A new article by Andrea Westermann explores how, in the late 19th century, Swiss geologist Albert Heim conceptualized the movement and deformation of large masses of rock based on his earlier studies of glaciers. Westermann argues that Heim’s Alpine locale provided him with a deep, first-hand experience of both glaciers and rock formations that showed evidence of past large-scale, fluid-like movement. His work on rock deformation laid foundations for the study of tectonics—the construction of landforms—decades before Alfred Wegener’s arguments positing the movement of entire continents.

Heim’s research was unusual in that, by his era, early speculations about the mechanisms that shaped the Earth’s surface had long since yielded to more humble, empirically grounded studies of rock stratigraphy. Heim’s and others’ glacier studies carved out a model for a return to theorization, combining field observations, workbench studies of material analogues, laboratory studies of material properties, and conceptual analyses grounded in the principles of mathematical physics. Drawing on this methodology and his understanding of glacier ice’s properties, he turned his attention to rock in his 1878 two-volume monograph Untersuchungen über den Mechanismus der Gebirgsbildung, or in English, Investigations into the Mechanism of Mountain Building.

Unraveling the mysteries of glacier flow

Heim was born in Zürich in 1849 and remained in the city for his university education, researching glaciers under geologist Arnold Escher von der Linth at the Federal Polytechnic School (now ETH Zürich). He completed his work in 1869 and went on a study tour that brought him to Berlin, Scandinavia, and Italy. He completed his habilitation degree at the Zürich polytechnic in 1871, remained there as a Privatdozent, and, after Escher’s death in 1873, succeeded him as professor. He served as president of the Swiss Geological Commission from 1894 to 1926, retired from his professorship in 1911, and died in 1937.

Per Westermann, Alpine geologists flocked to the study of glaciers in the 1830s as the idea of a past ice age gained popularity. The key problem was to understand both the long-term behavior of glaciers as well as the nature of their movement, as ice clearly demonstrated a plasticity in its ability to flow down mountain valleys, yet could also crack and burst into pieces like a rigid solid. In the 1840s, Escher studied the flow of the Aar glacier in the Bernese Alps as well as moraines from apparently retreating glaciers, which were the key evidence in favor of ice age theory.

Visiting the Alps in the 1850s, physicist John Tyndall staked out a reputation as a mountain climber as well as a researcher in the movement and material properties of glaciers. In 1857, he published a paper with Thomas Henry Huxley considering the behavior of ice under pressure. It focused on a process of “regelation,” or melting and refreezing of small grains of ice, which allowed them to change their relative position and, thus, allowed the large bodies of ice they were a part of to move in an apparently viscous flow. Tyndall and Huxley also addressed the active question of whether the “laminar,” that is the veined or layered, structure of glacial ice has any relationship to the material property of “cleavage” found in rocks like slate, arguing that pressure played a role in both phenomena.

When Heim took on the study of glaciers about a decade later, he took multiple approaches that included fieldwork, deforming ice in the laboratory, and building model glaciers using a particular mash of gypsum that exhibited the appropriate characteristics of motion. Based on his research and the work of Tyndall and Albert Mousson, a physics professor at the polytechnic, Heim analyzed glacier flow as a composite motion in which the ice deforms under its own weight and its motion is lubricated by melting at its base. Reasoning counterfactually, Heim argued that without these mechanisms glaciers would move more like sand or gravel.

In the early 1870s, Heim told his students that most problems around the physics of glaciers had been solved as “by now all forms of movements of the glaciers can be derived from the properties of ice in general; properties that we can study in any ice specimen in the laboratory.” Writing a handbook on glacier science in 1885, he seemed less sure, noting that “not only are there new questions emerging … but doubts arise regarding things you believed were crystal clear.” Of course, Westermann points out, glacier motion remains a subject of very active research in the 21st century.

From ice to rock

Westermann observes that Heim’s studies of glaciers were built on the established presumption that their physics and their rheology (the material conditions of their flow and deformation) could be directly accessed through field study and laboratory experiments. Rock masses were a different matter because they exhibited no measurable motion, and so any conclusions would have to be deemed speculative—an accusation many geologists were reluctant to invite.

This aversion to speculation arose as a reaction to the adventurousness of 18th-century natural philosophy, which offered up ideas such as that the irregularity of the Earth’s crust could be explained by its cooling from a molten state. Illustrating the reaction’s motivations, Westermann quotes physics professor and satirist Georg Christoph Lichtenberg’s biting admonition from 1793, “There are already close to fifty theories of the Earth, of which certainly nine-tenths tell us more about the history of the human mind than the history of the Earth. … Just like we find marine creatures on the mountain tops without any trace of a sea far and wide, we are surprised to find conclusions in our minds without any trace of robust premises as far as the eye can see.”

By the second half of the 19th century, some geologists saw a renewed need for speculation about unseen and unfamiliar processes. Among them were the Austrian Edward Suess, who argued in his 1857 book Die Entstehung der Alpen (The Origin of the Alps) that horizontal rather than vertical forces had been responsible for the mountains’ uplift. Like Suess, Heim could plainly see dramatically folded strata on display in the Alps, not unlike the ice formations often seen on glaciers.

In addressing the problem of rock tectonics, Heim used a multifaceted approach similar to the one he used for glaciers. For instance, in addition to pointing to deformation of large-scale rock formations visible in the field, he also examined rock samples under the microscope that had been prepared by Rudolf Fuess in his optical-mechanical workshop in Berlin, where Heim had studied during his tour. These samples showed similar evidence of plasticity at the smallest scales.

To explain such phenomena, which could not be produced in a laboratory, Heim turned to the same concept of cleavage that had been a clue to the importance of pressure in understanding the properties of glacier ice. He argued, “In a glacier, the greater the deforming force of pressure becomes, the denser the network of fractures and the smaller the ice grains. This phenomenon corresponds closely to the breakdown of rocks due to deformation during orogenesis [mountain building]. … When the deforming process becomes so large that instead of at a few thousand fracture points, they can overcome the strength of the rock at every single point, the net of cracks becomes infinitely fine. The mechanical units of movement become one of continuous deformation without fracture.”

Heim held that deep within the Earth’s crust there is a multidirectional pressure, akin to hydrostatic pressure in water, as well as higher temperatures that allow rock to reveal a “latent” plasticity. Whereas glacier flows were caused by gravity, he surmised that the folding of rock in such environments was driven by a one-sided horizontal compression, echoing Suess’s view and anticipating explanations later provided by plate tectonics. Following the implications of his physical model, Heim further posited that, were rock more brittle like ice, or were the force of Earth’s gravity stronger, mountains would not be stable objects and might flow downward as glaciers in fact do.

Westermann observes that, in the late 19th century, the physics-grounded methods advanced by Tyndall and Heim were not yet commonly used among geologists, nor were they as yet capable of providing quantifiable conclusions. They did, though, provide a means of tying multiple lines of evidence and reasoning together into a persuasive body of arguments, and in doing so created an environment in which speculative theorization could find new acceptance. Accordingly, Heim was able to secure a reputation as a respected authority on glaciers and metamorphic rock, and as a founding figure in the nascent study of tectonics.

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William Thomas
American Institute of Physics
wthomas@aip.org

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