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the sheeted dike complex. These dikes are former pathways
where magma rose to feed lava flows on the ocean floor.
 Layer 4: The lowest unit is mainly gabbro, the coarse-grained
equivalent of basalt, which crystallized deeper in the crust
without erupting. This sequence of layers 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.
The molten rock that goes into the making of new oceanic
crust originates from partial melting of the mantle rock peridotite
at depths greater than 40 km. This process generates a melt having
the composition of basalt, which is less dense than the surrounding
solid rock. The newly formed melt rises through the upper mantle
along thousands of tiny conduits that feed into a few dozen larger,
elongated channels, perhaps 100 m or more wide. These structures,
in turn, feed lens-shaped magma chambers located directly beneath
the ridge crest. With the addition of melt from below, the pressure
inside the chambers steadily increases. As a result, the rocks above
these reservoirs periodically fracture, allowing the melt to ascend
into the young oceanic crust above.
The molten rock surges upward along numerous vertical
fractures that develop in the ocean crust. Some cools and solidifies
to form dikes. New dikes intrude older dikes, which are still warm
and weak, to form a sheeted dike complex. This portion of the
oceanic crust is usually 1 to 2 km thick.
Roughly 10 % of the melt eventually erupts on the ocean
floor. Because the surface of a submarine lava flow is chilled
quickly by seawater, it generally travels no 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 repeatedly, as molten basalt is
extruded—like toothpaste from a tightly squeezed tube. The result

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