th鈥檚 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掳f。 no oneknows exactly how hot the earth鈥檚 core is； but estimates range from something over 7；000掳fto 13；000掳f鈥攁bout 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鈥檚 core revolves in away that makes it； in effect； an electrical motor； creating the earth鈥檚 magnetic field。 theassumption is that the convecting fluids in the earth act somehow like the currents in wires。
exactly what happens isn鈥檛 known； but it is felt pretty certain that it is connected with the corespinning and with its being liquid。 bodies that don鈥檛 have a liquid core鈥攖he moon and mars；for instance鈥攄on鈥檛 have magnetism。
we know that earth鈥檚 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鈥�37 million years appears to be the longest stretch鈥攁nd 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鈥檛 have any real idea why。 it has been called 鈥渢hegreatest unanswered question in the geological sciences。鈥�
we may be going through a reversal now。 the earth鈥檚 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 dna in useless tatters。 when the magnetic field is working；these rays are safely herded away from the earth鈥檚 surface and into two zones in near spacecalled the van allen belts。 they also interact with particles in the upper atmosphere to createthe bewitching veils of light known as the auroras。
a big part of the reason for our ignorance； interestingly enough； is that traditionally therehas been little effort to coordinate what鈥檚 happening on top of the earth with what鈥檚 going oninside。 according to shawna vogel： 鈥済eologists and geophysicists rarely go to the samemeetings or collaborate on the same problems。鈥�
perhaps nothing better demonstrates our inadequate grasp of the dynamics of the earth鈥檚interior than how badly we are caught out when it acts up； and it would be hard to e upwith a more salutary reminder of the limitations of our understanding than the eruption ofmount st。 helens in washington in 1980。
at that time； the lower forty…eight united states had not seen a volcanic eruption for oversixty…five years。 therefore the government volcanologists called in to monitor and forecast st。
helens鈥檚 behavior primarily had seen only hawaiian volcanoes in action； and they； it turnedout； were not the same thing at all。
st。 helens started its ominous rumblings on march 20。 within a week it was eruptingmagma； albeit in modest amounts； up to a hundred times a day； and being constantly shakenwith earthquakes。 people were evacuated to what was assumed to be a safe distance of eightmiles。 as the mountain鈥檚 rumblings grew st。 helens became a tourist attraction for the world。
newspapers gave daily reports on the best places to get a view。 television crews repeatedlyflew in helicopters to the summit； and people were even seen climbing over the mountain。 onone day； more than seventy copters and light aircraft circled the summit。 but as the dayspassed and the rumblings failed to develop into anything dramatic； people grew restless； andthe view became general that the volcano wasn鈥檛 going to blow after all。
on april 19 the northern flank of the mountain began to bulge conspicuously。 remarkably；no one in a position of responsibility saw that this strongly signaled a lateral blast。 theseismologists resolutely based their conclusions on the behavior of hawaiian volcanoes；which don鈥檛 blow out sideways。 almost the only person who believed that something reallybad might happen was jack hyde； a geology professor at a munity college in taa。 hepointed out that st。 helens didn鈥檛 have an open vent； as hawaiian volcanoes have； so anypressure building up inside was bound to be released dramatically and probablycatastrophically。 however； hyde was not part of the official team and his observationsattracted little notice。
we all know what happened next。 at 8：32 a。m。 on a sunday morning； may 18； the northside of the volcano collapsed； sending an enormous avalanche of dirt and rock rushing downthe mountain slope at 150 miles an hour。 it was the biggest landslide in human history andcarried enough material to bury the whole of manhattan to a depth of four hundred feet。 aminute later； its flank severely weakened； st。 helens exploded with the force of five hundredhiroshima…sized atomic bombs； shooting out a murderous hot cloud at up to 650 miles anhour鈥攎uch too fast； clearly； for anyone nearby to outrace。 many people who were thought tobe in safe areas； often far out of sight of the volcano； were overtaken。 fifty…seven people werekilled。 twenty…three of the bodies were never found。 the toll would have been much higherexcept that it was a sunday。 had it been a weekday many lumber workers would have beenworking within the death zone。 as it was； people were killed eighteen miles away。
the luckiest person on that day was a graduate student named harry glicken。 he had beenmanning an observation post 5。7 miles from the mountain； but he had a college placementinterview on may 18 in california； and so had left the site the day before the eruption。 hisplace was taken by david johnston。 johnston was the first to report the volcano exploding；moments later he was dead。 his body was never found。 glicken鈥檚 luck； alas； was temporary。
eleven years later he was one of forty…three scientists and journalists fatally caught up in alethal outpouring of superheated ash； gases； and molten rock鈥攚hat is known as a pyroclasticflow鈥攁t mount unzen in japan when yet another volcano was catastrophically misread。
volcanologists may or may not be the worst scientists in the world at making predictions；but they are without question the worst in the world at realizing how bad their predictions are。
less than two years after the unzen catastrophe another group of volcano watchers； led bystanley williams of the university of arizona； descended into the rim of an active volcanocalled galeras in colombia。 despite the deaths of recent years； only two of the sixteenmembers of williams鈥檚 party wore safety helmets or other protective gear。 the volcano erupted； killing six of the scientists； along with three tourists who had followed them； andseriously injuring several others； including williams himself。
in an extraordinarily unself…critical book called surviving galeras； williams said he could鈥渙nly shake my head in wonder鈥潯hen he learned afterward that his colleagues in the worldof volcanology had suggested that he had overlooked or disregarded important seismic signalsand behaved recklessly。 鈥渉ow easy it is to snipe after the fact； to apply the knowledge wehave now to the events of 1993；鈥潯e wrote。 he was guilty of nothing worse； he believed； thanunlucky timing when galeras 鈥渂ehaved capriciously； as natural forces are wont to do。 i wasfooled； and for that i will take responsibility。 but i do not feel guilty about the deaths of mycolleagues。 there is no guilt。 there was only an eruption。鈥�
but to return to washington。 mount st。 helens lost thirteen hundred feet of peak； and 230square m