IGNEOUS ROCKS

Introduction Igneous rock (derived from the Latin word igneus meaning of fire, from ignis meaning fire) is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava. This magma can be derived from partial melts of pre-existing rocks in either a planet'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 having formed beneath the surface of Earth's crust. These have diverse properties, depending on their composition and how they were formed.

Magma is a mixture of liquid rock, crystals, and gas Magma is a mixture of liquid rock, crystals, and gas. Characterized by a wide range of chemical compositions, with high temperature, and  properties of a liquid. Magmas are less dense than surrounding rocks, and will therefore move upward. If magma makes it to the surface it will erupt and later crystallize to form an extrusive or volcanic rock. Because cooling of the magma takes place at a different rate, the crystals that form and their interrelationship (texture) exhibit different properties.

Fast cooling on the surface results in many small crystals or quenching to a glass. Gives rise to aphanitic texture (crystals cannot be distinguished with the naked eye), or obsidian (volcanic glass).

Slow cooling at depth in the earth results in fewer much larger crystals, gives rise to phaneritic texture.

Porphyritic texture develops when slow cooling is followed by rapid cooling.

Intrusive igneous rocks Morphology and Setting In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic), extrusive (volcanic) or hypabyssal. Intrusive igneous rocks Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are coarse grained. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. The mineral grains in such rocks can generally be identified with the naked eye.

Intrusive rocks can also be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, stocks, laccoliths, sills and dikes.

Batholiths Batholiths are large bodies of intrusive igneous rock. Formed when magma cools and crystallizes beneath Earth's surface, batholiths are the largest type of pluton. By definition, a batholith must cover at least 100 km2, although most are even larger. Many batholiths cover hundreds to thousands of square miles.

Stocks Stocks are smaller bodies that are likely fed from deeper level batholiths. 

Laccoliths Laccoliths are somewhat large intrusions that result in uplift and folding of the pre-existing rocks above the intrusion.

Dikes Dikes are small (<20 m wide) shallow intrusions that show a discordant relationship to the rocks in which they intrude.  Discordant means that they cut across preexisting structures

A sub-horizontal sheet intrusion of molten or solidified magma Sills Sills are also small (<50 m thick) shallow intrusions that show a concordant relationship with the rocks that they intrude.  Sills usually are fed by dikes, but these may not be exposed in the field.  A sub-horizontal sheet intrusion of molten or solidified magma

Sills

Hypabyssal Igneous rocks Extrusive Igneous Rocks Extrusive igneous rocks, or volcanics, form when magma makes its way to Earth's surface. The molten rock erupts or flows above the surface as lava, and then cools forming rock. The lava comes from the upper mantle layer, between 50 km and 150 km below the Earth's surface. Hypabyssal Igneous rocks Hypabyssal igneous rocks are formed at a depth in between the plutonic and volcanic rocks.

Classification of Igneous rocks