Page 122 - Fundamentals of Geomorphology

LARGE-SCALE TECTONIC AND STRUCTURAL LANDFORMS 105

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