Reading the rocks

Originally published 1 July 1985

In his book Con­ver­sa­tions with the Earth, Ger­man geol­o­gist Hans Cloos described the moment when he “became a geol­o­gist for­ev­er.” It did not hap­pen at uni­ver­si­ty. It did not hap­pen with the pass­ing of an exam or the award­ing of a degree. It hap­pened one morn­ing in Naples, Italy, when Cloos opened the win­dow of his hotel room and saw the smok­ing cone of Vesu­vius loom­ing above the still-sleep­ing city. At that moment he had the real­iza­tion that moti­vat­ed a life­time of cre­ative work in geol­o­gy: The Earth is alive.

Cloos had a spe­cial gift for read­ing the sto­ry of the past in the rocks of the present. I thought of him — and of the liv­ing Earth — as I inspect­ed some mas­sive sand­stone boul­ders on a hill near my home in Eas­t­on, Massachusetts.

The sand­stone was lay­ered in del­i­cate alter­ations of a fine red sand, known to geol­o­gists as the Wamsut­ta for­ma­tion, and a coars­er gray sand, known as the Rhode Island for­ma­tion. The sands had been deposit­ed in a low-lying basin near­ly 300 mil­lion years ago, and sub­se­quent­ly con­sol­i­dat­ed into stone.

On the face of one of the largest boul­ders was a beau­ti­ful exam­ple of “cross-bed­ding,” lay­ers of sand that did not lie flat in par­al­lel planes, but instead swept upward in banked curves that were trun­cat­ed by oth­er lay­ers curv­ing upwards in quite dif­fer­ent direc­tions. Cross-bed­ding of this sort spoke of the depo­si­tion of the sands by cur­rents, at a place where spir­it­ed streams gave up their sus­pend­ed sed­i­ments and tum­bled grav­els to the slug­gish waters of a lake or shal­low sea, quite pos­si­bly at the delta of an ancient riv­er that flowed from high mountains.

Tropical New England

The Wamsut­ta and Rhode Island sand­stones con­tain interbed­ded coals and black slaty shales of a veg­e­ta­tive ori­gin: the place of their depo­si­tion was swampy. The image that came to mind was wild, wet, and trop­i­cal, more like the basin of the Ama­zon than present-day New England.

In fact, 300 mil­lion years ago New Eng­land lay in equa­to­r­i­al lat­i­tudes. North Amer­i­ca was part of a uni­fied land mass that embraced all of the present con­ti­nents, a super­con­ti­nent geol­o­gists call Pan­gaea (all-Earth). The super­con­ti­nent had been assem­bled south of the equa­tor by col­li­sions of drift­ing con­ti­nen­tal frag­ments, and was itself slip­ping slow­ly north­ward as part of the ongo­ing move­ments of the “eggshell” plates that make up the Earth­’s crust. The mighty ances­tral Appalachi­ans stood along the suture where the com­po­nent land mass­es had been joined, and from these moun­tains vig­or­ous streams car­ried sand and grav­el into swampy basins.

How can geol­o­gists be so con­fi­dent in telling this sto­ry of a time when New Eng­land lay athwart the equa­tor? The answer has to do with the Earth­’s mag­net­ic field.

The lines of force of the mag­net­ic field come out of the Earth near the south mag­net­ic pole, sweep around the curve of the Earth above the equa­tor, and plunge back into the Earth near the north mag­net­ic pole. Near the poles, the incli­na­tion of the field is per­pen­dic­u­lar to the sur­face of the Earth. Near the equa­tor, the field lies par­al­lel to the sur­face. In gen­er­al, the incli­na­tion of the field varies with latitude.

Rocks reveal latitude

Vol­canic rocks have a way of pre­serv­ing a record of the ori­en­ta­tion of the Earth­’s mag­net­ic field at the time they are formed. When the rocks are still in the molten state, iron min­er­als in the rock line up with the Earth­’s field, like iron fill­ings near a mag­net. When the rock solid­i­fies, the mag­net­ic field is frozen into place.

Today, geol­o­gists col­lect sam­ples of vol­canic rock from the Earth­’s con­ti­nents. These spec­i­mens, drilled from the rock, are about the size of a roll of quar­ters. They are returned to a prop­er­ly equipped lab and the direc­tion of their mag­net­ism is mea­sured. The result gives an approx­i­mate lat­i­tude for the rocks at the time of their for­ma­tion. The 300 mil­lion-year-old vol­canic rocks of North Amer­i­ca speak elo­quent­ly of hor­i­zon­tal fields and, there­fore, trop­ic latitudes.

So, the Earth is alive. The con­ti­nents move. Cli­mates change. Moun­tains are raised by col­lid­ing con­ti­nents and are worn away by water and time. Moun­tains are tum­bled into low­land basins. Moun­tains are crum­bled into sand and deposit­ed as cross-bed­ded sed­i­ments in swampy riv­er deltas. Con­ti­nents shift and bend and buck­le, and ancient trop­i­cal swamps are lift­ed onto the tops of north­ern hills, where they lie today.