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f 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 dna in useless tatters。 when the magnetic field is working；these rays are safely herded away from the earth’s 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’s happening on top of the earth with what’s going oninside。 according to shawna vogel： “geologists and geophysicists rarely go to the samemeetings or collaborate on the same problems。”
perhaps nothing better demonstrates our inadequate grasp of the dynamics of the earth’sinterior 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’s 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’s 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’t 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’t 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’t 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