Movement of the Earth’s Surface from Within - Southern England - page 34

FIG 31. Scale diagram comparing average thicknesses of oceanic and continental crust
and lithosphere.
In other cases the plate boundary is divergent , where the neighbouring plates
move apart and new material from deeper within the Earth rises to fill the space cre-
ated. The new oceanic crust is created by the arrival and cooling of hot volcanic mater-
ial from below. The mid-Atlantic ridge running through Iceland, with earthquakes and
volcanic activity, is one of the nearest examples to Britain of this sort of plate bound-
ary.
Other plate boundaries mainly involve movement parallel to the plate edges and
are sometimes called transform boundaries. The Californian coast zone is the classic
example but there are many others, such as the transform boundary between the Afric-
an and Antarctic plates. In some areas, plate movement is at an oblique angle to the
suture and there are components of divergence or convergence as well as movement
parallel to the boundary.
Britain today sits in the stable interior of the western Eurasian plate, almost
equidistant from the divergent mid-Atlantic ridge boundary to the west and the com-
plex convergent boundary to the south where Spain and northwest Africa are colliding.
In its earlier history the crust of Britain has been subjected to very direct plate
boundary activity: the results of convergent activity in Devonian and Carboniferous
times (between 416 and 299 million years ago) are visible at the surface in southwest
England, and in Ordovician to Devonian times (between 490 and 360 million years
ago) in Wales, northwest England and Scotland.