Page 387 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 12 The Dynamic Planet 351
Oceanic crust 3.0 g . cm–3
Crust
Upper mantle
Continental –3 crust 2.7 g . cm
Lithosphere
Mohorovicic´ discontinuity
but remains solid because of tremendous pressure. The iron is not pure, but probably is combined with silicon and possibly oxygen and sulfur. The outer core is mol- ten, metallic iron with lighter densities than the inner core. The high temperatures keep the outer core in a liq- uid state, and the flow of this material generates Earth’s magnetic field (discussed just ahead).
Earth’s outer core is separated from the mantle by a transition zone several hundred kilometres wide at an average depth of about 2900 km (see Figure 12.2b). This zone is a discontinuity, or a place where physical differ- ences occur between adjoining regions in Earth’s interior. By studying the seismic waves of more than 25000 earth- quakes, scientists determined that this transition area, the Gutenberg discontinuity, is uneven, with ragged peak- and-valley-like formations.
Together, the lower and upper mantle represent about 80% of Earth’s total volume. The mantle is rich in iron and magnesium oxides and in silicates, which are dense and tightly packed at depth, grading to lesser densities toward the surface. Temperatures are highest at depth and decrease toward the surface; materials are thicker at depth, with higher viscosity, due to increased pressure. A broad transition zone of several hundred kilometres, centred between 410 and 660 km below the surface, sepa- rates the lower mantle from the upper mantle. Rocks in the lower mantle are at high enough temperature that they become soft and are able to flow slowly, deforming over time scales of millions of years.
The boundary between the uppermost mantle and the crust above is another discontinuity, known as the M o h o r o v i cˇ i c´ d i s c o n t i n u i t y , o r M o h o f o r s h o r t . I t i s named for the Yugoslavian seismologist who determined that seismic waves change at this depth due to sharp con- trasts in material composition and density.
Earth’s Crust
Above the Moho is Earth’s outer layer, the crust, which makes up only a fraction of Earth’s overall mass. The crust also makes up only a small portion of the overall distance from Earth’s centre to its surface (Figure 12.3). The distances through Earth’s interior compared with surface distances in North America provide a sense of size and scale: An airplane flying from Anchorage, Alaska, to Fort Lauderdale, Florida, would travel the
◀Figure 12.2 earth in cross section. (a) Cutaway showing Earth’s interior. (b) Earth’s interior in cross section, from the inner core to the crust. (c) Detail of the structure of the lithosphere and its relation to the asthenosphere. (For comparison to the densities noted, the den- sity of water is 1.0 g·cm−3, and mercury, a liquid metal, is 13.0 g·cm−3.)
(c)
Lithosphere
70 km
250 km
660 km transition
2900 km transition
Distance below the surface
5150 km transition
Inner core
Solid iron 13.5 g . cm–3
6370 km Earth’s centre
Thickness
2230 km
2250 km
(b)
1230 km
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