Earth’s geological structure and processes - Fundamentals of the Physical Environment - page 204

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 moun-
tains 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.
The motion of the plates relative to each other is of
great interest to us and occurs 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 10.6 ). 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 deforma-
tion 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 expan-
sion 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

Worldwide tectonic activity, involving the creation and
destruction of lithosphere, impresses itself on land-
forms 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 geomorphological processes with gravita-
tional energy. Plate architecture - literally, the style, design
and construction of plate structures - is the key to global
morphotectonic landforms and the rock cycle . Construc-
tive, destructive and conservative styles of plate margins
are translated into mid-ocean ridge , subduction zone and
transform faults and related structures in the oceans and
continents. The logical place to start is where new lithos-
phere is created at mid-ocean ridges, but this is actually
preceded by continental rifting in the Wilson cycle, which
charts the birth and eventual death of the ocean.

Rift formation and development
Rifting involves the splitting and separation of crustal
lithosphere under high shear stresses. Sustained stress
propagates or extends rifts, often along major lineaments
or existing linear weaknesses such as sutures and faults.
Continental rift valleys and oceanic rifting, in the form of
mid-ocean ridges, develop with symmetrical separation
on both sides of the rift. Structural basins form on a
symmetrical crustal extension ( Figure 10.7 ). Active rifting
occurs over mantle plumes and leads eventually to the
emergence of new crust ( Plate 10.1 ). Rifting may still
occur in their absence, in which case the necessary crustal
extension for passive rifting must occur mechanically in
various ways. Subduction and associated slab pull on the
far side of a continent may trigger a corresponding trench
suction force , and thereby extensional rifting in the
continental lithosphere on the near side. Surface erosion
may have a similar extensional effect. In that case the
reduced mass of upper, brittle crust requires an isostatic
adjustment. It is achieved by 'inflow' of underlying ductile
(pliable) crust which undermines adjacent brittle crust.
Extension then causes faulting as lithosphere is stretched
beyond its brittle strength limits and the rift, or graben ,
is formed as crust subsides between inward-facing faults.
New crust will form in the rift only if faulting penetrates
the entire lithosphere and/or there is insufficient resis-
tance to magma flow from underlying asthenosphere
( Figure 10.7 ).