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FIGURE 1.2 Layers and regions within the Earth's interior from the rigid lithospheric crusts of the continents and ocean basins through the molten asthenosphere into the dense core toward the center of the planet, 6370 kilometers below (Chapter 6: Spreading Planet). Modified from Wylie (1976).

Although continental and oceanic crusts both are composed of silicate compounds (made up of silicon and oxygen), there are distinct differences between these two crustal types. Continental granite also contains aluminum, sodium, and potassium, whereas oceanic basalt has large amounts of iron and magnesium. The relatively low density of continental granite (2.7 grams per cubic centimeter), compared to oceanic basalt (3.2 grams per cubic centimeter) is the primary reason why the continents stand above the oceans. Similarly, pure water floats on the Earth's crust because its density only is 1.0 gram per cubic centimeter (Table 1.1).

Earth's crust is extruded from the underlying mantle, which extends to 2900 kilometers depth. Temperatures in the mantle range from around 1200° to 3700°C, causing partial melting along with magma formation. With an average density of 4.5 grams per cubic centimeter, the mantle constitutes two-thirds of the Earth's mass and more than 80% of its volume.

Beneath the mantle is the iron-nickel core of the Earth, which extends to the center of the planet, 6370 kilometers below the surface, with densities exceeding 10 grams per cubic centimeter. The molten core of the Earth has temperatures above 5000°C, which is nearly as hot as the Sun. Consequently, even though the core accounts for less than 16% of the planet's volume, it constitutes more than 30% of its mass. As can be seen, from the core of the planet outward to the hydrogen layer at the edge of outer space, all of the concentric spheres in the Earth system are related across a gradient based on their densities.

Why are there concentric spheres in the Earth system from the outermost regions of the atmosphere to the center of the planet?

TABLE 1.1 Common Conversion Units

Metric

Standard

1 liter (l) equals 1000 millimeters (ml) or 1000 cubic centimeters (cm3 or cc)

Degrees Celsius or centigrade (°C)"-b

Length

Weight

Volume

0.62 miles 3.28 feet 0.39 inches

0.03 ounces 2.20 pounds

0.26 gallons

Temperature degrees Fahrenheit (°F) equals 9/5°C + 32

a Named after Anders Celsius from Sweden (1701-1744).

b Absolute temperature scale in degrees Kelvin (°K), named after William Thompson, Lord Kelvin, from the United Kingdom (1824-1905), is °C + 273.15.

water and life

Relative to the expanse of the Earth system, across more than 15,000 kilometers from the top of the atmosphere to the center of the core (Figs. 1.1 and 1.2), planetary surfaces extend across an extremely thin zone above and below sea level (Fig. 1.3). Actually, from the highest point on land at the top of Mt. Everest to the

Earth's Surface Area (100,000,000 km2) 12 3 4

Earth's Surface Area (100,000,000 km2) 12 3 4

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