Page 54 - Environment: The Science Behind the Stories

Juan de
Fuca Plate
Eurasian
North Plate
American Arabian Philippine
Plate Appalachian Plate Plate
San Andreas Mountains Himalaya Pacific
Fault Caribbean Mountains Plate
Plate
Cocos Caroline
Plate African Plate
South Plate
American
Plate Mid-Atlantic Ridge
Nazca
Pacific Plate Andes Mountains Indian-
Plate Australian Plate
Scotia
Plate
Antarctic
Plate Divergent boundary
Transform plate boundary
Convergent boundary

Figure 2.16 Earth’s crust consists of roughly 15 major
plates (above) that move very slowly by the process of
plate tectonics. Today’s continents were joined together in the
landmass Pangaea (left) about 225 million years ago.
P A N G A E A

the site of Los Angeles will eventually reach that of modern- it and dives downward into the asthenosphere in a process
day San Francisco. called subduction. As the lithospheric plate descends, it slides
Convergent plate boundaries, where two plates con- beneath a neighboring plate that is less dense, forming a con- CHAPTER 2 • E ART h’s Physi CAL
verge or come together, can give rise to different outcomes vergent plate boundary. The subducted plate is heated and
(Figure 2.17c). As plates of newly formed lithosphere push out- pressurized as it sinks, and water vapor escapes, helping to
ward from divergent plate boundaries, this oceanic lithosphere melt rock (by lowering its melting temperature). The molten
gradually cools, becoming denser. Eventually (after millions rock rises, and this magma may erupt through the surface at
of years), it becomes denser than the asthenosphere beneath volcanoes (pp. 58–60).

Continental Collision
Trench crust mountains
Rift Oceanic
Ridge Strike-slip fault crust s ys TE m s: mATTER , E NER gy, AN d

Asthenosphere Subduction Volcano

Magma Magma
(a) Divergent plate boundary (b) Transform plate boundary (c) Convergent plate boundary
Figure 2.17 There are three types of boundaries between tectonic plates, generating different gE o L ogy
geologic processes. At a divergent plate boundary, such as a mid-ocean ridge on the seafloor (a), the two
plates move gradually away from the boundary in the manner of conveyor belts, and magma from beneath the
crust may extrude as lava. At a transform plate boundary (b), two plates slide alongside one another, creating
friction that leads to earthquakes. Where plates collide at a convergent plate boundary (c), one plate is sub-
ducted beneath another, leading to either volcanism or the formation of mountain ranges. 53

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