just the rightspeed and cool down with the necessary swiftness to bee a diamond。 it was such a pipethat made johannesburg the most productive diamond mining city in the world； but there maybe others even bigger that we don’t know about。 geologists know that somewhere in thevicinity of northeastern indiana there is evidence of a pipe or group of pipes that may be trulycolossal。 diamonds up to twenty carats or more have been found at scattered sites throughoutthe region。 but no one has ever found the source。 as john mcphee notes； it may be buriedunder glacially deposited soil； like the manson crater in iowa； or under the great lakes。
so how much do we know about what’s inside the earth？ very little。 scientists aregenerally agreed that the world beneath us is posed of four layers—rocky outer crust； amantle of hot； viscous rock； a liquid outer core； and a solid inner core。
1we know that thesurface is dominated by silicates； which are relatively light and not heavy enough to accountfor the planet’s overall density。 therefore there must be heavier stuff inside。 we know that togenerate our magnetic field somewhere in the interior there must be a concentrated belt ofmetallic elements in a liquid state。 that much is universally agreed upon。 almost everythingbeyond that—how the layers interact； what causes them to behave in the way they do； whatthey will do at any time in the future—is a matter of at least some uncertainty； and generallyquite a lot of uncertainty。
even the one part of it we can see； the crust； is a matter of some fairly strident debate。
nearly all geology texts tell you that continental crust is three to six miles thick under theoceans； about twenty…five miles thick under the continents； and forty to sixty miles thickunder big mountain chains； but there are many puzzling variabilities within thesegeneralizations。 the crust beneath the sierra nevada mountains； for instance； is only aboutnineteen to twenty…five miles thick； and no one knows why。 by all the laws of geophysics thesierra nevadas should be sinking； as if into quicksand。 （some people think they may be。）1for those who crave a more detailed picture of the earths interior； here are the dimensions of the variouslayers； using average figures： from 0 to 40 km （25 mi） is the crust。 from 40 to 400 km （25 to 250 mi） is theupper mantle。 from 400 to 650 km （250 to 400 mi） is a transition zone between the upper and lower mantle。
from 650 to 2；700 km （400 to 1；700 mi） is the lower mantle。 from 2；700 to 2；890 km （1；700 to 1；900 mi） is the〃d〃 layer。 from 2；890 to 5；150 km （1；900 to 3；200 mi） is the outer core； and from 5；150 to 6；378 km （3；200 to3；967 mi） is the inner core。
how and when the earth got its crust are questions that divide geologists into two broadcamps—those who think it happened abruptly early in the earth’s history and those who thinkit happened gradually and rather later。 strength of feeling runs deep on such matters。 richardarmstrong of yale proposed an early…burst theory in the 1960s； then spent the rest of hiscareer fighting those who did not agree with him。 he died of cancer in 1991； but shortlybefore his death he “lashed out at his critics in a polemic in an australian earth science journalthat charged them with perpetuating myths；” according to a report inearth magazine in 1998。
“he died a bitter man；” reported a colleague。
the crust and part of the outer mantle together are called the lithosphere （from the greeklithos； meaning “stone”）； which in turn floats on top of a layer of softer rock called theasthenosphere （from greek words meaning “without strength”）； but such terms are neverentirely satisfactory。 to say that the lithosphere floats on top of the asthenosphere suggests adegree of easy buoyancy that isn’t quite right。 similarly it is misleading to think of the rocksas flowing in anything like the way we think of materials flowing on the surface。 the rocksare viscous； but only in the same way that glass is。 it may not look it； but all the glass on earthis flowing downward under the relentless drag of gravity。 remove a pane of really old glassfrom the window of a european cathedral and it will be noticeably thicker at the bottom thanat the top。 that is the sort of “flow” we are talking about。 the hour hand on a clock movesabout ten thousand times faster than the “flowing” rocks of the mantle。
the movements occur not just laterally as the earth’s plates move across the surface； but upand down as well； as rocks rise and fall under the churning process known as convection。
convection as a process was first deduced by the eccentric count von rumford at the end ofthe eighteenth century。 sixty years later an english vicar named osmond fisher prescientlysuggested that the earth’s interior might well be fluid enough for the contents to move about；but that idea took a very long time to gain support。
in about 1970； when geophysicists realized just how much turmoil was going on downthere； it came as a considerable shock。 as shawna vogel put it in the book naked earth： thenew geophysics： “it was as if scientists had spent decades figuring out the layers of theearth’s atmosphere—troposphere； stratosphere； and so forth—and then had suddenly foundout about wind。”
how deep the convection process goes has been a matter of controversy ever since。 somesay it begins four hundred miles down； others two thousand miles below us。 the problem； asdonald trefil has observed； is that “there are two sets of data； from two different disciplines；that cannot be reconciled。” geochemists say that certain elements on earth’s surface cannothave e from the upper mantle； but must have e from deeper within the earth。
therefore the materials in the upper and lower mantle must at least occasionally mix。
seismologists insist that there is no evidence to support such a thesis。
so all that can be said is that at some slightly indeterminate point as we head toward thecenter of earth we leave the asthenosphere and plunge into pure mantle。 considering that itaccounts for 82 percent of the earth’s volume and 65 percent of its mass； the mantle doesn’tattract a great deal of attention； largely because the things that interest earth scientists andgeneral readers alike happen either deeper down （as with magnetism） or nearer the surface （aswith earthquakes）。 we know that to a depth of about a hundred miles the mantle consistspredominantly of a type of rock known as peridotite； but what fills the space beyond isuncertain。 according to a nature report； it seems not to be peridotite。 more than this we donot know。
beneath the mantle are the two cores—a solid inner core and a liquid outer one。 needless tosay； our understanding of the nature of these cores is indirect； but scientists can make somereasonable assumptions。 they know that the pressures at the center of the earth aresufficiently high—something over three million times those found at the surface—to turn anyrock there solid。 they also know from earth’s history （among other clues） that the inner coreis very good at retaining its heat。 although it is little more than a guess； it is thought that inover four billion years the temperature at the core has fallen by no more than 200°f。 no oneknows exactly how hot the earth’s core is； but estimates range from something over 7；000°fto 13；000°f—about as hot as the surface of the sun。
the outer core is in many ways even less well understood； though everyone is in agreementthat it is fluid and that it is the seat of magnetism。 the theory was put forward by e。 c。
bullard of cambridge university in 1949 that this fluid part of the earth’s core revolves in away that makes it； in effect； an electrical motor； creating the earth’s magnetic field。 theassumption is that the convecting fluids in the earth act somehow like the currents in wires。
exactly what happens isn’t known； but it is felt pretty certain that it is connected with the corespinning and with its being liquid。 bodies that don’t have a liquid core—the moon and mars；for instance—don’t have magnetism。
we know that earth’s magnetic field changes in power from time to time： during the age ofthe dinosaurs； it was up to three times as strong as now。 we also know that it reverses itselfevery 500；000 years or so on average； though that average hides a huge degree ofunpredictability。 the last reversal was about 750；000 years ago。 sometimes it stays put formillions of years—37 million years appears to be the longest stretch—and at other times it hasreversed after as little as 20；000 years。 altogether in the last 100 million years it has reverseditself about two hundred times； and we don’t have any real idea why。 it has been called “thegreatest unanswered question in the geological sciences。”
we may be going through a reversal now。 the earth’s magnetic field has diminished byperhaps as much as 6 percent in the last century alone。 any diminution in magnetism is likelyto be bad news； because magnetism； apart from holding notes to refrigerators and keeping ourpasses pointing the right way； plays a vital role in keeping us alive。 space is full ofdangerous cosmic rays that in the absence of magnetic protection would tear through ourbodies； leaving much of our dn