Geoscience Reference
In-Depth Information
Vertical movements by expansion or contraction
Changing the temperature of the crust and lithosphere is an inevitable result of
many of the processes active within the Earth, because they often involve the trans-
fer of heat. In particular, rising plumes of hot material in the Earth's mantle, often in-
dependent of the plate boundaries, are now widely recognised as an explanation for
various areas of intense volcanic activity (for example beneath Iceland today). These
plumes are often referred to as 'hot spots' (see Fig. 32). Heating and cooling leads to
expansion or contraction of the lithosphere and can cause the surface to rise or sink, at
least locally.
An example of this is the way that Southern England was tilted downwards to
the east about 60 million years ago. At about this time, eastern North America moved
away from western Europe as the North American and Eurasian plates diverged. The
divergence resulted in large volumes of hot material from deep within the Earth being
brought to the surface and added to the crust of western Southern England. It is be-
lieved that the heating and expansion of the crustal rocks in the west has elevated them
above the rocks to the east, giving an eastward tilt to the rock layers and exposing the
oldest rocks in the west and the youngest ones in the east. This sequence has important
implications for the scenery of England's south coast (see Chapter 5).
HOW CAN LOCAL SURFACE MOVEMENTS BE DETECTED?
Having just reviewed some of the processes that cause vertical movements of the
Earth's surface, it is useful to consider the practical difficulties of how such movements
are measured.
For present-day applications, it seems natural to regard sea level as a datum
against which vertical landscape movements can be measured, as long as we remember
to allow for tidal and storm variations. However, much work has demonstrated that
global sea level has changed rapidly and frequently through time, due to climate fluc-
tuations affecting the size of the polar icecaps and changing the total amount of liquid
water present in the oceans and seas. It has also been shown that plate tectonic move-
ments have an important effect on global sea level by changing the size and shape of
ocean basins.
Attempts have been made to develop charts showing how sea level, generalised
for the whole world, has varied through time. However, it has proved very difficult to
distinguish a worldwide signal from local variations, and the dating of the changes is
often too uncertain to allow confident correlation between areas.
