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GEOLOGICAL SURVEY CIRCULAR 532 /

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INTERIOR 

OF THE EARTH

/ AN El/EMEI^TARY xDESCRrPNTION

The Interior of the Earth

An Elementary Description

By Eugene C. Robertson

GEOLOGICAL SURVEY CIRCULAR 532

Washington 1966
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
CONTENTS

Page

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

ILLUSTRATIONS

COVER. The interior of the earth.

Page

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

iii

THE INTERIOR OF THE EARTH

By Eugene C. Robertson

ABSTRACT

Evidence on the structure and composition of the earth's 

interior comes from (1) observations of surface rocks, (2) 

geophysical data from earthquakes, flow of heat from the 

interior, the magnetic field, and gravity, (3) laboratory experiments on surface rocks and minerals, and (4) comparison of the earth with other planets, the sun, stars, and 

meteorites.

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. The 

crust and upper mantle are known to vary in physical and 

chemical characteristics, both horizontally and vertically; the 

lower mantle and core are generally assumed to be uniform 

because their diagnostic geophysical phenomena are masked 

by the physical-properties of the upper layers.

INTRODUCTION

Idealized, the inside of the earth can be described as being made up of layers, in a sequence 

of concentric shells as illustrated on the cover. The 

fact that the earth is not a homogeneous, structureless body has been realized since the time of Isaac 

Newton, who noted in a discussion of the planets 

that the average density of the earth is five to six 

times that of water. The average density of the 

earth actually is 5.5 g per em3 (grams per cubic 

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 

earth. We infer from this and other data that 

there is a heavy central part in the earth. In fact, 

during the last 60 years, geophysieists and geologists have estimated with increasing confidence the 

thickness and character of each of the successive 

layers in the earth, including variations within 

some layers.

Man has actually looked into the earth in deep 

mines and drill holes only a very small distance 

about 5 miles of the 4,000-mile distance to the 

earth's center. Furthermore, man will likely never 

be able to make a hole into the deep interior, so 

what we learn about the interior has to be from 

indirect evidence. At present, this evidence consists of (1) direct observation of rocks at the surface, (2) secondary observations based on geophysical phenomena (including waves through the 

earth from earthquakes and explosive sources, 

planetary motions of the earth, flow of heat from 

the interior, the magnetic field, and gravitational 

attraction), (3) laboratory experiments on surface 

rocks and minerals, and (4) comparison of the 

earth with the other planets, the sun, stars, and 

meteorites, which may be fragments of a disintegrated former planet. Our present understanding 

of the structural features and the composition of 

the earth is obtained from all these sources.

In the following discussion of the interior of the 

earth, we will use facts from these sources to consider layering, the existence of openings, and the 

physical state and composition of the rocks and 

minerals presumed to occur in the earth. Our estimates of composition are not much better than conjectures at present, but they are reasonably consistent with geophysical observations.

SURFACE OBSERVATIONS

Let us start from the earth's surface with features that we know and progress deeper and 

deeper inside the earth, using a series of scaled 

illustrations to comprehend better the sizes of 

structures in the earth. First, we look at the rocks 

under foot at a familiar scale and then at diagram-
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 

feature is reduced in size 10 times from one 

drawing to the next and compared with a new feature : 1 inch in the first figure will equal 10 million 

inches in the last. In all drawings, the vertical 

scale is equal to the horizontal scale, with no vertical exaggeration.

In the first drawing, figure 1, let us compare the 

length of a man's legs, about 2}/2 feet, with the 

depth of a gravel pit, about 25 feet; one length is 

10 times the other. In figure 2 the gravel pit and

steam shovel are shown reduced 10 times in size 

from figure 1, and a room in Carlsbad Caverns in 

New Mexico is sketched at this new, 10-timessmaller scale.

Openings Underground in Various Rocks

The existence of such large rooms as in the 

Carlsbad Caverns brings up the question of how 

large an opening at a given place underground 

can be supported by the surrounding rock. Sand 

and gravel like that shown in the pit in figure 1
are too loose and unconsolidated to support a very 

large hole, and this is true of the mud or sand 

deposited as sediment at the bottom of a sea, lake, 

or river. However, mud or sand, buried beneath 

the earth's surface by a thick deposit of similar 

material may be consolidated by pressure, heat, 

and chemical action into a sedimentary rock such 

as shale, sandstone, or limestone which would support a large opening.

At the relatively shallow depth of a thousand 

feet, huge natural caves, hundreds of feet in extent, larger than the one illustrated in figure 2, can 

be dissolved out of limestone by percolating 

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 

the other hand, less strong sedimentary rocks, such 

as shale or poorly cemented sandstone, would not 

support smaller openings (say, 40 ft across) under 

a load of a thousand feet of overlying rock.

Just as in limestone caves, large openings at a 

1,000-foot depth can also stand in unfractured igneous rocks, which are composed of hard silicate 

minerals and which were emplaced in a molten 

state and then cooled to form strong, massive bodies. Similar openings can stand in unfractured metamorphic rocks, which are sedimentary or igneous 

rocks that have been very highly compressed and

heated and which are now also strong and solid. 

Igneous or metamorphic rock can support openings 

even at 1-mile depth, as in mine workings; however, in very deep mines like the South African 

gold mines at 2 miles down, spalling of the walls 

and cave-ins can cause serious problems.

Immense chambers are sometimes postulated to 

exist very deep in the earth, but actually a 10-foot 

opening would not stand unsupported even in the 

strongest rock at a depth of 10 miles or more. The 

pressure at 10 miles exceeds the strength of the 

rock for a 10-foot span, and so it is merely speculation to predict larger openings at greater depths.

Diamond Pipes and Salt Domes

The underground diamond mines of South Africa and Siberia do not extend to as great a depth as 

the gold mines, but the diamond "pipes" themselves (fig. 3), in which the mining is done, extend 

as tubular conduits (hence the name "pipe") deep 

into the earth. This geologic feature is funnel 

shaped, decreasing in diameter to a narrow neck at 

a depth of a few thousand feet. We know that the 

neck extends beneath the surface about 100 miles 

because laboratory experiments show that high 

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.
Sedimentary

rocks:

sandstones

and

shales

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 

diamonds. Disseminated through the rock in the 

pipe are not only the sparse diamonds (about 1 

carat per ton), but also sporadic inclusions of eclogite and peridotite, the two dark-colored dense igneous rocks which we think make up the outer 

mantle of the earth. The mantle and its constituents are discussed below.

In the next drawing (fig, 4), at a scale 10 times 

smaller than the previous figure, we compare the 

diamond pipe with a salt dome, which has an upside-down funnel shape and is filled with salt. An 

adjacent drill hole is sketched in figure 4 because 

oil-well holes, as much as 5 miles deep, are commonly drilled near salt domes to tap the oil found 

in porous sandstones in the upturned beds around 

the domes, as for example, in the deep basin of 

sedimentary rock near the Gulf of Mexico.

A salt dome is presumed to form because the 

weight of 1 to 8 miles of rock on the thick horizontal parent salt bed causes the lower density relatively plastic salt to force its way upward through 

a crack in the rocks, sometimes to the surface. The 

intruding salt enlarges the crack and turns up the 

adjacent sedimentary beds. The salt rises until the 

thickness and strength of the overlying rock prevent further movement.

At depths of 5 to 10 miles only small openings 

(perhaps 0.1 in.) can stand in unconsolidated sedimentary rocks, such as those in the Gulf of Mexico 

basin, and only slightly bigger openings (perhaps 

1 in.) can persist in massive hard rocks.

THE CRUST 

Earthquakes and the Earth's Crust

When we remember that a great thickness of 

sedimentary rocks was uplifted 6 miles above sea 

level to form the Himalaya Mountains, we are 

made aware that tremendous forces are available 

inside the earth to move its surface up and down 

and laterally. We presume that local sources of 

heat deep inside the earth provide the energy to 

build mountains and to lower basins. The surface 

expressions of the release of mechanical energy of 

mountain building and other earth movements are 

earthquakes.

Geologists, in mapping the rocks at the earth's 

surface, locate the faults (fractures) and folds 

(plastic bends) which are evidence of the mechanical deformation of rocks in mountain building. 

Sudden displacement of rocks on either side of a 

fault produces a shaking of the earth called an 

earthquake; earth tremors are also produced by 

motion of the moken rock under volcanoes. The 

association of faults with earthquakes is a matter 

of historical record for the San Andreas fault 

along the coast of California and for faults in 

Chile, in Turkey, and elsewhere.

The violent breaking of the rock at a fault sets 

up vibrations, and waves of motion propagate 

through the whole earth, much like the waves produced in a pond when a stone is dropped into it.
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 

are of two types: (1) compressional waves, P, in 

which motion of solid particles is back and forth, 

parallel to the direction of travel, and (2) shear 

waves, S, in which particle motion is across, transverse to the direction of travel.

Geophysicists study the velocity and the paths 

of earthquake waves to learn about the earth's interior. One of them, a Yugoslav named Mohorovicic (pronounced Mo-ho-ro-vee-chich), in 1909 discovered a discontinuity between what we now call 

the crust and the mantle by observing a sharp increase in velocity of earthquake waves going from 

crust to mantle. The discontinuity has been named 

after him; it is sometimes called the M discontinuity, or more colloquially the "Moho"; a hole proposed to be drilled through the discontinuity has 

been dubbed the "Mohole."

Oceanic and Continental Crust

The crust of the earth ranges in thickness from 

about 5 miles at some places under the oceans to 

about 30 miles under high mountains; it forms a 

thin skin around the earth as shown on the cover. 

Below is the thick shell of the mantle. Oceanic and 

continental crustal thicknesses and topography are 

shown in the cross sections of figures 5 and 6, plotted at a scale that is 10 times smaller than the 

previous figure. In these figures in which the vertical and horizontal scales are equal, the craggy 

slopes of Mount Whitney in California and of 

Mauna Loa on Hawaii Island are not so impressive 

when compared with the thickness and the lateral 

extent of the earth's crust below. Relatively great 

thicknesses of crust, like the 34-mile thickness 

under the Sierra Nevada, are common under 

mountain ranges and are very much more than the 

4-mile thickness of crust (covered by 2y2 miles of 

water) at the Mohole site.

We believe that the continental crust is largely 

made up of (1) dense light-colored igneous rocks, 

such as granite or quartz diorite, in the upper part 

and (2) basalt, a dark and slightly denser igneous 

rock (commonly erupted from volcanoes), in the 

lower part. The oceanic crust appears to be composed almost entirely of basalt. The relative'thicknesses of the probable rocks occurring in four representative oceanic and continental locations (figs. 

5 and 6) are given in figure 7. Notice that the 

composition of the crust as well as its thickness 

varies laterally; the crust is not just a homogeneous, flat layer.

The acclaimed purpose for drilling the Mohole is 

to get a sample of rock from the mantle. It is widely believed that the discontinuity in earthquake 

wave velocity at the Moho is due to a change of 

rock type, from basalt in the lower part of the 

crust to the more dense eclogite or peridotite in the 

mantle. From the deep-sea sediments, from other 

crustal rocks, and from the mantle rock obtained
Continental i Coast Ranges |

Shelf I l 

1 I

i !

Sea level i ,_ ^ _ ^ _ _.- .. .. LJ n.-^-^sr

1 

Great Valley j 

of California I

1 

1 

I

Sierra Nevada 

Mount Whitney

50 Miles

I

FIGURE 6. Continental crust under California, showing a 21,500-foot drill hole for comparison with drill holes in figures 4 and 5.

 CQ E 3=

= '=

Mauna Loa Mohole site Sacramento Mount Whitney

Ocean water

	EXPLANATION 

LS Loose sediment 

CS Consolidated sediment 

G Granite 

QD Quartz diorite 

B Basalt

E-P Eclogite or peridotite

M Mohorovicic' discontinuity

if

Sea

QD

M 

E-P

level

10

15

20

25

30

35

FIGURE 7. Generalized geologic columns, showing rock types at two places each in figures 5 and 6.
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 

history of life, of the oceans, and of the earth itself.

THE AAANTLE

Earthquake wave velocities are faster in the upper mantle than in the crust, but the velocities 

decrease a little at a depth of about 100 miles in 

what is called the low-velocity layer (fig. 8). Although it is shown with constant thickness in the 

cover drawing and in figures 8 and 9, the lowvelocity layer may not extend uniformly around the 

earth; there is conflicting evidence for its existence 

under the central parts of the continents and 

oceans. In fact, it may be restricted to the mountainous regions, especially along continental margins, where volcanic activity and earthquakes occur. Considering the probable rise of temperature 

with depth in the earth, it is feasible for the temperature of the rock in this layer to be near the 

melting point.; this would explain the reduction in 

the wave velocities and would provide a source 

for the lava erupted from volcanoes or extruded 

in diamond pipes.

Below, in the transition zone, the earthquake 

wave velocities increase markedly again. They taper off as the zone grades into the lower mantle but 

continue to rise gradually to a maximum at the 

core boundary. (See fig. 10.) The boundaries of

these layers in the mantle are gradational; and although their depths are widely accepted, the details of their composition and structure are not 

known.

The possible openings in the mantle must be 

very small. We know from the occurrence of deepfocus earthquakes (as deep as 450 miles) that 

stresses may build up almost to the bottom of the 

transition zone, and this indicates that the rock 

there may have some strength. However, it is not 

likely that openings larger than 0.01 inch could 

stand in the low-velocity layer, owing to the weakness of rock near its melting point, or could stand 

in the transition zone or lower mantle, owing to 

the very high pressure in relation to rock strength.

The composition of the mantle has been conjectured from studies of volcanic lava, diamond 

pipes, and meteorites and from experiments on 

minerals and rocks. These studies indicate that the 

upper few hundred miles may be composed of eclogite and peridotite, which are composed mostly of 

iron and magnesium silicate minerals and some 

complex calcium, sodium, and aluminum silicate 

minerals. In the transition zone, denser forms of 

these minerals are presumed to exist. In the lower 

mantle, because of the very high pressure, only the 

simple oxides of iron, magnesium, and silicon are 

thought to be present. We are far from being sure 

of the composition of the mantle, which undoubtedly varies laterally as well as vertically.
THE CORE

Below the mantle is the earth's core, discovered 

by R. D. Oldham in 1906 from a study of earthquake records. The core itself is divided into an 

outer part and an inner part, as shown at a 10 

times further reduced scale in figure 9. The outer

Focus of earthquake

SKS

SKP

PP

PKP

PPP

Transition zone 

Low-velocity layer 

Crust and upper mantle

5000 Miles

FIGURE 9. Cross section of the whole earth, showing the paths of 

some earthquake waves. Compressional waves (P, solid lines) are 

refracted sha-ily by the core, as shown by the PKP trajectory. 

Shear waves (5, dashed lines), as such, end at the core, although 

they may be converted to P waves, traverse the core as a compressional wave (K), and emerge in the mantle again as P and S 

waves, that is, SKP and SKS. The waves may be reflected back 

into the earth at the surface and be picked up further on, as in 

trajectories PP, PPP, and SS.

core is presumed to be liquid because it does not 

transmit shear waves (8), and because it sharply 

reduces the velocity of compressional waves (P). 

(The P, and '8 waves are the two earthquake waves 

that go through the earth.) The inner core, discovered by its higher P-wave velocity in 1936 by Miss 

I. Lehmann, is considered to be solid. In figure 9 

the core looks about as big as the mantle, but actually the core occupies only 15 percent of the 

earth's volume, whereas the mantle occupies 84 

percent. The crust occupies the remaining 1 percent.

As shown in figure 9, the ray paths of earthquake waves from the focus, where the fault displacement or other disturbance initiates the earthquake, are curved in passing through the earth; 

the curvature is a refraction due to the increase in 

velocity with depth. Earthquake records, called 

seismograms, require great care in interpretation 

to disentangle the various waves; many more wave 

paths are observed than are shown in figure 9.

At the outer boundary of the core, there is a 

sharp discontinuity in both the P-wave velocity 

and the density, as shown in figure 10. These very 

striking changes in properties occur in a narrow 

border zone between the mantle and core, perhaps 

less than 10 miles thick. It is certain that there is a 

marked compositional difference between the mantle and core.

(

9fU°

160 

570

1800

3140 

 jofin

) 2 4 6 8 10 12 14 (

\ \ Transition zone ^^~~~^~>-^^

\ \ - - - - ^Nr 

\ v Low-velocity layer \

* Crust and upper mantle \

\

~A___

) 2 4 6 8 10 12

i i i 1 . i i I i 1 i I i

^\ Surface

Lower mantle \

Outer core \

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' 

boundary.

8
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 planetary motions of the earth, and by knowing the 

earth's average density (5.5 g per cm3 ), the density 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 material to account for the earth's magnetic field. The 

inner core is probably pure iron.

EARTH AND MOON

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 visualize 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 probably dispersed and not in a central core, a conclusion 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 interior of the earth as well.

QUESTIONS AND ANSWERS

Geologists, geochemists, and geophysicists have 

contributed to our understanding of the earth's FIGURE IK -Earth and moon system, showing the earth's core for 

comparison.
internal structure and composition, but questions 

about the earth's interior are still very much more 

numerous than answers: What are the causes and 

mechanisms of earthquakes, mountain building, 

and subsidence of large basins? What is the energy 

source for these phenomena? Why do volcanoes 

erupt, and what is their source? How do large 

granite bodies melt and move into place? How old 

is the earth? What have the structure and composition of the earth and its temperature and magnetism been like throughout geologic time? Why is 

the crust thinner under the oceans than under the 

continents?

Geological Survey scientists are actively searching for answers by measuring the velocities of 

earthquake waves in the mantle and crust, by ascertaining the variation of temperature with depth 

and the amount of heat flowing from the earth's 

interior, by determining the positions of the 

earth's magnetic poles millions of years ago as recorded in the rocks and compared with the present 

magnetic field, and by estimating the thickness of 

the crust under mountains and plains from the 

gravitational attraction. Survey geologists observe 

the occurrence and character of volcanic rocks to 

learn about the chemical processes of the mantle

and crust, and they map the folds and faults in 

rocks the external effects, demonstrated by every 

earthquake, of the mechanical forces deep in the 

earth. In Survey laboratories geophysicists and 

geochemists study the properties and composition 

of rocks and minerals under high pressure and 

temperature, simulating the conditions inside the 

earth. The interior of the earth is a fascinating 

topic for further study.

Earth layers on layers 

Up quaking, round ringing 

Deep secrets revealing 

To us unawares.

Anon. (ca. 19 5)

SUGGESTED READING

Hurley, P. M., 1959, How old is the earth f: Garden City, 

N.Y., Doubleday and Co., Inc., 160 p.

Jacobs, J. A., 1963, The earth's core and geomagnetism: 

New York, Macmillan Co., 137 p.

Scientific American, 1957, The planet earth: New York, 

Simon and Schuster, Inc., 168 p.

White, J. F., ed., 1962, Study of the earth: Englewood 

Cliffs, N.J., Prentice-Hall Internat., Inc., 408 p.

 6 U.S. GOVERNMENT PRINTING OFFICE: 1980 O 311-344-72

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