What is the interior of the earth?

Contributed by:
The major structural components in the earth that are separated by sharp discontinuities are the crust, the mantle,
and the core. The crust forms a very thin surface skin, the mantle is a thick shell that extends half the radius down
into the earth, and the core occupies the central part.
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3. The Interior of the Earth
An Elementary Description
By Eugene C. Robertson
Washington 1966
4. United States Department of the Interior
CECIL D. ANDRUS, Secretary
Geological Survey
H. William Menard, Director
First printing 1966
Second printing 1967
Third printing 1969
Fourth printing 1970
Fifth printing 1972
Sixth printing 1976
Seventh printing 1980
Free on application to Branch of Distribution, U.S. Geological Survey
1200 South Eads Street, Arlington, VA 22202
Abstract ......................................................... 1
Introduction ..................................................... 1
Surface observations .............................................. 1
Openings underground in various rocks .......................... 2
Diamond pipes and salt domes .................................. 3
The crust ............................................... f ........ 4
Earthquakes and the earth's crust ............................... 4
Oceanic and continental crust .................................. 5
The mantle ...................................................... 7
The core ......................................................... 8
Earth and moon .................................................. 9
Questions and answers ............................................. 9
Suggested reading ................................................ 10
COVER. The interior of the earth.
FIGURES 1-6. Sketches of
1. Man at the edge of a gravel pit ................... 2
2. Gravel pit compared with Bat Cave
in Carlsbad Caverns ........................... 2
3. Diamond pipe .................................. 3
4. Drill hole near salt dome ......................... 4
5. Oceanic crust at Hawaii ......................... 5
6. Continental crust under California ................ 6
7. Generalized geologic columns .......................... 6
8. Sketch of upper mantle and crust between
Hawaii and California ............................ 7
9. Cross section of the whole earth, showing the
paths of some earthquake waves ................... 8
10. Graph of earthquake compressional wave
velocity and density in the earth ................... 8
11. Diagram of earth and moon system ..................... 9
By Eugene C. Robertson
ABSTRACT Man has actually looked into the earth in deep
Evidence on the structure and composition of the earth's mines and drill holes only a very small distance
interior comes from (1) observations of surface rocks, (2) about 5 miles of the 4,000-mile distance to the
geophysical data from earthquakes, flow of heat from the earth's center. Furthermore, man will likely never
interior, the magnetic field, and gravity, (3) laboratory ex-
periments on surface rocks and minerals, and (4) compari-
be able to make a hole into the deep interior, so
son of the earth with other planets, the sun, stars, and what we learn about the interior has to be from
meteorites. indirect evidence. At present, this evidence con-
The major structural components in the earth that are sists of (1) direct observation of rocks at the sur-
separated by sharp discontinuities are the crust, the mantle, face, (2) secondary observations based on geo-
and the core. The crust forms a very thin surface skin, the physical phenomena (including waves through the
mantle is a thick shell that extends half the radius down
into the earth, and the core occupies the central part. The
earth from earthquakes and explosive sources,
crust and upper mantle are known to vary in physical and planetary motions of the earth, flow of heat from
chemical characteristics, both horizontally and vertically; the the interior, the magnetic field, and gravitational
lower mantle and core are generally assumed to be uniform attraction), (3) laboratory experiments on surface
because their diagnostic geophysical phenomena are masked rocks and minerals, and (4) comparison of the
by the physical-properties of the upper layers.
earth with the other planets, the sun, stars, and
meteorites, which may be fragments of a disinte-
grated former planet. Our present understanding
INTRODUCTION of the structural features and the composition of
the earth is obtained from all these sources.
Idealized, the inside of the earth can be de-
In the following discussion of the interior of the
scribed as being made up of layers, in a sequence
earth, we will use facts from these sources to con-
of concentric shells as illustrated on the cover. The
sider layering, the existence of openings, and the
fact that the earth is not a homogeneous, structure-
physical state and composition of the rocks and
less body has been realized since the time of Isaac
minerals presumed to occur in the earth. Our esti-
Newton, who noted in a discussion of the planets
mates of composition are not much better than con-
that the average density of the earth is five to six
times that of water. The average density of the jectures at present, but they are reasonably con-
earth actually is 5.5 g per em3 (grams per cubic sistent with geophysical observations.
centimeter), and as the average density of surface
rocks is only about 2.8 g per em3, there must be a
large mass of material of higher density inside the SURFACE OBSERVATIONS
earth. We infer from this and other data that
there is a heavy central part in the earth. In fact, Let us start from the earth's surface with fea-
during the last 60 years, geophysieists and geolo- tures that we know and progress deeper and
gists have estimated with increasing confidence the deeper inside the earth, using a series of scaled
thickness and character of each of the successive illustrations to comprehend better the sizes of
layers in the earth, including variations within structures in the earth. First, we look at the rocks
some layers. under foot at a familiar scale and then at diagram-
8. 25 Feet
0 0.005 Mile
FIGURE 1. Man at the edge of a gravel pit.
250 Feet
0 0.05 Mile
FIGURE 2. Gravel pit compared with Bat Cave in Carlsbad Caverns. Contiguous caves not shown are found to 1,100 feet below the
surface; the Big Room at an 830-foot depth is half a mile long and over 250 feet high.
matic cross sections of the earth; each structural steam shovel are shown reduced 10 times in size
feature is reduced in size 10 times from one from figure 1, and a room in Carlsbad Caverns in
drawing to the next and compared with a new fea- New Mexico is sketched at this new, 10-times-
ture : 1 inch in the first figure will equal 10 million smaller scale.
inches in the last. In all drawings, the vertical
scale is equal to the horizontal scale, with no verti- Openings Underground in Various Rocks
cal exaggeration. The existence of such large rooms as in the
In the first drawing, figure 1, let us compare the Carlsbad Caverns brings up the question of how
length of a man's legs, about 2}/2 feet, with the large an opening at a given place underground
depth of a gravel pit, about 25 feet; one length is can be supported by the surrounding rock. Sand
10 times the other. In figure 2 the gravel pit and and gravel like that shown in the pit in figure 1
9. are too loose and unconsolidated to support a very heated and which are now also strong and solid.
large hole, and this is true of the mud or sand Igneous or metamorphic rock can support openings
deposited as sediment at the bottom of a sea, lake, even at 1-mile depth, as in mine workings; how-
or river. However, mud or sand, buried beneath ever, in very deep mines like the South African
the earth's surface by a thick deposit of similar gold mines at 2 miles down, spalling of the walls
material may be consolidated by pressure, heat, and cave-ins can cause serious problems.
and chemical action into a sedimentary rock such Immense chambers are sometimes postulated to
as shale, sandstone, or limestone which would sup- exist very deep in the earth, but actually a 10-foot
port a large opening. opening would not stand unsupported even in the
At the relatively shallow depth of a thousand strongest rock at a depth of 10 miles or more. The
feet, huge natural caves, hundreds of feet in ex- pressure at 10 miles exceeds the strength of the
tent, larger than the one illustrated in figure 2, can rock for a 10-foot span, and so it is merely specula-
be dissolved out of limestone by percolating tion to predict larger openings at greater depths.
ground water, and they will stand open because
most limestone is strong. The weight of the rock
above is not great enough to collapse the caves. On Diamond Pipes and Salt Domes
the other hand, less strong sedimentary rocks, such
as shale or poorly cemented sandstone, would not The underground diamond mines of South Afri-
support smaller openings (say, 40 ft across) under ca and Siberia do not extend to as great a depth as
a load of a thousand feet of overlying rock. the gold mines, but the diamond "pipes" them-
Just as in limestone caves, large openings at a selves (fig. 3), in which the mining is done, extend
1,000-foot depth can also stand in unfractured ig- as tubular conduits (hence the name "pipe") deep
neous rocks, which are composed of hard silicate into the earth. This geologic feature is funnel
minerals and which were emplaced in a molten shaped, decreasing in diameter to a narrow neck at
state and then cooled to form strong, massive bod- a depth of a few thousand feet. We know that the
ies. Similar openings can stand in unfractured met- neck extends beneath the surface about 100 miles
amorphic rocks, which are sedimentary or igneous because laboratory experiments show that high
rocks that have been very highly compressed and pressure and temperature, such as would be found
2500 Feet
0 0.5 Mile
FIGURE 3. Diamond pipe, like the Kimberley in South Africa, compared with Bat Cave, Carlsbad Caverns, N. Mex.
10. Sedimentary
5 miles
FIGURE 4. Drill hole, 25,000 feel deep, near a salt dome, like the domes found in Texas, Louisiana, Mississippi, and the Gulf of Mexico.
in the earth at such a depth, are required to form THE CRUST
diamonds. Disseminated through the rock in the
pipe are not only the sparse diamonds (about 1 Earthquakes and the Earth's Crust
carat per ton), but also sporadic inclusions of eclo-
gite and peridotite, the two dark-colored dense ig- When we remember that a great thickness of
neous rocks which we think make up the outer sedimentary rocks was uplifted 6 miles above sea
mantle of the earth. The mantle and its constitu- level to form the Himalaya Mountains, we are
ents are discussed below. made aware that tremendous forces are available
In the next drawing (fig, 4), at a scale 10 times inside the earth to move its surface up and down
smaller than the previous figure, we compare the and laterally. We presume that local sources of
diamond pipe with a salt dome, which has an up- heat deep inside the earth provide the energy to
side-down funnel shape and is filled with salt. An build mountains and to lower basins. The surface
adjacent drill hole is sketched in figure 4 because expressions of the release of mechanical energy of
oil-well holes, as much as 5 miles deep, are com- mountain building and other earth movements are
monly drilled near salt domes to tap the oil found earthquakes.
in porous sandstones in the upturned beds around Geologists, in mapping the rocks at the earth's
the domes, as for example, in the deep basin of surface, locate the faults (fractures) and folds
sedimentary rock near the Gulf of Mexico. (plastic bends) which are evidence of the mechani-
A salt dome is presumed to form because the cal deformation of rocks in mountain building.
weight of 1 to 8 miles of rock on the thick horizon- Sudden displacement of rocks on either side of a
tal parent salt bed causes the lower density rela- fault produces a shaking of the earth called an
tively plastic salt to force its way upward through earthquake; earth tremors are also produced by
a crack in the rocks, sometimes to the surface. The motion of the moken rock under volcanoes. The
intruding salt enlarges the crack and turns up the
association of faults with earthquakes is a matter
adjacent sedimentary beds. The salt rises until the
of historical record for the San Andreas fault
thickness and strength of the overlying rock pre-
along the coast of California and for faults in
vent further movement.
At depths of 5 to 10 miles only small openings Chile, in Turkey, and elsewhere.
(perhaps 0.1 in.) can stand in unconsolidated sedi- The violent breaking of the rock at a fault sets
mentary rocks, such as those in the Gulf of Mexico up vibrations, and waves of motion propagate
basin, and only slightly bigger openings (perhaps through the whole earth, much like the waves pro-
1 in.) can persist in massive hard rocks. duced in a pond when a stone is dropped into it.
11. Hawaii Island
,^^^*r^^ CRUST..-:.:::;:>
FIGURE 5. Oceanic crust at Hawaii Island. The proposed site of the 34,000-foot Mohole is 150 miles north of Hawaii
although shown closer in the figure.
Earthquake waves transmitted through the earth tical and horizontal scales are equal, the craggy
are of two types: (1) compressional waves, P, in slopes of Mount Whitney in California and of
which motion of solid particles is back and forth, Mauna Loa on Hawaii Island are not so impressive
parallel to the direction of travel, and (2) shear when compared with the thickness and the lateral
waves, S, in which particle motion is across, trans- extent of the earth's crust below. Relatively great
verse to the direction of travel. thicknesses of crust, like the 34-mile thickness
Geophysicists study the velocity and the paths under the Sierra Nevada, are common under
of earthquake waves to learn about the earth's in- mountain ranges and are very much more than the
terior. One of them, a Yugoslav named Mohorovi- 4-mile thickness of crust (covered by 2y2 miles of
cic (pronounced Mo-ho-ro-vee-chich), in 1909 dis- water) at the Mohole site.
covered a discontinuity between what we now call We believe that the continental crust is largely
the crust and the mantle by observing a sharp in- made up of (1) dense light-colored igneous rocks,
crease in velocity of earthquake waves going from such as granite or quartz diorite, in the upper part
crust to mantle. The discontinuity has been named and (2) basalt, a dark and slightly denser igneous
after him; it is sometimes called the M discontin- rock (commonly erupted from volcanoes), in the
uity, or more colloquially the "Moho"; a hole pro- lower part. The oceanic crust appears to be com-
posed to be drilled through the discontinuity has posed almost entirely of basalt. The relative'thick-
been dubbed the "Mohole." nesses of the probable rocks occurring in four rep-
resentative oceanic and continental locations (figs.
5 and 6) are given in figure 7. Notice that the
Oceanic and Continental Crust composition of the crust as well as its thickness
varies laterally; the crust is not just a homogene-
The crust of the earth ranges in thickness from ous, flat layer.
about 5 miles at some places under the oceans to The acclaimed purpose for drilling the Mohole is
about 30 miles under high mountains; it forms a to get a sample of rock from the mantle. It is wide-
thin skin around the earth as shown on the cover. ly believed that the discontinuity in earthquake
Below is the thick shell of the mantle. Oceanic and wave velocity at the Moho is due to a change of
continental crustal thicknesses and topography are rock type, from basalt in the lower part of the
shown in the cross sections of figures 5 and 6, plot- crust to the more dense eclogite or peridotite in the
ted at a scale that is 10 times smaller than the mantle. From the deep-sea sediments, from other
previous figure. In these figures in which the ver- crustal rocks, and from the mantle rock obtained
12. 1
Continental i Coast Ranges | Great Valley j Sierra Nevada
Shelf I l of California I
1 I
i ! 1
Mount Whitney
Sea level i ,_ ^ _ ^ _ _.- .. .. LJ n.-^-^sr
50 Miles
FIGURE 6. Continental crust under California, showing a 21,500-foot drill hole for comparison with drill holes in figures 4 and 5.
= '=
E 3=
Mauna Loa Mohole site Sacramento Mount Whitney
Ocean water
LS Loose sediment 25
CS Consolidated sediment
G Granite
QD Quartz diorite
B Basalt 30
E-P Eclogite or peridotite
M Mohorovicic' discontinuity
FIGURE 7. Generalized geologic columns, showing rock types at two places each in figures 5 and 6.
13. FIGURE 8. Upper mantle and crust between Hawaii and California. The crust is very thin relative to the rest of the earth.
from the Mohole, we will learn about the origin and these layers in the mantle are gradational; and al-
history of life, of the oceans, and of the earth it- though their depths are widely accepted, the de-
self. tails of their composition and structure are not
THE AAANTLE The possible openings in the mantle must be
very small. We know from the occurrence of deep-
Earthquake wave velocities are faster in the up- focus earthquakes (as deep as 450 miles) that
per mantle than in the crust, but the velocities stresses may build up almost to the bottom of the
decrease a little at a depth of about 100 miles in transition zone, and this indicates that the rock
what is called the low-velocity layer (fig. 8). Al- there may have some strength. However, it is not
though it is shown with constant thickness in the likely that openings larger than 0.01 inch could
cover drawing and in figures 8 and 9, the low- stand in the low-velocity layer, owing to the weak-
velocity layer may not extend uniformly around the ness of rock near its melting point, or could stand
earth; there is conflicting evidence for its existence in the transition zone or lower mantle, owing to
under the central parts of the continents and the very high pressure in relation to rock strength.
oceans. In fact, it may be restricted to the moun- The composition of the mantle has been conjec-
tainous regions, especially along continental mar- tured from studies of volcanic lava, diamond
gins, where volcanic activity and earthquakes oc- pipes, and meteorites and from experiments on
cur. Considering the probable rise of temperature minerals and rocks. These studies indicate that the
with depth in the earth, it is feasible for the tem- upper few hundred miles may be composed of eclo-
perature of the rock in this layer to be near the gite and peridotite, which are composed mostly of
melting point.; this would explain the reduction in iron and magnesium silicate minerals and some
the wave velocities and would provide a source complex calcium, sodium, and aluminum silicate
for the lava erupted from volcanoes or extruded minerals. In the transition zone, denser forms of
in diamond pipes. these minerals are presumed to exist. In the lower
Below, in the transition zone, the earthquake mantle, because of the very high pressure, only the
wave velocities increase markedly again. They tap- simple oxides of iron, magnesium, and silicon are
er off as the zone grades into the lower mantle but thought to be present. We are far from being sure
continue to rise gradually to a maximum at the of the composition of the mantle, which undoubt-
core boundary. (See fig. 10.) The boundaries of edly varies laterally as well as vertically.
14. THE CORE core is presumed to be liquid because it does not
transmit shear waves (8), and because it sharply
Below the mantle is the earth's core, discovered reduces the velocity of compressional waves (P).
by R. D. Oldham in 1906 from a study of earth- (The P, and '8 waves are the two earthquake waves
quake records. The core itself is divided into an that go through the earth.) The inner core, discov-
outer part and an inner part, as shown at a 10 ered by its higher P-wave velocity in 1936 by Miss
times further reduced scale in figure 9. The outer I. Lehmann, is considered to be solid. In figure 9
the core looks about as big as the mantle, but ac-
Focus of earthquake
tually the core occupies only 15 percent of the
earth's volume, whereas the mantle occupies 84
percent. The crust occupies the remaining 1 per-
As shown in figure 9, the ray paths of earth-
quake waves from the focus, where the fault dis-
placement or other disturbance initiates the earth-
quake, are curved in passing through the earth;
the curvature is a refraction due to the increase in
PP velocity with depth. Earthquake records, called
PKP Transition zone seismograms, require great care in interpretation
Low-velocity layer to disentangle the various waves; many more wave
PPP Crust and upper mantle paths are observed than are shown in figure 9.
At the outer boundary of the core, there is a
5000 Miles
sharp discontinuity in both the P-wave velocity
and the density, as shown in figure 10. These very
FIGURE 9. Cross section of the whole earth, showing the paths of
some earthquake waves. Compressional waves (P, solid lines) are striking changes in properties occur in a narrow
refracted sha-ily by the core, as shown by the PKP trajectory.
Shear waves (5, dashed lines), as such, end at the core, although border zone between the mantle and core, perhaps
they may be converted to P waves, traverse the core as a com- less than 10 miles thick. It is certain that there is a
pressional wave (K), and emerge in the mantle again as P and S
waves, that is, SKP and SKS. The waves may be reflected back marked compositional difference between the man-
into the earth at the surface and be picked up further on, as in
trajectories PP, PPP, and SS. tle and core.
() 2 4 6 8 10 12 14 () 2 4 6 8 10 12
9fU° i i i 1 . i i I i 1 i I i
\ \ Transition zone ^^~~~^~>-^^ ^\ Surface
570 \ \ - - - - ^Nr
\ v Low-velocity layer \
* Crust and upper mantle \ Lower mantle \
Outer core \
jofin ~A___ Inner core Center v \
FIGURE 10. Variation of earthquake P-wave velocity and of density with depth in the earth, illustrating the short decrease in P in the
low-velocity layer and the sharp rise in P in the transition zone, as well as the abrupt changes in P and in density at the core-mantle'
15. By analyzing seismograms for the variation of P
and 8 wave velocities with depth and for the free
oscillations of the earth, by observing the plane-
tary motions of the earth, and by knowing the
earth's average density (5.5 g per cm3 ), the den-
sity of the core has been determined to be about 11
g per cm3. Iron is the most likely constituent of the
core for the following reasons: Iron has about the
right density; it has the right P-wave velocity; its
abundance in the earth would be about the same as
in the sun and stars; it occurs in large proportions
in meteorites, which are presumed to be fragments
of a disintegrated planet, analogous to the earth; it
would be properly molten at the core temperature
and pressure (8,000°F and 3 million atmospheres);
and even though molten, iron is a satisfactory mate-
rial to account for the earth's magnetic field. The
inner core is probably pure iron.
A view of the earth and moon at about average
separation is shown in figure 11, drawn at the
final, smallest scale; the size of the moon here is
about equal to the size of the man's head in figure
1, although the moon is actually 10 million times
greater in diameter. The diameter of the moon is
just half that of the earth's core and one-fourth
that of the whole earth. When next you look at the
moon, these proportions will enable you to visual-
ize the size of the earth's core and of the earth
itself at a distance of 240,000 miles.
Judging from the moon's average density (3.3 g
per cm3 ), we can hypothesize that its composition
is much like the earth's crust and upper mantle.
Recent data from American and Soviet satellites
show that the moon's magnetic field is very small,
which indicates that there is only a small amount
of iron in the moon; furthermore, the iron is prob-
ably dispersed and not in a central core, a conclu-
sion based on the motions of the moon. Samples of
the moon's surface brought back by astronauts will
not only give us a first estimate of the composition
of the moon but may also tell us something about
the origin of the solar system and about the interi-
or of the earth as well.
Geologists, geochemists, and geophysicists have
FIGURE IK -Earth and moon system, showing the earth's core for
contributed to our understanding of the earth's comparison.
16. internal structure and composition, but questions and crust, and they map the folds and faults in
about the earth's interior are still very much more rocks the external effects, demonstrated by every
numerous than answers: What are the causes and earthquake, of the mechanical forces deep in the
mechanisms of earthquakes, mountain building, earth. In Survey laboratories geophysicists and
and subsidence of large basins? What is the energy geochemists study the properties and composition
source for these phenomena? Why do volcanoes of rocks and minerals under high pressure and
erupt, and what is their source? How do large temperature, simulating the conditions inside the
granite bodies melt and move into place? How old earth. The interior of the earth is a fascinating
is the earth? What have the structure and compo- topic for further study.
sition of the earth and its temperature and mag-
netism been like throughout geologic time? Why is Earth layers on layers
the crust thinner under the oceans than under the Up quaking, round ringing
continents? Deep secrets revealing
Geological Survey scientists are actively search- To us unawares.
ing for answers by measuring the velocities of Anon. (ca. 19 5)
earthquake waves in the mantle and crust, by as-
certaining the variation of temperature with depth
and the amount of heat flowing from the earth's SUGGESTED READING
interior, by determining the positions of the
Hurley, P. M., 1959, How old is the earth f: Garden City,
earth's magnetic poles millions of years ago as re- N.Y., Doubleday and Co., Inc., 160 p.
corded in the rocks and compared with the present Jacobs, J. A., 1963, The earth's core and geomagnetism:
magnetic field, and by estimating the thickness of New York, Macmillan Co., 137 p.
the crust under mountains and plains from the Scientific American, 1957, The planet earth: New York,
gravitational attraction. Survey geologists observe Simon and Schuster, Inc., 168 p.
the occurrence and character of volcanic rocks to White, J. F., ed., 1962, Study of the earth: Englewood
learn about the chemical processes of the mantle Cliffs, N.J., Prentice-Hall Internat., Inc., 408 p.