Igneous Rocks

Igneous rocks originate in masses of hot fluid that circulate deep within the Earth. This molten rock, called magma, may consist of part of the Earth's crust that has melted as a result of tectonic or mountain-building activity, or it may arise from the mantle (the layer immediately below the crust).

Magma Rocks

Igneous rocks are formed when molten magma cools and solidifies. Different types of igneous rocks may be produced out of the same mass of magma. As it cools, the components of the magma solidify in a set sequence.

The first minerals to crystallize out of the melt are the high-temperature minerals - the olivines and pyroxenes, which are silicates of magnesium and iron. They tend to be denser than the magma and so they sink, leav­ing the remaining fluid deficient in magnesium and iron.

The next group of minerals to solidify are the feldspars (silicate minerals of potassium, sodium, calcium and aluminum - the lighter metallic elements); the magma thus loses its metal­lic constituents first.

Finally, any remaining silica crystallizes out as quartz. The entire solidification process, or differentiation as it is called, therefore results in dense iron- and magnesium-rich rocks and less dense silica-rich rocks from the same original fluid. This is dramatically exemplified in the rare outcrops in which the different types of rock can be seen as layers in the same rock mass - as occurs in the 300m thick Palisade Sill in New Jersey in the United States, which has an olivine-rich layer at the bottom and rocks with progres­sively less olivine above. Usually, however, an outcrop consists of only one type of igneous rock.

Geologists classify igneous rocks according to their composition. Those that have a low silica content (and are also usually rich in iron and magnesium) are called basic rocks; those with a high silica content are termed acid rocks.

Basic rocks, such as gabbro, tend to be dark in color because their constituent minerals are dark, whereas acid rocks (such as granite) are light in color because they contain white and pink feld­spars and glassy quartz.

Igneous rocks are also categorized according to their origin: intrusive rocks, formed from magma that solidified beneath the surface, and extrusive rocks, from magma that solidified above the surface.

Igneous Rock Textures

When hot magma cools slowly, the minerals in it have sufficient time to grow large crystals and hence form coarse rock. The mineral crystals in rocks that have cooled quickly, on the other hand, are often too small to be seen with the naked eye.

The coarseness of a rock depends on where it was formed. Very coarse-grained rocks, such as the gabbros and granites, solidified deep under­ground and therefore cooled slowly. Volcanic rocks, such as basalts and trachites, were formed from magma that cooled rapidly on the surface of the Earth and are therefore fine-grained. The fin­est-grained igneous rocks originated from vol­canoes that erupted underwater or beneath glaciers, as a result of which the lava (magma ejected by a volcano) cooled extremely rapidly.

Occasionally, such igneous rocks are so fine-grained that no crystalline structure is visible, resulting in a natural glass called obsidian.

Sometimes an igneous rock has two textures. It may have large crystals (called phenocrysts) embedded in a matrix of very small ones. This type of two-textured rock forms when magma begins to differentiate slowly then, when some of the crystals have formed, solidifies much more rapidly - probably because it was forced into a cooler location. This texture is known as porphyritic, and the rock is called a porphyry.

The texture and composition of a rock can be studied by cutting a sample into thin transparent slices and examining them with a microscope. The rock's constituents can then be determined by viewing the sections using polarized light, a tech­nique that causes each mineral crystal to appear as a different color. This method reveals that the minerals which formed first have well-defined crystal shapes, whereas those that grew later tend to be distorted.

Igneous Rock Structures

It is not possible to observe an igneous rock while it forms (except volcanic rocks, in which the crys­tallization and solidification can be particularly spectacular) because most igneous rocks form deep under the surface of the Earth in structures called intrusions. From these intrusions the magma can push its way through cracks, forcing aside or melting the surrounding rocks, and the resultant structures reflect this action.

The largest igneous intrusions are called batholiths and they form deep below the surface in active mountain chains. They may extend over hundreds of square kilometers. Underground cracks may fill with magma, forming sheets of igneous rock when the magma solidifies. The sheets are known as sills if they lie parallel to the strata of the surrounding rocks, or dykes if they cut across the strata. Igneous rocks may also form cylindrical structures - called stocks if they are broad and necks if they are narrow - which may once have led to volcanoes on the surface.