Igneous rocks form from the cooling and solidification of molten material. They are categorized by the location where this process takes place: intrusive (formed beneath the surface) or extrusive (solidified on the surface). This classification helps geologists understand the conditions under which a rock crystallized. The specific nature of peridotite, a dense and relatively uncommon rock, is determined by this system.
The Difference Between Intrusive and Extrusive Rocks
The distinction between the two types of igneous rock is determined by the rate at which molten rock, known as magma or lava, cools. Intrusive igneous rocks, also called plutonic rocks, form when magma crystallizes slowly deep within the Earth’s crust. The insulating effect of the surrounding rock allows cooling to take place over thousands or even millions of years. This prolonged cooling provides ample opportunity for mineral grains to grow into large, visible crystals, resulting in a phaneritic texture.
Conversely, extrusive igneous rocks, or volcanic rocks, form when magma is erupted onto the surface as lava and cools rapidly. Contact with the atmosphere or water causes the lava to solidify quickly, often within hours or days. This fast cooling prevents the formation of large mineral crystals, resulting in a fine-grained texture described as aphanitic. For example, granite is an intrusive rock with large crystals, while basalt is its fine-grained extrusive equivalent.
Defining Peridotite: Composition and Characteristics
Peridotite is a dense, dark-colored igneous rock classified as ultramafic due to its unique composition. It is rich in magnesium and iron, containing less than 45% silica by weight. Its color typically ranges from green to greenish-black, a hue derived from its dominant mineral.
The rock is composed primarily of the silicate minerals olivine and pyroxene, with olivine often making up 40% or more of the volume. The name peridotite is derived from the gemstone peridot, which is the gem-quality variety of olivine. It may also contain minor amounts of spinel, garnet, or chromite, depending on formation conditions. Peridotite exhibits a coarse-grained texture, meaning its interlocking mineral crystals are visible to the unaided eye.
Peridotite’s Classification and Formation Depth
Based on its coarse-grained, phaneritic texture, peridotite is classified as an intrusive, or plutonic, igneous rock. This texture is the direct result of the slow cooling process that occurs when the molten material solidifies deep underground. The rock originates in the Earth’s upper mantle, forming under immense pressure and high temperatures, typically between 30 and 200 kilometers below the surface.
The composition of ultramafic magma, the parent material for peridotite, reinforces its intrusive nature. This magma has a high melting temperature, making it viscous and difficult to move. Due to this high viscosity, the magma rarely ascends rapidly enough to the surface before crystallizing within the mantle or lower crust. Consequently, the cooling process is insulated and slow, confirming peridotite’s status as a deep-seated intrusive rock.
The Geological Significance of Mantle Rocks
Peridotite holds importance for geologists because it is the primary rock type making up the Earth’s upper mantle. Studying samples offers direct insight into the composition and processes occurring deep within our planet, which is otherwise inaccessible. It serves as the source material for basaltic magmas that form the oceanic crust and fuel much of the world’s volcanic activity.
The rock only reaches the surface through specific tectonic mechanisms. These include large-scale tectonic uplift, where sections of the mantle are thrust onto continental crust in formations called ophiolites. It can also be brought up as fragments, known as xenoliths, carried within faster-moving magmas like those found in basalt flows or kimberlite pipes. Although peridotite is intrusive, a rare extrusive equivalent called komatiite exists, which required extremely high temperatures common only during the Earth’s early Archean Eon.