The portion of the earth’s crust that lies underneath the oceans tends to be newer than that of the continents. The fast spreading from the ocean ridges creates new oceanic crust so fast that it is measurable even in human terms: one to twenty centimetres a year. Because of its relative youth, oceanic crust is thinner than continental, usually less than ten kilometres thick. Varied as the oceans are the basic crust structure is the same.
On the simplest level there are three layers above the mantle. The top layer consists of sediments of both organic and inorganic origin. Below this is the basaltic level and finally the gabbro. Aside from the sediments, which are not strictly speaking part of the crust, the lithosphere consists of igneous rocks produced by the volcanic activity of the spreading ridges.
The basaltic layer is divided into two. Basaltic lava on top and basaltic dykes below. Dykes don’t reach the ocean floor before solidifying, forming solid sheets of rock. Lava is magma that did reach the ocean floor and spread before solidifying. Characteristic of the ocean crust are pillow lavas, which are unique to the ocean.
Their distinctive bulbous appearance is due to the difference between the cooling properties of water and air. Water has far more thermal conductivity and cooling is rapid. This means that once magma gets to the sea floor it develops a skin. This skin holds the approximate shape but is still flexible to change shape and expand as more magma flows up.
Huge piles of pillow lavas have been found, up to hundreds of metres thick. Near the ocean ridges, where there is little if any sediment, the sea floor can clearly be seen have this lumpy, hummocky appearance from the pillow lavas. Individual pillows get quite big, more than a metre across in some cases.
The gabbro level is similar to the balsaltic dykes but with an intriguing difference that has not yet been fully explained. The crystals in this layer are much larger. This must be due to slow cooling of magma and it was once theorised the magma collected in large chambers before crystallising. Now it is thought that the crystals begin to form as the magma moves towards the crust. Basically the magma begins to settle as a crystal mush (which is also the name of the resultant rock).
The relationship between the rate of spreading and the magma supply that create the ocean crust is a complex one and there are still a large number of questions not completely answered yet and major new discoveries still being made. As with most of the earth sciences some of the fastest moving and most exciting work in geology occurs in the depths of the sea.
