Page 86 - Earth's Climate Past and Future

62 PART II • Tectonic-Scale Climate Change

Divergent margin
Divergent margin (ocean ridge)
(ocean ridge) Convergent margin Transform
fault margin

Plate
motion
Lithospheric
plates

Asthenosphere

FIGURE 4-4 Plate margins Earth’s tectonic plates move apart at ocean ridges (divergent
margins), slide past each other at faults (transform fault margins), and push together at
convergent margins. (Modified from F. Press and R. Siever, Understanding Earth, 2nd ed., © 1998 by
W. H. Freeman and Company.)

continental collision of landmasses such as India and These natural compasses were frozen into the rocks
Asia, which can create massive high-elevation regions shortly after they cooled from a molten state. Today,
such as the Tibetan Plateau. they give scientists studying paleomagnetism a way to
Plates also can slide past each other at transform reconstruct past positions of continents and ocean
fault margins (see Figure 4-4 right), moving hori- basins with respect to the pole of rotation.
zontally along faults such as the San Andreas Fault in The best rocks to use as ancient compasses are
western California. Sliding of plates at transform basalts, which are rich in highly magnetic iron. Basalts
faults involves not just the upper 30 km of continen- form the floors of ocean basins and are also found on
tal crust but also the underlying 70 km of upper land in actively tectonic regions. They form from
mantle. molten lavas, which cool quickly after being extruded
Even though geoscientists do not yet know the bal- onto Earth’s surface. As the molten material cools, its
ance of forces that caused past movements of plates and iron-rich components align with Earth’s magnetic field
produced their present distributions, they can accu- like a compass. After the lava turns into basaltic rock
rately measure the way these processes have changed (when its temperature drops below 1200°C), continued
Earth’s surface during the last several hundred million cooling to temperatures near 600°C allows the “fos-
years. With this knowledge, the tectonic changes can silized” magnetic compasses to become fixed in position
be compared with changes in climate over the same in the rock. Also locked in the basalts are radioactive
interval in order to evaluate how tectonic changes have minerals such as potassium (K). Their slow decay
influenced Earth’s climate. (Chapter 2) can date the time when each basalt cooled
and acquired its magnetic compass.
Paleomagnetism is used to reconstruct changes in
4-2 Evidence of Past Plate Motions
the configuration of Earth’s surface in two ways. (1)
A broad range of evidence reveals the past effects of plate Back to about 500 Myr ago, paleomagnetic compasses
tectonics in rearranging Earth’s geography. The most recorded in continental basalts can be used to track
important evidence starts with the fact that Earth has a movements of landmasses with respect to latitude. (2)
magnetic field. Molten fluids circulating in Earth’s liquid Over the last 175 million years, paleomagnetic changes
iron core today create a magnetic field analogous to that recorded in basaltic oceanic crust are used to recon-
of a bar magnet (Figure 4-5). Compass needles today struct movements of plates and (over part of that inter-
point to magnetic north, which is located a few degrees of val) rates of spreading of the seafloor.
latitude away from the geographic North Pole, which Paleomagnetic Determination of Past Locations
marks Earth’s axis of rotation. of Continents Because ocean crust is constantly being
This link between “magnetic north” and Earth’s destroyed at convergent plate boundaries, no crust older
north polar axis of rotation is assumed to have held in than 175 Myr survives. For older intervals back to about
the past. Some of Earth’s once-molten rocks contain 500 Myr ago, paleomagnetism must rely on basalts found
“fossil compasses” that record its past magnetic field. on the continents. The orientations of the magnetic