Geoscience Reference
In-Depth Information
Chapter 10
The continental lithosphere
10.1 Introduction
10.1.1 Complex continents
We have seen something of the general simplicity of the Earth's internal structure
and the detailed complexity of the motions of tectonic plates and convective
systems. The clues to this simplicity and complexity come from the oceans,
the study of whose structures has led to an understanding of the plates, of the
mantle beneath and, to some extent, of the core, via its magnetic properties.
Although complex details must be sorted out and theories may change slightly,
we can now be reasonably confident that the oceans are understood in their broad
structure. In contrast, the continents are not understood at all well. Yet we need
to understand the continents because in their geological record lies most of the
history of the Earth and its tectonic plates, from the time that continental material
first formed over 4400 Ma ago (see Section 6.10). The oldest oceanic crust is only
about 160 Ma old, so the oceanic regions can yield no earlier information.
In the broadest terms, the continents are built around ancient crystalline crust,
flanked by younger material representing many events of mountain building,
collision, rifting and plate convergence and subsidence. Figure 3.30 shows the
recent motions of the plates, illustrating how continents have collided and been
torn asunder.
A major problem in the geological and geophysical study of continents is
that we can observe only what is exposed at or near the surface. To extend
that knowledge to tens of kilometres deep, let alone to hundreds of kilometres,
demands conjecture that cannot be tested directly. Oil and mineral exploration
companies have developed sophisticated techniques for surveying the upper few
kilometres of the crust in search of deposits and have significantly advanced our
knowledge of sedimentation, oil maturation and ore genesis. The proof of the
pudding is in the eating - oil companies are accountable, and they finally have to
drill to verify their interpretations and conclusions. If they are wrong too often,
they become bankrupt. Their methods must be good since the results are tested.
In our study of the Earth we are at present unable to sample directly the
deep interior and so are at the disadvantage of being unable to test our models
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