Page 55 - Environment: The Science Behind the Stories

When one plate of oceanic lithosphere is subducted Only in the last several decades have scientists learned
beneath another plate of oceanic lithosphere, the resulting about plate tectonics—this environmental system of such
volcanism may form arcs of islands, such as Japan and the fundamental importance was completely unknown to
Aleutian Islands of Alaska. Subduction zones may also cre- humanity just half a century ago. Amazingly, our civiliza-
ate deep trenches, such as the Mariana Trench, our planet’s tion was sending people to the moon by the time we were
deepest abyss. When oceanic lithosphere slides beneath coming to understand the movement of land under our
continental lithosphere, this leads to the formation of vol- very feet.
canic mountain ranges that parallel coastlines (a process
shown in the left-hand drawing of Figure 2.17c). The Cas-
cades in the Pacific Northwest, where Mount St. Helens The rock cycle alters rock
erupted violently in 1980 (see Figure 17.11, p. 475) and
renewed its activity in 2004, are fueled by magma from Just as plate tectonics shows geology’s dynamism at a large
subduction. Another example is South America’s Andes scale, the rock cycle shows it at a smaller one. We tend to
Mountains, where the Nazca Plate slides beneath the South think of rock as pretty solid stuff. Yet over geologic time,
American Plate. rocks and the minerals that comprise them are heated, melted,
When two plates of continental lithosphere meet, the cooled, broken down, and reassembled in a very slow process
continental crust on both sides resists subduction and instead called the rock cycle (Figure 2.18).
crushes together, bending, buckling, and deforming layers A rock is any solid aggregation of minerals. A mineral,
of rock from both plates in a continental collision (shown in turn, is any naturally occurring solid element or inorganic
in the right-hand drawing of Figure 2.17c). Portions of the compound with a crystal structure, a specific chemical com-
accumulating masses of buckled crust are forced upward position, and distinct physical properties. The type of rock in a
as they are pressed together, and mountain ranges result. given region affects soil characteristics and thereby influences
The Himalayas, the world’s highest mountains, result from the region’s plant community. Understanding the rock cycle
the Indian-Australian Plate’s collision with the Eurasian enables us to better appreciate the formation and conservation
Plate beginning 40–50 million years ago, and these moun- of soils, mineral resources, fossil fuels, groundwater sources,
tains are still rising today as these plates converge. The
Appalachian Mountains of the eastern United States, once
the world’s highest mountains themselves, resulted
from a more-ancient collision with the edge of Cooling and
what is today Africa. crystallization

Magma
Tectonics produces Earth’s and lava
landforms Melting

In these ways, the processes of plate (a) Igneous rock
tectonics create the landforms around Heating and pressure
us. Tectonic movements build moun-
tains; shape the geography of oceans, Weathering, erosion,
islands, and continents; and give rise transport, deposition
to earthquakes and volcanoes.
The topography created by tec- Weathering, erosion, transport, deposition
tonic processes, in turn, shapes cli- (c) Metamorphic rock
mate by altering patterns of rainfall,
wind, ocean currents, heating, and
cooling—all of which affect rates of
weathering and erosion and the ability Weathering, erosion,
of plants and animals to inhabit differ- Heating and transport, deposition
pressure
ent regions. Thus, the locations of biomes Sediments
(pp. 111–117) are influenced by plate tec- Lithification
tonics. Moreover, tectonics has affected the
history of life’s evolution; the convergence of
landmasses into supercontinents such as Pangaea is
thought to have contributed to wide- (b) Sedimentary rock
spread extinctions by reducing the Figure 2.18 The rock cycle. Igneous rock (a) is formed when rock melts and the resulting
area of species-rich coastal regions magma or lava then cools. Sedimentary rock (b) is formed when rock is weathered and eroded
and by creating an arid continental and the resulting sediments are compressed to form new rock. Metamorphic rock (c) is formed
interior with extreme temperature when rock is subjected to intense heat and pressure underground. Through these processes,
54 swings. each type of rock can be converted into either of the other two types.

M02_WITH7428_05_SE_C02.indd 54 12/12/14 2:53 PM