Geology Reference
In-Depth Information
The relative motion of adjacent plates primarily
creates the many tectonic forces in the lithosphere.
Indeed, relative plate motions underlie almost all sur-
face tectonic processes. Plate boundaries are particu-
larly important for understanding geotectonics. They
are sites of strain and associated with faulting, earth-
quakes, and, in some instances, mountain building
(Figure 4.5). Most boundaries sit between two adja-
cent plates, but, in places, three plates come into con-
tact. This happens where the North American, South
American, and Eurasian plates meet (Figure 4.2). Such
Y-shaped boundaries are known as
triple junctions
.
Three plate-boundary types produce distinctive tectonic
regimes:
overridden rather than underridden the continent.
This process, called
obduction
, has produced the
Troödos Mountain region of Cyprus. Collisions of
continental lithosphere result in crustal thickening
and the production of a mountain belt, but little
subduction. A fine example is the Himalaya, pro-
duced by India's colliding with Asia. Divergence and
convergence may occur obliquely. Oblique diver-
gence is normally accommodated by transform off-
sets along a mid-oceanic ridge crest, and oblique
convergence by the complex microplate adjustments
along plate boundaries. An example is found in the
Betic cordillera, Spain, where the African and Iberian
plates slipped by one another from the Jurassic to
Tertiary periods.
3
Conservative
or
transform plate boundaries
occur
where adjoining plates move sideways past each
other along a transform fault without any conver-
gent or divergent motion. They are associated with
strike-slip tectonic regimes and with shallow earth-
quakes of variable magnitude. They occur as fracture
zones along mid-ocean ridges and as
strike-slip
fault zones
within continental lithosphere. A prime
example of the latter is the San Andreas fault system
in California.
1
Divergent plate boundaries
at construction sites,
which lie along mid-ocean ridges, are associated with
divergent tectonic regimes involving shallow, low-
magnitude earthquakes. The ridge height depends
primarily on the spreading rate. Incipient diver-
gence occurs within continents, including Africa,
and creates rift valleys, which are linear fault sys-
tems and, like mid-ocean ridges, are prone to shallow
earthquakes and volcanism (p. 143). Volcanoes at
divergent boundaries produce basalt.
2
Convergent plate boundaries
vary according to the
nature of the converging plates. Convergent tectonic
regimes are equally varied; they normally lead to par-
tial melting and the production of granite and the
eruption of andesite and rhyolite. An oceanic trench,
a volcanic island arc, and a dipping planar region of
seismic activity (a Benioff zone) with earthquakes
of varying magnitude mark a collision between two
slabs of oceanic lithosphere. An example is the
Scotia arc, lying at the junctions of the Scotia and
South American plates. Subduction of oceanic litho-
sphere beneath continental lithosphere produces two
chief features. First, it forms an oceanic trench, a dip-
ping zone of seismic activity, and volcanicity in an
orogenic mountain belt
(or
orogen
) lying on the
continental lithosphere next to the oceanic trench
(as in western South America). Second, it creates
intra-oceanic arcs
of volcanic islands (as in parts
of the western Pacific Ocean). In a few cases of
continent-ocean collision, a slab of ocean floor has
Tectonic activity also occurs within lithospheric plates,
and not just at plate edges. This is called
within-
plate tectonics
to distinguish it from plate-boundary
tectonics.
Volcanic and plutonic processes
Volcanic forces are either intrusive or extrusive forces.
Intrusive forces
are found within the lithosphere and
produce such features as batholiths, dykes, and sills. The
deep-seated, major intrusions - batholiths and stocks -
result from plutonic processes, while the minor, nearer-
surface intrusions such as dykes and sills, which occur
as independent bodies or as offshoots from plutonic
intrusions, result from hypabyssal processes.
Extrusive
forces
occur at the very top of the lithosphere and lead
to exhalations, eruptions, and explosions of materials
through volcanic vents, all of which are the result of
volcanic processes.
