An Ophiolite is a typical piece of sea floor

A comparison of oceanic crust and an ophiolite sequence. (A) Structure of oceanic crust, determined from seismic studies and drilling. Gabbro and ultramafic rock drilled and dredged from seafloor fault blocks may be from Layer 3 and the mantle. (B) Typical ophiolite sequence found in mountain ranges on land. Thickness is approximate—sequence is usually highly faulted. (C) Pillow basalt from the upper part of an ophiolite, northern California. These rocks formed as part of the sea floor, when hot lava cooled quickly in cold seawater.
(C) ©David McGeary

The top layer (Layer 1), of variable thickness and character, is marine sediment. In an abyssal fan or on the continental rise, Layer 1 may consist of several kilometers of terrigenous sediment. On the upper flanks of the mid-oceanic ridge, there may be less than 100 meters (328 feet) of pelagic sediment. An average thickness for Layer 1 might be 0.5 kilometer (1,640 feet).

Beneath the sediment is Layer 2, which is about 1.5 kilometers (about 1 mile) thick. It has been extensively sampled. This layer consists of pillow basalt overlying dikes of basalt. The basalt pillows, rounded masses that form when hot lava erupts into cold water (figure 18.24), are highly fractured at the top of Layer 2. The dikes in the lower part of Layer 2 are closely spaced, parallel, vertical dikes (“sheeted dikes”). Widely sampled by drilling and dredging, Layer 2 has also been observed directly by geologists in submersibles diving into the rift valley of the mid-oceanic ridge during the FAMOUS expedition.

The lowest layer in the crust is Layer 3. It is about 5 kilometers (3.1 miles) thick and thought to consist of sill-like gabbro bodies. The evidence for this interpretation is suggestive but not conclusive. Geologists have drilled through 2 kilometers 451(1.2 miles) of pillow basalt and basaltic dikes in oceanic crust but have not yet been able to reach gabbro by drilling through basalt. Gabbro (and other rocks) are exposed on some fault blocks in rift valleys and on some steep submarine cliffs in fracture zones; 0.5 kilometer of gabbro has been drilled on one such cliff in the southern Indian Ocean. Some geologists presume that the gabbro represents Layer 3 exposed by faulting, but other geologists are not so sure. Some evidence exists that rocks exposed in fracture zones may not be representative of the entire sea floor. Even if the drilled gabbro does represent the upper one-tenth of Layer 3, no one has yet sampled the lower nine-tenths of the layer.


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