Geology Reference
In-Depth Information
my present purpose and a separate account is required that is explicitly directed to
geologists.
I use the term
geologist
in the broad sense of anyone who studies geological
processes. This includes not only field geologists of various kinds, such as structural
geologists, but also petrologists, geochronologists, geochemists, ore geologists,
sedimentologists, palaeontologists and so on. It also includes geophysicists. For
example, seismologists are often so specialised in the intricacies of their discipline
that they have little understanding of other aspects of geophysics.
So, returning to the relevance of mantle convection to geology, there are two
energy sources driving geological processes. Solar energy drives surficial processes
related to the weather and life, such as weathering, erosion and sediment transport.
All other geological processes are driven by the Earth's internal heat. These include
mountain building, or more generally tectonics, magma generation, water flows in
the deeper crust, metamorphism and much mineral deposition.
Tectonics, meaning the movements of the crust that result in mountains, rifts,
faults, folds and so on, is driven by the Earth's internal heat and is obviously
fundamental to a large proportion of geological processes. The connection between
heat and tectonics is through convection: convection in the mantle is driven by the
Earth's internal heat, and it generates the movements that manifest as tectonics.
Plate tectonics is the primary agent of tectonics, and its existence is now widely
accepted. Volcanic hotspots (by which I mean surface features like the Hawaiian
and Icelandic volcanic centres) and their relatives, the flood basalts, are a secondary
tectonic agent, and their existence and basic features are also widely recognised.
There may be other tectonic agents, but evidently they are minor.
This much is well known and widely understood. However, the causes of plate
motions and volcanic hotspots still do not seem to be clearly understood in the broad
geological community. They are usually understood to involve mantle convection,
but the relationship of plates and plumes to that convection seems to be understood
only vaguely, and often with some basic misconceptions - or so it seems from my
encounters with non-specialist colleagues and students.
To many non-specialists, mantle convection seems to be something rather mys-
terious that happens 'down there'. Its relationship with plates is not very clear. The
idea of mantle plumes is hotly disputed by a few, and plumes are not uncommonly
regarded as arbitrarily adjustable to fit circumstances and therefore not real science.
Other confusions may exist, such as between crust and lithosphere, and miscon-
ceptions are not uncommon, such as that there are warmer upwellings rising under
mid-ocean ridges, that 'plume tectonics' is an alternative to plate tectonics, or that
mantle plumes are molten.
Yet there are now straightforward and well-quantified physical theories that
account for the main features of plate movements, volcanic hotspots and flood
