Geoscience Reference
In-Depth Information
FIG
32.
Diagram illustrating the movement processes of plates (not to scale).
UNDERSTANDING SURFACE MOVEMENTS
We have been considering the large movement systems that originate within the Earth.
There are also more local movement systems operating on the Earth's surface, which
are linked to a very variable degree to the large-scale movements of plate tectonics.
To explore this complex linkage further, it will be helpful to look now at different pro-
cesses that may combine to cause particular local movements.
Horizontal movements as part of convergence, divergence or lateral transfer
Tectonic plates are recognised by their rigidity, so there is relatively little horizontal
movement between points within the same plate compared to the deformation seen in
plate boundary zones. This extreme deformation may involve folding and fracturing of
the rock materials, addition of new material from below, or absorption of material into
the interior during subduction.
Nonetheless, deformation is not restricted solely to plate boundaries, and does oc-
cur to a lesser extent within the plates. In some cases, major structures that originally
formed along a plate boundary can become incorporated into the interior of a plate
when prolonged collision causes two plates to join. Southern England includes the re-
mains of a former convergent plate boundary and contains many examples of struc-
tures of this sort (particularly around Dorset and the Isle of Wight). These structures
have often been reactivated long after they first formed in order to accommodate forces
along the new plate boundary via deformation within the plate. Conversely, changes of
internal stress patterns can sometimes lead to the splitting of a plate into two, forming
a new, initially divergent plate boundary. Many of the oil- and gas-containing features
of the North Sea floor originated when a belt of divergent rift faults formed across a
previously intact plate.
It needs to be stressed that the patterns of deformation (fracturing and folding)
due to these plate motions occur at a wide range of different scales, from centimetres
