Geology Reference
In-Depth Information
basalts. Furthermore you don't have to be a rocket scientist to understand the
essence of the physical theories - they can be understood fairly readily without a
lot of mathematics or a PhD in physics.
The main purpose of this topic is to focus on the questions of why plates move
and what causes volcanic hotspots and flood basalts, and to present answers in a
way that any geologist should be able to understand. In the view presented here,
moving plates and volcanic hotspots are manifestations of convection in the Earth's
mantle. Convection can be understood on the basis of two kinds of basic physical
process (heat conduction and viscous fluid flow), which are explained through
simple examples. Putting these two physical processes together allows one, for
example, to calculate fairly simply how fast plates should move, and to get an
answer that is close to what we observe. In the process, the relationship between
plates and convection becomes clear, the likelihood of narrow, warm, columns
rising through the mantle becomes evident, and other major features of Earth's
tectonic system fall into place.
The topic then goes on to look at two kinds of implication. The first is the
evolution of the mantle, and how that might relate to tectonic evolution. The second
is how geochemistry might fit into the physical and dynamical picture. This has
been a vexed issue for some time, but refractory trace elements have by now been
plausibly and quantitatively incorporated, and the noble gases might now also be
finding a place. This is important because the geochemistry provides information
not available just from the physics.
I should be clear that in offering 'answers' I don't mean 'the truth', I mean
theories well based in physics that can account for many of the main features we
observe. As always in science, this does not mean that better theories might not
emerge, nor that alternative theories do not exist. Choosing between alternative
theories is not entirely a rational process, it also involves judgements, and this
topic reflects my own judgements. In my experience, some of the extant criticisms
of the theories presented here reflect a lack of clear understanding of what the
theories actually are. So there are two roles this presentation can play: to inform
the professionally curious, and to focus debates more constructively on real issues
instead of misunderstandings.
