Peridotite is a dense, coarse-grained rock that rarely appears on the Earth’s surface, yet it is a fundamental component of our planet’s structure. Its dark olive-green to yellowish-green color results directly from its high magnesium content. This coloration comes from the dominant mineral, olivine, which also forms the gem peridot. Originating deep within the Earth, peridotite provides scientists with a window into the composition of the planet’s interior.
Classification as an Ultramafic Rock
Peridotite is classified as an intrusive igneous rock, meaning it formed from magma that cooled and solidified slowly beneath the Earth’s surface. This slow cooling allows large mineral crystals to develop, giving the rock a characteristic coarse-grained texture known as phaneritic. Peridotite falls into the category of ultramafic rocks, defined by their specific chemical makeup.
Ultramafic rocks are chemically defined by a low silica content (less than 45%) and a high concentration of iron and magnesium. This composition is the opposite of the silica-rich rocks that make up most of the continental crust, such as granite. The high proportion of iron and magnesium-bearing minerals (often over 90% of the rock’s volume) gives peridotite its density.
Essential Mineral Components
The definition of peridotite hinges on its mineral composition, requiring olivine as the primary component. Olivine must constitute more than 40% of the rock’s total volume, and this magnesium-rich mineral imparts the rock’s signature green color. Olivine is an iron-magnesium silicate with a high melting temperature, making it stable under the conditions of the deep Earth.
The remaining portion of peridotite is made up of the pyroxene family of minerals, which are generally darker. The precise ratio between olivine, orthopyroxene, and clinopyroxene determines the specific name of a peridotite subtype. For example, lherzolite contains significant amounts of both pyroxenes, making it the most common and “fertile” variety found in the upper mantle.
If the rock is nearly pure olivine (often exceeding 90% of the mineral content), it is classified as dunite. Harzburgite, another common type, is composed mostly of olivine and orthopyroxene but contains little clinopyroxene, representing a more “depleted” composition. These variations allow geologists to interpret the specific conditions and processes the rock experienced deep within the Earth.
Origin and Global Geological Context
Peridotite is the dominant rock type of the Earth’s upper mantle, extending from the base of the crust down to about 410 kilometers. It forms the source material for most basaltic magma, created when peridotite undergoes partial melting and the melt rises toward the surface. Partial melting leaves behind a more chemically depleted peridotite residue, such as harzburgite, which is enriched in minerals with a higher melting temperature.
Since peridotite forms at immense depths, its appearance on the surface is a rare geological event requiring powerful tectonic forces. One mechanism is the rapid transport of rock fragments, known as xenoliths, which are pieces of the mantle carried upward within erupting magmas. These xenoliths are found embedded in volcanic rocks like basalts and kimberlites, providing direct samples of the deep mantle from depths sometimes exceeding 200 kilometers.
Sections of peridotite are also brought to the surface through obduction, a process which thrusts fragments of oceanic crust and the underlying mantle onto continental plates. These large, exposed slabs of mantle peridotite and associated oceanic crust are called ophiolites, offering extensive cross-sections for study. Studying these surface exposures allows scientists to analyze the composition, structure, and chemical evolution of the mantle, which drives plate tectonics and volcanism.
Alteration and Economic Importance
When peridotite is exposed to water, especially at low temperatures, it undergoes a fundamental chemical change called serpentinization. This process converts the primary minerals, olivine and pyroxene, into a new group of hydrated minerals known as serpentine. Serpentinization can cause a significant increase in rock volume, and the resulting rock, serpentinite, is much softer than the original peridotite.
Peridotite also plays a role in the global carbon cycle, as its magnesium and iron-rich minerals can react with carbon dioxide in a process called carbonation. This natural reaction captures gaseous carbon dioxide and locks it away in solid carbonate minerals, offering a potential mechanism for long-term carbon sequestration. The rock’s chemical makeup makes it a host for valuable resources, giving it economic importance.
Peridotite is the source rock for valuable metals, including nickel, chromium, and the platinum group elements. Chromite, the only ore mineral of chromium, is often found in association with dunite, a peridotite subtype. Furthermore, the kimberlite variety of peridotite is the host rock in which many of the world’s natural diamonds are found.