Geoscience Reference
In-Depth Information
10
CHAPTER TEN
acts as the conveyor belt in plate motion. The persistence
of plate tectonics over geological time accounts for the
extreme youth of ocean crust, with a mean age of only 55
Ma and none older than 200 Ma, and the relative youth
of Earth's crust as a whole, with 98 per cent less than
2·5 Ga and 90 per cent less than 0·6 Ga old. Continental
crust is, on average, fifteen to twenty times older than
oceanic crust because it is recycled more slowly. Fragments
of Archaean Earth, 3·7-4·3 Ga old, survive in parts of
Canada, Greenland, Australia and South Africa. Modern
continents are a collage of quite different crustal
terranes
,
or fragments of widely dispersed origin and form, which
reflect the repeated accretion and break-up of older crust
A
supercontinental cycle
- the
Wilson cycle
- is at
work. Continents converge and coalesce during one phase
of Earth history and subsequently rift apart through
relentless plate motion. Supercontinents surrounded by a
single global ocean become fragmented continents
separating several smaller oceans (
Figure 10.2
).
Earth is
small enough for rifted fragments to reassemble eventually
elsewhere. Moving on average at 10
1
cm yr
-1
today -
fingernail growth rates! - the cycle may seem impercep-
tibly slow but can be completed within 500 Ma. This is
short enough to have occurred eight to ten times during
Earth's history, especially as greater heat flow may have
driven the cycle faster during Archaean times.
Earth's present crust is half-way through such a cycle,
which commenced with the rifting of the supercontinent
ORIGIN AND DYNAMICS
Infant Earth, dominated by heat-generating accretion
around a dense core, would be unrecognizable today.
After an initial hot phase, cooling formed an outer crust
violently pockmarked by outgassing of volatile gases
and pulverized by
planetesimals
and other space debris.
This dramatic
Hadean
aeon, named after Hades - the
underworld of Greek mythology - was short-lived and
Earth's essential structure was in place 4·4 Ga ago.
Meteorite impacts, occasionally large enough to form
craters, and cooling accompanied by volcanic activity still
occur but on a reduced scale. Most geological activity is
now confined to the crust and upper mantle within 150
km of the surface. The present form of the continents and
oceans is less than 200 Ma old, which allows us to
concentrate on just 7 per cent of global rock mass and 4
per cent of Earth history.
In the intervening 4·2 Ga continuous but uneven
cooling developed a process of crustal evolution which
acts as the radiator to Earth's internal engine. New crust
forms over hot spots and old, cold crust sinks and is
recycled elsewhere, actively venting geothermal energy as
well as passively emitting it to space. Mobile crust in
transit between these zones takes the form of semi-rigid
plates, and their boundaries coincide with global-scale
landforms, earthquake and volcanic belts. Crustal forma-
tion differentiates between lighter, granitic continental
rafts 'floating' above heavier, basaltic oceanic crust which
