Chapter Overview

The only rocks that geologists can study directly in place are those of the crust; and Earth's crust is but a thin skin of rock, making up less than 1% of Earth's total volume. Mantle rocks brought to Earth's surface in basalt flows, in diamond-bearing kimberlite pipes, and also the tectonic attachment of lower parts of the oceanic lithosphere to the continental crust, give geologists a glimpse of what the underlying mantle might look like. Meteorites also give clues about the possible composition of the core of the Earth. But, to learn more about the deep interior of Earth, geologists must study it indirectly, largely by using tools of geophysics - that is, seismic waves and the measurement of gravity, heat flow, and earth magnetism.

The evidence from geophysics suggests that Earth is divided into three major compositional layers - the crust on Earth's surface, the rocky mantle beneath the crust, and the metallic core at the center of Earth. The study of plate tectonics has shown that the crust and uppermost mantle can be mechanically divided into the brittle lithosphere and the ductile or plastic asthenosphere.

You will learn in this chapter how gravity measurements can indicate where regions of the crust and upper mantle are being held up or held down out of their natural position of equilibrium. We will discuss Earth's magnetic field and its history of reversals. We will show how magnetic anomalies can indicate hidden ore and geologic structures. The chapter closes with a discussion of the distribution and loss of Earth's heat.

Expanded Readings From Chapter 17

Mantle Xenoliths - A Peek at the Deep

Learning Objectives

1. Seismic reflection is the return of some energy to the earth's surface from rock boundaries. Seismic refraction is the bending of waves as they pass from one rock layer to another. Both provide information about the earth's internal layers.

2. The earth's interior contains three main zones: thin crust, thick mantle, central core. P waves pass through oceanic crust at 7 km/sec, indicating that it is mafic, composed of basalt (upper portion) or gabbro (lower portion). P waves travel through continental crust at 6 km/sec indicating that it is felsic, or"granitic." Crust is thin (7 km) under ocean basins, thick (30-50 km) under continents, and thickest (up to 70 km) under the roots of young mountain ranges. Seismic waves speed-up at the Mohorovicic discontinuity or Moho, that separates the crust and mantle.

3. The mantle seems to be composed of ultramafic rocks because P waves travel through it at 8 km/sec. The lithosphere combines the crust and uppermost mantle and forms the tectonic plates. The asthenosphere extends from the lithosphere to 200 km as a low seismic velocity zone indicating rocks close to their melting point. It may generate magmas and lubricates the movement of lithospheric plates. A chemical change at 670 km, also the limit to earthquakes, separates the upper and lower mantle.

4. P wave refraction (producing the P wave shadow zone) provides the size and shape of the core. The S wave shadow zone indicates that the outer core is liquid, and P wave refraction indicates a solid inner core.

5. Earth's density is 5.5 gm/cm³. Data from density studies (core must be very dense since the crust and mantle are not), meteorites, and the magnetic field indicate that the core is a mixture of mostly iron, with some nickel, and lighter elements.

6. The core-mantle boundary is marked by increased seismic velocity (the D" layer), density, and temperature. The undulating border of the boundary is the ultra-low velocity zone (ULVZ) that seems to represent either partial melting at the base of the mantle or a chemical reaction between the core and mantle. Convection occurs at the core-mantle boundary producing mantle plumes. Seismic tomography and isotopic studies suggesting that hot spot mantle plumes feeding Hawaii have a core signature.

7. Isostasy is the equilibrium between crustal blocks"floating" on the upper mantle. Mountain ranges have a root extending into the mantle to provide isostatic balance. Isostatic adjustment involves rising or sinking of crustal blocks and the depth of equal pressure balances the blocks. Plastic flow in the asthenosphere accommodates isostatic adjustment. Crustal rebound is isostatic adjustment after continental ice sheet removal.

8. Positive gravity anomalies, measured by a gravity meter, indicate areas of high density rock (such as ore bodies), and regions above isostatic equilibrium. Negative gravity anomalies indicate areas of low density rock, and regions below isostatic equilibrium, such as ocean trenches.

9. The earth's magnetic field is bipolar and inclined 11 1/2 degrees to the axis of rotation. It is thought to be generated by convection within the core. Paleomagnetic studies of stacked lava flows indicate periods of normal and reversed polarity during the earth's history. Reversals may be caused by changes in convection and could account for extinctions. Positive magnetic anomalies, measured by a magnetometer, may indicate ore bodies, intrusions, or basement highs. Negative magnetic anomalies indicate thick sedimentary fill over grabens.

10. The geothermal gradient is 25 degrees C/km through the upper crust, but decreases sharply to about 1 degree C/km below that point. The core-mantle boundary is about 3800 degrees C, increasing to 6300 degrees C at the outer-inner core boundary, and 6400 degrees C at the center of the earth (hotter than the surface of the sun).

11. The gradual loss of heat through the earth's surface is heat flow. That heat may be from the earth's formation or the result of radioactive decay, and it is the same between continents and the sea floor. High heat flow indicates rising mantle rocks due to convection.

Related Readings

Bloxham, J., and D. Gubbins. 1989. The Evolution of the Earth's Magnetic Field. Scientific American 261(6): 68-75.

Bolt, B. A. 1982. Inside the Earth. New York: W. H. Freeman.

Burchfiel, B. C. 1983. The Continental Crust. Scientific American 249(3): 86-98.

Carlowicz, M. 1996. Spin Control. Earth 12(21): 62-63.

Carrigan, C. R., and D. Gubbins. 1979. The Source of the Earth's Magnetic Field. Scientific American 240(2): 118-30.

Fowler, C. M. R. 1990. The Solid Earth. New York: Cambridge University Press.

Jacobs, J. A. 1992. The Deep Interior of the Earth. New York: Chapman and Hall.

Jeanloz, R. 1990. The Nature of the Earth's Core. Annual Review of Earth and Planetary Sciences 18:357-86.

Jeanloz, R., and T. Lay. 1993. The Core-Mantle Boundary. Scientific American 268(5): 48-55.

Kerr, R. A. 1991. Do Plumes Stir Earth's Entire Mantle? Science vol. 275:613-15.

McKenzie, D. P. 1983. The Earth's Mantle. Scientific American 249(3): 50-62.

Wyession, M. E. 1996. Journey to the Center of the Earth. Earth 12:46-49.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 17:

Boxed Readings