Page 84 - Earth's Climate Past and Future

60 PART II • Tectonic-Scale Climate Change

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0 North America, Eurasia density (2.7 g/cm ). This thick, low-density crust stands
much higher than the floor of the ocean basins lying
Antarctica
some 4000 m below sea level. Ocean crust is 5–10 km
thick, has an average composition like that of basalt, and
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is higher in density (3.2 g/cm ). Below each of these
100
crustal layers lies the mantle, which is richer in heavy
elements like iron (Fe) and magnesium (Mg) and has an
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even higher density (> 3.6 g/cm ). The mantle extends
2890 km into the Earth’s interior, almost halfway to the
200 center of the Earth at a depth of 6370 km.
But these large differences in elevation, crustal
Myr ago Antarctica, India, thickness, and composition are deceptive: they are not
the primary explanation for the fact that continents (and
South America
300 Australia, South Africa, ocean basins) move. The critical reason for this mobility
lies in the way different layers of rock behave.
Two rock layers characterized by very different
long-term behavior exist well below Earth’s surface
(Figure 4-2). The outer layer, called the lithosphere, is
400 100 km thick and generally behaves the way the word
“rock” suggests: as a hard, rigid substance. The lithos-
North Africa
phere encompasses not just the crustal layers (oceanic
and continental) but also the upper part of the underly-
ing mantle.
500 Below the lithosphere is a layer of partly molten yet
Glaciation
More extensive mostly solid rock called the asthenosphere. This layer
Less extensive
FIGURE 4-1 Icehouse intervals Three major intervals of
glaciation occurred during the last 500 Myr.
0 km

Plate Tectonics 0 km
Continental Ocean crust
In 1914 the German meteorologist Alfred Wegener pro- crust
posed that continents have slowly moved across Earth’s Lithosphere
surface for hundreds of millions of years. He based his 30 km (rigid plates)
hypothesis in part on the obvious fact that continental Mantle
margins such as those of eastern South America and
western Africa fit together like pieces of a jigsaw puzzle. 100 km
Research in the last half of the twentieth century showed
that Wegener was correct in claiming that these conti- Asthenosphere
nents were once together and have since moved apart
but that he underestimated the mobility of Earth’s outer
surface. In fact, Earth’s entire surface is on the move. (350 km)

(2890 km)
4-1 Structure and Composition of Tectonic Plates Physical
Wegener’s assumption that continents move in relation Chemical behavior
to ocean basins had a reasonable basis. The contrast composition
between the elevated continents and the submerged FIGURE 4-2 Earth’s structure Earth’s outer layers can be
ocean basins is the most obvious division on Earth’s subdivided in two ways. The basalts of the ocean crust and the
surface. It also reflects the large difference in thickness granites in continental crust differ from each other and from
and composition of the crustal layers that make up the the underlying mantle in chemical composition. The other
continents and ocean basins (Figure 4-2). division is physical behavior: the lithosphere that forms the
Continental crust is 30–70 km thick, has an aver- tectonic plates is a hard, rigid unit, whereas the underlying
age composition like that of granite, and is low in asthenosphere is softer and capable of flowing slowly.