Relentless change is Earth鈥檚 signature tune: plates move, icecaps grow and
melt, species come and go. It鈥檚 a turmoil unmatched by almost anything else,
except perhaps the curricula of university earth science courses.
Before the revolution of plate tectonics in the 1960s, geology students were
often taught long lists of facts, the names of fossils, minerals and rocks.
There was little context for all this information. But plate tectonics provided
the key framework for linking these facts together. Separate evolution of land
animals in Australia relates to the continent鈥檚 separation from Africa, the
building of mountains in Asia can be matched with the opening of the Indian
ocean. Through making these types of links, today鈥檚 geology students are far
more accustomed to drawing together the strands of different, complex arguments,
than memorising facts.
Since the heady days of revolution the treatment in textbooks of tectonics,
the study of a dynamic and restless Earth, has become rather staid. It is giving
way to a shapeless field of study centred on environmental concerns as
university degree courses seek ever greater human relevance. Admittedly,
geologists have been a bit quirky up to now by tending to concentrate on the
planet鈥檚 crust to the exclusion of its depths. But going by what鈥檚 seen in the
bookshops and in university curricula, they now seem intent on going shallower
still, and not worrying too much about anything deeper than groundwater.
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Not so. What we have instead is a lot of pioneering basic research that
simply hasn鈥檛 filtered through to the textbooks. Since the mid-1980s, geologists
have been busy looking at the world beneath the plates. As a result, we now know
that the liquid outer core convects, and that the continuous flow of molten iron
generates the Earth鈥檚 magnetic field. We are finally getting a picture of the
solid mantle as a giant heat engine that not only drives the plates but does a
whole lot more besides.
Unfortunately for students, not to mention the textbook writers,
understanding convection in solid rocks isn鈥檛 easy. Many students think that
convection only happens in liquids and presume that the plates float about on
molten mantle. While melting explains what happens in the outer core, it is not
the answer for the mantle. Although it is extremely hot in the mantle, the
pressure is too high for the rocks to melt. Like ice, solid rock can flow, but
ten thousand times more slowly than the ice of a glacier. The flow can speed
up鈥攁s it does when the rocks weaken during chemical reactions鈥攐r
break up entirely.
At present, textbooks focus on how flow happens to the rocks in the Earth鈥檚
crust. Over the coming years, watch out for books that dig deeper, into rocks in
the mantle. We know the mantle flows, but debate continues about whether
convection overturns the mantle as a whole, or whether it occurs in layers
(seeNew 杏吧原创, 17 October 1998, p 39).
It is this that holds the key to how the plates move.
Seismic tomography is giving us a critical breakthrough in understanding
convection in the mantle. This technique uses the energy from earthquakes to map
out its upwelling and sinking regions: hot, upwelling mantle transmits seismic
energy more slowly that cold, sinking mantle. Making these maps requires
powerful computers to handle the signals of thousands of earthquakes recorded
from all over the earth. The results are stunning images of plates from the
surface down almost to the base of the mantle, about 2900 kilometres below. It鈥檚
revolutionary stuff, but some accessible accounts have already surfaced (鈥淎 lava
lamp model for the deep earth鈥�, Science, 19 March 1999, vol 283, p
1826).
Moving plates are not the only surface manifestation of unrest in the deep
mantle. The vast lava fields that erupt periodically over geological time, such
as the Deccan Traps of western India or the ocean island hot-spot chain that
makes up Hawaii, have their source in the rapid upwelling of hot rock from deep
in the mantle. These so-called 鈥減lumes鈥� appear to originate in the boundary
between core and mantle. The process seems to be something like this: solid
mantle rises swiftly towards the surface, decompresses and melts. Lavas are then
thrown out. Interestingly, these show some chemical signatures of ancient
sediments and seawater, recycled from subducted oceanic plates (see 鈥淯nmixing
Hawaiian cocktails鈥�, Nature, 24 June 1999, p 733). This finding
confirms the old idea that the core-mantle boundary is the graveyard of plates.
Seismic imaging has begun to show us where the graveyards are
thickest鈥攑oints where, perhaps, new mantle plumes are emerging.
All this adds up to a picture that鈥檚 less and less like a jigsaw puzzle made
up of two-dimensional plates. Geology is the study of the whole Earth, after
all, so we need a third dimension. It鈥檚 becoming more like 3D chess, as played
by our favourite Vulcan, Star Trek鈥檚 Mr Spock. To play the game,
geology students need to 鈥渞ead鈥� the spectacular new images of the deep. Today鈥檚
3D models of the mantle circulation, of rising plumes and subducting plates,
show a restless interior pushing and pulling at our planet鈥檚 skin.
And this brings us back to our starting point鈥攖he surface.
Ultimately, the processes deep in the mantle are one of the forces changing
our climate. As oceans change shape, so do their currents, which in turn affect
climate. The shifting of continents can cause ice ages, shrink the shallow seas
that host much of life on our planet and, by isolating gene pools, promote
evolution. Mantle plumes can trigger huge volcanic eruptions, cooling the
climate within months. Competing convection between the core and mantle can even
change the lengths of days. Even in what has become the environmentally
conscious world of earth science teaching, the next generation of students will
have to be ready to dig deep.
The Core-Mantle Boundary Region
edited by Michael Gurnis and others, American Geophysical Union, Geodynamics
series, volume 28, AGU Washington, $65, ISBN 0875905307.
A formidable array of expert and sometimes controversial essays covering
modern research on the deep Earth and models of mantle convection.
Fault-Related Rocks
edited by Arthur Snoke, Jan Tullis, Victoria Todd, Princeton University
Press, 拢75, ISBN 0691012202.
Some stunning images make this library reference book a superb account of
what happens to solid rocks when they flow or break apart on faults.
Encyclopedia of Geochemistry
edited by Clare Marshall and Rhodes Fairbridge, Kluwer Academic, Dordrecht,
拢280, ISBN 0412755009.
A library-priced reference book, not encyclopedic, but a collection of
succinct chapters on disparate topics, from activation energies to the
geochemistry of zirconium.
Sedimentology and Stratigraphy
by Gary Nichols, Blackwell Science, 拢26.50, ISBN 0632035781.
This year鈥檚 textbook on sedimentary rocks is readable and wide-ranging. But
soon there鈥檒l be a textbook by every sedimentologist in the country.
Geological Maps: An Introduction
(2nd edition) by Alex Maltman, John Wiley and Sons, 拢19.99, ISBN
0471976962.
A welcome reappearance of this highly regarded basic text. Shows how
essentially two-dimensional observations can yield a three dimensional
view鈥攁 key skill.
Low-Grade Metamorphism
by Martin Frey and Doug Robinson, Blackwell, 拢44.50, ISBN
0632047569.
Discover the processes which make muds into slate and how the water that
escapes from black smokers has changed basalts beneath the sea floor.