Page 56 - Environment: The Science Behind the Stories

(a) Intrusive igneous rock: Granite at Yosemite National Park (b) Extrusive igneous rock: Basalt in the Canary Islands

(c) Sedimentary rock: Sandstone in Arizona (d) Metamorphic rock: Gneiss in Utah

Figure 2.19 Examples of rock types. The towering rock formations of Yosemite National Park are made
of granite (a), a type of intrusive igneous rock. The lava flows on the Canary Islands form basalt (b), a type CHAPTER 2 • E ART h’s Physi CAL
of extrusive igneous rock. The layered formation in Paria Canyon, Arizona, is an example of sandstone (c),
a type of sedimentary rock. Gneiss (pronounced “nice”) (d), at Antelope Island, Utah, is a type of metamor-
phic rock.

geothermal energy sources, and other natural resources—all from a volcano, it cools quickly, so minerals have little time
of which we discuss in later chapters. to grow into coarse crystals. This class of igneous rock is
called extrusive igneous rock, and its most common repre-
Igneous rock All rocks can melt. At high enough tem- sentative is basalt, the principal rock type of the Japanese
peratures, rock will enter the molten, liquid state called islands (Figure 2.19b).
magma. If magma is released through the lithosphere (as in
a volcanic eruption), it may flow or spatter across Earth’s Sedimentary rock All exposed rock weathers away s ys TE m s: mATTER , E NER gy, AN d
surface as lava. Rock that forms when magma or lava cools with time. The relentless forces of wind, water, freezing,
is called igneous rock (from the Latin ignis, meaning “fire”) and thawing eat away at rocks, stripping off one tiny grain
(Figure 2.18a). (or large chunk) after another. Through weathering (p. 237)
Igneous rock comes in two main classes because magma and erosion (pp. 240–241), particles of rock blown by wind or
can solidify in different ways. When magma cools slowly washed away by water come to rest downhill, downstream, or
and solidifies while it is below Earth’s surface, it forms downwind from their sources, eventually forming sediments.
intrusive igneous rock. This process created the famous rock Alternatively, some sediments form chemically from the pre-
formations at Yosemite National Park (Figure 2.19a). Granite cipitation of substances out of solution. gE o L ogy
is the best-known type of intrusive rock. A slow cooling Sediment layers accumulate over time, causing the
process allows minerals of different types to aggregate weight and pressure of overlying layers to increase.
into large crystals, giving granite its multicolored, coarse- Sedimentary rock (Figure 2.18b) is formed as sediments are
grained appearance. In contrast, when molten rock is ejected physically pressed together (compaction) and as dissolved 55

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