Seismic studies have been used to determine the thickness of the oceanic
crust on a worldwide basis. The thickness of the basaltic oceanic crust has
a nearly constant value of about 6 km throughout much of the area of the
oceans. Exceptions are regions of abnormally shallow bathymetry such as
the North Atlantic near Iceland, where the oceanic crust may be as thick
as 25 km. The near-constant thickness of the basaltic oceanic crust places
an important constraint on mechanisms of partial melting beneath the ridge
crest. If the basalt of the oceanic crust represents a 20% partial melt, the
thickness of depleted mantle beneath the oceanic crust is about 24 km.
However, this depletion is gradational so the degree of depletion decreases
with depth.