CHAPTER 4 - THE EARTH'S INTERIOR
Overview
Mines, oil wells and scientific wells have penetrated no more of 12 km of the
crust. Samples of mantle rocks have been brought to the surface by basalt
flows, kimberlite pipes, and at some convergent plate boundaries. This is a
minute sample, and most of what is know about the earth's interior is based
on studies (the branch of geology called geophysics) of seismic waves, the
magnetic field, gravity and heat. Seismic reflection and refraction reveal
boundaries of rock layers comprising the three main zones of the earth's
interior: crust, mantle and core.
Seismic P waves travel faster through oceanic crust (7 km/sec) than
continental crust (6 km/sec). Samples suggest that the upper oceanic crust is
basalt, and the lower part is gabbro (mafic). Continental crust is called
"granitic" (felsic), but includes many different rock types. Continental crust is
thicker than oceanic crust and thickest under mountain ranges that have a
root. The Mohorovicic discontinuity separates the crust and mantle (Figure
4.6).
The mantle is thought to be composed of ultramafic rocks. The crust and
uppermost mantle form the lithosphere, about 70 km thick beneath the oceans
and as much as 250 km thick under the continents. The asthenosphere
extends from the lithosphere to 200 km and is a low velocity seismic zone.
Controversy exists concerning the thickness and even presence of
asthenosphere beneath continents. Plates of lithosphere move over this layer
capable of plastic flow and near its melting point. The upper mantle-lower
mantle boundary lies at 670 km, the limit of earthquakes, and may represent
a chemical as well as physical change.
The S wave shadow zone indicates that the outer core is liquid (Figure 4.9).
Refraction at the P wave shadow zone (Figure 4.8) allows calculation of the
size and shape of the core, and suggests that the inner core is solid. Density
studies (earth's density is 5.5 gm/cm3), meteorites (Box 1.8), and the
magnetic field indicate that the core is a mixture of mostly iron, with nickel
and a small amount of lighter elements. 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, Figure
4.10) 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 developing mantle plumes that feed hot spots like
Hawaii.
Isostasy is the equilibrium between crustal blocks "floating" on the mantle that
is controlled by the depth of equal pressure. Mountains are "rooted"
supporting their height, and isostatic adjustment occurs through the rising or
sinking of crustal blocks by plastic mantle flow. Gravity meters measure
gravitational attraction and can be used to find ore bodies and determine
whether regions are in isostatic equilibrium. Positive gravity anomalies indicate
a region higher than isostatic equilibrium, while small ones may indicate ore
bodies. Negative gravity anomalies indicate a region lower than isostatic
equilibrium.
The earth's magnetic field is formed by core convection. It is a dipole inclined
11 1/2 degrees to the axis of rotation and measured with a magnetometer.
Studies of paleomagnetism in stacked lava flows indicate intervals of normal
and reversed polarity occurring through geologic time. Positive magnetic
anomalies may indicate ore bodies, intrusions, or areas of high basement
(Figure 4.25). Negative magnetic anomalies indicate grabens and thick
sedimentary fill.
The geothermal gradient is 25 degrees C per kilometer in the outer crust, but
decreases sharply with depth in the earth's interior. The temperature of the
earth's center is approximately 6400 degrees C (hotter than the surface of the
sun!)(N.B. still stated as 6900 degrees C in chapter summary), decreasing to
6300 degrees C at the inner core-outer core boundary, and 3800 degrees C at
the core-mantle boundary. Heat flow is the loss of heat through the earth's
surface. High heat flow over an extensive area may indicate rising mantle
rock. Average heat flow from continents is about the same as the sea floor.
Regional patterns of high and low heat flow on the ocean floor can be
explained by convection of mantle rock.
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,
which 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 outerinner 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.
Boxes
4.1 - IN GREATER DEPTH - DEEP DRILLING ON CONTINENTS - Most drilling
on the continents has been in search of oil from sedimentary rocks occupying
deep basins, and little of the 40 km thick continental crust has ever been
penetrated. Russian geologists drilled the world's deepest (12 km) borehole
into the Precambrian basement on the Kola Peninsula. The KTB second
deepest borehole (10 km) was drilled in southeastern Germany. Drilling of
both holes advanced technology. The Kola hole also demonstrated that seismic
surveys of continental crust are being misinterpreted as to the presumed
location of"basaltic" crust. Unexpectedly, both holes encountered both gas and
mineralized waters circulating through open fractures in spite of the high
bottom hole temperatures and pressures. It appears that there is more to
learn about the characteristics of continental crust.
4.2 IN GREATER DEPTH – CANADIAN LITHOPROBE PROJECT – The
Lithoprobe Project is an enormous scientific effort which aims to increase
understanding of major geological terranes in the Canadian Shield and the
boundaries that separate them. One of the techniques used in this project is
seismic reflection where vibrations generated by huge vibroseis trucks
(‘dancing elephants’) are sent into the ground and are reflected when they
pass through rocks with different physical properties. Geophones record these
seismic reflections when they return to the Earth’s surface. Computer analysis
of the seismic reflection data recorded by the geophones allows complex
images of the internal structure of the crust to be drawn. By improving our
understanding of the Canadian Shield the Lithoprobe Project will help
geoscientists unravel the long history of development of the North American
continent.
4.3 - IN GREATER DEPTH - A CAT SCAN OF THE MANTLE - Hot rocks within
the earth's interior slow seismic waves, while cold rocks speed up seismic
waves. Seismic tomography, similar to the familiar CAT (computerized axial
tomography) scan in medicine, can be used to study these changes in seismic
velocities at depth within the earth to provide insight into its character.
Velocities at 100 km display a pattern consistent with sea floor spreading: hot
under the ridges, cold for the rest of the sea floor and continents. At 300 km,
the continents are still cold, indicating very deep roots. Some ridges are hot at
100 km but cold at 300 km, while the reverse is true in others. Mantle plumes
originate at various depths in the mantle, and the Hawaiian hot spot contains
material from the crust, mantle, and core. Depth of subduction is also variable.
Some slabs extend to the core-mantle boundary, while others stop at the 670
km mantle boundary. Density may be the controlling factor. Older, denser
plates may be capable of deeper subduction (core-mantle boundary), while
younger, less dense plates are limited to the 670 km boundary layer.
4.4 IN GREATER DEPTH – DIAMONDS – A WINDOW INTO THE MANTLE –
Diamonds are found in kimberlites pipes, carrot-shaped bodies of igneous rock
found only on the oldest parts of continents. The carbon that forms diamonds
is thought to have originated from carbon-bearing rocks found on ocean plates
that were subducted at collisional margins. Extreme heat and pressure in the
mantle below continents transformed the carbon into diamond and subsequent
eruption of kimberlites through the vents of kimberlite pipes brought the
diamonds to the Earth’s surface. Diamonds are thought to have formed early
in Earth history but were trapped in the mantle for long time periods and
erupted during episodes of continental breakup. Diamonds are found on most
ancient continents and are now being mined in the Northwest Territories of
Canada.
4.5 - IN GREATER DEPTH - EARTH'S SPINNING INNER CORE - A computer
model of outer core convection by circulating metallic fluids has been able to
simulate a magnetic field similar to Earth's. The model predicts that the inner
core spins faster (one full lap every 150 years) than the rest of the Earth
generating the magnetic field and causing reversals. Analyses of seismic
waves indicate a preferred pathway of slightly faster arrival times in the inner
core that has recently (1967-1995) changed its orientation 10o to the spin
axis. This change in pathway for seismic waves supports the interpretation of a
faster rotation, but further work on seismic waves will be needed for
confirmation.
Short Discussion/Essay
1. What seismic waves would a seismograph located at 102o record for an
earthquake generated at the north pole?
2. Why is the mantle thought to be ultramafic?
3. Why is continental crust so much thicker than oceanic crust?
4. What is the difference between a solid and liquid, i.e. if the asthenosphere
is capable of flowing, why isn't it a liquid?
5. What is the problem with trying to relate periods of extinction to magnetic
reversals?
Longer Discussion/Essay
1. Briefly review how geologists have determined the thickness of the crust,
mantle, outer core and inner core using seismic waves.
2. Explain why geophysicists believe that the Earth has a solid inner core.
3. Why is the presence of a solid inner core necessary to explain the Earth's
magnetic field?
4. How does isostatic adjustment explain why the Rocky Mountains are
tectonically active, while the Appalachian Mountains are not?
5. Why do the Earth's density measurements require an iron-nickel core?
Antarctic Meteorite Thin Sections
NSF, the Smithsonian Institution, and NASA have prepared a suite of 12
polished thin sections of representative meteorites recovered from Antarctica
(described in Boxes 12.4 - Mars on a Glacier and 17.1 - Meteorites). The thin
sections are accompanied by booklets that describe the thin sections and
introduce meteorite science. Applications for the package should include name,
institution, department, institutional address, telephone, a brief statement of
planned use and desired dates of loan. Applications should be sent to:
Curator
Mail Code SN2
Lyndon B. Johnson Space Center
Houston, TX 77058
There is no cost for this loan other than return by registered mail.
Selected Readings
The September, 1983 issue of Scientific American was devoted to"The
Dynamic Earth." While a little dated, it is a good general reference and also
provides a look at the progress of geology and geophysics in this area over the
subsequent decade. It contains the following articles:
Siever, R. - The dynamic earth
Jeanloz, R. - The earth's core
McKenzie, D.P. - The earth's mantle
Francheteau, J. - The oceanic crust
Burchfiel, B.C. - The continental crust
Bell, R.E. 1998."Gravity gradiometry," Scientific American 278: 74-79.
Jacobs, J.A. 1992. The Deep Interior of the Earth. New York: Academic Press.
Jeanloz, R. and Lay, T. 1993."The core-mantle boundary," Scientific American
268: 48-55.
Lowman, P.D., Jr. 1996."Twelve key 20th century discoveries in the
geosciences," Journal of Geoscience Education 44: 485- 502.
Moores, E.M. and Twiss, R.J., 1995. Tectonics. W.H. Freeman and Co.
310pp. One of the best detailed treatments of how the Earth works.
Newsom, H.E. and Jones, J.H., eds. 1990. Origin of the Earth. New
York:Oxford University Press.
Olson, P., Silver, P.G. and Carlson, R.W. 1990."The large-scale structure of
convection of the Earth's mantle," Nature 344: 209-215.
Poirier, Jean-Paul. 1991. Introduction to the Physics of the Earth's Interior.
Cambridge: Cambridge University Press.
Wysession, M. 1995."The inner workings of the earth," American Scientist
83(2): 134-147