Intrusive Igneous Rocks Help

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Intrusive Igneous Rocks

The cooled, crystallized magma that forces its way into the surrounding unmelted rock masses deep in the Earth is called intrusive igneous rock . It can be identified by its interlocking large crystals (grains 1 mm or larger), which grow slowly as magma cools over time. These are the large crystals Hutton observed. Commonly, large mineral grain igneous rock is known as phanerites .

When this type of rock is formed several kilometers below the surface of the Earth, it is known as plutonic rock . All masses of intrusive igneous rock, whether large or small, are called plutons . They are created on the slow boat of movement to the surface and have a lot of time to solidify and develop their individual unique compositions.

Plutons are given specific names depending on their size and shape. Dikes are areas of intrusive igneous rock that thrust up through other rock layers. They are generally perpendicular to the layers above, found at any depth, and trace the last push of a finger of upward rising magma. A volcanic neck is different from a dike in that it is discordant. It forms the feeder pipe just below a volcanic vent. The 400 m upthrusting mass of igneous rock known as Shiprock, New Mexico, United States is an ancient volcanic neck.

Sills are areas of intrusive igneous rock that are parallel to the layering of other intruded rock layers. They form flat, horizontal pockets between piles of rock layers at shallow depths and lower overlaying pressure. A laccolith is a mass of intrusive igneous rock that has pushed up between rock layers and been stopped to form a dome-shaped mound that looks like a blister.

Dikes and sills are often found together as part of a larger pluton network of intrusive igneous rock called a batholith.

Large plutons with outcrop exposures (rock sticking up through the ground) of greater than 100 km 2 are called batholiths and are huge compared to dikes and sills. They cover thousands of square kilometers and stretch across big parts of states and even between countries. One such monstrous batholith is the Coast Range Batholith that stretches from southern Alaska down the western coast of British Columbia, Canada to end in the state of Washington. It is roughly 1500 km in length.

When a batholith has outcrop exposure of less than 100 km 2 in length, it is called a stock . A stock of igneous rock is often found as a minor collection of rock located near the main batholith or as part of a mostly worn-away batholith.

Batholiths are huge masses of intrusive igneous rock, usually granite, with an exposed surface of larger than 100 km 2 and formed in the subduction zone along continental plate borders.

Granite, an example of intrusive igneous rock that crystallized slowly from magma below the Earth’s surface, makes up a large portion of plutons and batholiths. Geologists’ measurements of large batholiths have recorded depths of between 15 and 30 km thick. Figure 6-1 shows the differences between the creation of igneous intrusive and extrusive rock.

Igneous Rock


Fig. 6-1. The rate of speed with which magma rises to the surface affects crystal formation.

A single magma can crystallize into an assortment of igneous rock types. It doesn’t solidify into one compound like water does when it freezes into ice. When magma solidifies, it forms many different minerals, which all crystallize at different temperatures. The different crystals solidify from the liquid magma, when their crystallization temperature is reached. Like a row of dominoes, as the temperature drops, crystals form one after another. Cooling magma then contains some fluid rock and some rock that has already hardened into crystals. When this happens, the concentration of certain minerals in the remaining magma is increased.

Sometimes when crystals are forming from an isolated chamber of magma, they are denser than the surrounding fluid. When this happens, they sink to the bottom of the chamber and form a separate crystalline layer with characteristics different from the remaining magma. As cooling, crystallization, and sinking of minerals continues, many crystal layers with different compositions are formed.

In the early 1900s, geologist Norman Bowen was the first person to understand the importance of the temperature and the formation of separate crystals from magma. His studies at the Geophysical Laboratory in Washington, DC, which focused on the melting and crystallization properties of minerals, showed that as magma cooled at different temperatures, the composition of later formed crystals was very different. Bowen found that early forming crystalline rock had a lot more calcium, than later formed crystalline rock. As time went on, other geologists got interested in Bowen’s ideas and the process of separating crystalline fall out from liquid magma became known as magmatic differentiation by fractional crystallization .

Practice problems of this concept can be found at: Igneous Rock Practice Test

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