Ultramafic rocks are igneous and meta-igneous materials characterized by an exceptionally low silica content, typically less than 45% by weight, and a high concentration of magnesium and iron oxides. They are dark and dense, composed of more than 90% ferromagnesian minerals, primarily olivine and pyroxene. These unique compositions offer geologists direct physical samples and chemical clues about the deep interior of the Earth. Understanding their origin and how they reach the surface is central to deciphering the planet’s internal dynamics.
The Primary Formation Environment
The vast majority of the Earth’s ultramafic material constitutes the upper mantle, deep beneath the crust. The dominant rock type is peridotite, a coarse-grained rock composed mainly of olivine and pyroxene. Peridotite is classified into two main types: lherzolite and harzburgite. Lherzolite is considered the most primitive ultramafic rock, containing significant amounts of olivine, orthopyroxene, and clinopyroxene.
This environment is characterized by immense lithostatic pressure, which prevents the rock from melting despite high temperatures. The stability of the peridotite’s mineral assemblage changes with depth and pressure.
As depth increases, the aluminous phases within the peridotite transform due to increasing pressure. At relatively shallow depths, plagioclase is the stable phase, forming plagioclase peridotite. This changes to spinel peridotite at depths up to around 90 kilometers. Below 100 kilometers, the pressure is high enough to form pyrope garnet, creating garnet peridotite, which remains stable down to the transition zone near 400 kilometers.
Harzburgite is the second major type of mantle peridotite and is considered a residue of partial melting from the more fertile lherzolite. When the mantle partially melts to produce basaltic magma, the clinopyroxene component is preferentially removed. This leaves behind a refractory rock rich in olivine and orthopyroxene. The mantle’s ultramafic composition thus reflects a history of both primitive material (lherzolite) and material depleted by magma generation (harzburgite).
Formation Through Magmatic Differentiation
Ultramafic rocks also form in a secondary location: large, stable magma chambers through magmatic differentiation. This process involves fractional crystallization, where minerals crystallize sequentially as the magma cools. Because ferromagnesian minerals like olivine and pyroxene have the highest crystallization temperatures, they form first.
These early-forming, dense crystals then settle under gravity to the floor of the magma chamber, accumulating to form distinct layers known as cumulates. This gravitational settling creates layered mafic intrusions. In these massive bodies, the composition changes vertically from ultramafic layers at the bottom to more silica-rich layers near the top.
In these layered intrusions, ultramafic rocks are represented by monomineralic layers such as dunite, which consists almost entirely of accumulated olivine crystals. Pyroxenite layers, composed dominantly of pyroxene, form as the temperature drops and pyroxene begins to crystallize and settle. The Bushveld Igneous Complex in South Africa is a prime example of this formation. These accumulated layers do not represent the composition of the original magma, but rather the material that separated from it.
Tectonic Settings for Surface Exposure
Although most ultramafic material resides deep within the planet, several tectonic processes bring these high-pressure rocks to the Earth’s surface. The most common mechanism is obduction, which involves the thrusting of oceanic crust and underlying mantle onto continental margins. This process creates an ophiolite, a complex slice of oceanic lithosphere that exposes the mantle section, typically composed of harzburgite and dunite, on land.
Obduction occurs when the oceanic plate is pushed over the edge of a continental plate during a collision, instead of sinking in a subduction zone. The Samail Ophiolite in Oman is a well-preserved example where deep-seated peridotite has been exposed, providing a natural cross-section of the upper mantle.
A second mechanism is the formation of kimberlite pipes, which are vertical volcanic conduits that rapidly transport mantle fragments to the surface. These volatile-rich, silica-poor magmas originate at depths of 150 to 450 kilometers, within the garnet peridotite stability field. The extreme pressure from dissolved volatiles, such as carbon dioxide and water vapor, drives the magma’s rapid ascent.
The rapid transport prevents deep-mantle rock fragments, known as xenoliths, and any entrained diamonds from being reabsorbed or transformed. This explosive eruption creates a carrot-shaped volcanic structure, delivering samples of the deep Earth to the crust. Finally, large-scale, deep-seated faulting can expose ultramafic rocks, often serpentinized variants. Transpressional forces along major fault systems, such as the Sagaing fault in Myanmar, can uplift slivers of hydrated, mantle-derived rock, bringing them to the surface as serpentine bodies in tectonic windows.