Page 53 - Environment: The Science Behind the Stories

merely a snapshot in our changing planet’s long history. We where the asthenosphere’s molten rock approaches to within
can begin to grasp this long-term dynamism as we consider a few miles of the surface, we can harness geothermal energy
two processes of fundamental importance—plate tectonics and by drilling boreholes into the crust (see Figure 21.17, p. 616).
the rock cycle. After examining the geologic processes that In fact, we don’t even need to drill deep or find geysers to
shape our planet, we will close our chapter by exploring the take advantage of geothermal energy. Anywhere in the world,
catastrophic hazards we sometimes face from these processes. the soil and rock just below the surface are fairly constant in
temperature, cooler than the air above the surface in summer
Earth consists of layers and warmer than the air in winter. Because of this, you can
use a geothermal heat pump (also called a ground-source heat
Most geologic processes take place near Earth’s surface, but pump) to heat and cool a home (Figure 21.19, p. 618). Over
our planet consists of multiple layers (Figure 2.15). At Earth’s 600,000 U.S. homes now use these highly efficient systems.
center is a dense core consisting mostly of iron, solid in the The heat from the inner layers of Earth also drives con-
inner core and molten in the outer core. Surrounding the core vection currents that flow in loops in the mantle, pushing the
is a thick layer of less dense, elastic rock called the mantle. A mantle’s soft rock cyclically upward (as it warms) and down-
portion of the upper mantle called the asthenosphere contains ward (as it cools), like a gigantic conveyor belt system. As
especially soft rock, melted in some areas. The harder rock the mantle material moves, it drags large plates of lithosphere
above the asthenosphere is what we know as the lithosphere. along its surface. This movement of lithospheric plates is
The lithosphere includes both the uppermost mantle and the known as plate tectonics, a process of extraordinary impor-
entirety of Earth’s third major layer, the crust, the thin, brit- tance to our planet.
tle, low-density layer of rock that covers Earth’s surface. The
intense heat in the inner Earth rises from core to mantle to Plate tectonics shapes Earth’s geography
crust, and it eventually dissipates at the surface. In regions
Our planet’s surface consists of about 15 major tectonic plates,
which fit together like pieces of a jigsaw puzzle (Figure 2.16).
Crust Imagine peeling an orange and then placing the pieces of peel
Oceanic back onto the fruit; the ragged pieces of peel are like the lith-
Continental
ospheric plates riding atop Earth’s surface. However, the plates
are thinner relative to the planet’s size, more like the skin of an
Lithosphere apple. These plates move at rates of roughly 2–15 cm (1–6 in.)
Uppermost mantle per year. This slow movement has influenced Earth’s climate
and life’s evolution throughout our planet’s history as the con-
tinents combined, separated, and recombined in various con-
Asthenosphere figurations. By studying ancient rock formations throughout
~100 km (62 mi) Upper mantle
the world, geologists have determined that at least twice, all
landmasses were joined together in a “supercontinent.” Sci-
entists have dubbed the one that occurred about 225 million
~250 km (155 mi)
years ago Pangaea (see Figure 2.16).

There are three types of plate boundaries

The processes that occur at the boundaries between plates
have major consequences. We can categorize these boundaries
Lower into three types: divergent, transform, and convergent plate
mantle
boundaries.
Outer core At divergent plate boundaries, tectonic plates push apart
Inner core from one another as magma (rock heated to a molten, liquid
state) rises upward to the surface, creating new lithosphere as
~600 km (370 mi) it cools (Figure 2.17a). An example is the Mid-Atlantic Ridge,
2900 km (1800 mi) part of a 74,000-km (46,000-mi) system of divergent plate
boundaries slicing across the floors of the world’s oceans.
5150 km (3190 mi)
Where two plates meet, they may slip and grind along-
6370 km (3950 mi)
side one another, forming a transform plate boundary
(Figure 2.17b). This movement creates friction that generates
Figure 2.15 Earth’s three primary layers—core, mantle, and earthquakes (p. 58) along strike-slip faults. Faults are frac-
crust—are themselves layered. The inner core of solid iron is
surrounded by an outer core of molten iron, and the rocky mantle tures in Earth’s crust, and at strike-slip faults, each landmass
includes the molten asthenosphere near its upper edge. At Earth’s moves horizontally in opposite directions. The Pacific Plate
surface, dense and thin oceanic crust abuts lighter, thicker conti- and the North American Plate, for example, slide past one
nental crust. The lithosphere consists of the crust and the upper- another along California’s San Andreas Fault. Southern Cali-
52 most mantle above the asthenosphere. fornia is slowly inching its way northward along this fault, and

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