Geology Reference
In-Depth Information
The ascent of internal energy originating in the Earth's
core impels a complicated set of geological processes.
Deep-seated lithospheric, and ultimately baryspheric,
processes and structures influence the shape and dynam-
ics of the toposphere. The primary surface features of
the globe are in very large measure the product of
geological processes. This primary tectonic influence is
manifest in the structure of mountain chains, volcanoes,
island arcs, and other large-scale structures exposed at
the Earth's surface, as well as in smaller features such as
fault scarps.
Endogenic landforms may be tectonic or structural
in origin (Twidale 1971, 1).
Tectonic landforms
are
productions of the Earth's interior processes without
the intervention of the forces of denudation. They
include volcanic cones and craters, fault scarps, and
mountain ranges. The influence of tectonic processes
on landforms, particularly at continental and large
regional scales, is the subject matter of
morphotecton-
ics
.
Tectonic geomorphology
investigates the effects of
active tectonic processes - faulting, tilting, folding, uplift,
and subsidence - upon landforms. A recent and prolific
development in geomorphology is the idea of '
tectonic
predesign
'. Several landscape features, patently of exo-
genic origin, have tectonic or endogenic features stamped
on them (or, literally speaking, stamped under them).
Tectonic predesign arises from the tendency of erosion
and other exogenic processes to follow stress patterns
in the lithosphere (Hantke and Scheidegger 1999). The
resulting landscape features are not fashioned directly
by the stress fields. Rather, the exogenic processes act
preferentially in conformity with the lithospheric stress
(see p. 138). The conformity is either with the direc-
tion of a shear or, where there is a free surface, in the
direction of a principal stress.
Few landforms are purely tectonic in origin: exoge-
nous forces - weathering, gravity, running water, glaciers,
waves, or wind - act on tectonic landforms, picking
out less resistant rocks or lines of weakness, to pro-
duce
structural landforms
. An example is a volcanic
plug, which is created when one part of a volcano is
weathered and eroded more than another. A breached
anticline is another example. Most textbooks on geomor-
phology abound with examples of structural landforms.
Even in the Scottish Highlands, many present landscape
features, which resulted from Tertiary etching, are closely
adjusted to underlying rock types and structures (Hall
1991). Such passive influences of geological structures
upon landforms are called
structural geomorphology
.
PLATE TECTONICS AND VOLCANISM
The outer shell of the solid Earth - the
lithosphere
-
is not a single, unbroken shell of rock; it is a set of
snugly tailored
plates
(Figure 4.2). At present there are
seven large plates, all with an area over 100 million km
2
.
They are the African, North American, South American,
Antarctic, Australian-Indian, Eurasian, and Pacific
plates. Two dozen or so smaller plates have areas in
the range 1-10 million km
2
. They include the Nazca,
Cocos, Philippine, Caribbean, Arabian, Somali, Juan de
Fuca, Caroline, Bismarck, and Scotia plates, and a host of
microplates
or
platelets
. In places, as along the western
edge of the American continents, continental margins
coincide with plate boundaries and are
active margins
.
Where continental margins lie inside plates, they are
pas-
sive margins
. The break-up of Pangaea created many
passive margins, including the east coast of South Amer-
ica and the west coast of Africa. Passive margins are
sometimes designated rifted margins where plate motion
has been divergent, and sheared margins where plate
motion has been transformed, that is, where adjacent
crustal blocks have moved in opposite directions. The dis-
tinction between active and passive margins is crucial to
interpreting some large-scale features of the toposphere.
Earth's tectonic plates are continuously created at mid-
ocean ridges and destroyed at subduction sites, and are
ever on the move. Their motions explain virtually all
tectonic forces that affect the lithosphere and thus the
Earth's surface. Indeed, plate tectonics provides a good
explanation for the primary topographic features of the
Earth: the division between continents and oceans, the
disposition of mountain ranges, and the placement of
sedimentary basins at plate boundaries.
Plate tectonic processes
The plate tectonic model currently explains changes
in the Earth's crust. This model is thought satisfacto-
rily to explain geological structures, the distribution and
