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In the final few chapters we shall consider the plan-et earth. We shall make observations about it and applyto them the natural laws and the physical and chemicalprinciples we have learned previously. From these datawe shall build models (or theories) that explain how theearth came to be the way it is. If our models are partic-ularly good, they will not only explain the past but alsoallow us to predict some things about the future of ourplanet.

The models cannot be arbitrary, however; theymust be consistent with what we observe, both now andin the future. No crystal ball permits us to foreseeobservations that future scientists will make, but thosefuture observations are the data from which refinementsto our models will be made. Theories that require toomuch revision may simply be discarded and replaced bybetter ones, but they will at least have served as step-ping-stones to more refined theories.

In general terms, we know quite well what the earthlooks like and how it behaves today, and the theories webuild to reconstruct its development must lead to a plan-et that looks and behaves as ours does. Whether the the-ories do that successfully will depend in large measureon how we envision the beginning of the earth. If wechoose the wrong sort of beginning, then correct ideasabout the development of the earth may not predict itspresent state; whereas theories that do predict its presentstate may, nonetheless, be incorrect. Moreover, theoriesabout the evolution of the earth should also be capableof dealing with the development of other planets, per-haps with different conclusions, owing to the variety ofconditions that prevail on those bodies. Alternatively,we would have to believe that each planet had an inde-pendent origin, and it would then probably be beyondour ability to understand how it all came about. Wemust first, then, consider the origin of the sun and itscompanions in space—the solar system.

The Solar System

Our sun is a rather ordinary star—compared toother stars, neither very large nor very small, neithervery bright nor very dim. Like other stars, the sun isthought to have formed from an accreting cloud of dustand gas, condensing under the influence of gravity into

a protostar, and finally becoming a star as nuclear reac-tions in its core began converting hydrogen to helium.The sun certainly was not among the first generation ofstars to form after the Big Bang: First, a star of its sizewould long since have exhausted its fuel if it were thatold; and second, it contains enough of the heavier ele-ments that, according to our current models of stellarevolution, it is enriched by material disseminated bysupernovae of earlier generations.

The sun is surrounded by a system of planets (seeFig. 28.1) and other, smaller bodies with a number ofcharacteristics that suggest a common origin. Forexample, the densities of the planets show a generaldecrease with distance from the sun; that is, the densestplanets are nearest to the sun. All of the planets revolvein the same direction about the sun and, with the excep-tion of Pluto, all of them have orbits that are nearly cir-cular and nearly in the same plane, which is called theplane of the ecliptic. With one exception, the spin axesof all the planets are inclined less than 30° to the planeof the ecliptic, and all but two rotate around those axesin the same direction: Venus spins on its axis in theopposite direction from the others, and Uranus is tippedso that its rotation axis is almost in the plane of theecliptic (that is, it rolls, rather than spins, around thesun). Other evidence that points to a common, coherentorigin for the solar system could be cited, but the pointis that our models of the very early earth must alsoinclude the sun and the other planets—and must, inci-dentally, be flexible enough to account for exceptionslike the inclination of the orbit of Pluto.

During the middle of the 18th century, two theoriesabout the origin of the solar system emerged: the cata-strophe theory and the nebular theory. Comte Georges-Louis Leclerc de Buffon (French naturalist, 1707-1788)first suggested that the material that formed the planetswas ejected from the sun when it was struck by a comet.(Early ideas about the sizes and compositions of cometswere not very accurate!) A variation on this theme wasthat a passing star drew the material from the sun by theforce of gravity. These sorts of theories, collectivelycalled catastrophe theories, required unusual and vio-lent events to explain the solar system; and, inasmuch assuch events would be very infrequent in the galaxy, theypredicted the existence of planets to be exceptional.

28. Planet Earth

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The nebular hypothesis, which was initially proposedby Immanuel Kant (German philosopher, 1724-1804)and independently some years later by the MarquisPierre-Simon de Laplace (French mathematician, 1749-1827), viewed planetary formation as a natural part ofnormal stellar evolution and so predicted that planetsshould, with high probability, accompany most stars.

The nebular theory is the currently preferredmodel, but it did not reach that status without difficulty.The theory originally proposed that a star formed froma slowly rotating mass of gas and dust. As gravitypulled material toward the center of mass, the rotationrate increased for the same reason that a skater rotatesfaster when she pulls in her arms. (The principle iscalled the conservation of angular momentum and isa conservation law similar to those we covered earlier inthis text. If a spinning object contracts in a directionperpendicular to the axis of rotation, then it must spin

faster to satisfy this conservation law. Distributing themass further from the axis of rotation results in a slow-er rate of spin.) As the rotation rate of the condensingstar increased, rings of matter were thought to bethrown off and eventually to condense to become plan-ets. Formulated in this way, the nebular theory wasdoomed for at least two reasons. First, calculationsshow that matter thrown off a spinning star would dissi-pate into the interstellar medium rather than condense toform planets. Second, the theory predicts that most ofthe angular momentum of the solar system shouldreside in the sun; in fact, most of it resides in the plan-ets, and the sun spins too slowly for the solar system tohave formed in this way.

The current formulation of the nebular hypothesisalso begins with the condensing cloud of gas and dust,but then diverges from the old theory (see Fig. 28.2). Asthe cloud rotated, there would be no restriction on its

Pluto

Neptune

Uranus

Saturn

Mars

Earth

Venus

MercurySun

Jupiter

Figure 28.1. A view of the solar system. Planetary orbits are drawn to scale, but the sizes of the planets and sun are not.

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flattening along the axis of rotation, and so it wouldbecome a rotating disc. Within that disc, random varia-tions in density would naturally occur, and gas and dustparticles rotating about the protosun would begin toform clumps. These would grow by gravitationalattraction of nearby clumps until finally the central starwould be surrounded by a disc of orbiting chunks ofmatter called planetesimals, each one perhaps hundredsof kilometers across, which would further condense toform protoplanets. Finally, a few relatively large bod-ies—the planets—would sweep up all of the planetesi-mals within their gravitational reach. Some of thesmaller chunks might have become satellites (moons)for the larger ones. The problem with angular momen-tum is not completely overcome by this new view of thenebular hypothesis, but the discovery of the solar wind(a stream of particles constantly spewed outward by thesun) provides one known way in which angular momen-tum is transferred away from the sun. There may alsobe other ways. It is quite possible that the early sunrotated considerably faster than it does at present (oncein 25 earth-days) and that it is slowing to conserveangular momentum as it ejects material.

There is some observational evidence—admittedlytentative, but nonetheless tantalizing—to support thenebular hypothesis. From the southern hemisphere onecan see a star named beta Pictoris (a Greek letter fol-lowed by the name of the constellation it is in). Figure28.3 shows how that star appears when the bright lightfrom it and others in the field of view has been blockedout; a faint disk of matter, seen edge-on, is clearly visi-ble. This looks very similar to what we would expect tosee in a forming star with planets, according to the neb-ular theory. In addition, a few other nearby stars seemto wobble ever so slightly or to display distinctivelybroadened lines in their spectra, suggesting that they arepulled by the gravity of unseen objects that accompanythem—possibly planets.

Figure 28.3. Beta Pictoris, a star seen from the southernhemisphere of the earth, has a disk of gas that surroundsit and that is visible by a special technique that blocksout interfering light from stars. From our location in thegalaxy, we see the disc edge-on. It extends from thecentral star over ten times the distance from the sun toPluto. (After B. A. Smith and R. J. Terrile, Science, vol.226, p. 1422.)

In the following section, we shall take a brief tour ofthe planets, ending with the earth. More has beenlearned about these bodies from space probes in the lastseveral decades than was known in all previous time.Not only shall we see that the nebular hypothesisaccounts for many of the properties of these planets, butalso that it is flexible enough to accommodate some verysingular catastrophic events that must be invoked toexplain some of what we see. In this sense, the current-

Figure 28.2. A series of sketches depicting the leadingtheory for the formation of the solar system. Theyshow, schematically, the interstellar dust and gas con-densing to form a protostar that flattens, transferringangular momentum to the surrounding disk of material.The disk proceeds from clumps of aggregated dust toplanetesimals to protoplanets, finally resulting in a starsurrounded by planets.

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ly accepted model of the formation of the solar system isa blend of the nebular and the catastrophe theories.

A Brief Tour of the Planets

The planets are generally divided into two groups:the terrestrial planets and the Jovian planets. The ter-restrial planets are the four that are closest to the sun(Mercury, Venus, Earth, and Mars); they are rocky incomposition, denser than the other planets, and relative-ly small. The Jovian planets are the next four out fromthe sun (Jupiter, Saturn, Uranus, and Neptune); they arelargely gaseous in composition, have low densities, andare very large. Pluto, the planet with the largest orbit, ismuch like the terrestrial planets; it is thus an anomaly,but one that can be explained by current ideas about itsformation. Color Plates 9 through 12 show some of theplanets.

Mercury is the planet nearest the sun. If we definean astronomical unit (AU) as the average distance ofthe earth from the sun, then the average distance ofMercury from the sun is 0.4 AU. Less than half thediameter of the earth, it is a small, rocky planet pockedby craters (Color Plate 9) that are remnants of the plan-etesimal bombardment that occurred as the planets wereforming. Because Mercury has a high temperature anda small mass (and hence a weak gravitational field), ithas retained neither water nor, for all practical purpos-es, any atmosphere; thus erosion has never occurred onMercury, and these early craters have never been wornaway as they have on earth. There are also features thatappear to be lava flows, suggesting that Mercury onceunderwent an episode of intense heating and partialmelting.

Venus is a mysterious planet because it is coveredby a dense atmosphere that prohibits direct observationof the surface. It is about the same size as the earth andabout 0.7 AU from the sun. The atmosphere consists ofdense clouds of carbon dioxide with sulfuric acid andvery small amounts of water vapor. The greenhouseeffect (the rise in temperature when heat [infrared radi-ation] produced by incoming sunlight is unable toescape because carbon dioxide is opaque to infraredradiation) has resulted in a surface temperature of about475 °C—very uncomfortable indeed! Radar mapsmade by a space probe that orbited Venus for over adecade reveal the presence of large plateaus, craters,volcanoes, and several types of exotic features notentirely like any found on earth. The planet rotates in adirection opposite to that of the other planets, and a sat-isfactory explanation can only be guessed; perhapsVenus had an off-center collision with a very large plan-etesimal during its formative stages and this set it rotat-ing “backward.” (Such ideas may be correct, but theyrequire speculation about unusual and unpredictableevents, and such speculation inevitably makes scientists

uncomfortable because there is no way to either proveor disprove that the events ever happened.)

Skipping the earth for the moment, we proceed out-ward to Mars, the red planet, which revolves about thesun at an average distance of 1.5 AU (Color Plate 10).Because it is smaller than the earth and has only aboutone-tenth the mass, Mars has retained only a thin atmos-phere consisting mostly of carbon dioxide. The Martianlandscape possesses sand dunes, impact craters, gigan-tic canyons, and extinct volcanoes that dwarf any onearth; the largest, Olympus Mons (Fig. 28.4), wouldcover the state of Utah and rise 25 kilometers (15 miles)above it! Among the most intriguing features are somethat so resemble stream channels that no other explana-tion seems reasonable. With an average surface tem-perature of -53 °C, there is no liquid surface water onthe planet now (though there is frozen water under thesurface), but during its younger days, Mars must haveexperienced erosion from running water. Speculationabout life on Mars has always been popular, especiallybecause some areas of the surface get darker during theMartian summer, but none of the three unmanned space-craft (one Soviet, two U.S.) that landed on Mars during

Figure 28.4. Olympus Mons, a Martian volcano thatrises 25 kilometers (15 miles) above the surface of theplanet. The outline of the State of Utah is superimposedfor scale.

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the 1970s detected any signs of life in the atmosphere orin the soil. However, an announcement was made in thesummer of 1996 that a meteorite found on earth, butwhich originated in a large impact on Mars, containedpossible evidence of fossil bacteria—an intriguing hintabout what might once have been. Whereas the twoinnermost planets have no satellites, Mars is circled bytwo small, rocky moons.

Beyond Mars, we move outward to the Jovian plan-ets—giants compared to the terrestrial planets and dif-ferent from them in nearly every significant way.Jupiter, the first, is 5.2 AU from the sun and more thaneleven times the diameter of the earth. If it were thesame density as earth, it would be 1400 times moremassive than our planet; in fact, it is only about 300times more massive because its density is less than one-fourth that of earth and only a little more than that ofwater. During the formation of the solar system,according to the nebular theory, the lightest gases—hydrogen and helium—would have been driven awayfrom the inner planets by the young, hot star. At the dis-tance of Jupiter, energy from the sun would not havebeen intense enough to strip away these gases; soJupiter, with the other Jovian planets, consists mostly ofhydrogen (with lesser amounts of helium and other ele-ments) compressed by the intense gravitational fieldproduced by that much mass. Below a thin skin of gas,the hydrogen is probably in a liquid state. Roughlyhalf-way to the center of the planet, pressures and tem-peratures are such that metallic hydrogen, a formunknown on earth, should form. There may well be arocky or metallic core, but it does not account for muchof the total mass of the planet. Seen through a telescopeor in photographs sent back by the Voyager spacecraft(Color Plate 11), Jupiter is a beautiful sight with its vari-colored cloud belts spread out parallel to the equator byvery rapid planetary rotation (once in only ten hours).

Jupiter has 16 satellites, four of which are largeenough to be seen from earth with a good pair of binoc-ulars. These four are roughly the size of earth’s moon,and two of them have cratered areas much like our satel-lite’s. There are other features that are not similar to anyon our moon, though. One of Jupiter’s satellites hasareas covered with curious grooves the size of theAppalachian Mountains. Another has a smooth surfaceof ice covered with cracks. The one closest to Jupiter isdeformed by enormous tides, and molten sulfur fromactive sulfur volcanoes covers its surface. Strangeworlds, these.

Next comes Saturn, arguably the most beautifulsight in the solar system (Color Plate 12). It is nearlytwice as far from the sun (9.5 AU) as Jupiter and hasabout one-third the mass and half the density. With adiameter nearly ten times that of earth, Saturn is certain-ly among the giant planets. Besides at least 18 moons, itis circled by a system of rings that look solid and opaque

(yet somehow delicate) from earth but comprises asmany as 1000 individual ringlets, each one consisting, inturn, of rock and ice in chunks a few meters across todust-size. The ring system is extremely thin, only two tofive kilometers thick, and over 400,000 kilometers frominner to outer edge. (Put another way, if Saturn’s ringswere only as wide as this page, they would be less thanthree one-hundredths as thick!) Like Jupiter, Saturn iscomposed mostly of hydrogen, liquid not far below thesurface and probably becoming solid and metallic deepin the interior. The moons of Saturn, like those ofJupiter, show some strange phenomena (like water-icevolcanoes), but its largest satellite (Titan) has a featureknown on only one other satellite (Triton, circlingNeptune) in the solar system: an atmosphere. Nitrogen(the same gas that makes up four-fifths of our ownatmosphere) comprises nearly all of it, with a littlemethane and some other hydrocarbons.

At 19.2 AU from the sun is Uranus, a planet so faraway that it looks like a faint, structureless, greenishdisk even in a good telescope, although Voyager 2 pro-vided us some much better views. Though half the sizeof Saturn, it is still one of the four giant planets. Theoddest thing about Uranus is that its axis of rotation(that is, the line between its north and south poles) isinclined only about 8° to the plane of the ecliptic; it layson its side and rolls around the sun. In 1977 Uranuspassed in front of a distant star and revealed an unex-pected phenomenon: As the planet approached theimage of the star, the star blinked out five times beforepassing behind it, then it reappeared on the other sideand blinked out five more times as the planet movedaway. The interpretation of this behavior was clear—Uranus is surrounded by a system of thin rings, invisi-ble but not undetectable from earth. Subsequent occul-tations and the Voyager 2 photographs have revealedthat there are a total of ten rings in the Uranian system.(Jupiter, incidentally, has a single narrow ring, too, thatwas discovered during the Voyager 1 mission.)

Besides Uranus’ five major satellites that wereknown from earth-based observation, the Voyager 2encounter revealed ten others, all small and within theorbit of the innermost major moon. Like the moons ofJupiter and Saturn that were also investigated duringVoyager missions, these satellites are different from oneanother; one of them has a strangely wrinkled andscarred surface unlike anything so far seen elsewhere inthe solar system. It seems that each new encounter by aspace probe reveals surprising images that challengeour conservative, terrestrial prejudices about howworlds ought to be.

Little is known of the eighth planet, Neptune, cir-cling the sun at 30 AU. Although it was discovered in1846 by applying Newton’s Laws to irregularities in theorbit of Uranus, a dim, small, blue-green dot was all thatwe saw until Voyager 2 reached the planet in August

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1989. It is about the size of Uranus and very likely sim-ilar in composition—mostly hydrogen, methane, heli-um, water ice, and ammonia ice. It has two moonsknown from earth-based observations: one about thesize of our moon and traveling in a nearly circular orbit,but circling Neptune in a backward direction relative tothe planet’s rotation; and the other quite small and in avery elongated elliptical orbit. These orbital curiositiessuggest that both were probably stray bodies capturedby the gravity of Neptune, rather than bodies thatformed along with it. As on one of Saturn’s moons, anatmosphere has been detected on the larger satellite ofNeptune. The Voyager 2 spacecraft detected six othersatellites and a system of three thin rings aboutNeptune. Neptune is the windiest of the planets, withgales reaching speeds near that of sound.

Pluto, on the edge of the solar system and not dis-covered until 1930, is shrouded in mystery. Even in thelargest telescopes, it appears as only a point of lightmoving extremely slowly among the “fixed” stars.Pluto is only about two-thirds the diameter of earth’smoon and is thus more like the terrestrial than theJovian planets. It travels about the sun in a very ellipti-cal orbit that averages 39.5 AU in radius, but actuallycomes closer to the sun at its closest approach than doesNeptune. In fact, during the last two decades of the 20thcentury Pluto is the eighth planet from the sun andNeptune is the ninth. Besides its strange shape, the orbitis inclined to the plane of the ecliptic by 17°, over 10°more than any other planet. Pluto’s single known satel-lite circles the planet in a plane nearly perpendicular tothe planetary orbit itself. These facts led to the once-popular hypothesis (speculation might be a more appro-priate term) that Pluto and its single known satellite aremoons that escaped from Neptune. The escape couldhave been caused by the nearby passage of some ran-dom large body moving through the early solar system;but such a body would have left no trace of its exis-tence, and it is unlikely that such a theory could ever beproven. Besides, recent calculations predict that suchan occurrence would likely have disintegrated both bod-ies. Unfortunately, neither Voyager 2 nor any otherplanned space probe will fly by Pluto, so we may neverknow more than we can learn by looking from earth.

The Moon

Before we end our tour by returning to our ownplanet, the focus of the remaining chapters, we stop toconsider our nearest celestial neighbor, the moon.Considering how long we have been looking at themoon, even traveling there, it might seem as if weshould know much more about it than we know aboutthe more distant planets. Often in science, though, theavailability of more data simply generates more ques-tions, and the answers come slowly. We know a great

deal about the moon, but the answers to some of themost interesting questions—those about its origin—arestill elusive. Before considering what we don’t know,we’ll look at what we do know.

The moon is bone dry and has no atmospherebecause its gravity is insufficient to hold gases. Thus,there is nothing to moderate the temperature, whichranges from over 100°C when the sun is overhead tolower than -100°C during the lunar night. The moonrevolves once on its axis in the same time that it rotatesonce about the earth, so it always keeps the same sidetoward us, and its days and nights are both nearly 15earth days long. (When the moon is “full,” it is essen-tially opposite the sun from us, and it is midday for thecenter of the side toward us. When the moon is “new,”it is up during the day, and thus on the same side of theearth as the sun; the same features that we saw at the“lunar midday” two weeks before are now at “lunarmidnight.”)

From the earth, two strikingly different kinds oftopography are visible even without a telescope orbinoculars. Some areas are comparatively bright andwhite, while others are darker gray. The brighter areasare the lunar highlands and they are the most ancientlunar terrains. The darker regions are called maria (sin-gular, mare—Latin for sea, although they contain nowater), and they consist of relatively thin veneers oflava erupted into large, shallow basins after the forma-tion of the highlands. A good pair of binoculars or asmall telescope will reveal that the highlands are heavi-ly cratered (Color Plate 13), and the maria are consider-ably smoother. Even the maria contain some craters,though, and some of these have rays that emanate fromthem—like the spokes of a wheel—and cover the mariaaround them. These observations permit the followinggeneralization of lunar history.

Like the other planets and satellites in the solar sys-tem, the moon formed by accretion of planetesimals(although the different models of lunar origin, which weshall discuss later, attribute the accretion to differentcauses). Early in the moon’s history, planetesimals ofall sizes rained down upon its surface, generally pulver-izing the landscape and adding mass to the young moon.During this time the surface became literally saturatedwith craters. In each collision, the moon absorbed thekinetic energy of an incoming planetesimal and con-verted it to internal (thermal) energy in the same waythat pounding on a nail heats it. Eventually enough heatwas generated to melt the crust and form a molten“ocean” over the entire surface. The less dense volatilecompounds, like water and carbon dioxide, came to thesurface and were lost to space, and the less volatileremainder solidified. The last stage of the bombard-ment phase was still going on, of course, though it wastapering off, so the new crust was re-cratered, and a fewvery large meteor impacts created shallow basins many

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tens of kilometers across. (Some meteors are chunks ofrock that orbit the sun as remnants of the early solar sys-tem—pieces that were not swept up by the formation ofthe larger bodies—while others are the icy remains ofdisintegrated comets.) Later, molten rock, perhapsreheated by radioactive decay, pushed its way throughthe thin crust over the large, shallow basins and floodedthem, creating the maria. Finally, the last of the craters,including those in the maria, were formed; the rays thatemanate from some of these consist of debris“splashed” out from the impact site. Although smallmeteors must still occasionally hit the moon, no newcraters large enough to be seen from earth have beenobserved since the telescope was invented. (Manymeteors hit the earth’s atmosphere, but they are burnedup by friction, becoming “shooting stars.”)

Five theories purporting to explain the origin of themoon have been popular at one time or another—froma scientific point of view, at least. One is that the moondeveloped as an equatorial bulge on a very rapidly spin-ning young, molten earth, and was “thrown off” intoorbit (in the sense of Newton’s first law—it simplyescaped from the earth when gravity could not hold itin). Several drawbacks—problems with conservationof angular momentum, small differences in the chem-istry of moon rocks and earth rocks, and others—makethis theory, the fission model, a very unlikely possibili-ty.

The binary accretion model holds that the earthand the moon were accreted separately from the samecloud of material in the primitive solar nebula. Minordifferences in the chemical compositions of the twobodies are difficult to explain in terms of this idea, butthe fact that the ratios of the various oxygen isotopes arethe same for both the earth and the moon suggests thatthe earth and moon formed in at least the same region ofthe solar nebula.

According to the capture model the moon was awanderer that came within the earth’s gravitational pulland was caught; some difficulties in the detailed dynam-ics of such a capture, as well as the oxygen-isotopicsimilarities, constitute problems for this idea.

A fourth idea, the tidal disintegration model,requires incoming planetesimals to be disintegrated bythe gravity of the enlarging earth and reaccreted to formthe moon; this theory suffers from problems involvingangular momentum.

Finally, there is recent enthusiasm for a collisionalmodel in which the early earth is rammed by a large(Mars-size) planetesimal, and so the material of themoon comes partly from that body and partly from theearth. This theory looks promising (as have the others atvarious times), but many of the theoretical details havenot yet been worked out sufficiently to test them. Noneof the theories appear fully satisfactory at this point, andit is possible that the truth lies in some combination of

them or in some idea that no one has yet suggested.

The Earth

As we leave the moon, our final destination capturesour attention. Looming large in the black lunar sky, theearth looks unlike any planet or satellite we have seen inthe solar system. It obviously possesses an atmosphere,for we can see clouds as white swirls against the back-ground of the planet’s surface. Unlike clouds we haveseen elsewhere, however, these are made of watervapor—a substance absolutely indispensable to life, yetnotably scarce throughout the rest of the solar system.Most of the atmosphere is nitrogen, but one-fifth isanother gas uncommon in the solar system—oxygen,contributed virtually entirely by living plants.

We are struck by how blue the earth is (Color Plate14). The blue, of course, is liquid water in the oceans.Nowhere else have we encountered a body on which liq-uid water is stable at the surface. Earth happens to liejust within the inner edge of the continuously habitablezone (CHZ), a shell around the sun inside of which aplanet would lose its water because of strong solar radi-ation and outside of which temperatures would be toolow to sustain life. If the earth were just 5 percent clos-er to the sun, we could not live here. The CHZ probablyextends to just beyond the orbit of Mars; but for reasonswe can’t go into here, a planet as small as Mars cannotproduce an atmosphere capable of supporting complexlife forms. A planet as large as earth might, but it wouldbe unable to support human life at that distance from thesun. Hence, the size of the earth and its distance fromthe sun have combined to yield a planet uniquely suitedto life. In making this observation we do not suggest inany way that this is happenstance or accident. We dosuggest that, intentional as this uniqueness is, it wasbrought to pass by means that are essentially natural and,incomplete as our current theories may be, they at leastpropel us in the direction of understanding.

As we near the earth, it is apparent that there aretwo essentially different types of surfaces. One is theliquid surface of the oceans (the hydrosphere, or“water sphere”), and the other is the solid surface of thecontinents (the lithosphere, the brittle outer shell). Forour present purposes, we are not particularly interestedin the water of the oceans but rather in the solid rocksthat make up the floors of them, so we shall ignore thehydrosphere. Each type of surface—the continents andthe ocean floors—has features not found in the other,and, while it may be a few chapters further before weunderstand all of what we see, it is worth pointing outthese features here so that we at least know some of thequestions we ought to be asking.

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The Continents

We have become accustomed to seeing impactcraters on virtually any solid surface in the solar system;on earth they seem strangely rare, although a searchturns up a few. Meteor Crater in Arizona is a beautifulexample of such a feature (Fig. 28.5). A couple of oth-ers—or at least features that may be very large andancient meteor impact craters—are discernible inCanada in the vicinity of Hudson Bay. How different isthe surface of the earth from that of the moon! It is thisvery comparison, though, that provides insight into thedifference. The moon has no atmosphere and no waterto alter the surface. On the earth, erosion is a continualprocess that wears away surface features. There is noreason for us to believe that the earth did not experiencea period of intense bombardment in its formative era,just as the moon did, but the evidence is largely wornaway. This leads to a question that needs to be investi-gated in subsequent chapters: If the process of erosionwears away high areas of the continents, why are therestill mountains? Is erosion so slow that there has notbeen time to wear the mountains away, or are thereprocesses that oppose erosion and build mountains?

Figure 28.5. Meteor Crater, Arizona. Theoretical mod-els of the early history of the earth suggest that our plan-et was once as cratered as the moon but that erosion hasobliterated the direct evidence.

There are three genetically different sorts of rockspresent on earth—igneous, sedimentary, and meta-morphic—and each kind consists of natural chemicalcompounds called minerals. Most of the common min-erals are silicates (that is, they contain the SiO4

4– mole-cular ion) or carbonates (containing the CO3

2– molecu-lar ion). The names for the rocks imply somethingabout the processes that produced them. The English

word igneous comes from the Latin word for fire;igneous rocks result when molten rock solidifies.Molten rock is called magma when it is underneath thesurface of the earth and lava when it is erupted onto thesurface. So igneous rocks are formed when magma orlava crystallize. While we are accustomed to thinkingof freezing as a process associated with cold, freezingfor molten rock is anywhere from 600 °C to 1200 °C,depending on the chemical composition of the melt.Typical igneous rocks that you might have seen aregranite and basalt (Fig. 28.6). The minerals that formthe basalt are too small to be seen without magnifica-tion, but the different white, gray, and black mineralsthat constitute the granite are visible.

Figure 28.6. Typical igneous rocks: A granite (left) anda basalt (right). Notice the separate minerals that con-stitute the granite. The basalt is also made of distinctminerals, but they are too small to be seen without mag-nification.

Sedimentary rocks are formed when debris erodedfrom other rocks is transported (mostly by runningwater), accumulates in basins, and is gradually buriedby later sediment. Eventually the pressure of overlyingsediment compacts the deep material, and it is cement-ed together by minerals precipitated from groundwater.The nature of this process most often results in layeredrocks, like some kinds you may have seen in canyonsand mountains. Alternatively, some kinds of sedimen-tary rocks form by chemical precipitation from bodiesof water. Some common sedimentary rocks are sand-stone, shale, and limestone (see Fig. 28.7).

Sometimes igneous or sedimentary rocks are burieddeep in the earth and subjected to intense heat and pres-sure. Under such conditions the minerals of which theyare made may become unstable and undergo chemicalreactions. The products of those reactions (which gener-ally also involve fluids) are other minerals that are stableunder the new conditions. The resulting rocks are calledmetamorphic rocks because they have been changed.(Metamorphic rocks themselves can be re-metamor-phosed, too.) Some metamorphic rocks you might have

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heard of are slate, gneiss, and marble; these are, respec-tively, metamorphosed shale, granite, and limestone.Figure 28.8 shows how these typically appear.

Despite the widely differing appearances andchemical compositions of all the sorts of rocks found onthe continents, the average rock type is essentially gran-ite. This means that if one put representative amountsof all the continental rocks into a crucible and allowedthem to crystallize after being melted, the result wouldbe something very much like granite.

The continents all have different shapes, but allhave essentially the same sort of anatomy, and general-izations we make about one will apply to all of the oth-ers. Because most of us are familiar with the geograph-ical names and features of North America, we shall useNorth America as our model continent. We find thatNorth America has two essentially different major phys-iographic regions (Fig. 28.9): the craton (consisting ofthe shield and the stable platform) and the fold moun-tain belts. The craton is that part of the interior of a con-tinent that has been geologically quiet for a long time(meaning the last several hundreds of millions of yearsof earth history). Thus, it has long been relatively freefrom earthquakes and the sort of deformation that haveproduced the present mountain ranges.

The Continental Shield

In the northern part of North America, encirclingHudson Bay, is a region of very ancient, very erodedrocks known as the shield. It gets its name from the factthat its profile would somewhat resemble the gentlyrounded shape of a warrior’s shield—high in the centerand sloping toward the edges. It is not in the geograph-ic center of North America, but in a geologic sense it isthe core of the continent. The oldest rocks to be foundon the continent are in the shield. Structurally, it con-sists mostly of igneous and metamorphic rocks that are

the roots of ancient mountains long since worn away.The rocks are now a vast lowland, mostly less than 50meters above sea level. Some continents have morethan one shield (which tells us something about howcontinents formed in the first place, but we shall discussthat more in another chapter).

The Stable Platform

To the west and south of the Canadian shield is apart of the craton in which the shield is covered by aveneer of sedimentary rocks. This is the stable plat-form. The sediment has come from erosion of theshield itself and from the mountain ranges that surroundthe stable platform. The rocks are approximately 1000meters thick on the average, and except for gentle warp-ing that produced broad, shallow basins and broad, lowdomes, they are essentially flat and horizontal. Only ina few places do rocks of the shield protrude through thesedimentary cover, but deep wells assure us that it isthere, nonetheless. Like the shields, the existence ofstable platforms is common to all continents.

Fold Mountain Belts

Bordering the stable platform are the fold mountainbelts. They are called fold mountains because therocks that constitute them are warped into folds, in thesame way that a carpet shoved against a wall would bewarped into alternating “up-folds” and “down-folds.”(We point out that the mountains themselves are notnecessarily the “up-folds” and the valleys the “down-folds.” The folding is seen in the internal structure ofthe mountain belts, not necessarily in their topography.)They are called “belts” because they occur in long, lin-ear (or mildly curved) ranges. The AppalachianMountains stretch from Georgia into Maine andbeyond; the Rocky Mountains extend from Alaska

Figure 28.7. Typical sedimentary rocks: (a) sandstone,(b) shale, and (c) limestone.

Figure 28.8. The metamorphic rocks: (a) slate, (b)gneiss, and (c) marble.

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south into Central America, and they even appear tocontinue down the west coast of South America as theAndes Mountains. You are familiar, too, with the longbelt of mountains that begins as the Pyrenees betweenSpain and France, continues eastward as the Alps, thenbecomes the Caucasus, the Zagros, and finally theHimalayas. Some mountain ranges are higher and morerugged than others, but the geologic importance of afold mountain belt is not in its topography; it is in thestructure of the rocks, which indicates that they havebeen folded and deformed by huge horizontal (com-pressional) forces in the lithosphere over long expansesof time and are not as rigid and immobile as they appearduring the short duration of human experience.

The Continents Reconsidered

Based on the observations of continental anatomywe have just made, some obvious questions come tomind. Even if the shields are not in the geometrical cen-ters of the continents, they are surrounded by the stableplatforms, and those are rimmed by fold mountains.

This suggests some sort of predictable scenario for theformation of continents. What is it, and why are therecontinents at all? What makes the continents differentfrom the ocean basins, other than their elevations withrespect to sea level? Why are the mountains distributedin the way they are, instead of at random all over thesurfaces of continents? Why don’t mountain belts crossthrough the cratons? What is the origin of forces largeenough to produce fold mountain ranges? We shalladdress such questions in the succeeding chapters, butnot until we have first considered the geologic featuresthat characterize the ocean basins.

The Ocean Basins

Despite their appearance on a map of the world, thecontinents do not actually stop at the shorelines. If thewater of the oceans were removed, we would find thatthe rocks just seaward of the present shorelines are con-tinuations of the rocks of the continent and are quite dif-ferent from the rocks of the ocean basins themselves.This continental border that happens to be below pre-

Shield

Stable platform

Fold mountain belts

Figure 28.9. The major physiographic divisions of the North American continent. The shield and the stable platformtogether make up the craton. Other continents possess the same features, although the numbers of them vary from onecontinent to another.

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sent-day sea level is called the continental shelf.Toward the ocean from the continental shelf is a

region transitional to the ocean floor called the conti-nental slope. Beyond the continental slope are therocks that make up the ocean basin. They are basalt.Notice that we did not say “basaltic,” in the sense thatcontinental rocks were “granitic “ (granite only on theaverage). Except for the thin layer of sediment that cov-ers them and has nothing at all to do with the origin ofthe ocean floor, these rocks are basalt, period. To besure, they vary slightly in chemical composition fromlocation to location in the world, but their similarity ismuch more striking than any differences.

We shall discuss the issue of geologic time in a laterchapter and learn how it is possible to determine theages of rocks. For now, let us just postulate that such athing can be done. When the ages of the rocks of theocean floor are determined, a surprising result isobtained: None of the rocks are older than the mostrecent 5 percent of earth history. In contrast, the ages ofcontinental rocks span nearly all of earth history, from

the newest rocks formed in recent volcanic eruptions tothe most ancient rocks known on earth. Put anotherway, if all the years of earth history were represented bya hundred-meter dash at a track meet, the rocks of thecontinents would be strewn all along the track; but therocks of the ocean floors would be found only within 5meters of the finish line.

Figure 28.10 depicts the locations of the principalphysiographic features of the ocean floor; in outwardappearance they are at once similar to and differentfrom the features of the continents. There are large flatareas and hills called abyssal plains and abyssal hills,respectively. There is a major mountain chain that islong and linear but does not consist of fold mountains;this is the oceanic ridge. Island chains are more com-mon than single, isolated islands and are of two types:island arcs that are gently curved and always accompa-nied by deep oceanic trenches, and linear islandchains that often continue underwater at one end asseamounts.

Midocean ridge Trench Island arc

Linear island chain

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Figure 28.10. Locations of the major physiographic features of the ocean floor. Linear island chains includeseamounts. Abyssal hills are omitted for clarity but extend for hundreds of kilometers on either side of the oceanicridge.

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The Deep Ocean Floor

Most of the ocean floor is covered by small hills onlya few hundred meters higher than the surroundingseafloor. These are known as the abyssal (meaning “verydeep”) hills, and they are actually the most common land-forms on earth. They become smaller and smaller withdistance away from the oceanic ridge because the lowareas between them become filled with sediment. Insome places (particularly where a great deal of sedimentfrom a nearby continent is available), they are complete-ly covered over to give the remarkably flat abyssal plains.

The Oceanic Ridge

Winding around the globe like a giant snake is arange of mountains called the oceanic ridge. (Note thatit is not always in the middle of the ocean; but it is oftencalled the “midocean ridge” anyway, because it wasfirst discovered in the Atlantic Ocean where it is in themiddle.) This is actually the longest mountain chain inthe world and is considered by many geologists to bethe single most important geologic feature on the plan-et. It projects above sea level only at a few islands likeIceland. Like the rest of the seafloor, the oceanic ridgeis made of basalt; and unlike the continental mountainchains, it is not the product of compressional forces inthe lithosphere. Most of the heat that leaves the interi-or of the earth does so through the oceanic ridge system.

The ocean ridge is much broader than it is tall(about 1400 kilometers wide and only 3 kilometers tallat most), and a central rift valley runs lengthwise downits center. It is cut by many fractures that are essential-ly perpendicular to its length. Thus, it is a great curvi-linear swell that encircles the planet.

Island Arcs and Other Island Chains

Perhaps you have noticed that island chains aremore common than isolated islands, particularly in theCaribbean Sea and the Pacific Ocean. Some of thesechains, such as the Aleutians off Alaska, the Japaneseislands, and the Marianas, are broadly curved, or arc-shaped and are called island arcs. These are all vol-canic islands, and they are invariably flanked on oneside by deep oceanic trenches, the deepest surface fea-tures of the earth. The most well-known is the Marianastrench, which descends to over 11 kilometers (nearly 7miles!) below sea level. The island arcs are the sites ofnumerous, frequent, and strong earthquakes.

Another type of island chain is represented by theHawaiian Islands and others that are nearly parallel to itin the Pacific Ocean. These are not arc-shaped, they arenever accompanied by deep ocean trenches, and theyare not prone to large earthquakes. But they are vol-canic. Often these linear island chains continue under-

water at one end as former islands that are now belowsea level owing to erosion and subsidence of theseafloor around them.

The Oceans Reconsidered

As in the case of the continents, our observations ofthe ocean basins have generated a number of questions.How could such a vast expanse of nearly identical rockhave developed? Why are the rocks of the continents sovaried in comparison? Why are island arcs shaped asthey are, and why are they always adjacent to deeptrenches? What is the difference between an island arc,with its associated trench, and a linear chain, like theHawaiian Islands? What is the origin of the oceanicridge? Why are the ocean floors of uniformly younggeologic age? Were there any oceans during the ancienteras of geologic time? If so, what happened to the rocksof their floors? If not, then why are there oceans now?

Summary

Never in all of history has there been a more intel-lectually stimulating time to live. We have sent expedi-tions to the moon and other planets in our solar systemand have begun to find evidence of other planetary sys-tems in the universe. We have seen worlds that arebizarre (to say the least) when measured against our ter-restrial expectations. But not all of our discoveries haveconcerned worlds alien to our own. We have learnedmuch more about the earth than any previous generationhas known and have found it to be different in somevery important ways from other planets we know about.Its oxygen-rich atmosphere and surface water areunique in our solar system.

The earth appears to be geologically quite “alive.”Active volcanoes dot its surface, earthquakes occur inlarge numbers, and mountain ranges are evidently pro-duced by active processes that require gigantic horizon-tal forces—forces that must be absent on bodies like themoon and Mars, where there are no fold mountains andthe numerous craters are all circular and undeformed.The similarities among continents, their interior cratonsand surrounding mountain belts, suggest that theyundergo predictable developmental stages. The oceanfloors contrast sharply in both structure and age with thecontinents and are characterized by abyssal plains andhills, the oceanic ridge, trenches, island arcs, and linearisland chains. Moreover, the sorts of rocks that make upthe continents and ocean basins are strikingly different.

The earth is like a gigantic engine, with the energyrequired to run some parts of it coming from within andthe energy required to run other processes coming fromwithout. To understand how the engine works and tobegin uncovering the answers to some of the questionsthat have been posed in this chapter, we need to first

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establish the appropriate time scale for understandingthe development of our planet and then to determinewhat the interior of the earth is like and how it affectswhat we see on the outside.

STUDY GUIDEChapter 28: Planet Earth

A. FUNDAMENTAL PRINCIPLES1. Conservation of Angular Momentum: In the

absence of any net torque (“twist”), the angularmomentum of an object revolving about an axis isconserved, i.e., the product of the mass, the speed,and the distance from the object to the axis of rev-olution is a constant in time. See Chapter 7.

B. MODELS, IDEAS, QUESTIONS, OR APPLICA-TIONS

1. The Nebular Hypothesis as a Model for theFormation of the Solar System: The model inwhich a condensing cloud of gas and dust flattensinto a disc as it condenses. Inside this disc randomvariations in density would form planetesimalsorbiting around the protostar. These planetesimalswould condense by gravitational attraction to formprotoplanets which would eventually form planets.The core of material at the center of the systembecomes a star.

2. The Fission Model of Lunar Formation: A theo-ry of the formation of the moon in which it splitsfrom a rapidly spinning earth, much like the fis-sioning of a nucleus.

3. The Binary Accretion Model of LunarFormation: Theory in which the earth and moonseparately accrete in close proximity within theoriginal nebula of the solar system.

4. The Capture Model of Lunar Formation:Theory in which the moon forms elsewhere in thenebula, but wanders into proximity and is capturedby the earth’s gravity.

5. The Tidal Disintegration Model of LunarFormation: Theory in which infalling planetesi-mals are disintegrated by the gravitational field ofthe enlarging earth and then reaccreted to form themoon.

6. The Collisional Model of Lunar Formation:Theory in which the early earth is struck by a largeplanetesimal, whereupon the ejected debris fromboth planetesimal and earth condense to form themoon.

7. What are the main features of the moon?8. What are the main features of the continents and

the ocean basins?9. What are the three broad classes of rocks found on

the earth?

C. GLOSSARY1. Abyssal Hill: Small hills on the flanks of an

oceanic ridge.2. Abyssal Plain: Large, flat area on the ocean floor

where layers of sediment have covered the originalseafloor topography.

3. Astronomical Unit (AU): A unit of measurementof distance. An astronomical unit is the averagedistance of the earth from the sun, about93,000,000 miles.

4. Atmosphere: All of the mixture of gases (air) sur-rounding a planet.

5. Catastrophe Theory: A class of theories thatseeks to explain phenomena in terms of an event ofgigantic proportions and relatively short duration.The collision theory of the moon’s formation is acatastrophe theory.

6. Continent: The granitic part of the earth’s crust.The continent is divided into three major structuralparts: shield, stable platform, and folded mountainbelts.

7. Continental Shelf: The continental border whichhappens to be below present-day sea level.

8. Continental Shield: The region of very ancient,very eroded rock on the continent. The shield con-sists mostly of igneous and metamorphic rocks thatare the roots of ancient mountain belts that haveworn away.

9. Continental Slope: The transitional region fromthe continental shelf to the ocean floor.

10. Continuously Habitable Zone (CHZ): An imag-inary shell centered on a star in which the condi-tions for life are thought to exist.

11. Deep Oceanic Trench: The deepest surface fea-ture on the earth, a very deep, arc-shaped channelin the ocean floor. Example: The Marianas Trench.

12. Fold Mountain Belt: Long, linear or mildly curv-ing regions of the continent where the rocks arewarped into folds by huge compressional forces inthe lithosphere over long expanses of time.

13. Greenhouse Effect: The trapping of the incomingenergy of sunlight within the atmosphere of a planet,causing the surface temperature of the planet toincrease. The high surface temperature of Venus isthought to be a consequence of the greenhouse effect.

14. Hydrosphere: All of the water on or near the sur-face of a planet.

15. Igneous Rock (examples: granite, basalt): Rocksthat have been deposited on the surface of the earthby volcanic activity (extrusion) or that have crys-tallized beneath the surface from molten magmathat has been intruded into surrounding rock.

16. Island Arc: A broadly curved, or arc-shaped, chainof volcanic islands flanked on one side by deepoceanic trenches. Examples: The Aleutian Islands,Japan.

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17. Jovian Planets: The large, gaseous outer planetsof the solar system (Jupiter, Saturn, Uranus,Neptune).

18. Lava: Molten rock on the surface of the earth.19. Linear Island Chain: Volcanic islands that form a

fairly straight line, often continuing underwater atone end as former islands that were eroded as theseafloor subsided. Example: Hawaiian Islands.

20. Lithosphere: The rigid, brittle outer shell of theearth.

21. Lunar Highlands: The brighter, ancient, heavily-cratered terrain of the earth’s moon.

22. Magma: Molten rock underneath the surface ofthe earth.

23. Maria: The darker terrain of the earth’s moon,which consists of relatively thin veneers of lavaerupted into large shallow basins on the moon’ssurface.

24. Metamorphic Rock (examples: slate, gneiss,marble): Rocks that have been subjected tointense heat and pressure which cause the mineralsin them to undergo chemical reactions; the result isa “new” rock different from its precursor.

25. Meteors: Chunks of rock that orbit the sun as rem-nants of the early solar system. Meteors that enterthe earth’s atmosphere are heated by friction andcalled shooting stars. Remnants of meteors thatreach the earth’s surface are called meteorites.Large meteorites leave meteor impact craters onthe surface of a planet or moon.

26. Mineral: A naturally occurring, inorganic, crys-talline solid.

27. Moon: A small “planet” revolving around a largerplanet—a natural satellite.

28. Ocean Basin: The ocean floor. The ocean floor iscomposed primarily of basalt. The ocean floor hasfive structural parts: oceanic ridge, abyssal hills,abyssal plains, continental rises, and continentalslopes.

29. Oceanic Ridge: A range of mountains on theocean floor caused by upwelling of magma.

30. Plane of the Ecliptic: The plane (defined by theearth’s motion about the sun) in which, to goodapproximation, all planets of the solar system(except Pluto) revolve.

31. Planetesimal: Chunks of matter hundreds of kilo-meters across that orbit around a protosun, accord-ing to the nebular hypothesis.

32. Protoplanet: Small planet-sized bodies of matterformed when planetesimals collide and adhere toone another by gravitational attraction.

33. Protostar (or Protosun): The early stage of a starwhere no fusion is yet taking place, the only lightcoming from electromagnetic processes.

34. Rayed Crater: Impact craters appearing on themaria of the moon which have rays (debris in the

shape of spokes of a wheel) emanating from them.35. Sedimentary Rock (examples: sandstone, shale,

limestone): Rocks formed when (1) debris erodedfrom other rocks is transported, accumulates inbasins, and is gradually buried by later sedimentthat compacts it; (2) chemical precipitation fromwater occurs; or (3) organic material accumulates.These rocks have been deposited in layers andburied.

36. Solar System: A solar system consists of one ormore stars surrounded by a system of planets andother smaller bodies such as comets and asteroids.

37. Solar Wind: A stream of particles (such as pro-tons, electrons, etc.) constantly spewed outwardinto the solar system by the sun.

38. Stable Platform: The region of the continentwhere the shield is covered by a veneer of sedi-mentary rock.

39. Terrestrial Planets: The small, rocky inner plan-ets of the solar system (Mercury, Venus, Earth,Mars).

D. FOCUS QUESTIONS1. Consider the earth:

a. Outline the main elements of the nebularhypothesis leading to the formation of the earth andthe other planets.b. Describe the main features of both the conti-nents and the ocean basins.c. Describe the three main kinds of rocks foundon the earth and give at least one example of eachkind.

E. EXERCISES28.1. Which of the following lists contains a fea-

ture not found in the oceans?(a) trenches, abyssal hills, ocean ridges(b) ocean ridges, island arcs, trenches(c) abyssal plains, stable platforms, ocean ridges(d) island arcs, trenches, linear island chains

28.2. Explain the difference between catastrophetheories and nebular theories of the origin of the solarsystem.

28.3. When angular momentum is conserved, (a) stationary objects begin spinning.(b) spinning objects always spin faster.(c) spinning objects may either speed up or slowdown.(d) large objects spin faster than smaller ones.

28.4. How do the origins of igneous, sedimentary,and metamorphic rocks differ?

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28.5. Describe the essential anatomy of a conti-nent.

28.6. Continents contain(a) granitic rocks.(b) folded rocks.(c) stable platforms.(d) all of the above.(e) none of the above.

28.7. Describe the spatial relationships among themajor features of the ocean floor.

28.8. Compare the compositions of the oceanbasins and the continents.

28.9. The Jovian planets(a) are rocky in composition.(b) are larger than the terrestrial planets.(c) are closer to the sun than the terrestrial planets.(d) are denser than the terrestrial planets.

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