Page 388 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

352 part III The Earth–Atmosphere Interface
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ALASKA Anchorage
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160°
CANADA
Regina OUTER CORE
Calgary
Seattle Portland
Duluth
Detroit Chicago
Pittsburgh
Atlanta
UPPER MANTLE
ATLANTIC OCEAN
LITHOSPHERE
CONTINENTAL 70° CRUST
INNER CORE
PACIFIC OCEAN
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LOWER MANTLE Kansas City
Denver
Fort Lauderdale
Vancouver
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New Orleans Dallas
Houston
ASTHENOSPHERE
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90°
0
UNITED STATES
San Francisco
▲Figure 12.3 Distances from core to crust. The distance from Anchorage, Alaska, to Fort Lauderdale, Florida, is the same as the distance from Earth’s centre to its outer crust. The distance from Boca raton, Florida, to Fort Lauderdale (30 km) represents the thickness of the continental crust.
same distance as that from Earth’s centre to its surface. The last 30 km of that journey represent the thickness of Earth’s crust.
The thickness of Earth’s crust varies over the extent of the planet. Crustal areas beneath mountain masses are thicker, extending to about 50–60 km, whereas the crust beneath continental interiors averages about 30 km in thickness. Oceanic crust averages only 5 km in thick- ness. Drilling through the crust and Moho discontinuity (the crust–mantle boundary) into the uppermost mantle remains an elusive scientific goal.
Just eight natural elements make up over 98% of Earth’s crust by weight, and just two of these—oxygen and silicon—together account for 74.3% (Table 12.1). Oxygen is the most reactive gas in the lower atmosphere, readily combining with other elements. For this reason, the percentage of oxygen is higher in the crust (at 46%) than in the atmosphere, where it makes up 21%. The internal differentiation process, in which less-dense ele- ments are nearer the surface, explains the relatively large percentages of elements such as silicon and aluminum in the crust.
Continental crust differs greatly from oceanic crust in composition and texture, and the difference has a bearing on the dynamics of plate tectonics and continen- tal drift discussed later in the chapter.
• Continental crust is relatively low in density, aver- aging 2.7 g · cm−3 (or 2700 kg · m−3), and is composed mainly of granite. It is crystalline and high in silica, aluminum, potassium, calcium, and sodium. Some- times continental crust is called sial, shorthand for the dominant elements of silica and aluminum.
• Oceanic crust is denser than continental crust, aver- aging 3.0 g·cm−3 (or 3000 kg·m−3), and is composed of basalt. It is granular and high in silica, magnesium, and iron. Sometimes oceanic crust is called sima, shorthand for the dominant elements of silica and magnesium.
The Asthenosphere and Lithosphere
The interior layers of core, mantle, and crust are dif- ferentiated by chemical composition. Another way to distinguish layers within the Earth is by their rigid or plastic character. A rigid layer will not flow when a force acts on it; instead, it will bend or break. A plastic layer will slowly flow when a force is present. Using this criterion, scientists divide the outer part of Earth into two layers: the lithosphere, or rigid layer (from the Greek lithos, or “rocky”), and the asthenosphere, or plastic layer (from the Greek asthenos, meaning “weak”).
Georeport 12.2 Earth on the Scales
How much does our planet weigh? A revised estimate of Earth’s mass, or weight, calculated in 2000 set its weight at 5.972 sextillion metric tons (5972 followed by 18 zeros).
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