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The
Faulted Earth
As
mentioned previously, the lithosphere (the outer rocky cover of the
Earth, which is 65-70 km thick under the oceans and 100 -150 km thick under
the continents) is broken up by deep rift systems into separate plates that
vary greatly in both dimensions and shape. Each of these rigid, outer rocky
covers of the Earth floats on the semi-molten, plastic, weak zone of the
Earth's mantle (the asthenosphere) and moves freely away from, towards or
past adjacent plates.
At
the diverging boundary of each plate, molten magma rises and solidifies to
form strips of new ocean floor, and at the opposite boundary (the converging
boundary) the plate dives (subducts) underneath the adjacent plate to be
gradually consumed in the underlying asthenosphere, at exactly the same rate
of sea-floor spreading on the opposite boundary.
An
ideal, rectangular, lithospheric plate would thus have one edge growing at a
mid-oceanic rift zone (diverging boundary), the opposite edge being consumed
into the asthenosphere, under the over-riding plate (converging or
subduction boundary) and the other two edges sliding past the adjacent
plates along a transform fault (transcurrent or transform fault boundaries,
sliding or gliding boundaries).
In
this way, the lithospheric plates are constantly shifting their positions on
the surface of the Earth, despite their rigidity, and as they are carrying
continents with them, such continents are also constantly drifting away or
towards each other. As a plate is forced under another plate and gets
gradually consumed by melting, magmatic activity is set into action. More
viscous magmas are intruded, while lighter and more fluid ones are extruded
to form island arcs that eventually grow into continents, are plastered to
the margins of nearby continents or are squeezed between two colliding
continents. Traces of what is believed to have been former island arcs are
now detected along the margins and in the interiors of many of today's
continents.
The
processes of both divergence and convergence of lithospheric plates are not
only confined to ocean basins, but are also active within continents and
along their margins. This can be demonstrated in both the Red Sea and the
Gulf of California through which lie extensions of Oceanic rifts that are
currently widening at a rate of 3cm/year in the former case and 6cm/year in
the latter. Again, the collision of the Indian Plate with the Eurasian Plate
(which is a valid example of continent/continent collision after the
consumption of the oceanic plate which was separating them) has resulted in
the formation of the Himalayan chain, with the highest peaks currently found
on the surface of the Earth.
“Lest
it Should Shake With You” (16:15)
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The
Himalayan chain resulted from the collision of the Indian and Eurasian
plates.
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Earthquakes
are common at all plate boundaries, but are most abundant and most
destructive along the collisional ones. Throughout the length of the
divergent plate boundary, earthquakes are mostly shallow seated, but along
the subduction zones, these come from shallow, intermediate and deep foci
(down to a depth of 700 km), accompanying the downward movement of the
subducting plate below the over-riding one. Seismic events also take place
at the plates transcurrent fault boundaries where it slides past the
adjacent plates along transform faults. Plate movements along such fault
planes do not occur continuously, but in interrupted, sudden jerks, which
release accumulated strain.
Moreover, it has to be mentioned that both the pattern and the speed of
movement of lithospheric plates vary from one case to another. Where the
plates are rapidly diverging, the extruding lava in the plane of divergence
spreads out over a wide expanse of the ocean bottom and heaps up to form a
deeply rifted, broad mid-oceanic ridge, with gradually sloping sides (e.g.
the East Pacific Rise). Contrary to this, slow divergence of plates gives
time for the erupting lava flows to accumulate in much higher heaps with
steep sides (e.g. the Mid-Atlantic Ridge). The rates of plate movements away
from their respective spreading axes (rift zones) can be easily calculated
by measuring the distances of each pair of magnetic anomaly strips on both
sides of the axial plane of spreading. Such strips can be easily identified
and dated, the distance of each from its spreading axial plane can be
measured, and hence the average spreading rate can be calculated.
Spreading
rates at mid-oceanic ridges are usually given as half-rates, while plate
velocities at trenches are full rates. This is simply because the rate at
which one lithospheric plate moves away from its spreading center represents
half the movement at the center as the full spreading rate is the velocity
differential between the two diverging plates which were separated at the
axial plane of spreading (the mid-oceanic ridge rift or its axial plane of
rifting).
In
studying the pattern of motion of plates and plate boundaries, nothing is
fixed, as all velocities are relative. Spreading rates vary from about 1
cm/year in the Arctic Ocean to about 18 cm/year in the Pacific Ocean, with
the average being 4-5 cm/year. Apparently, the Pacific Ocean is now
spreading almost ten times faster than the Atlantic (cf. Dott and Batten,
1988, p. 167).
Rates
of convergence between plates at oceanic trenches or at mountain belts can
be computed by vector addition of known plate rotations (c.f. Le Pichon,
1968). These can be as high as 9cm/year at oceanic trenches and 6cm/year
along mountain belts (Le Pichon, op. Cit.). Rates of slip along the
transform fault boundaries of the lithospheric plates can also be calculated
once the rates of plate rotation are known.
Both
the patterns of magnetic anomaly strips and the sediment thickness on top of
such strips suggest that the spreading pattern and velocities of oceanic
lithospheric plates have been different in the past, and that the volcanic
activity along mid-oceanic ridges varies in both space and time.
Consequently such ridges appear, migrate and disappear with time.
Spreading
from the Mid-Atlantic rift zone began between 200 and 150 MYBP (million
years before present) from the north-western Indian Ocean rift zone between
100 and 80 MYBP, while both Australia and Antarctica did not separate until
65 MYBP (cf. Dott and Batten, loc. it.).
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