On plate tectonics, old Ben got it right 200 years ago

On plate tectonics, old Ben got it right 200 years ago

Benjamin Franklin at his desk • Mezzotint by engraver Edward Fisher, ca. 1763

Originally published 13 September 1993

Ben­jamin Franklin is usu­al­ly depict­ed as a grand­fa­ther­ly fel­low, port­ly and genial, who would be at home by the fire of an 18th-cen­tu­ry tav­ern, with a long-stemmed pipe in one hand and a tankard of porter in the oth­er. In the famil­iar por­trait of Franklin by his con­tem­po­rary Edward Fish­er, the great man seems fixed to his chair by a kind of weary contentment.

We tend to think of Franklin as a tin­ker­er and dab­bler, occu­pa­tions befit­ting a man in repose: inven­tor of bifo­cals and the Franklin stove, and quaint apho­rist of Poor Richard’s Almanack.

In fact, the man behind the pop­u­lar iconog­ra­phy and myths was ener­getic, insa­tiably curi­ous, and prodi­gious­ly active. Even in the midst of stren­u­ous diplo­mat­ic labors on the part of the infant Amer­i­can repub­lic, Franklin squeezed sci­ence into every free moment.

His volu­mi­nous cor­re­spon­dence with sci­en­tists world­wide includes obser­va­tions of clouds, storms, ocean cur­rents, tides, rivers, sunspots, whirl­winds, lighter-than-air bal­loons, lead poi­son­ing, day­light sav­ings time, and the com­mon cold. There is lit­tle that escaped his rav­en­ous attention.

In Europe — today as in his own time — he is best known as the author of Exper­i­ments and Obser­va­tions on Elec­tric­i­ty, a book that helped lay the foun­da­tions of elec­tri­cal sci­ence. It was Franklin, for exam­ple, who dis­cov­ered that elec­tri­cal charge was of two kinds, which he called pos­i­tive and negative.

Not so wide­ly known are some extra­or­di­nary rumi­na­tions by the diplo­mat-sci­en­tist that were detailed by Ronald Clark in his biog­ra­phy of Franklin. These may qual­i­fy Franklin to be called the “Father of Plate Tectonics.”

Plate tec­ton­ics is the rev­o­lu­tion­ary the­o­ry that turned geol­o­gy upside down in the 1960s. The the­o­ry assumes that the Earth­’s crust is egg-shell thin and floats upon the almost-flu­id rock of the inte­ri­or. The inte­ri­or rock is in con­vec­tive motion, dri­ven by heat and grav­i­ty. These motions work upon the crust, frac­tur­ing it and drag­ging it about. Con­ti­nents move and col­lide, and ocean basins are cre­at­ed and destroyed.

Those of us who were edu­cat­ed only a gen­er­a­tion ago were taught that the Earth is rigid to its core — as sol­id as a bowl­ing ball — and that any large-scale motions of the crust were up-and-down, not side­wards. The new slip-and-slide egg-shell mod­el of the Earth seemed with­out antecedent.

But Ben Franklin had been there first.

In July 1747, he wrote to Jared Eliot, a Con­necti­cut cler­gy­man: “The great Appalachi­an Moun­tains, which run from York Riv­er back of these Colonies to the Bay of Mex­i­co, show in many Places near the high­est Parts of them, Stra­ta of Sea Shells, in some Places the Marks of them are in the sol­id Rocks. ‘Tis cer­tain­ly the Wreck of a World we live on!”

And what caused this “wreck” that heaved the floors of oceans high into the air, lift­ing sea shells to moun­tain peaks? Franklin found oth­er clues dur­ing his trav­els in Britain.

In a coal mine at White­haven in north­ern Eng­land he observed the leaves and branch­es of ferns impressed upon slates which formed the nat­ur­al roof of the mine, deep beneath the present sur­face of the Earth. Else­where in Eng­land he found oys­ter shells mixed with the rocks of a moun­tain top. Evi­dent­ly, sur­face marsh­es had been depressed and the ocean floor thrust upwards.

Franklin wrote: “Such changes in the super­fi­cial parts of the globe seemed to me unlike­ly to hap­pen, if the earth were sol­id to the cen­ter. I there­fore imag­ined, that the inter­nal parts might be a flu­id more dense, and of greater spe­cif­ic grav­i­ty than any of the solids we are acquaint­ed with, which there­fore might swim in or upon that flu­id. Thus the sur­face of the globe would be a shell, capa­ble of being bro­ken and dis­or­dered by the vio­lent move­ments of the flu­id on which it rested.”

This is an accu­rate descrip­tion of the fun­da­men­tal notion of plate tec­ton­ics, writ­ten a gen­er­a­tion before James Hut­ton found­ed the sci­ence of geol­o­gy with his The­o­ry of the Earth in 1785, and more than two hun­dred years before the the­o­ry of plate tec­ton­ics changed our way of think­ing about the Earth.

Of course, a sci­en­tif­ic the­o­ry is more than a pre­co­cious intu­ition; it must be backed by obser­va­tion and exper­i­ment. In Franklin’s time there was no way to ascer­tain the phys­i­cal state of the Earth­’s inte­ri­or; it was not until the age of com­put­ers and inter­na­tion­al seis­mo­graph net­works that this became possible.

But Franklin was clever enough to rec­og­nize that the wreck of the world was caused by “some inter­nal mighty force” (as he wrote his broth­er Peter), that this force was capa­ble of caus­ing large-scale hor­i­zon­tal motions, and that the drift and crash of con­ti­nents could best be explained by allow­ing the crust to move upon a tur­bu­lent fluid.

Two cen­turies were to pass before sci­ence caught up with Franklin’s astute intuition.

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