Geology Reference
In-Depth Information
Plate construction
Plate destruction
Volcano
Volcano
Hotspot
Ocean sediments
Continental
lithosphere
Oceanic lithosphere
Mantle
wedge
Asthenosphere
Mantle
plume
Mesosphere
Residues
Mafic lower crust
Sediments
Oceanic crust
Hotspot sources
Enriched mantle type II
Mafic lower crust
modified slab
Enriched mantle type I
High- mantle
Reservoir of chemically modified
slab components (oceanic crust
and sediments) and delaminated
mafic lower arc crust
Hotspot
sources
Residues
Core
Figure 4.3
Interactions between the asthenosphere, lithosphere, and mesosphere. The oceanic lithosphere gains material
from the mesosphere (via the asthenosphere) at constructive plate boundaries and hotspots and loses material to the
mesosphere at destructive plate boundaries. Subduction feeds slab material (oceanic sediments derived from the
denudation of continents and oceanic crust), mantle lithosphere, and mantle wedge materials to the deep mantle.
These materials undergo chemical alteration and accumulate in the deep mantle until mantle plumes bear them to
the surface where they form new oceanic lithosphere.
Source:
Adapted from Tatsumi (2005)
consists of buoyant low-density crust (the tectosphere)
and relatively buoyant upper mantle. It therefore floats
on the underlying asthenosphere. Continents break up
and reassemble, but they remain floating at the surface.
They move in response to lateral mantle movements,
gliding serenely over the Earth's surface. In breaking
up, small fragments of continent sometimes shear off;
these are called
terranes
. They drift around until they
meet another continent, to which they become attached
(rather than being subducted) or possibly are sheared
along it. As they may come from a different conti-
nent from the one they are attached to, they are called
exotic
or
suspect terranes
(p. 113). Most of the western
seaboard of North America appears to consist of these
exotic terranes. In moving, continents have a tendency
to drift away from mantle hot zones, some of which they
may have produced: stationary continents insulate the
underlying mantle, causing it to warm. This warming
may eventually lead to a large continent breaking into
several smaller ones. Most continents are now sitting on,
or moving towards, cold parts of the mantle. An excep-
tion is Africa, which was the core of Pangaea. Continental
drift leads to collisions between continental blocks and
to the overriding of oceanic lithosphere by continental
lithosphere along subduction zones.
Continents are affected by, and affect, underlying
mantle and adjacent plates. They are maintained against
erosion (rejuvenated in a sense) by the welding of
