Ophiolites are slices of oceanic crust that have been uplifted and exposed, making it possible to examine a cross-section of a tectonic plate. This provides a valuable opportunity to study the different layers of the Earth. There are many ophiolites, the most famous probably being those found in Oman and Cyprus. These were emplaced onto continental crust through obduction and are in some cases tilted or rotated.
The uppermost ~5 kilometers of the brittle oceanic crust can thus be examined in cross-section. At the top lies a layer of pillow lavas, beneath which are sheeted dikes, followed by a gabbro layer extending down to the Moho discontinuity. In favorable cases, ophiolites also expose portions of the underlying mantle.
At the Moho, there is typically a transition zone composed mainly of two rock types: wehrlite just below the gabbro, followed by dunite. Beneath this lies the lithospheric mantle, which is composed mainly of harzburgite. This is a type of peridotite that remains after the original lherzolite of the asthenosphere has undergone partial melting to produce basalt.
Basalt originates from the asthenosphere at depths of around 120 km. The temperature of basalt at eruption at the Earth’s surface is surprisingly close to the temperature at which it originally formed. In Iceland, basalt may represent up to about 20% partial melting of the original lherzolite.
One particularly important feature revealed by the exposure of mantle below the Moho is the presence of conduits through which partial melt (basaltic magma) has traveled. These conduits consist of dunite, composed almost entirely of olivine. They form vertical channels leading up toward the Moho. This indicates that partially molten material can flow upward relatively rapidly, entering the gabbroic section immediately above the Moho transition zone.
The gabbro zone is commonly divided into two parts, with the lower portion showing layered structures. Within this zone, basaltic magma can accumulate in sills and magma chambers, where it may partially crystallize before continuing its ascent. When conditions allow, the magma rises again, typically vertically, through the sheeted dike complex, forming successive dikes.
As frequently observed in Iceland, such processes can eventually lead to volcanic eruptions at the surface. On the ocean floor, these eruptions typically produce pillow lavas.
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