Great question! How does oceanic lithosphere
end up on the continents? Oceanic lithosphere is
occasionally exposed in mountain belts in units
call ophiolites. Ophiolites are sections of
oceanic crust and lithospheric mantle that have
been obducted during a collisional plate
There are several models
for how ophiolite subduction works, but the
basis of each comes down to density differences
between the oceanic lithosphere and the
underlying asthenosphere. subduction shows a very
simplified cross section of a subduction zone.
Oceanic lithosphere is subducting into the
asthenosphere because it has become denser than
the underlying asthenosphere (~3300
kg/m3) through cooling and thickening
(as asthenospheric mantle turns into
lithospheric mantle at the base of the plate). A
mobile terrane (e.g., Japan), ocean island
(e.g., Hawaii), or a seamount on the subducting
plate may plug of the subduction zone. Because
the column of seamount + oceanic lithosphere is
less dense than the asthenosphere, it may float
and be thrust up or obducted onto the margin and
preserved as an ophiolite.
figure ophiolite (modified by Keary et
al., 2009 from by Wakabayashi & Dilek, 2000)
shows many more details of one of the competing
models or ophiolite subduction.
from your question here, but I should mention
that ophiolites are incredibly useful to
geologists. They allow us to directly study the
structure of oceanic crust and lithospheric
mantle. They also provide us samples of mantle
peridotite, which are relatively rare on the
surface of the earth (also occasionally brought
to the surface as xenoliths in mantle-derived
Keary, P., Klepeis, K.A.,
Vine, F.J. (2009). Global Tectonics. Oxford:
Wakabayashi, J. & Dilek, Y.
(2000) Spatial and temporal relationships
between ophiolites and their metamorphic soles:
a test of models of forearc ophiolite genesis.
In Dilek, Y. et al. (eds) Ophiolites and
Oceanic Crust: new insights from field studies
and ocean drilling program. Geo. Soc. Am. Sp
Paper 349, 53-64.