Plate Tectonics: A Unifying Theory - The Changing Earth: Exploring Geology and Evolution - page 40

Deep-sea sediments Mid-ocean ridge

Ocean lithosphere

Asthenosphere Magma

Subduction

◗

Figure 2.14 Deep-Sea Sediments and Seafl oor Spreading The total thickness of deep-sea
sediments increases away from oceanic ridges. This is because oceanic crust becomes older away
from oceanic ridges, and there has been more time for sediment to accumulate.

igneous bodies), lava fl ows, and thick sedimentary sequences
within rift valleys, all features that are preserved in the geo-
logic record. The Triassic fault basins of the eastern United
States are a good example of ancient continental rifting (see
Figure 22.7). These fault basins mark the zone of rifting that
occurred when North America split apart from Africa. The
basins contain thousands of meters of continental sediment
and are riddled with dikes and sills (see Chapter 22).
Pillow lavas, in association with deep-sea sediment,
are also evidence of ancient rifting. The presence of pillow
lavas marks the formation of a spreading ridge in a narrow

linear sea. A narrow linear sea forms when the continental
crust in the rift valley finally breaks apart, and the area is
fl ooded with seawater. Magma, intruding into the sea along
this newly formed spreading ridge, solidifi es as pillow lavas,
which are preserved in the geologic record, along with the
sediment being deposited on them.

Convergent Boundaries
Whereas new crust forms at divergent plate boundaries, older
crust must be destroyed and recycled in order for the entire

Eurasian
plate

Eurasian
plate

1.8

North American
plate

5.4

Arabian
plate

2.3

7.9 3.7

6.9

Pacific
plate

Caribbean
plate

5.5

3.0

2.0

11.7

Cocos
plate

2.0 2.5

10.5

10.1

South
American
plate

3.8

7.0

17.2

4.0

African
plate Nazca

plate

Indian-Australian
plate

6.2

11.1

7.1

7.4

18.3

7.3

4.1

7.2

1.7

10.3

3.7 1.3

3.3

7.7

5.7

Antarctic plate

Hot spot
◗ Figure 2.15 Earth's Plates A world map showing Earth's plates, their boundaries, their relative
motion and rates of movement in centimeters per year, and hot spots.

Ridge axis Subduction zone Direction of movement