EARTH AS AN EVOLVING PLANETARY SYSTEM Part 1 ppt

Although this book began life in 1976 with the title Plate Tectonics and Crustal Evolution, the subject matter has gradually changed focus with subsequent editions, and especially since

Trang 2

EARTH AS AN EVOLVING PLANETARY SYSTEM

Trang 5

525 B Street, Suite 1900, San Diego, CA 92101-4495, USA

84 Theobald’s Road, London WC1X 8RR, UK

This book is printed on acid-free paper

Cover image courtesy of Tasa Graphic Arts, Inc., Taos, NM

http://www.tasagraphicarts.com

No part of this publication may be reproduced or transmitted in any form or by any means, electronic

or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone: (+44) 1865 843830, fax: (+44) 1865 853333, e-mail:

permissions@elsevier.com.uk You may also complete your request on-line via the Elsevier homepage (http://elsevier.com), by selecting “Customer Support” and then “Obtaining Permissions.”

Library of Congress: Application submitted

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

ISBN: 0-12-088392-9

For information on all Academic Press publications

visit our Web site at www.books.elsevier.com

Printed in the United States of America

04 05 06 07 08 09 9 8 7 6 5 4 3 2 1

Trang 6

Although this book began life in 1976 with the title Plate Tectonics and Crustal

Evolution, the subject matter has gradually changed focus with subsequent editions, and

especially since the third edition in 1989 In the last decade it has become increasingly

clear that the various components of Earth act as a single, interrelated system, often

referred to as the Earth System One reviewer of the fourth edition pointed out that the

title of the book was no longer appropriate, since plate tectonics was not a major focus

This is even more so in this fifth version, and thus, I introduce a new title for the book:

Earth as an Evolving Planetary System

Since the first edition in 1976, which appeared on the tail end of the plate tectonics

revolution of the 1960s, our scientific database has grown exponentially and continues to

grow—in fact, much faster than we can interpret it If one compares the earlier editions

of the book with this edition, a clear trend is apparent Plate tectonics is now assimilated

into geological textbooks and is part of the vernacular The changing emphasis over the

last 30 years is from how one system in our planet works (plate tectonics) to how all

systems in our planet work, how they are related, and how they have governed the

evolu-tion of the planet As scientists continue to work together and share informaevolu-tion from

many disciplines, this trend should continue for many years into the future

Today, more than at any time in the past, we are beginning to appreciate the fact that

to understand the history of our planet requires understanding the various interacting

components and how they have changed with time Although science is made up of

spe-cialties, to learn more about how Earth operates requires input from all of these

special-ties—geology alone is not sufficient In this new book the various subsystems of the

Earth are considered as vital components in the evolution of our planet Subsystems

include such components as the crust, mantle, core, atmosphere, oceans, and life

As with previous editions, the book is written for advanced undergraduate and

gradu-ate students and assumes a basic knowledge of geology, biology, chemistry, and physics

that most students in the Earth Sciences acquire during their undergraduate education

It also may serve as a reference book for specialists in the geological sciences who want

Trang 7

to keep abreast of scientific advances in this field I have attempted to synthesize andincorporate data from the fields of oceanography, geophysics, paleoclimatology, geology,planetology, and geochemistry, and to present this information in a systematic manneraddressing problems related to the evolution of Earth over the last 4.6 billion years The book includes an introduction to some of the new and exciting topics in the EarthSciences Among these are results from increased resolution of seismic tomography bywhich plates can be tracked into the deep mantle High resolution U-Pb zircon isotopicdating now permits us to better constrain the timing of important events in Earth history.

We have detrital zircons with ages up to 4.4 Ga, suggesting the presence of felsic crustand water on the planet by this time New information on the core provides us with abetter understanding of how the inner and outer core interact and how the Earth’smagnetic field is generated

Two expanding areas of knowledge have necessitated new chapters Exciting work onthe origin of life and the possibilities of life beyond the Earth is summarized in a newchapter on Living Systems Also, the continuing saga of mass extinctions and the role ofimpacts and mantle plume events have required more coverage And a contentious newdiscussion on the Snowball Earth enters the picture Did the Earth really freeze up some600-700 million years ago? The episodic nature of crustal production, stable isotopeanomalies, black shale deposition, giant dyke swarms and other phenomena have beenwell documented in the last few years, so much so that a new chapter has been added tocover this subject Also included in this chapter are new ideas and results from the super-continent cycle and on the possibility of global mantle plume events as the driving force

of episodic events

In addition, a new version of an interactive CD ROM by the author, titled “PlateTectonics and How the Earth Works (version 1.2)” is available to accompany this book.This CD, with animations and interactive exercises, can be obtained from Tasa GraphicArts Inc., Taos, NM at http://www.tasagraphicarts.com

Kent C CondieDepartment of Earth & Environmental ScienceNew Mexico Tech

Socorro, New Mexico

Trang 8

Preface v

1 Earth Systems 1

Earth as a Planetary System 1

Structure of the Earth 3

Plate Tectonics 5

Is the Earth Unique? 8

Interacting Earth Systems 9

Further Reading 405

References 407

Index 443

Trang 16

Earth Systems

Earth as a Planetary System

A system is an entity composed of diverse but interrelated parts that function as a

whole (Kump et al., 1999a) The individual parts, often called components, interact with

each other as the system evolves with time Components include reservoirs of matter or

energy, described by mass or volume, and subsystems, which behave as systems within

a system In recent years, the Earth has been considered a complex planetary system that

evolved over 4.6 billion years of time It includes reservoirs, such as the crust, mantle,

and core, and subsystems, such as the atmosphere, hydrosphere, and biosphere Because

many of the reservoirs in the Earth interact with each other and with subsystems, such as

the atmosphere, there is an increasing tendency to consider most or all of the Earth’s

reservoirs as subsystems

The state of a system is characterized by a set of variables at any time during the

evolution of the system For the Earth, temperature, pressure, and various compositional

variables are most important The same thing applies to subsystems within the Earth

A system is at equilibrium when nothing changes as it evolves If, however, a system is

perturbed by changing one or more variables, it responds and adjusts to a new equilibrium

state A feedback loop is a self-perpetuating change and a response in a system to a

change If the response of a system amplifies the change, it is known as a positive feedback

loop, whereas if it diminishes or reverses the effect of the disturbance, it is a negative

feedback loop.As an example of positive feedback, if volcanism pumps more CO2into

a CO2-rich atmosphere of volcanic origin, this should promote greenhouse warming and

the temperature of the atmosphere would rise If the rise in temperature increases

weath-ering rates on the continents, this would drain CO2from the atmosphere causing a drop

in temperature, an example of negative feedback Because a single subsystem in the Earth

affects other subsystems, many positive and negative feedback loops occur as the Earth

attempts to reach a new equilibrium state These feedback loops may be short lived over

hundreds to tens of thousands of years, such as short-term changes in climate, or they

Trang 17

may be long lived over millions or tens of millions of years, such as changes in climaterelated to the dispersal of a supercontinent.

The driving force of planetary evolution is the thermal history of a planet, more fullydescribed in Chapter 10 The methods and rates by which planets cool, either directly orindirectly, control many aspects of planetary evolution In a silicate-metal planet likeEarth, thermal history determines when and if a core will form (Fig 1.1) It determineswhether the core is molten, which in turn determines whether the planet will have aglobal magnetic field (generated by dynamo-like action in the outer core, as explained inChapter 5) The magnetic field, in turn, interacts with the solar wind and with cosmicrays, and it traps high-energy particles in magnetic belts around the planet This affectslife because life cannot exist in the presence of intense solar wind or cosmic radiation Planetary thermal history also strongly influences tectonic, crustal, and magmatic his-tory (Fig 1.1) For instance, only planets that recycle lithosphere into the mantle by sub-duction, as the Earth does, appear capable of generating continental crust and thus havingcollisional orogens Widespread felsic and andesitic magmas can only be produced in aplate tectonic regime In contrast, planets that cool by mantle plumes and lithospheredelamination, as perhaps Venus does today, should have widespread mafic magmas withlittle felsic to intermediate component They also appear to have no continents

Solar wind Cosmic rays

CLIMATIC HISTORY

THERMAL HISTORY Core

Tectonic history Crustal evolution Magmatic history Life

Impact

Magnetic field

Figure 1.1 Major

rela-tionships between thermal

and climatic histories of the

Earth.

Trang 18

So where does climate come into these interacting histories? Climate reflects complex

interactions of the ocean–atmosphere system with tectonic and magmatic components, as

well as interactions with the biosphere In addition, solar energy and asteroid or cometary

impacts can have severe effects on climatic evolution (Fig 1.1) The thermal history of a

planet affects directly or indirectly all other systems in the planet, including life The

Earth has two kinds of energy sources: those internal to the planet and those external to

the planet In general, internal energy sources have long-term (>106years) effects on

planetary evolution, whereas external energy sources have short-term (<106years) effects

Gradual increases in solar energy over the last 4.6 Gy have also influenced the Earth’s

climate on a long timescale The most important extraterrestrial effects on planetary

evo-lution, and especially on climate and life, are asteroid and cometary impacts, the effects

of which usually last less than 106years

Many examples of interacting terrestrial systems are described in later chapters

However, before describing these systems and their interactions, I first need to review the

basic structure of the Earth as determined primarily from seismology

Structure of the Earth

The internal structure of the Earth is revealed primarily by compressional waves (primary

waves, or P-waves) and shear waves (secondary waves, or S-waves) that pass through the

planet in response to earthquakes Seismic-wave velocities vary with pressure (depth),

temperature, mineralogy, chemical composition, and degree of partial melting Although

the overall features of seismic-wave velocity distributions have been known for some

time, refinement of data has been possible in the last 10 years Seismic-wave velocities and

density increase rapidly in the region between 200 and 700 km deep Three first-order

seismic discontinuities divide the Earth into crust, mantle, and core (Fig 1.2): the

Mohorovicic discontinuity, or Moho, defining the base of the crust; the core–mantle

interface at 2900 km; and the inner-core–outer-core interface around 5200 km The

core composes about 16% of the Earth’s volume and 32% of its mass These

discontinu-ities reflect changes in composition, phase, or both Smaller but important velocity

changes at 50 to 200 km, 410 km, and 660 km provide a basis for further subdivision of

the mantle, as described in Chapter 4

The major regions of the Earth can be summarized as follows (Fig 1.2):

1 The crust consists of the region above the Moho and ranges in thickness from

about 3 km at some oceanic ridges to about 70 km in collisional orogens

2 The lithosphere (50–300 km thick) is the strong outer layer of the Earth—including

the crust, which reacts to many stresses as a brittle solid The asthenosphere,

extend-ing from the base of the lithosphere to the 660-km discontinuity, is by comparison a

weak layer that readily deforms by creep A region of low seismic-wave velocity and

of high attenuation of seismic-wave energy, the low-velocity zone (LVZ), occurs at the

top of the asthenosphere and is from 50 to 100 km thick Significant lateral variations

in density and in seismic-wave velocity are common at depths of less than 400 km

Structure of the Earth

Trang 19

3 The upper mantle extends from the Moho to the 660-km discontinuity and

includes the lower part of the lithosphere and the upper part of the asthenosphere

The region from the 410-km to the 660-km discontinuity is known as the

transi-tion zone.These two discontinuities, further described in Chapter 4, are caused bytwo important solid-state transformations: from olivine to wadsleyite at 410 kmand from spinel to perovskite with the addition of magnesiowustite at 660 km

4 The lower mantle extends from the 660-km to the 2900-km discontinuity at the

core–mantle boundary It is characterized mostly by rather constant increases invelocity and density in response to increasing hydrostatic compression Between

200 and 250 km above the core–mantle interface, a flattening of velocity and density

gradients occurs in a region known as the D′′ layer, named after the seismic wave used to define the layer The lower mantle is also referred to as the mesosphere, a

region that is strong but relatively passive in terms of deformational processes

5 The outer core will not transmit S-waves and is interpreted to be liquid It extends

from the 2900-km to the 5200-km discontinuity

6 The inner core, which extends from the 5200-km discontinuity to the center of the

Earth, transmits S-waves—although at very low velocities, suggesting that it is asolid near the melting point

There are only two layers in the Earth with anomalously low seismic-velocity gradients:the lithosphere and the D′′ layer just above the core (Fig 1.2) These layers coincide withsteep temperature gradients; hence, they are thermal boundary layers in the Earth Bothlayers play an important role in the cooling of the Earth Most cooling (>90%) occurs by

LITHOSPHERE ASTHENOSPHERE

MESOSPHERE

Lower Mantle Upper

2000

6000 8000

4000

Figure 1.2 The

distribu-tion of average

compres-sional-wave, or P-wave

(V p ), and shear-wave, or

S-wave (V S ), velocities and

the average calculated

den-sity (ρ) in the Earth Also

shown are temperature

dis-tributions for whole-mantle

convection (T W ) and

lay-ered mantle convection

(T L ) LVZ, low-velocity

zone.

Trang 20

plate tectonics as plates are subducted deep into the mantle The D′′ layer is important in

that steep thermal gradients in this layer may generate mantle plumes, many of which

rise to the base of the lithosphere, thus bringing heat to the surface (<10% of the total

Earth cooling)

Considerable uncertainty exists regarding the temperature distribution in the Earth It

depends on features of the Earth’s history such as the initial temperature distribution in

the planet, the amount of heat generated as a function of both depth and time, the nature

of mantle convection, and the process of core formation Most estimates of the temperature

distribution in the Earth are based on one or a combination of two approaches: Models

of the Earth’s thermal history involving various mechanisms for core formation, and

models involving redistribution of radioactive heat sources in the Earth by melting and

convection processes

Estimates using various models seem to converge on a temperature at the core–mantle

interface of about 4500 ± 500° C and a temperature at the center of the core from 6700

to 7000° C Two examples of calculated temperature distributions in the Earth are

shown in Figure 1.2 Both show significant gradients in temperature in the LVZ and the

D′′ layer The layered convection model also shows a large temperature change near the

660-km discontinuity, because this is the boundary between shallow and deep convection

systems in this model The temperature distribution for whole-mantle convection,

pre-ferred by most scientists, shows a rather smooth decrease from the top of the D′′ layer to

the LVZ

Plate Tectonics

Plate tectonics, which has so profoundly influenced geologic thinking since the early

1970s, provides valuable insight into the mechanisms by which the Earth’s crust and

mantle have evolved as well as into how the Earth has cooled Plate tectonics is a

uni-fying model that attempts to explain the origin of patterns of deformation in the crust,

earthquake distribution, supercontinents, and midocean ridges and that provides a

mech-anism for the Earth to cool Two major premises of plate tectonics are as follows:

1 The lithosphere behaves as a strong, rigid substance resting on a weaker asthenosphere

2 The lithosphere is broken into numerous segments or plates that are in motion with

respect to one another and are continually changing shape and size (Fig 1.3)

The parental theory of plate tectonics, seafloor spreading, states that new lithosphere

is formed at ocean ridges and moves from ridge axes with a motion like that of a

con-veyor belt as new lithosphere fills the resulting crack or rift The mosaic of plates, which

ranges from 50 to more than 200 km thick, is bounded by ocean ridges, subduction zones

(partly collisional boundaries), and transform faults (boundaries along which plates slide

by each other) (Fig 1.3, cross-sections) To accommodate the newly created lithosphere,

oceanic plates return to the mantle at subduction zones so that the surface area of the

Earth remains constant

Plate Tectonics

Trang 21

M id -Ind

Caspian Sea

Baikal Rift

Japan Sea

Sea of Okhotsk

India

East African Rift System

Sunda Arc

Tasman Sea

Caroline Ridge

Kermadec Arc

Alpine Fault

Mariana Arc

Izu-Bonin Arc

Emperor-Hawaiian Chain

▲

▲ ▲ ▲

▲ ▲

▲ ▲ ▲

▲ ▲ ▲ ▲

▲

Philippine Plate

ANTARCTIC PLATE

Figure 1.3 Map of the major lithospheric plates on Earth Arrows are directions of plate motion Filled barbs are convergent plate

bound-aries (subduction zones and collisional orogens); single lines are divergent plate boundbound-aries (ocean ridges) and transform faults Cross-sections show details of typical plate boundaries Artwork by Dennis Tasa courtesy of Tasa Graphic Arts.

Many scientists consider the widespread acceptance of the plate tectonic model as a revolution in the earth ences As pointed out by J Tuzo Wilson in 1968, scientific disciplines tend to evolve from a stage primarily of datagathering, characterized by transient hypotheses, to a stage at which a new unifying theory or theories are proposedthat explain a great deal of the accumulated data Physics and chemistry underwent such revolutions around thebeginning of the twentieth century, whereas earth sciences entered such a revolution in the late 1960s As with

Trang 22

▲ ▲ ▲ ▲ ▲ ▲

Gulf of Mexico

Antilles Arc

Rio Grande Rift

Galapagos Ridge

San Andreas Fault

Chile Ridge NORTH AMERICAN PLATE

ANTARCTIC PLATE

Divergent boundary Convergent boundary

Transform boundary

Divergent plate boundaries

Convergent plate boundaries

Transform plate boundaries

Scotia Plate

Nazca Plate

PACIFIC PLATE

SOUTH AMERICAN PLATE

Caribbean Plate

Figure 1.3, cont’d.

Trang 23

scientific revolutions in other fields, new ideas and interpretations do not invalidate earlierobservations On the contrary, the theories of seafloor spreading and plate tectonics offerfor the first time a unified explanation for heretofore seemingly unrelated observations inthe fields of geology, paleontology, geochemistry, and geophysics.

The origin and evolution of the Earth’s crust is a tantalizing question that has lated much speculation and debate since the early part of the nineteenth century Some ofthe first problems recognized, such as how and when the oceanic and continental crustformed, remain a matter of considerable controversy Results from the Moon and otherplanets indicate that the Earth’s crust may be a unique feature in the solar system.The rapid accumulation of data in the fields of geophysics, geochemistry, and geologysince 1970 has added much to our understanding of the physical and chemical nature ofthe Earth’s crust and of the processes by which it evolved Evidence favors a source forthe materials composing the crust from within the Earth Partial melting of the mantleproduces magma that moves to the surface and forms the crust The continental crust,being less dense than the underlying mantle, rises isostatically above sea level and issubjected to weathering and erosion Eroded materials are partly deposited on continen-tal margins and partly returned to the mantle by subduction to be recycled and perhapsagain become part of the crust at a later time

stimu-Is the Earth Unique?

There are many features of our planet indicating that it is unique among planets in theSolar System and certainly among planets discovered so far around other stars Consider,for instance, the following characteristics:

1 The Earth’s near-circular orbit results in a more or less constant amount of heat fromthe Sun If the orbit were more elliptical, the Earth would freeze over in the winterand roast in the summer In such a case, higher forms of life could not survive

2 If the Earth were much larger, the force of gravity would be too strong for higherlife forms to exist

3 If the Earth was much smaller, water and oxygen would escape from the phere and higher life forms could not survive

atmos-4 If the Earth was only 5% closer to the Sun, the oceans would evaporate and house gases would cause the surface temperature to rise too high for any life toexist (like on Venus today)

green-5 If the Earth was only 5% farther from the Sun, the oceans would freeze over andphotosynthesis would be greatly reduced, leading to a decrease in atmosphericoxygen Again, higher life forms could not exist

6 If the Earth didn’t have plate tectonics, there would be no continents; thus, largenumbers of subaerial higher life forms could not exist

7 If the Earth did not have a magnetic field of just the right strength, lethal cosmicrays would kill most or all life forms (including humans) on the planet

Định dạng
Số trang	47
Dung lượng	1,64 MB