Page 389 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 12 The Dynamic Planet 353
Table 12.1
Common Elements in Earth’s Crust
element
Percentage of earth’s Crust by Weight
Oxygen (O) Aluminum (Al) Calcium (Ca) Potassium (K) All others
46.6 8.1 3.6 2.6 1.5
Silicon (Si)
Iron (Fe)
Magnesium (Mg)
2.1
Total
100.0
The lithosphere includes the crust and the upper- most mantle, to about 70 km in depth, and forms the rigid, cooler layer at Earth’s surface (Figure 12.2c). Note that the terms lithosphere and crust are not the same; the crust makes up the upper portion of the lithosphere.
The asthenosphere lies within the mantle from about 70 km to 250 km in depth. This is the hottest re- gion of the mantle—about 10% of the asthenosphere is molten in uneven patterns. The movement of convec- tion currents in this zone in part causes the shifting of lithospheric plates, discussed later in the chapter.
Adjustments in the Crust
We discussed the buoyancy force in Chapter 7 with re- gard to parcels of air: if a parcel of air is less dense than the surrounding air, it is buoyant and will rise. In essence, buoyancy is the principle that something less dense, such as wood, floats in something denser, such as water. The balance between the buoyancy and gravitational forces is the principle of isostasy, which explains the elevations of continents and the depths of ocean floors as determined by vertical movements of Earth’s crust.
Earth’s lithosphere floats on the denser layers be- neath, much as a boat floats on water. If a load is placed on the surface, such as the weight of a glacier, a moun- tain range, or an area of sediment accumulation (rock material that has been transported by exogenic pro- cesses), the lithosphere tends to sink, or ride lower in the asthenosphere (Figure 12.4, page 354). When this happens, the rigid lithosphere bends, and the plastic
A quartz crystal (SiO2) consists of Earth’s two most abundant elements, silicon (Si) and oxygen (O). [Stefano Cavoretto/ Shutterstock.]
asthenosphere flows out of the way. If the load is re- moved, such as when a glacier melts, the crust rides higher and the asthenosphere flows back toward the region of uplifting lithosphere. The uplift after removal of surface load is known as isostatic rebound. The en- tire crust is in a constant state of isostatic adjustment, slowly rising and sinking in response to weight at the surface.
In southeast Alaska, the retreat of glacial ice follow- ing the last ice age about 10000 years ago removed weight from the crust. Researchers today, using an array of GPS receivers to measure isostatic rebound, expected to find a slowed rate of crustal rebound occurring now in south- eastern Alaska, as compared to the more rapid response in the distant past when the ice first retreated. Instead, they detected some of the most rapid vertical motion on Earth, averaging about 36 mm per year. Scientists attri- bute this isostatic rebound to the loss of modern glaciers in the region, especially in the areas from Yakutat Bay and the Saint Elias Mountains in the north, through Gla- cier Bay and Juneau in the south. This rapid rebound is attributable to climate change—causing glacial melt and retreat over the past 150 years—and correlates with re- cord warmth across Alaska.
Earth’s Magnetism
As mentioned earlier, Earth’s fluid outer core generates most (at least 90%) of Earth’s magnetic field and the mag- netosphere that surrounds and protects Earth from solar wind and cosmic radiation. One hypothesis explains
27.7
5.0
Sodium (na)
2.8
Georeport 12.3 Deep-drilling the Continental Crust
The Kola Borehole in russia, north of the Arctic Circle, is 12.23 km deep, drilled over a 20-year period purely for
exploration and science. This is the deepest drilling attempt for scientific purposes in continental crust; drilling for oil wells has gone deeper—the record is 12,289 m in the Al Shaheen oil field in Qatar. The Kola Borehole reached rock 1.4 billion years old at 180°C in the Earth’s crust, and for two decades was the deepest borehole ever drilled (see www-icdp.icdp-online.org/front_ content.php?idcat=695).
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