5.7 What is an Ophiolite Complex? The four distinct layers of oceanic crust

5.7 What is an Ophiolite Complex? The four distinct layers of oceanic crust

One of the most interesting aspects of the oceanic crust is that its thickness and structure is remarkably consistent throughout the entire ocean basin. Seismic soundings indicate that it averages only about 5 miles in thickness. Furthermore, it is composed almost entirely of mafic basaltic rocks that are underlain by a layer of the ultramafic rock peridotite, which forms the lithospheric mantle.

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Although most oceanic crust forms out of view, far below sea level, geologists have been able to examine the structure of the ocean floor first hand. In such locations as Newfoundland, Cyprus, Oman, and California, slivers of oceanic crust have been thrust high above sea level. From these exposures, researchers conclude that the ocean crust consists of four distinct layers.

Layer 1
The upper layer consists of a sequence of unconsolidated sediments. Sediments are very thin near the axes of oceanic ridges but may be several kilometers thick next to continents.

Layer 2
Below the layer of sediment is a rock unit composed mainly of basaltic lava that contains abundant pillow-like structures called pillow basalts.

Layer 3
The middle rocky layer is made up of numerous interconnected dikes having a nearly vertical orientation called a sheeted dike complex. These dikes are former pathways where magma rose to feed lava flows on the ocean floor.

Layer 4
The lower unit is made up mainly of gabbro, the coarse grained equivalent of basalt, which crystallized in a magma chamber below the ridge axis.

This sequence of rocks composing the oceanic crust is called an ophiolite complex. From studies of various ophiolite complexes around the globe and related data, geologists have pieced together a scenario for the formation of the ocean floor.

How does oceanic crust form?

Recall that the basaltic magma needed to create new ocean crust originates from partially melted mantle rock called peridotite. Being partially molten and less dense than the surrounding solid rock, the magma gradually moves upward, where it enters a magma chamber that is thought to be less than 10 km wide and located only 1 or 2 km below the ridge crest. Seismic studies conducted along the East Pacific Rise have identified magma chambers along 60% of the ridge. Hence, these structures appear to be relatively permanent features, at least along fast spreading centers. However, along slow spreading ridges, where the rate of magma production is less, magma chambers are small and appear to form intermittently.

As seafloor spreading proceeds, numerous vertical fractures develop in the ocean crust that lies above these magma chambers. Molten rock is injected into these fissures, where some of it cools and solidifies to form dikes. New dikes intrude older dikes, which are still warm and weak, to form the sheeted dike complex. This portion of the oceanic crust is usually 1 to 2 km thick.

Roughly 10% of the magma entering the reservoirs eventually erupts on the ocean floor. Because the surface of a submarine lava flow is chilled quickly by seawater, it rarely travels more than a few kilometers before completely solidifying. The forward motion occurs as lava accumulates behind the congealed margin and then breaks through. This process occurs over and over, as molten basalt is extruded, like toothpaste out of a tightly squeezed tube. The result is tube shaped protuberances resembling large bed pillows stacked one atop the other. Hence the name pillow basalts. In some settings, pillow lavas may build into volcano size mounds that resemble shield volcanoes, whereas in others they form elongated ridges tens of kilometers long. These structures will eventually be cut off from their supply of magma as they are carried away from the ridge crest by seafloor spreading.

The lowest unit of the ocean crust develops from crystallization within the central magma chamber itself. The first minerals to crystallize are olivine, pyroxene, and occasionally chromite (chromium oxide), which fall through the magma to form a layered zone near the floor of the reservoir. The remaining magma tends to cool along the walls of the chamber and forms massive amounts of coarse grained gabbro. This unit makes up the bulk of the oceanic crust, where it may account for as much as 70% of its total thickness.

In this manner, the processes at work along the ridge system generate the entire sequence of rocks found in an ophiolite complex. Since the magma chambers are periodically replenished with fresh magma rising from the asthenosphere, the oceanic crust is continuously being generated.

Interactions between seawater and oceanic crust

In addition to serving as a mechanism for the dissipation of Earth’s internal heat, the interaction between seawater and the newly-formed basaltic crust alters both the seawater and the crust. Because submarine lava flows are very permeable and the upper oceanic crust is highly fractured, sea water can penetrate to a depth of 2 to 3 km. As sea water circulates through the hot crust, it is heated and alters the basaltic rock by a process called hydrothermal metamorphism. This alteration causes the dark silicates, olivine and pyroxene, found in basalt to form new minerals such as chlorite and serpentine.

In addition to the basaltic crust being altered, so is the sea water. As the hot sea water circulates through the newly-formed rock, it dissolves ions of silica, iron, copper, and sometimes silver and gold from the hot basalt. Once the water is heated to several hundred degrees Celsius, it buoyantly rises along fractures and eventually spews out at the surface. Studies conducted by submersibles along the Juan de Fuca Ridge have photographed these metallic-rich solutions as they gush from the seafloor to form particle filled clouds called black smokers. As the hot liquid mixes with the cold, mineral-laden sea water, the dissolved minerals precipitate to form massive, metallic sulfide deposits, some of which are economically important. Occasionally these deposits grow upward to form large, chimney-like structures as tall as skyscrapers.