Geology Reference
In-Depth Information
9
Evolution and tectonics
Decaying heat sources and evolution of the system. Is there enough
heat generation to keep the system cooking? Complications, and alter-
native controls on evolution. Implications for tectonic history. Archaean
plates? Episodes?
Mantle convection is likely to have changed over the course of Earth's history. The
mantle was probably hotter when the Earth formed, because of the large releases of
gravitational energy during its accretion and the separation of the mantle and core.
Also the radioactivity that drives the system now would have been stronger in past
times, because of radioactive decay. Thus we might expect that mantle convection
used to be more vigorous. But how much more vigorous? We can answer that
question by building on the understanding we have already established.
It turns out that there may have been changes other than just a slowing down as
the mantle cooled. For example, the melting and differentiation that accompanies
mantle convection generate compositional density differences, and these may affect
the course of the convection. Rheology may also have important effects. In fact
there may have been some quite drastic changes in mantle convection, especially
in the first half of Earth's history.
The geological record preserved in continents tells us that there have been
changes in geological and tectonic processes over the past several billion years.
Quite what the tectonic changes were is not yet very clear, but it is clear that the
character of fragments of continental crust preserved from 3.5 Gyr ago is different
in important ways from more modern crust. It seems also that the Earth's tectonic
processes have proceeded in bursts, interspersed with quieter periods.
The ancient geological record is of course quite fragmented and incomplete, yet
exploration continues, and chemical analysis techniques keep improving, extracting
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