Igneous Rock.

« 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 asanorthite).

The composition of these early-formed mineral crystals will be different from the initial composition of the magma.

Consequently, as these growing crystalstake 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 expelsthe 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-formedminerals 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 thetemperature 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 dependson three factors: the initial composition of the magma, the degree to which already-formed crystals separate from the magma, and the speed of cooling. IV 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 wouldform 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 areoften 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 Muskoxintrusion 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 firstdescribed 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. V 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 themagma 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 themagma 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 andfloods 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 theColumbia 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 ofinterconnected 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, orbasaltic, 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 compositevolcanoes.

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 veryexplosive 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) VI PLATE TECTONICS AND IGNEOUS ROCKS The advent of the theory of plate tectonics in the 1960s provided a theoretical framework for understanding the worldwide distribution of different types of igneousrocks.

According to the theory of plate tectonics, the surface of the earth is covered by about a dozen large plates.

Some of these plates are composed primarily ofbasalt and are called oceanic plates, since most of the ocean floor is covered with basalt.

Other plates, called continental plates because they contain the continents, arecomposed of a wide range of rocks, including sedimentary and metamorphic rocks, and large amounts of granite. Where two plates diverge (move apart), such as along a mid-ocean ridge, magma rises from the mantle to fill the gap.

This material is mafic in composition and formsbasalt.

Where this divergence occurs on land, such as in Iceland, flood basalts are formed. When an oceanic plate collides with a continental plate, the heavier oceanic plate subducts , or slides, under the lighter continental plate.

Some of the subducted material melts and rises.

As it travels through the overriding continental plate, it melts and mixes with the continental material.

Since continental material, on average, is morefelsic than the mafic basalt of the oceanic plate, this mixing causes the composition of the magma to become more mafic.

The magma may become intermediate incomposition and form andesitic volcanoes.

The Andes Mountains of South America are a long chain of andesitic volcanoes formed from the subduction of the Pacific Plateunder the South American plate.

If the magma becomes mafic, it may form rhyolitic volcanoes like Mount Saint Helens.

Magma that is too viscous to rise to the surfacemay instead form granitic batholiths. VII ECONOMIC IMPORTANCE OF IGNEOUS ROCKS Many types of igneous rocks are used as building stone, facing stone, and decorative material, such as that used for tabletops, cutting boards, and carved figures.

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