Page 85 - Earth's Climate Past and Future

CHAPTER 4 • Plate Tectonics and Long-Term Climate 61

lies entirely within the upper section of Earth’s mantle Plates move apart at divergent margins, the crests
at depths between 100 and 350 km. Compared to the of ocean ridges like the one that runs down the middle
rigid lithosphere, this deeper layer behaves like a soft, of the Atlantic Ocean (see Figure 4-3). This motion
viscous fluid over long intervals of time and flows more allows new ocean crust to be created, and the new crust
easily. The behavior of this “softer” deeper layer allows spreads away from the ridge (see Figure 4-4 left). Plates
the overlying lithosphere to move. diverging at ocean ridges carry not just the near-surface
The lithosphere consists of a dozen tectonic layer of ocean crust but also a much thicker layer of
plates, each drifting slowly across Earth’s surface mantle lying underneath.
(Figure 4-3). These plates move at rates ranging from Plates come together at convergent margins (see
less than 1 up to 10 cm per year, about the same as the Figure 4-4 left). At these locations, the ocean crust
rate of growth of a fingernail. Over a time span of plunges deep into Earth’s interior at ocean trenches in a
100 Myr, 5 cm of plate motion per year adds up to process called subduction. The subducting ocean crust
5000 km, enough to create or destroy an entire ocean rides on top of a much thicker layer of upper mantle
basin. that also moves downward.
Most tectonic plates consist not simply of conti- Some convergent margins occur along continent-
nents or ocean basins but of combinations of the two. ocean boundaries, such as the western coast of South
For example, the South American plate consists of the America. In this case, narrow mountain chains such as
continent of South America and the western half of the Andes form on the adjacent continents because of
the South Atlantic Ocean, all moving as one rigid the compressive (squeezing) forces produced when the
unit. two plates move together. Subduction can also occur
These rigid tectonic plates have three basic types of within the ocean, where the ocean crust of one plate
edges, or margins. Most tectonic deformation on Earth plunges under another and forms volcanic ocean
(earthquakes, faulting, and volcanoes) occurs at these islands, such as those in the western Pacific. A less com-
plate margins (Figure 4-4). mon but important example of converging plates is the

EURASIAN EURASIAN PLATE
EURASIAN
EURASIAN PLATE
PLATE
PLATE
NORTH AMERICAN PLATE
NORTH AMERICAN PLATE
ANATOLIAN
ANATOLIAN
JUAN DE FUCA PLATE
JUAN DE FUCA
PLATE
PLATE
PLATE
CARIBBEAN
CARIBBEAN
ARABIAN
PHILIPPINE PLATE ARABIAN
PLATE
PHILIPPINE
PLATE
PLATE PLATE
PLATE
COCOS
COCOS AFRICAN PLATE
AFRICAN PLATE
PLATE
PLATE
PACIFIC PLATE
PACIFIC PLATE
SOUTH
SOUTH SOMALI
SOMALI
NAZCA AMERICAN SUB-PLATE
NAZCA
AMERICAN
PLATE PLATE SUB-PLATE
PLATE
INDIAN-AUSTRALIAN PLATE
INDIAN-AUSTRALIAN
PLATE
PLATE
ANTARCTIC PLATE
ANTARCTIC PLATE
FIGURE 4-3 Tectonic plates Earth’s lithosphere is divided into a dozen major tectonic plates
and several smaller plates, which move as rigid units in relation to one another, as the arrows
indicate. (F. Press and R. Siever, Understanding Earth, 2nd ed., © 1998 by W. H. Freeman and
Company. False-color topography courtesy of Peter Schloss, NGDC, Boulder, Colo.)