My experiments with volcanoes

by Thomas Augustus Jaggar

This oration is introduction to the next series of Harvard experiments, which dealt with squeezing and wrinkling strata in imitation of the folds and faults of the Appalachian Mountains. Bailey Willis, at the Geological Survey, made a press of wax models of strata. A heavy oak piston was advanced by a screw crank. The models were waxes mixed with plaster for hard strata and waxes mixed with Venice turpentine for soft strata. They were cast to imitate actual successions of hard, thick limestones; less hard sandstones; soft mudstones; or slates. The piston advanced at a measured rate against one end of the model, the other end being a fixed box, the strata lying horizontally. The elongate Appalachian basin had a continent (the piston) to the east; a wide flat fill of limestones or sea bottom to the west (the box); and the deepest trough of pebbles, sands, and muds on the east, toward the rivers of the eroding continent of that ancient time. The heavy limestone tapered from the west into these thinner beds and made a stiff rib in their midst. The final result of their wrinkling was linear folds with axes north and south parallel to the trough, and close set at the east. The folds overturned toward the west, the overturns developing into overthrust fractures westward when the beds ruptured. Also, the folds became bigger, flatter, and wider apart westward under the deeper sea, the famous one being the Cincinnati arch.

The evidence in the middle eastern states is that the trough bottom sank as the heavy shore sediments were dumped by rivers into the sea. The west-central states received a wide flat of limestone. Uplift of the continent shallowed the ocean and pushed it, narrower, over to the great plains. So there were left a deep trough of weak beds, a massive limestone, and an overlap of continental wash across the uplifted later continent of the present time. The problems to be studied in Willis’ models were how folding would affect such a pile, what transmitted the wrinkling force, what started a single fold, and how soft and hard strata behaved under horizontal pressure.

He found that hard, thick layers of limestone transmitted the push farthest. That soft beds piled up on each other near the piston. That these beds showed beautiful overthrust faults inclined away from the piston. And that the start of individual folds was favored by very small initial bends in a transmitting layer. These downbends away from the continent would be made as the trough bottom sank through the ages. The nature of this sinking in upright slices of the bottom rock is probably downfaulting. Each vertical slice would make a step-bend as it sank.

The bottom of Willis’ box did not admit of down motion by underflow, nor did the piston pressure create an opposed horizontal force that might have come from the ocean area. In restraining up motion over the folds that formed, Willis piled bags of shot on top of the model to represent downweighting. The folding in the Appalachians was down at the bottom of the heap where things were hot and compressed, and heat could extend individual strata.

In our pressure chest we extended the Willis conception. We made two pistons at opposite ends of an oaken box, with thick plate glass panes at one side, so as to watch the folding. The two pistons would distribute the end pressure better and admit the possibility that all the pressure did not come from the continent. The bottom under the model was an inner box that could move down, hung on heavy spring balances. These could be screwed up to a pressure upward to compensate the load of shot. Thus the first fold could arch downward as well as upward. This imitated a possible lowered trough bottom. The piston rate of advance was controlled by metronome, one man at each screw.

For examples, models E, F, and G had four white and four black layers, all alike in substance, at fast, medium, and slow rates. The quickest was shortened one inch in five minutes. The slowest was one inch in an hour and three-quarters. The quick-squeeze model flexed smoothly, all folds seemed to flow, and the model held together compactly. The slow-squeeze model shortened the same amount, cracked in many places, was brittle, and did not hold together compactly. This appeared to prove that slow motion will fracture where quicker motion will hold strata intact, under otherwise identical conditions of substance, of folding and shortening, and of vertical confinement.

We verified Willis’ conclusions that stiff and thick beds transmit the pressure farthest and that overthrust tends to form in soft beds, which thicken near a piston. In one model we got overthrusts in opposite directions on opposite sides of the model along a single-fold axis, with a twist in between. While an experiment was in progress, the chest creaked occasionally, the equivalent of an earthquake. One model was cast to represent overlap of strata near shore, like a coastal plain. When squeezed, it made a group of overthrusts away from the piston acting as shore rock.

In burial of strata there is a possibility whereby they wrinkle, and wrinkle most in one direction, which piston pressure does not imitate. That is the heating by burial and expansion or lengthening of controlling layers. In a long basin like the Appalachians, the wrinkling under expansion across the greatest length is easiest, because the axis of stiffness is parallel to the long trough. Transitions off the coastal line from one sediment to the next—sand to mud, mud to lime—will be weaknesses to start bends when expansion pressure takes place under burial along the layers separately heated. These bends develop into wrinkles and the wrinkles, into propagated folds, with the axis parallel to the initial change of weaknesses. Expansion lengthwise on folds, once begun, may make long flat arches pitching in one direction. This heating by burial distributes the folding better and farther than pushing abutments, and makes initial bends. All bottom strata heat and expand in all directions. The direction of easiest yielding to a folding impulse is across the weak transition belts. After that the motion is taken up by linear folds and fractures in one direction.

The models, after continuous or intermittent squeezing, were removed from the chest and sliced with a hot wire for sectioning and photographing. In one, brittle, broken series of folds in a hard layer, the model was taken apart on that layer and the surface photographed. The crest of the folds showed jointing or regular cracks. One set paralleled the fold axes as would be expected; the other set crossed the slopes diagonally and in curves. These last indicated the strains of a twisting nature on a single layer between a downfold and an upfold.

What makes the end thrust, or piston push, in nature? According to the old idea, it was contraction of the inner earth by loss of heat. Willis wrote that the basin sank, isostasy or deep flow was at right angles to the length of the basin, and general contraction took effect by reason of the deep flow. The deep flow was toward the lighter continent, from which the sands were originally lost.