June 2002 AGGMAN Geology

By Bill Langer

Author’s Note: For the past few years this column has taken on issues of concern to the aggregate industry and has described how geology relates to those issues. While most folks who read this column probably have some knowledge of geology, it is a science with a history worth sharing. These next few articles will provide brief descriptions of some of the theories that have evolved into the science of geology.

Roman naturalist Gaius Plinius Secundus (Pliny the Elder—23-79 A.D.) began Book XXXVI of his encyclopedia Naturalis Historiae by writing “Mountains…were made by Nature for herself to serve as a kind of framework for holding together the inner parts of the earth… We quarry these mountains and haul them away for a mere whim; and yet there was a time when it seemed remarkable even to have succeeded in crossing them. Our forefathers considered the scaling of the Alps by Hannibal…to be almost unnatural. Now these selfsame Alps are quarried into marble (which to Pliny was any kind of stone) of a thousand varieties.”
We still obtain “thousands of varieties” of stone from these magnificent landforms. But how did the mountains get there? Mountains are one of the most spectacular manifestations of geologic processes and have been a source of awe and inquiry since mankind became aware of his surroundings. Scientists have long struggled to identify the forces that could create such splendid features, but it was not until the late 20th century that all the pieces were put together to create the modern theory of mountain-building.
Writers in classical times frequently mention mountains, but very few speculated on the forces that created them. Greek writer Pythagoras (circa 569-475 B.C.) was one of the authors who did speculate when he wrote “the wild forces of the winds, shut up in dark regions underground…puffed out and stretched the ground, just as one inflates a bladder with his breath… That swelling in the ground…still has the appearance of a high hill and has hardened as the years went by.” Greek philosopher Aristotle (384-322 B.C.) also described a hill as swelling up into a mound that finally burst asunder by “winds” escaping from the earth’s interior.
During medieval times, European writers had little to say about mountain building. However, in Persia, Islamic philosopher and physician ibn Sina, who is commonly known by his Latin name Avicenna (980-1037 A.D.), appears to have combined geologic observations of earthquakes with the beliefs of men like Aristotle. Ibn Sina postulated that mountains were created when “the wind which produces the earthquake raises a part of the ground and a height is suddenly formed.”
Toward the close of the 13th century, Ristoro d’Arezzo, in his writings on La Composizione del Mondo, proposed that the chief cause of mountain-making resided with the stars. Ristoro said the heavens have a mountain and valley character, and wherever there is a mountain in the heavens, there is a corresponding valley on the earth’s surface. Ristoro also recognized three lesser causes of mountains: the action of running water, which cuts into the land leaving behind hills or mountains; the actions of waves along the coast, which throw up hills of sand and gravel; and the Noachian deluge, which deposited great mountains of sedimentary rocks on various parts of the earth’s surface. And while some of Ristoro’s explanations are quaint, it appears that he had personally studied the geological structures of the mountains of Tuscany. While doing so, he identified layers of rocks that contained shells and bones of fish and recognized them to be of marine origin.
During the 17th century, scientists such as Nils Stensen (Steno–1638-1686) began basing theories of mountain building on close study and observations made in the field. Steno, a Danish medical doctor who was living in Italy, took a major step forward in the interpretation of the origin of mountains based on a study of the mountains of Tuscany—the same mountains visited by Ristoro centuries earlier. In his manuscript Provisional Dissertation on Solid Bodies Naturally Embedded in Other Solids, Steno explained the origin of the various horizontal and tilted rocks in the mountains of Tuscany by a series of cycles of deposition of horizontal strata, erosion of underground (not quite right!) cavities and collapse of the overlying strata. Although Steno’s theories were not completely correct, they were quite insightful, especially considering the state of the knowledge at the time.
In 1872, Benjamin Franklin expressed his opinion on the nature of the earth, an opinion that would be appreciated by later generations. He said the crust of the earth must be a shell floating on a fluid interior “thus the surface of the globe would be capable of being broken and disordered by the violent movements of the fluids on which it rested.”
Toward the close of the 18th century, James Hutton (1726-1797), a Scottish doctor, farmer and naturalist, found stringers of granite penetrating sedimentary rocks and saw them as evidence of subterranean fire and heat. And although he had not identified the right source of heat, Hutton properly concluded that subterranean heat had consolidated and compacted sediment that accumulated on the ocean floors. He also erroneously concluded that expansion caused by the heat had elevated the rock and overlying strata.
As recently as the early 1960s, the origin of mountains was still a mystery. But studies that took place during the last half of the 20th century, combined with ideas spawned during the 19th and 20th centuries, eventually led to what we have come to accept as the current theory of mountain building—a process called plate tectonics. British journalist and freelance writer Russell Miller nicely summed it up when he wrote that today’s view of mountains is that they are a “visible manifestation of an inch-by-inch movement of great slabs of the earth’s surface layers. This ages-long process not only pushes up mountains in crustal collisions of inconceivable force, it also creates and destroys oceans, and shatters and reshapes whole continents.” But that’s another story.

William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.

June 2002 Carved in Stone A Brief History of the Origin of Mountains				A Brief History of the Origin of Mountains By Bill Langer Author’s Note: For the past few years this column has taken on issues of concern to the aggregate industry and has described how geology relates to those issues. While most folks who read this column probably have some knowledge of geology, it is a science with a history worth sharing. These next few articles will provide brief descriptions of some of the theories that have evolved into the science of geology. Roman naturalist Gaius Plinius Secundus (Pliny the Elder—23-79 A.D.) began Book XXXVI of his encyclopedia Naturalis Historiae by writing “Mountains…were made by Nature for herself to serve as a kind of framework for holding together the inner parts of the earth… We quarry these mountains and haul them away for a mere whim; and yet there was a time when it seemed remarkable even to have succeeded in crossing them. Our forefathers considered the scaling of the Alps by Hannibal…to be almost unnatural. Now these selfsame Alps are quarried into marble (which to Pliny was any kind of stone) of a thousand varieties.” We still obtain “thousands of varieties” of stone from these magnificent landforms. But how did the mountains get there? Mountains are one of the most spectacular manifestations of geologic processes and have been a source of awe and inquiry since mankind became aware of his surroundings. Scientists have long struggled to identify the forces that could create such splendid features, but it was not until the late 20th century that all the pieces were put together to create the modern theory of mountain-building. Writers in classical times frequently mention mountains, but very few speculated on the forces that created them. Greek writer Pythagoras (circa 569-475 B.C.) was one of the authors who did speculate when he wrote “the wild forces of the winds, shut up in dark regions underground…puffed out and stretched the ground, just as one inflates a bladder with his breath… That swelling in the ground…still has the appearance of a high hill and has hardened as the years went by.” Greek philosopher Aristotle (384-322 B.C.) also described a hill as swelling up into a mound that finally burst asunder by “winds” escaping from the earth’s interior. During medieval times, European writers had little to say about mountain building. However, in Persia, Islamic philosopher and physician ibn Sina, who is commonly known by his Latin name Avicenna (980-1037 A.D.), appears to have combined geologic observations of earthquakes with the beliefs of men like Aristotle. Ibn Sina postulated that mountains were created when “the wind which produces the earthquake raises a part of the ground and a height is suddenly formed.” Toward the close of the 13th century, Ristoro d’Arezzo, in his writings on La Composizione del Mondo, proposed that the chief cause of mountain-making resided with the stars. Ristoro said the heavens have a mountain and valley character, and wherever there is a mountain in the heavens, there is a corresponding valley on the earth’s surface. Ristoro also recognized three lesser causes of mountains: the action of running water, which cuts into the land leaving behind hills or mountains; the actions of waves along the coast, which throw up hills of sand and gravel; and the Noachian deluge, which deposited great mountains of sedimentary rocks on various parts of the earth’s surface. And while some of Ristoro’s explanations are quaint, it appears that he had personally studied the geological structures of the mountains of Tuscany. While doing so, he identified layers of rocks that contained shells and bones of fish and recognized them to be of marine origin. During the 17th century, scientists such as Nils Stensen (Steno–1638-1686) began basing theories of mountain building on close study and observations made in the field. Steno, a Danish medical doctor who was living in Italy, took a major step forward in the interpretation of the origin of mountains based on a study of the mountains of Tuscany—the same mountains visited by Ristoro centuries earlier. In his manuscript Provisional Dissertation on Solid Bodies Naturally Embedded in Other Solids, Steno explained the origin of the various horizontal and tilted rocks in the mountains of Tuscany by a series of cycles of deposition of horizontal strata, erosion of underground (not quite right!) cavities and collapse of the overlying strata. Although Steno’s theories were not completely correct, they were quite insightful, especially considering the state of the knowledge at the time. In 1872, Benjamin Franklin expressed his opinion on the nature of the earth, an opinion that would be appreciated by later generations. He said the crust of the earth must be a shell floating on a fluid interior “thus the surface of the globe would be capable of being broken and disordered by the violent movements of the fluids on which it rested.” Toward the close of the 18th century, James Hutton (1726-1797), a Scottish doctor, farmer and naturalist, found stringers of granite penetrating sedimentary rocks and saw them as evidence of subterranean fire and heat. And although he had not identified the right source of heat, Hutton properly concluded that subterranean heat had consolidated and compacted sediment that accumulated on the ocean floors. He also erroneously concluded that expansion caused by the heat had elevated the rock and overlying strata. As recently as the early 1960s, the origin of mountains was still a mystery. But studies that took place during the last half of the 20th century, combined with ideas spawned during the 19th and 20th centuries, eventually led to what we have come to accept as the current theory of mountain building—a process called plate tectonics. British journalist and freelance writer Russell Miller nicely summed it up when he wrote that today’s view of mountains is that they are a “visible manifestation of an inch-by-inch movement of great slabs of the earth’s surface layers. This ages-long process not only pushes up mountains in crustal collisions of inconceivable force, it also creates and destroys oceans, and shatters and reshapes whole continents.” But that’s another story. William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.