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Igneous Rock Help

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

Introduction to Igneous Rock

Unlike plants and animals, most rocks have long histories. They seem ancient and never changing because within our lifetimes, they don’t change much. A rock is a rock is a rock.

Igneous rocks, however, are probably the only rocks that give us a window into new rock formation. Igneous rocks are formed from magma that is sent through volcanic activity to the surface. Depending on the speed and way magma reaches the surface, the hardened igneous rock that is formed can look very different. We will look more closely at the three major magma types when we study volcanoes.

There are three main rock types that come from magma: sedimentary, igneous, and metamorphic. Of these three, igneous is probably the most active and exciting. Igneous rock is created by exploding volcanoes and boiling undersea fissures. It has lots of distinct textures and colors depending on its chemical content and formation.

Six minerals make up nearly all of igneous rock. These minerals are quartz, feldspar, pyroxene, olivine, amphibole, and mica. The chemical elements that make up these minerals are silicon (Si), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), iron (Fe), aluminum (Al), hydrogen (H), and oxygen (O).

Rocks formed by the hardening of molten rock (magma), whether deep in the Earth or blasted out during an eruption, are called igneous rock.

Igneous rock is formed from the cooling and hardening of magma within the Earth’s crust. Over 95% of the top 10 miles of the crust is made up of igneous rock formed from lava eruptions. The root word ignis in Latin means fire. It is formed in temperatures of at least 700°C, the temperature needed to melt rock. The deepest magma in the mantle, next to the super-heated extreme heat of the outer core, has a different makeup from magma just beneath the crust and squeezed up through cracks or conduits.

The study of igneous rock is a study of magma , since igneous rock comes from cooled magma that has made its way to the Earth’s surface. But not all magma is created equally. Depending on the time of heating and method of getting to the surface, different cooled magmas form rocks that look very different from each other. When scientists started studying igneous rock in the laboratory, they found two simple ways to separate igneous rock samples, texture and composition .

Sitting around a campfire on a starry night, the encircling rocks around the fire don’t usually melt. It takes very high temperatures to melt rock. The type of rock-melting heat that affects igneous rock is a lot like that found on the Earth in its earliest days. From earlier chapters, we learned that the deeper into the Earth you go, the hotter the temperature. Sample temperatures taken at different depths commonly increase about 30°C per kilometer (90°F per mile). Of course, rock samples taken near magma lakes, along known fissures and near volcanoes, are a whole lot hotter.

The rate of temperature increase compared to depth is known as a geothermal gradient.

Pressure also has an effect on the melting of rock. The greater the pressure applied to a solid (rock), the more force is applied to its atoms. This force packs the rock into denser and denser structures. Rocks deep in the mantle are under a lot of pressure. When a tectonic plate shifts or a crustal fissure forms releasing some of the overlying pressure, tightly compressed rock structure loosens up. Atoms aligned and held in a certain pattern within the rock structure are then able to shift. Their movement becomes freer and a lot more like a liquid state.

For example, the compound albite melts at 1104°C at the Earth’s surface where the pressure is 1 bar. The melting temperature at 100 km, where the pressure is 35,000 times greater, is 1440°C. The extreme heat that couldn’t affect the deeply pressurized rock can melt the less-compressed rock at the surface, allowing it to flow as a fluid.

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