Composition of Oceanic Crust Part 2: Pillow Lava, Dikes, Gabbro, and Peridotite

Oceanic Crust Layers

In this section, we will discuss the different layers of oceanic crust and their composition.

Pillow Lava Layer

• Beneath the sediment is a layer of rock called pillow lava.

• At mid-ocean ridges where new oceanic crust is made, molten lava erupts from cracks in the crust.

• The rapidly quenched lava forms pillows or bulbous bodies which can then break open and exude more hot lava from beneath the surface to form additional pillows.

• This process repeats over and over, forming a thick layer of stacked pillow lavas.

Sheeted Dikes Layer

• Underneath the pillow basalt is a layer of rock called sheeted dikes.

• Dikes are underground plumbing systems that bring magma from the mantle upward toward the ocean floor.

• Sheeted dikes are formed when rising magma crystallizes within fracture-conduits instead of extruding from the ocean floor to form pillow lava.

• Over time, clusters of basalt columns form, creating a sheeted dike complex that looks like a row of several-meter-long dominos standing side-by-side.

Gabbro Layer

• Beneath the sheeted dike complex is a layer of coarser-grained igneous rock called gabbro.

• Gabbro has larger crystals because it cools much more slowly than pillow basalt as it is not near the ocean water.

• The gabbro layer is basically the crystallized remnants of the magma chamber that was responsible for feeding all the volcanism along the ridge.

• The upper portion of the gabbro has a uniform structure, while the bottom portion displays a layered or cumulate structure caused by dense crystals forming in the magma chamber and sinking down to the bottom.

Layered Peridotite

• Finally, at the very bottom of the lithosphere is rock called layered peridotite.

• The upper portion of this layer is composed of crystallized ultramafic partial melts of the mantle below, called wehrlite.

• As magma rises, it cools and crystallizes out the most refractory minerals first, which causes the magma to change composition as it rises, becoming more felsic or rich in silica. This process is called differentiation.

• Beneath the wehrlite is another cumulate layer composed mainly of olivine crystals that formed in the wehrlite magma chamber and sank to the bottom. Finally, at the very bottom is unmelted residuum of original mantle which is composed mainly of olivine-rich rocks harzburgite and dunite.

Ophiolites

• The entire sequence of rocks that comprise oceanic crust from layered peridotite on bottom to sediment on top is called an ophiolite.

• Ophiolites are typically last sliver of oceanic crust that evades subduction and gets smushed between two continents right before they collide.

• Semali ophiolite was formed along a 95-million-year-old mid-ocean ridge and one of largest and best-exposed ophiolites in world.

• Today scientists use Semali ophiolite as a point of reference for seismic measurements of oceanic crust.

Conclusion

• The different layers of the earth's crust have unique compositions and are formed through various geological processes.

• Understanding these layers is important in understanding the formation and evolution of the earth.