Geography Reference
In-Depth Information
larger-scale cycles of the Earth, but these processes can be affected
by human action.
TECTONICS: CONTINENTS AND OCEANS
Plate tectonic theory
The Earth is roughly spherical although it is 42 kilometres shorter
if you travel around the world via the poles compared to travelling
around the equator. The 1,200 kilometres thick, inner core of the
Earth is solid, hot (3,000°C) iron. The inner core is surrounded by
a 2,300 kilometres thick layer of liquid iron-rich material forming
the outer core (Figure 3.1). The next layer, as we move out from
the centre of the Earth, is called the mantle (2,900 kilometres
thick). The outer part of the mantle (180 kilometres) along with
the overlying crust is called the lithosphere and this is rigid and
floats on the more mobile asthenosphere. When rocks in the
mantle are subjected to different pressures and heat they flow
slowly. Large continents form a cold and rigid crust mainly made
from granite rock around 35 to 70 kilometres thick. However, the
rock on the ocean floors is made from basalt and forms a thinner, 6
to 10 kilometres thick, crust. Importantly (as we will see later), the
density of the rock on the ocean floors is slightly greater than that
of the continents.
The proof for plate tectonic theory has surprisingly only
emerged in the last half century. Earlier explorers had noted that
the shapes of the continents seemed like they would fit together
(e.g. South America and Africa) and there had been scientists who
studied fossil evidence during the nineteenth and early twentieth
centuries such as Alfred Wegener who had suggested that all of the
continents were once joined together in a single land mass called
Pangaea, which slowly broke and drifted apart. However, it was
only through surveys of the ocean floor, as part of naval submarine
and nuclear research during the 1950s and 1960s that data was pro-
duced that showed how the continents really did move slowly
around the Earth and that the oceans floors spread from their
centres.
Detailed maps of the ocean floor showed that there are large
mountain ranges running through the centre of the world's major
