Movement of the Earth’s Surface from Within - Southern England - page 30

CHAPTER 3
Movement of the Earth's Surface from
Within
WIDESPREAD MOVEMENTS OF THE EARTH'S SURFACE
T O UNDERSTAND THE CHANGES and movements affecting the appearance of the
landscape on large scales we need to review some geological systems, especially plate
tectonics. Many of the large changes that have created landscapes over long periods of
time can now be understood using this discovery.
Knowledge of the processes causing the movement of large (10-1,000 km length-scale)
areas of the Earth's surface has been revolutionised by scientific advances made over
the last 40 years. During this time, scientists have become convinced that the whole of
the Earth's surface consists of a pattern of interlocking tectonic plates (Fig. 29). The
word 'tectonic' refers to processes that have built features of the Earth's crust (Greek:
tektōn , a builder). The worldwide plate pattern is confusing - particularly when seen on
a flat map - and it is easier to visualise the plates in terms of an interlocking arrange-
ment of panels on the Earth's spherical surface, broadly like the panels forming the skin
of a football.
Tectonic plates are features of the lithosphere , the name given to the ≈125 km thick
outer shell of the Earth, distinguished from the material below by the strength of its ma-
terials (Greek: lithos , stone). The strength depends upon the composition of the material
and also upon its temperature and pressure, both of which tend to increase with depth be-
low the Earth's surface. In contrast to the mechanically strong lithosphere, the underly-
ing material is weaker and known as the asthenosphere (Greek: asthenos , no-strength).
Note that on figure 30 the crustal and outer mantle layers are shown with exaggerated
thickness, so that they are visible.