area of more than 100 sq km (39 sq mi), usually consisting of granite, granodiorite, and diorite. Deep batholiths are often concordant, while shallow batholiths are usually discordant. Deep batholiths can be extremely large; the Coast Range batholith of North America is 100 to 200 km (60 to 120 mi) wide and extends 600 km (370 mi) through Alaska and British Columbia, Canada.
Lopoliths are saucer-shaped concordant intrusions. They may be up to 100 km (60 mi) in diameter and 8 km (5 mi) thick. Lopoliths, which are usually basaltic in composition, are frequently called layered intrusions because they are strongly layered. Well-known examples are the Bushveld complex in South Africa and the Muskox intrusion in the Northwest Territories, Canada.
Laccoliths have a flat base and a domed ceiling, and are concordant with the neighboring rocks; they are usually small. The classic area from which they were first described is the Henry Mountains in the state of Utah.
Dikes and sills are sheetlike intrusions that are very thin relative to their length; sills are concordant and dikes are discordant. They are commonly fairly small features (a few meters thick) but can be larger. The Palisades Sill in the state of New York is 300 m (1000 ft) thick and 80 km (50 mi) long.
EXTRUSIVE BODIES
Many different types of extrusive bodies occur throughout the world. The physical characteristics of these bodies depend on their chemical composition and on how the magma from which they formed erupted. The chemical composition of the parent magma affects its viscosity, or its resistance to flow, which in turn affects how the magma erupts. Felsic magma tends to be thick and viscous, while mafic magma tends to be fluid. (See also Volcano)
Flood basalts are the most common type of extrusive rock. They form when highly fluid basaltic lava erupts from long fissures and many vents. The lava coalesces and floods large areas to considerable depths (up to 100 m/300 ft). Repeated eruptions can result in accumulated deposits up to 5 km (3 mi) thick. Typical examples are the Columbia River basalts in Washington and the Deccan trap of western India; the latter covers an area of more than 500,000 sq km (200,000 sq mi).
When basalt erupts underwater, the rapid cooling causes it to form a characteristic texture known as pillow basalt. Pillow basalts are lava flows made up of interconnected pillow-shaped and pillow-sized rocks. Much of the ocean floor is made up of pillow basalt.
Extrusive rocks that erupt from a main central vent form volcanoes, and these are classified according to their physical form and the type of volcanic activity. Mafic, or basaltic, lava is highly fluid and erupts nonexplosively. The fluid lava quickly spreads out, forming large volcanoes with shallow slopes called shield volcanoes. Mauna Loa (Hawaii) is the best-known example. Intermediate, or andesitic, magmas have a higher viscosity and so they erupt more explosively. They form steep-sided composite volcanoes. A composite volcano, or stratovolcano, is made up of layers of lava and volcanic ash. Well-known examples of composite volcanoes include Mount Rainier (Washington), Mount Vesuvius (Italy), and Mount Fuji (Japan).
Felsic (rhyolitic) magmas are so viscous that they do not flow very far at all; instead, they form a dome above their central vent. This dome can give rise to very explosive eruptions when pressure builds up in a blocked vent, as happened with Mount Saint Helens (Washington) in 1983, Krakatau (Indonesia) in 1883, and Vesuvius (Italy) in AD 79. This type of explosive behavior can eject enormous amounts of ash and rock fragments, referred to as pyroclastic material, which form pyroclastic deposits (See also Pyroclastic Flow)
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