In the previous tutorial we discussed the types of sediment that cover oceanic crust.
Now it’s on to the crust itself. 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,
where it almost instantly cools once in contact with the cold ocean water. 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. The pillow lavas are often called pillow basalts,
with the term basalt referring to the lava’s mafic composition and fine grain size. Basaltic
rocks typically lack visible crystals. Underneath the pillow basalt is a layer
of rock called sheeted dikes. Dikes are a sort of underground plumbing system that bring magma from
the mantle upward, toward the ocean floor. They are essentially fractures caused by extensional
faulting that act as magma conduits. Sheeted dikes are formed when the rising magma crystallizes
within the fracture-conduits, instead of extruding from the ocean floor to form pillow lava, and 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. Beneath the sheeted dike complex is a layer of
coarser grained igneous rock called gabbro, which has the same composition as the above
pillow basalt. Gabbro has larger crystals because it cools much more slowly, as it is not near
the ocean water. In fact, 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.
The basalt and gabbro of the upper oceanic crust is composed mainly of calcium-rich
plagioclase feldspar, or CaAl2Si2O8, augite, or CaMgSi2O6, and magnesium-rich olivine, or Mg2SiO4.
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. This material represents the first and most undifferentiated magma created
at the ridge. 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.
Wehrlite is rock that is composed mainly of augite and olivine, and as it rises,
much of the olivine crystallizes out, leaving behind a more silica-rich melt that eventually
forms the basaltic, feldspar-rich pillow lavas of the ocean floor. Beneath the wehrlite is another
cumulate layer, or layer composed mainly of olivine crystals that formed in the wehrlite
magma chamber and sank to the bottom. Finally, at the very bottom is the unmelted residuum
of the original mantle which is composed mainly of the olivine-rich rocks harzburgite and dunite.
The entire sequence of rocks that comprise oceanic crust, from the layered peridotite on the bottom
to the sediment on top, is called an ophiolite. Ophiolite outcrops can be found in Greece,
and most notably, Oman. The Semali ophiolite, which was formed along a 95-million-year-old
mid-ocean ridge, is one of the largest and best exposed ophiolites in the world.
As the ocean basin closed 80 million years ago, the entire section of oceanic crust was thrust
onto the southeastern corner of the Arabian Peninsula during the collision of the Arabian and
Eurasian plates. Ophiolites are typically the last sliver of oceanic crust that evades subduction and
gets smushed between the two continents right before they collide. Today, scientists use the
Semali ophiolite as a point of reference for seismic measurements of oceanic crust.
So, with that we conclude a broad assessment of all the layers of the earth. Now it’s time to zoom
in from this grander scale down to things that are much more familiar and readily observable. What
are rocks themselves made of? Let’s spend the next few tutorials answering this important question.