Page 120 - Fundamentals of Geomorphology

LARGE-SCALE TECTONIC AND STRUCTURAL LANDFORMS 103

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