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r the western united states there was a hugecauldron of magma； a colossal volcanic hot spot； which erupted cataclysmically every600；000 years or so。 the last such eruption was just over 600；000 years ago。 the hot spot isstill there。 these days we call it yellowstone national park。
we know amazingly little about what happens beneath our feet。 it is fairly remarkable tothink that ford has been building cars and baseball has been playing world series for longerthan we have known that the earth has a core。 and of course the idea that the continents moveabout on the surface like lily pads has been mon wisdom for much less than a generation。
鈥渟trange as it may seem；鈥潯rote richard feynman； 鈥渨e understand the distribution of matterin the interior of the sun far better than we understand the interior of the earth。鈥�
the distance from the surface of earth to the center is 3；959 miles； which isn鈥檛 so very far。
it has been calculated that if you sunk a well to the center and dropped a brick into it； it wouldtake only forty…five minutes for it to hit the bottom （though at that point it would beweightless since all the earth鈥檚 gravity would be above and around it rather than beneath it）。
our own attempts to penetrate toward the middle have been modest indeed。 one or two southafrican gold mines reach to a depth of two miles； but most mines on earth go no more thanabout a quarter of a mile beneath the surface。 if the planet were an apple； we wouldn鈥檛 yethave broken through the skin。 indeed； we haven鈥檛 even e close。
until slightly under a century ago； what the best…informed scientific minds knew aboutearth鈥檚 interior was not much more than what a coal miner knew鈥攏amely； that you could dig down through soil for a distance and then you鈥檇 hit rock and that was about it。 then in 1906；an irish geologist named r。 d。 oldham； while examining some seismograph readings from anearthquake in guatemala； noticed that certain shock waves had penetrated to a point deepwithin the earth and then bounced off at an angle； as if they had encountered some kind ofbarrier。 from this he deduced that the earth has a core。 three years later a croatianseismologist named andrija mohorovi？i鈥瞔 was studying graphs from an earthquake in zagrebwhen he noticed a similar odd deflection； but at a shallower level。 he had discovered theboundary between the crust and the layer immediately below； the mantle； this zone has beenknown ever since as the mohorovi？i鈥瞔 discontinuity； or moho for short。
we were beginning to get a vague idea of the earth鈥檚 layered interior鈥攖hough it really wasonly vague。 not until 1936 did a danish scientist named inge lehmann； studyingseismographs of earthquakes in new zealand； discover that there were two cores鈥攁n innerone that we now believe to be solid and an outer one （the one that oldham had detected） thatis thought to be liquid and the seat of magnetism。
at just about the time that lehmann was refining our basic understanding of the earth鈥檚interior by studying the seismic waves of earthquakes； two geologists at caltech in californiawere devising a way to make parisons between one earthquake and the next。 they werecharles richter and beno gutenberg； though for reasons that have nothing to do with fairnessthe scale became known almost at once as richter鈥檚 alone。 （it has nothing to do with richtereither。 a modest fellow； he never referred to the scale by his own name； but always called it鈥渢he magnitude scale。鈥潱﹖he richter scale has always been widely misunderstood by nonscientists； though perhapsa little less so now than in its early days when visitors to richter鈥檚 office often asked to seehis celebrated scale； thinking it was some kind of machine。 the scale is of course more anidea than an object； an arbitrary measure of the earth鈥檚 tremblings based on surfacemeasurements。 it rises exponentially； so that a 7。3 quake is fifty times more powerful than a6。3 earthquake and 2；500 times more powerful than a 5。3 earthquake。
at least theoretically； there is no upper limit for an earthquake鈥攏or； e to that； a lowerlimit。 the scale is a simple measure of force； but says nothing about damage。 a magnitude 7quake happening deep in the mantle鈥攕ay； four hundred miles down鈥攎ight cause no surfacedamage at all； while a significantly smaller one happening just four miles under the surfacecould wreak widespread devastation。 much； too； depends on the nature of the subsoil； thequake鈥檚 duration； the frequency and severity of aftershocks； and the physical setting of theaffected area。 all this means that the most fearsome quakes are not necessarily the mostforceful； though force obviously counts for a lot。
the largest earthquake since the scale鈥檚 invention was （depending on which source youcredit） either one centered on prince william sound in alaska in march 1964； whichmeasured 9。2 on the richter scale； or one in the pacific ocean off the coast of chile in 1960；which was initially logged at 8。6 magnitude but later revised upward by some authorities（including the united states geological survey） to a truly grand…scale 9。5。 as you will gatherfrom this； measuring earthquakes is not always an exact science； particularly wheninterpreting readings from remote locations。 at all events； both quakes were whopping。 the1960 quake not only caused widespread damage across coastal south america； but also set offa giant tsunami that rolled six thousand miles across the pacific and slapped away much ofdowntown hilo； hawaii； destroying five hundred buildings and killing sixty people。 similarwave surges claimed yet more victims as far away as japan and the philippines。
for pure； focused； devastation； however； probably the most intense earthquake in recordedhistory was one that struck鈥攁nd essentially shook to pieces鈥攍isbon； portugal； on all saintsday （november 1）； 1755。 just before ten in the morning； the city was hit by a suddensideways lurch now estimated at magnitude 9。0 and shaken ferociously for seven full minutes。
the convulsive force was so great that the water rushed out of the city鈥檚 harbor and returnedin a wave fifty feet high； adding to the destruction。 when at last the motion ceased； survivorsenjoyed just three minutes of calm before a second shock came； only slightly less severe thanthe first。 a third and final shock followed two hours later。 at the end of it all； sixty thousandpeople were dead and virtually every building for miles reduced to rubble。 the san franciscoearthquake of 1906； for parison； measured an estimated 7。8 on the richter scale andlasted less than thirty seconds。
earthquakes are fairly mon。 every day on average somewhere in the world there aretwo of magnitude 2。0 or greater鈥攖hat鈥檚 enough to give anyone nearby a pretty good jolt。
although they tend to cluster in certain places鈥攏otably around the rim of the pacific鈥攖heycan occur almost anywhere。 in the united states； only florida； eastern texas； and the uppermidwest seem鈥攕o far鈥攖o be almost entirely immune。 new england has had two quakes ofmagnitude 6。0 or greater in the last two hundred years。 in april 2002； the region experienceda 5。1 magnitude shaking in a quake near lake champlain on the new york鈥搗ermont