Geology Reference
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sedimentary prisms to continental margins through
metamorphism, by the stacking of thrust sheets, by the
sweeping up of microcontinents and island arcs at their
leading edges, and by the addition of magma through
intrusions and extrusions (Condie 1989). Geologists
have established the relative movement of continents over
the Phanerozoic aeon with a high degree of confidence,
although pre-Pangaean reconstructions are less reliable
than post-Pangaean reconstructions. Figure 4.4 charts
the probable break-up of Pangaea.
245 million years ago
160 million years ago
Diastrophic processes
Traditionally, tectonic (or geotectonic) forces are divided
into two groups: (1) diastrophic forces and (2) vol-
canic and plutonic forces. Diastrophic forces lead to
the folding, faulting, uplift, and subsidence of the litho-
sphere. Volcanic forces lead to the extrusion of magma
on to the Earth's surface as lava and to minor intru-
sions (e.g. dykes and sills) into other rocks. Plutonic
forces , which originate deep in the Earth, produce major
intrusions (plutons) and associated veins.
Diastrophic forces may deform the lithosphere
through folding, faulting, uplift, and subsidence. They
are responsible for some of the major features of the
physical toposphere. Two categories of diastrophism are
recognized: orogeny and epeirogeny, but these terms
are a source of much confusion (Ollier and Pain 2000,
4-8). Orogeny literally means the genesis of moun-
tains, and when first used it meant just that. Later, it
became associated with the idea of folding, and eventu-
ally it came to mean the folding of rocks in fold belts.
As mountain building is not associated with the fold-
ing of rocks, it cannot be synonymous with orogeny
(Ollier 2003). Epeirogeny is the upheaval or depression
of large areas of cratons without significant folding or
fracture. The only folding associated with epeirogeny is
the broadest of undulations. Epeirogeny includes iso-
static movements , such as the rebound of land after
an ice sheet has melted, and cymatogeny, which is the
arching, and sometimes doming, of rocks with little
deformation over 10-1,000 km. Some geomorphologists
believe that mountains result from the erosion of areas
uplifted epeirogenically (e.g. Ollier and Pain 2000, 8;
Ollier 2003; see Huggett 2006, 29-30).
95 million years ago
30 million years ago
Figure 4.4 Changing arrangement of continents over the
last 245 million years, showing the break-up of Pangaea,
during the Early Triassic period; during the Callovian age
(Middle Jurassic); during the Cenomanian age (Late
Cretaceous); and during the Oligocene epoch. All maps
use Mollweide's equal-area projection.
Source: Adapted from maps in Smith et al. (1994)
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