About Igneous Rock

Igneous rock (etymology from Latin ignis, fire) is one of the three main rock types (the others being sedimentary and metamorphic rock). Igneous rocks are formed by solidification of cooled magma (molten rock). They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. These have diverse properties, depending on their composition and how they were formed.

Igneous Rock Textures

Tell the three types of igneous rocks apart by their texture, starting with the size of the mineral grains. Extrusive rocks cool quickly (over periods of seconds to months) and have invisible or very small grains, or an aphanitic texture. Intrusive rocks cool more slowly (over thousands of years) and have small to medium-sized grains. Plutonic rocks cool over millions of years, deep underground, and can have grains as large as pebbles — even a meter across. Both intrusive and plutonic rocks have phaneritic texture.

Because they solidified from a fluid state, igneous rocks tend to have an equigranular texture, a uniform fabric without layers, and the mineral grains are packed together tightly. Think of the texture of a piece of bread as a similar example.

In many igneous rocks, large mineral crystals "float" in a fine-grained groundmass. The large grains are called phenocrysts, and a rock with phenocrysts is called a porphyry; that is, it has a porphyritic texture. Phenocrysts are minerals that solidified earlier than the rest of the rock, and they are important clues to the rock's history.

Some extrusive rocks have distinctive textures. Obsidian, formed when lava hardens quickly, has a glassy texture. Pumice and scoria are volcanic froth, puffed up by millions of gas bubbles giving them a vesicular texture. Tuff is a rock made entirely of volcanic ash, fallen from the air or avalanched down a volcano's sides. It has a pyroclastic texture. And pillow lava is a lumpy formation created by extruding lava underwater.

Pillow lava

Source : www.wikipedia.com, http://geology.about.com