Elsevier Encyclopedia of Geology

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results of a frequency analysis, which was run over a

10-My-long interval to better resolve the position of

individual peaks. The peaks correspond to the fre-

quencies given in Table 2.

Obliquity

The obliquity (tilt) e of Earth’s axis with respect to the

orbital plane (see Figure 1) is defined by the angle

between Earth’s spin vector s and that of the orbital

plane n, and can be computed as cos e ¼ n

s, using

unit vectors. As the inclination and orientation of the

orbital plane vary, the obliquity is not constant, but

oscillates due to the interference of the precession

frequency p and the orbital elements s

. As shown in

Table 3, if the variation in obliquity is approximated

by quasiperiodic terms, the result is a strong oscilla-

tion with a period of approximately 41 ky, with add-

itional periods around 54 and 29 ky. The 41-ky

period arises from the simultaneous variation in

Earth’s orbital inclination, given by s

, and the pre-

cession of Earth’s spin direction, expressed by p.

Table 3 also shows that the obliquity signal contains

contributions from the g

as well as the s

fundamental

frequencies, due to their combined effect on the change

of the orbital plane normal. The present day obliquity

of approximately 23.45



has varied between 22.25



and 24.5



during the past 1 million years. The main

climatic effect of variations in Earth’s obliquity is in

control of the seasonal contrast. The total annual

energy received on Earth is not affected, but the obli-

quity controls the distribution of heat as a function of

latitude, and is strongest at high latitudes.

It is important to note that all of the obliquity

frequency components contain the precession con-

stant p. Due to tidal dissipation, the frequency of the

precession constant p has been higher in the past, a

fact that can be shown from geological observations,

such as ancient growth rings in corals, and from tidal

laminations. Figure 5 illustrates the variation in obli-

quity over 1.2 million years. The oscillation is domin-

ated by a 41-ky period cycle, and a variation in

amplitude is also observed. This variation is due to

beats arising from the presence of additional 29- and

54-ky periods, which are just visible in the frequency

analysis shown on the right-hand side of Figure 5.

Climatic Precession

The precession of Earth’s spin axis has a profound

effect on Earth’s climate, because it controls the tim-

ing of the approach of perihelion (the closest ap-

proach to the Sun) with respect to Earth’s seasons.

At present, perihelion occurs on the 4 January, close

to the winter solstice. With respect to the stars, the

precessional movement of Earth’s spin axis traces out

a cone shape with a period of 25.8 ky. However,

due to the precession of the perihelion within the

orbital plane, the period of precession, measured

with respect to the Sun and the seasons, is shorter.

The motion of the perihelion is not steady but is

caused by a superposition of the different g

frequen-

cies. For this reason, the precession of the equinoxes

with respect to the orbital plane lurches with a super-

position of several periods around 19, 22, and

24 ky. The effect of the precession of the equinoxes

on the amount of solar radiation received by Earth

also depends on the eccentricity. If the eccentricity is

zero (i.e., the orbit of Earth follows a circle), the effect

of the precession of the equinoxes is also zero. From a

climatic point of view, the eccentricity and longitude

of the perihelion combine to create what is termed the

‘climatic precession’, defined as e sin(

o), where

e sin(

o) is the longitude of perihelion from the moving

equinox. This means that the climatic precession

index is modulated in amplitude by variations in

Earth’s eccentricity.

A quasiperiodic approximation of the climatic pre-

cession time series reveals the contribution from dif-

ferent frequency components, as shown in Table 4.

Table 3 Six leading terms for Earth’s obliquity

Term

Frequency

(

year

1

) Period (ky) Amplitude

p þ s

31.613 40.996 0.0112

p þ s

32.680 39.657 0.0044

p þ s

þ g

 g

32.183 40.270 0.0030

p þ s

24.128 53.714 0.0029

p þ s

 g

þ g

31.098 41.674 0.0026

p þ s

44.861 28.889 0.0015

Principal obliquity frequency components analysed over the

past 4 My. The frequency terms g

and s

refer to those given in

Table 1.

Data from Laskar J (1999) The limits of Earth orbital calculations

for geological time-scale use.

Philosophical Transactions of the

Royal Society of London, Series A, Mathematical, Physical and

Engineering Sciences

357(1757): 1735–1759.

Figure 5 Earth’s obliquity over time (1.2 million years) and fre-

quency analysis for a 10-My time-span. The peaks in the fre-

quency analysis correspond to the frequencies given in Table 3;

the numbers over the peaks represent the periods (in thousands

of years).

414 EARTH/Orbital Variation (Including Milankovitch Cycles)

Note that the components of climatic precession can

be constructed from the precession constant p and

the fundamental frequencies g

. Figure 6 illustrates

the variation in the climatic precession index, and

its modulation in amplitude by eccentricity, over

1.2 million years. The frequency analysis, shown on

the right-hand side of the plot, reveals three peaks

corresponding to frequencies given in Table 4.

Insolation

Conceptually, the actual forcing of Earth’s climate

by orbital variations is applied through the radiative

flux received at the top of the atmosphere at a par-

ticular latitude and time, which is then transferred

through oceanic, atmospheric, and biological pro-

cesses into the geological record. The integral of the

radiative flux over a specified interval of time, termed

‘insolation’ (French from the Latin insolare), can be

computed from the eccentricity e, the obliquity e, and

the climatic precession e sin(

o). The amount of

solar radiation received at a particular location

depends on the orientation towards the Sun of that

location. Calculation of insolation becomes complex

if it is to be calculated over a particular time interval.

Averaged over 1 year and the whole Earth, the only

factor that controls the total amount of insolation

received, apart from the solar constant, is the

changing distance from Earth to the Sun, which is

determined by Earth’s semi-major axis a and its

eccentricity e.

If insolation variations are computed for a particu-

lar latitude, and over a particular length of time, the

main contribution arises from the climatic precession

signal, with additional contributions from the vari-

ation in obliquity. The exact nature of the insolation

signal is complicated. Certain general statements can

be made, though. The signal arising from the climatic

precession is always present in insolation time series.

Also, if the obliquity signal is present, it typically

shows a larger amplitude towards high latitudes.

In addition, the climatic precession signal in the in-

solation calculation depends on the latitude at which

it is calculated, such that the signal in the southern-

hemisphere summer shows opposite polarity to that

in the northern-hemisphere summer (see Figure 7). If

the mean monthly insolation is computed for a par-

ticular latitude, each month corresponds to a differ-

ence in phase (i.e., a difference in time of a particular

insolation maximum or minimum) of approximately

2 ky (12 months approximately correspond to the (on

average) 24-ky-long climatic precession cycle).

It is unlikely that geological processes that encode

the insolation signal are driven by variations at the

same latitudes and times of the year throughout geo-

logical time. Depending on the latitude and the time

interval over which insolation quantities are com-

puted, the calculation can be very complex, and the

question of time lags and forcing can be resolved only

through the application of climate models. A very

revealing study to this effect was reported by Short

and colleagues in 1991. At the present level of under-

standing, it is probably appropriate to avoid a strict

Table 4 Five leading terms for Earth’s climatic precession

Term

Frequency

(

year

1

) Period (ky) Amplitude

þ g

54.7064 23.680 0.0188

p þ g

57.8949 22.385 0.0170

p þ g

68.3691 18.956 0.0148

p þ g

67.8626 19.097 0.0101

p þ g

56.0707 23.114 0.0042

Principal climatic precession components analysed over the

past 4 My. The frequency terms

refer to those given in Table 1.

Data from Laskar J (1999) The limits of Earth orbital calculations

for geological time-scale use. Philosophical Transactions of the

Royal Society of London, Series A, Mathematical, Physical and

Engineering Sciences

357(1757): 1735–1759.

Figure 6 Climatic precession index and eccentricity envelope over time (1.2 million years) and frequency analysis for a 10-My time-

span. The peaks in the frequency analysis correspond to the frequencies given in Table 4; the numbers over the peaks represent the

periods (in thousands of years).

EARTH/Orbital Variation (Including Milankovitch Cycles) 415

interpretation of Milankovitch’s theory, which would

imply that the ice-ages are best explained by the

summer insolation curve computed at 65



N. Instead,

a better understanding of the complex mechanisms

of the climate system will have to be achieved thro-

ugh the use of geological data providing boundary

conditions for climate models.

Amplitude Modulation Patterns: The

‘Fingerprint’ of Orbital Cycles

A very important feature of Earth’s eccentricity, obli-

quity, and climatic precession variations is that

they display modulations in amplitude and frequency.

These modulations provide a ‘fingerprint’ of a par-

ticular astronomical calculation at a given time. The

modulation terms arise through the interference of

individual cycles to produce ‘beats’, with periods

ranging from hundreds of thousands to millions of

years. The most prominent amplitude modulation

cycles are listed in Table 5. An excellent visual repre-

sentation of amplitude modulation cycles in astro-

nomical calculations can be obtained by computing

evolutionary or wavelet spectra, which show the vari-

ation in amplitude at different frequencies over time.

This is shown in Figure 8 for the past 10 million years.

Figure 7 Earth’s insolation over time (1.2 million years) at various latitudes and frequency analysis for a 10-My time-span (on a

logarithmic amplitude scale); the numbers over the peaks represent the periods (in thousands of years). The precession component is

out of phase between the northern-hemisphere summer and the southern-hemisphere summer. Note that the eccentricity component

in insolation computations is always very weak compared to the obliquity and precession components. Also note the latitudinal

dependence of the relative strength of the obliquity cycle in the insolation curves.

Table 5 Modulation terms

Type Interfering terms ‘Beat’ term Period

Short eccentricity amplitude modulation terms (g

 g

)  (g

 g

)

¼ (g

 g

Þ400 ky

(

 g

)  (g

 g

)



... ... ...

Short and long eccentricity amplitude modulation terms (

 g

)  (g

 g

)

¼ðg

 g

Þ2.4 My

(

 g

)  (g

 g

)



... ... ...

Climatic precession amplitude modulation terms

Identical to eccentricity frequencies and amplitude

modulation terms

Obliquity amplitude modulation terms (

p þ s

)  (p þ s

) ¼ (s

 s

)  1.2 My

(p þ s

þ g

 g

)  (p þ s

 g

þ g

) ¼ (2g

 2g

)  1.2 My

(p þ s

)  (p þ s

þ g

g

)

¼ðg

 g

Þ2.4 My

(

p þ s

)  (p þ s

þ g

g

)



(

p þ s

)  (p þ s

) ¼ (s

 s

)  173 ky

... ... ...

Origin of amplitude modulation terms that affect Earth’s eccentricity, obliquity, and climatic precession. Individual g

and s

terms

refer to those given in Table 1. The 100-ky (short) eccentricity cycles are modulated with a period of 400 ky by the long eccentricity

cycle. Both short and long eccentricity cycles are modulated with a period of 2.4 My, but with a phase difference of 180



(i.e., an

amplitude maximum of the 100-ky eccentricity coincides with an amplitude minimum of the 400-ky eccentricity). Because

eccentricity directly modulates the climatic precession, all eccentricity amplitude modulation terms are also present in the climatic

precession signal. The obliquity signal is weakly amplitude modulated with a period of 170 ky, and more strongly with a period of

1.2 My. This amplitude modulation cycle is dynamically linked with the 2.4-My cycle present in the eccentricity modulation. All period

cycles listed here are shown in Figure 8.

416 EARTH/Orbital Variation (Including Milankovitch Cycles)

The significance of amplitude modulation cycles

is twofold. First, if these cycles can be detected in

the geological record, they allow the placement of

geological data into a consistent framework within

these amplitude modulation envelopes, even in the

absence of individual cycles and in the presence of

gaps. The extraction of long amplitude modulation

cycles typically requires high-fidelity geological

records that are millions of years long. Of particular

value for the generation of geological time-scales

beyond the Neogene is the 400-ky-long eccentricity

cycle, because it is considered to be very stable. In

addition, if the eccentricity signal could be found in

the geological data directly, as well as through its

modulation of the climatic precession amplitude, it

might be possible to evaluate phase lags between the

astronomical forcing and the geological record. The

second significant use of amplitude modulation cycles

is that they are related to specific dynamical proper-

ties of a given astronomical model. These properties

are related to the chaotic nature of the Solar System,

and potentially allow the use of geological data to

provide constraints on the dynamical evolution of the

Solar System and astronomical models.

Tidal Dissipation and Dynamical Ellipticity

The mean fundamental orbital frequencies g

and s

as well as the precession constant p, are likely to have

changed throughout geological time. However,

whereas the changes in g

and s

have probably been

small, the precession constant p is likely to have

changed significantly. Changes are caused by the

effects of energy (tidal) dissipation as well as by redis-

tribution of mass (due, e.g., to waxing and waning

ice-caps and mantle convection). Changes in p are

related to changes of the length of day, which is

reflected in geological and palaeontological records.

In particular, Earth’s tidal response to the gravita-

tional pull from the Sun and the Moon is not instant-

aneous. This means that the tidal bulge that develops

on Earth (and on the Moon), is not aligned with the

direction of the Moon’s gravitational pull. This pull

exerts a torque on Earth, which leads to a gradual

decrease in its rotational velocity. In addition, conser-

vation of angular momentum leads to an increase in

the distance from Earth to the Moon over time, and a

change in Earth’s rotational velocity leads to a redis-

tribution of mass on Earth (‘dynamical ellipticity’).

The dynamical ellipticity of Earth can also be affected

by mantle convection and ice loading.

These processes modify Earth’s precession constant

p, the frequency of which has decreased over geo-

logical time. Because p is contained in the expressions

for obliquity and climatic precession (see Tables 3

and 4), the periods of obliquity and climatic preces-

sion also change. In 1994, Berger and Loutre esti-

mated possible values for changes of astronomical

periods, based on astronomical and geological obser-

vations. Their results are illustrated in Figure 9. The

effects of changing tidal dissipation and dynamical

ellipticity values have a large impact on astronomical

calculations, and have to be obtained from observa-

tion. Strictly speaking, astronomical calculations

cannot be performed independently of the chosen

Earth model, and particularly, there cannot be separ-

ate treatment of the Earth–Moon system. This is why

numerical computations are invaluable.

Chaos in the Solar System

Probably the most significant development of astro-

nomical theory in recent times has been the discov-

ery of the chaotic behaviour of the Solar System by

Jacques Laskar in 1990. Laskar established that the

dynamics of the orbital elements in the Solar System

are not fixed for all times, but rather are unpredict-

able over tens of millions of years. This is due to the

non-linear gravitational interaction of the different

bodies in the Solar System, which makes it theoretic-

ally impossible to calculate the exact movements of

celestial bodies from their present-day masses, veloci-

ties, and positions over long periods of time. This

feature poses limits on the use of astronomical theory

for the purposes of creating astronomically calibrated

Figure 8 Joint time–frequency analysis of an arbitrary mixture

of the eccentricity, obliquity, and climatic precession signal to

allow a better visual representation of the main climatically im-

portant orbital variations. The amplitude at a particular frequency

and time is colour coded, red corresponding to a high relative

amplitude. A selection of amplitude modulation terms is high-

lighted (boxes inside figure); these represent the ‘fingerprint’

of an astronomical model. The numbers near the fingerprints

represent periods (see Table 5).

EARTH/Orbital Variation (Including Milankovitch Cycles) 417

Figure 9 Estimated changes in the obliquity and climatic precession periods related to a changing distance between Earth and the

Moon over the past 440 My, according to calculations by Berger and Loutre in 1994. Note that the periods of obliquity and climatic

precession change due to a change of the precession constant p, whereas the eccentricity periods remain unaffected. Note that this

diagram is for illustrative purposes only, and the exact variation of orbital frequencies over tens of millions of years is not yet known in

detail.

Figure 10 Lithological magnetic susceptibility data from Leg 154 of the Ocean Drilling Program. These data, analysed by Shackleton

and colleagues in 1999, demonstrate the fidelity with which orbital variations can be encoded in the geological record. The magnetic

susceptibility parameter, plotted together with filters that extract the visible cycles, chiefly reflects variations between terrestrial

(clay-rich) and marine (carbonate-rich) sediments, being forced by a dominant obliquity signal.

418 EARTH/Orbital Variation (Including Milankovitch Cycles)

geological time-scales. Laskar found that the calcula-

tions of orbits diverge exponentially with time for a

given set of initial conditions. This implies, for

example, that if the present position of a planet is

known with a relative error of 10

10

, this error

increases to 10

9

after 10 My, and reaches the

order of 1 after 100 My. Despite these limitations,

the determination of astronomical parameters, such

as the planetary masses, velocities, and positions, is

improving due to additional satellite measurements.

Figure 11 (Top) Evolutionary spectrum (frequency vs age, amplitude colour coded) of Ocean Drilling Program Leg 154 lithological

data (combined magnetic susceptibility and colour reflectance) (cf. Figure 10). These data, analysed by Shackleton and colleagues

in 1999, and contrasted to astronomical calculations of Earth’s orbital variations, demonstrate that the geological data contain an

imprint of long-term amplitude modulations of astronomical forcing components. (Bottom) The astronomical solution provided

by J Laskar.

EARTH/Orbital Variation (Including Milankovitch Cycles) 419

Together with improved and more detailed astronom-

ical models, the limits of accuracy of astronomical

calculations are also likely to extend further back

in time.

A clearer representation of chaos in the Solar

System is provided by amplitude modulation terms.

The 2.4- and 1.2-My-long amplitude modulation

terms that occur in the calculations of eccentricity

and obliquity, respectively, are in resonance, and the

expression (s

 s

)  2(g

 g

) ¼ 0 can evolve into a

new state, where (s

 s

)  (g

 g

) ¼ 0. This implies

a change from a 1:2 resonance to a 1:1 resonance.

Laskar found this behaviour to be the main source

and representation of chaos in the Solar System. As

shown in Table 5, these terms are present in several

astronomical frequencies, and should be possible to

detect in the geological record.

Earth’s Orbital Variation Encoded in

Geological Data

The imprint of Earth’s orbital variation in geological

records, first statistically demonstrated in 1976 for

the recent geological past, in the seminal paper by

Hays and colleagues, has now been found throughout

parts of the Cenozoic and beyond, through variations

of stable isotope measurements that act as a proxy for

Earth’s climate system, as well as in a large number of

lithological parameters. A review of this body of data

is beyond the scope of the present discussion, but it is

illustrative to show at least one example of very high-

quality data that demonstrate the imprint of Earth’s

orbital variations in the rock record. Figure 10 shows

part of a record used to correlate geological data for

the past 30 My with astronomical calculations. The

record shows exceptionally well-encoded obliquity

and climatic precession cycles. Most importantly,

this record also demonstrates the consistent variation

in amplitude of the obliquity cycles. This can be

illustrated with the help of evolutionary spectral an-

alysis, whereby the relative amplitude at individual

frequencies is evaluated at different times. This is

shown in Figure 11.

Linking Earth’s orbital variations (Milankovitch

cycles) with geological records has caused controver-

sies as to whether the theory that orbital variations

driving Earth’s climate conditions can be correct. The

controversies stem from observations of palaeocli-

matic proxies from the recent past that reveal an

imprint that does not correspond to the expected

strength of orbital variations in insolation calcula-

tions, and instead suggest a nonlinear response of

the climate system at different orbital frequencies.

In particular, during the past 800 ky, the imprint of

eccentricity in stable isotope records has been much

stronger than expected. It is now becoming clear that

geological data show a wide variety of responses to

individual orbital frequencies, depending on factors

such as palaeolatitude, the prevailing oceanographic

system at the study site, global ice volume, etc., with

records showing much more variation than would

be expected from a simple insolation calculation.

A better understanding of the interaction between

Earth’s orbital variations and their imprint on the

climate system and geological records is likely going

to be gained from integrated Earth system model-

ling studies, making use of the growing body of ob-

servations that has been provided recently by ocean

drilling. As a final note, orbital variations also affect

the other planets of the Solar System, and recent

attempts have been made to link these to climatic

variations on Venus and Mars.

Further Reading

Berger A, Imbrie J, Hays J, Kukla G, and Saltzman B. (eds.)

(1984) Milankovitch and Climate: Understanding the

Response to Astronomical Forcing. Dordrecht and

Boston: D. Reidel Publishing Company.

Berger A and Loutre MF (1994) Astronomical forcing

through geological time. In: de Boer PL and Smith DG

(eds.) Orbital Forcing and Cyclic Sequences (IAS Special

Publication), vol. 19, pp. 15–24. Oxford: Blackwell

Scientific.

Berger A, Loutre MF, and Tricot C (1993) Insolation and

Earth’s orbital periods. Journal of Geophysical Research

98(D6): 10341–10362.

de Boer PL and Smith DG (eds.) (1994) Orbital Forcing and

Cyclic Sequences (IAS Special Publication), vol. 19.

Oxford: Blackwell Scientific Publications.

Croll J (1875) Climate and Time in their Geological Rela-

tions: A Theory of Secular Changes of the Earth’s

Climate. London: Daldy, Tsbister and Co.

Einsele G, Ricken W, and Seilacher A (eds.) (1991) Cycles

and Events in Stratigraphy. Berlin: Springer Verlag.

Gilbert GK (1895) Sedimentary measurement of Cret-

aceous time. Journal of Geology III: 121–127.

Hays JD, Imbrie J, and Shackleton NJ (1976) Variations in

the Earth’s orbit: pacemaker of the Ice Ages. Science

194(4270): 1121–1131.

420 EARTH/Orbital Variation (Including Milankovitch Cycles)

Hinnov LA (2000) New perspectives on orbitally forced

stratigraphy. Annual Review of Earth and Planetary

Science 28: 419–475.

House MR and Gale AS (eds.) (1995) Orbital Forcing

Timescales and Cyclostratigraphy, Geological Society

Special Publication, vol. 85. London: The Geological

Society.

Imbrie J and Palmer-Imbrie K (1979) Ice Ages; Solving

the Mystery. Short Hills, NJ/Cambridge, MA: Enslow

Publishers/Harvard University Press.

Lambeck K (1980) The Earth’s Variable Rotation:

Geophysical Causes and Consequences. Cambridge:

Cambridge University Press.

Laskar J (1990) The chaotic motion of the solar system – a

numerical estimate of the size of the chaotic zones. Icarus

88(2): 266–291.

Laskar J (1999) The limits of Earth orbital calculations

for geological time-scale use. Philosophical Transactions of

the Royal Society of London, Series A, Mathematical,

Physical and Engineering Sciences 357(1757): 1735–1759.

Milankovitch M (1941) Kanon der Erdbestrahlung

und seine Anwendung auf das Eiszeitenproblem, Special

Publication 132, vol. 33. Belgrade: Royal Serbian Sci-

ences. (‘Canon of Insolation and Ice Age Problem’,

English translation by Israe

l Program for Scientific Trans-

lation and published for the US Department of Com-

merce and the National Science Foundation,

Washington, DC, 1969).

Schwarzacher W (1993) Cyclostratigraphy and the Milan-

kovitch Theory. Amsterdam: Elsevier.

Shackleton NJ, Crowhurst SJ, Weedon GP, and Laskar J

(1999) Astronomical calibration of Oligocene–Miocene

time. Philosophical Transactions of the Royal Society

of London, Series A, Mathematical Physical and

Engineering Sciences 357(1757): 1907–1929.

Shackleton NJ, McCave IN, and Weedon GP (eds.) (1999)

Astronomical (Milankovitch) calibration of the geo-

logical timescale (9–10 December 1998, London) Philo-

sophical Transactions of the Royal Society of London,

Series A, Mathematical Physical and Engineering

Sciences 357(1757).

Short DA, Mengel JG, Crowley TJ, Hyde WT, and North

GR (1991) Filtering of Milankovitch cycles by Earth’s

geography. Quaternary Research 35(2): 157–173.

EARTH STRUCTURE AND ORIGINS

G J H McCall, Cirencester, Gloucester, UK

Introduction

The Earth is a planet of the inner zone of the solar

system, which is occupied by four ‘terrestrial’ planets

– Mercury, Venus, Earth, and Mars from the Sun

outwards. These planets are composed mainly of

rock and metal, whereas the outer planets – the so

called ‘gas-giants’ and ‘ice-giants’–have immense

gaseous envelopes. The four differ greatly in charac-

ter. Mercury has a dead cratered surface closely re-

sembling that of the Moon, though there is evidence

that it is compositionally very different. Venus is

shrouded by a dense greenhouse (carbon dioxide-

rich) atmosphere that is not visually penetrable, but

radar images reveal a largely volcanically moulded

surface. Mars, like Venus and Earth, has a very com-

plex surface, moulded by volcanism, impacts, and

wind action, and has polar ice caps. The Earth is

unique in possessing oceans that cover the majority

of the surface and form part of the hydrosphere,

which includes surface and underground water

bodies. The Earth is also unique in possessing an

oxygen-containing atmosphere dominated by nitro-

gen; Mars has a thin carbon dioxide-rich atmosphere

with negligible oxygen, and Venus has a thick carbon

dioxide-rich atmosphere. Mercury and the Moon

have negligible atmospheres dominated by inert

gases. The unique character of the Earth is well seen

from Space (Figure 1): the oceans appear blue in

reflected sunlight, and the single satellite is uniquely

Figure 1 The Earth and Moon as seen from Space (NASA

image kindly supplied by Sir Patrick Moore).

EARTH STRUCTURE AND ORIGINS 421

large compared with the mother planet, and uniquely

orbits it at the same rate as it rotates so that the same

lunar face is always Earthwards. Of the other inner

planets, only Mars has moons (two of them), and they

may be captured asteroids. The origin of the Earth’s

Moon remains highly controversial despite vigorous

championing of various models.

The Unique Biosphere

As far as we know, the Earth is unique in having a

biosphere on and below its surface, in its waters, and

in the air above it – a quite thin zone compared with

the planetary interior. Minute living organisms may

be carried naturally high in the atmosphere, and the

exact upper boundary of the biosphere is not closely

defined. In both the oceanic and continental realms,

bacteria can be found deep beneath the surface of the

rocks: 92% of the Earth’s bacteria occupy the deep

biosphere, and they can live at depths of up to 4 km

and at temperatures of up to 90



Properties

The essential properties of the Earth are given in

Table 1, where they are compared with those of the

other inner planets and the Moon (see Solar System:

Moon).

The Earth is not perfectly spherical but is an oblate

spheroid; this was recognized by Newton to explain

the precession of the equinoxes, the shift of the equi-

noctial point eastwards each year. The figure of 1/

298.25 is widely accepted for the degree of flattening

of the polar diameter compared with the equatorial

diameter. The Earth is subject to various secular

changes that affect the length of the day; these include

secular orbital changes and rotational changes,

amounting to several parts per 10

over time-scales

ranging from days to millennia. Besides the rate of

rotation, there are secular changes related to the ce-

lestial frame. Polar motion is a combination of the

effect of the Chandler Wobble (departure from pure

spin amounting to an axial deviation of 4

of arc with

a 428 day cycle) and a contributing effect of variation

of the distribution of mass within the Earth and in the

oceans and atmosphere. The precession of the

equinoxes mentioned above is due to lunar and solar

gravity acting on an oblate spheroid with an axis of

rotation inclined at 23.5



to the perpendicular. There

is also planetary precession, which reduces the obli-

quity of the axis to the ecliptic (the mean plane of the

orbit around the sun) by 0.5

per year. The combined

precession has a period of 25870 years. The Milan-

kovich model of climate change is generally accepted

to be related to orbital eccentricity, obliquity of the

ecliptic, and the precession of the equinoxes (see

Earth: Orbital Variation (Including Milankovitch

Cycles)), and this has a periodicities of 90 000–

110 000, approximately 42 000 and approximately

26 000 years. This model is believed to control glaci-

ations in the geological record. The world’s temperate

zone is advancing into the tropics, and this is the

Milankovich cycle in operation before our eyes. All

these secular changes complicate extremely accurate

calculations of time and geodetic surveying, and there

is the further complication of magnetism, in particu-

lar the Earth’s magnetic field, which is at present

subject to increasing variation (see below).

Earth Tides

The gravitational attraction within the Earth–Moon

system keeps them in orbit about the common centre

of mass, a point about 4670 km from the centre of

the Earth. The pull on the side of the Earth facing

the Moon at any time is strongest, and the result is a

tendency to pull the Earth into a prolate ellipsoid

aligned with the Earth–Moon axis (Figure 2). The

Table 1 The essential properties of the Earth, the other inner

planets, and the Moon

Radius (km)

Mass

(10

kg)

Density

(10

kg m

3

)

Mercury 2439 3300 5.4

Venus 6051 48700 5.3

Earth 6371 59970 5.517

Moon 1738 734.9 3.34

Mars 3394 6420 3.9

Figure 2 The pattern of tidal forces at the surface of the Earth

resulting from the gravitational pull of the Moon and Sun (repro-

duced with permission from F D Stacey (2000) Earth Tides. In:

Hancock PL and Skinner BJ (eds.)

Oxford Companion to the Earth,

pp. 280–281. Oxford: Oxford University Press.).

422 EARTH STRUCTURE AND ORIGINS

variation in gravitational pull imposed by the rota-

tion of the Earth causes Earth tides. There is also a

smaller contribution to Earth tides by the Sun.

Internal Configuration

What we know of the internal configuration of the

Earth comes from the following sources:

study of surface rocks and rocks brought up by

eruptive processes (kimberlite diatremes are par-

ticularly important with respect to the mantle as

they sample regions within it that would otherwise

be unknown to us directly. (see Volcanoes; Igneous

Rocks: Kimberlite);

analogies with meteorites (in particular be-

tween nickel–iron meteorites and the core, and

between carbonaceous chondrites and the mantle);

instrumental measurements (geophysical), especially

seismology (Figure 3); and

geochemical research (relating to the Earth, other

planetary bodies, and meteorites).

As a result of these approaches it is accepted that

the Earth is made up of a series of concentric shells,

each with its own physical and chemical properties

(Figure 4). The outermost is the crust, beneath it is the

mantle (divided into an upper mantle and lower

mantle), and at the centre is the core (divided into

the inner core and outer core). The plate-tectonics

paradigm also accepts the existence of a lithosphere,

which is not synonymous with the crust, but is a

solid and rigid zone extending into the upper part of

the mantle and bounded below by a transition zone.

This separates the strong elastic layer from the weak

and ductile asthenosphere, which is solid but very

slowly convecting. The crust is bounded below by a

discontinuity at which the velocity of seismic P waves

increases abruptly; this is the Mohorovicic discon-

tinuity and forms the lower boundary of the zone

of isostatic adjustment (see Earth: Mantle; Crust).

The properties of the various concentric zones of

the Earth are summarized in Table 2.

Magnetic Field

The Earth’s magnetic field is environmentally critical

because it forms a protective shield that blocks out

solar radiation and harmful incoming particles. It has

the character of a dipole, but has, throughout geo-

logical time, been subject to reversals, with the poles

Figure 3 Earthquake energy travels down and out from the

focus as shown, but at each internal boundary part of it is re-

flected or refracted and returns to the surface. Times of travel of

the energy waves and their behaviour have given us much of the

information that we have about the depths of the boundaries and

the nature of the material forming the concentric shells of the

Earth (reproduced from Van Amdel TJ (1994) (eds.)

New Views on

an Old Planet

, 2nd edn. Cambridge: Cambridge University Press,

with permission from the author and publisher).

Figure 4 The configuration of the Earth’s internal zones as

deduced from the four indirect sources listed in the text (repro-

duced with permission from C M R Fowler (2000) Mantle convec-

tion, plumes, viscosity and dynamics. In: Hancock PL and Skinner

BJ (eds.)

Oxford Companion to the Earth, pp. 649–652. Oxford:

Oxford University Press).

EARTH STRUCTURE AND ORIGINS 423

Elsevier Encyclopedia of Geology - vol I A-E

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