Olivine is a common rock-forming mineral, recognized by its distinctive green color. This mineral is a silicate, meaning its structure is built around silicon and oxygen atoms. It is found in a variety of geological settings, from deep within Earth’s mantle to the surfaces of other planetary bodies. Understanding how olivine forms helps clarify its presence in different types of igneous rocks.
Understanding Igneous Rock Formation
Igneous rocks originate from the cooling and solidification of molten rock, known as magma or lava. The location where this cooling occurs determines whether the rock is intrusive or extrusive. Intrusive igneous rocks form when magma cools slowly beneath Earth’s surface, insulated by surrounding rock. This slow cooling allows mineral crystals ample time to grow, often resulting in visible, coarse-grained textures.
Conversely, extrusive igneous rocks form when lava erupts onto the surface or cools rapidly underwater. The quick exposure to cooler temperatures causes the lava to solidify quickly, leaving little time for large crystals to develop. Consequently, extrusive rocks have fine-grained textures, where individual crystals are often too small to be seen without magnification. The rate of cooling directly impacts the size of the mineral grains within the resulting rock.
Olivine’s Primary Formation
Olivine primarily forms in extrusive igneous rocks, particularly those rich in magnesium and iron, known as mafic and ultramafic volcanic rocks. This mineral has a high crystallization temperature, meaning it is among the first minerals to solidify as molten rock cools. In volcanic eruptions, lava cools very quickly upon reaching Earth’s surface or interacting with water. This rapid cooling environment is ideal for olivine to crystallize swiftly before other minerals can form large structures.
While olivine can be a component in some intrusive rocks, such as gabbro or peridotite, it is predominantly associated with extrusive processes. Its most common and abundant occurrences are linked to rapid cooling scenarios. The rapid formation of small olivine crystals within fast-cooling lavas highlights its extrusive association.
Common Occurrences of Olivine
Olivine is a widespread mineral, frequently found in basaltic lavas, which are common extrusive rocks. Vast expanses of oceanic crust are composed of basalt, making olivine a significant component of the ocean floor. Active volcanic regions, such as the Hawaiian Islands, also feature abundant olivine in their basaltic lava flows.
Olivine can also be found in mantle xenoliths, which are fragments of Earth’s mantle brought to the surface during volcanic eruptions. These xenoliths provide direct evidence of olivine’s prevalence in the high-temperature conditions of the upper mantle. Olivine is also a common mineral in some meteorites, indicating its formation in high-temperature, rapidly cooling conditions in the early solar system.