Environmental Engineering Reference
In-Depth Information
(single or dashed lines). Arrows indicate general direction
and velocities of movement in mm yr
−1
.
Palaeozoic era
c
. 430 Ma ago formed the Caledonian mountains of north-west Europe
and eastern North America. Conversely, the Carboniferous basin of equatorial Pangaea is
now divided by the Atlantic Ocean. The Appalachian, Scottish and Norwegian mountains
are surviving Caledonian remnants dispersed as the Atlantic Ocean formed, and the East
African-Red Sea rift valley system may lead to the formation of a future ocean.
Plate motions relative to each other are of great interest to us and occur in three
principal ways.
Divergent
or spreading boundaries are associated with extension of the
crust, forming new oceanic plate at
constructive margins
. Spreading rates are typically
20-40 mm yr
−1
between the American and Eurasian plates in the mid-Atlantic Ocean and
20-100 mm yr
−1
between the Antarctic plate and the southern Indo-Australian and Pacific
plates (Figure 10.4).
Convergent
boundaries are associated with crustal compression and
old plate is consumed by subduction back into the mantle at
destructive margins
. Slab
pull accelerates subduction, with velocities of 60-100 mm yr
−1
around the western Pacific
margin and 50-120 mm yr
−1
on the eastern Pacific ocean plate boundaries with the
Americas. In its absence at the Africa-Eurasia boundary, velocities fall to 10-35 mm
yr
−1
.
Convection and gravity slide are unlikely to be uniform plate-wide, and motion must
also accommodate the spherical shape of Earth and drag-resistant, stable continental
lithosphere. As a result, plates may articulate internally along
transform faults
or slide
past each other at
transform margins
, which are
conservative
boundaries (since plate is
normally neither created nor destroyed) or meet at
triple junctions
(Figure 10.4). Most
plates also have an absolute motion about Earth, but parts or all of some, especially the
smaller plates, still have active margins even where they are caught like fixed 'eddies'
rotated by the 'stream' of larger plates. Despite their general rigidity, plates also
experience plastic deformation in the form of doming, bulging, subsidence and folding,
and brittle deformation through faulting. Tectonic deformation concentrated at
convergent plate margins forms mountains by
orogenesis
. More general uplift/subsidence
or
epeirogenesis
is associated with continental plate interiors. It, too, may be generated
thermally
by expansion over isolated hot spots or mantle plumes; or
mechanically
through crustal loading/subsidence by sediment deposition, ice sheets, rising sea level,
etc., or unloading/elevation by deglaciation, erosion, falling sea level, etc. This
isostatic
adjustment
slowly attempts to restore loading equilibrium to every part of the crust and
we shall see later the vertical rates at which plates also move.
PLATE ARCHITECTURE AND MORPHOTECTONIC LANDFORMS:
GLOBAL GEOMORPHOLOGY
World-wide tectonic activity, involving the creation and destruction of lithosphere,
impresses itself on landforms at all scales. Extremely slow average rates of motion,
which persist for 10
6-8
years, contrast with violent volcanic eruptions and earthquakes.
Even the thinness of the lithosphere (10
2-3
km) and low range of its surface elevation (20
km spans the deepest ocean to the highest mountain) are not eclipsed by the very large
area and horizontal dimensions of plates. The elevation of the land surface endows
