Why Are There Mountains?
Isostacy and Geosynclines

One of the greatest challenges in the development of the geological sciences was to discover the processes and forces that led to the development of mountain ranges. James Hutton had proposed that mountains were built by upheavals driven by heat escaping from the Earth's fiery interior. This was apparent in volcanoes, but what about great mountain ranges like the Appalachians, Alps, Himalayas, and Andes? In the most general sense he was correct that these were the result of heat transfer from the interior. But the specific processes took two centuries to discover.

Isostacy

The French mapping expeditions in South America in the mid eighteenth century and the Great Trigonometrical Survey begun by the British in 1802 in India observed that the force of gravity was less than expected in mountainous regions like the Andes and Himalayas. G.B. Airy proposed that the Earth's crust was made of relatively low density material lying above a more dense material that behaves as a fluid. We now know that the mantle which underlies the crust behaves as a plastic solid or extremely viscous fluid and it is about 20 percent more dense than continental crust. The low density crust in a real sense "floats" at the surface of the mantle just as icebergs, like the one that sunk the Titanic, float in water. Airy believed that mountainous regions were areas with thickened continental crust. They had deep crustal roots extending downward into the more dense mantle. The force of gravity over these low density roots would be less than the force of gravity over high density material.

 

 

Geosynclines 

James Hall (1857), an American geologist, noted that deformed (folded and faulted) Paleozoic strata in the Appalachians are 10 times the thickness of undeformed Paleozoic strata in the Mississippi valley. He proposed that the great sediment load in the Appalachians region caused crustal failure and downwarp. This warping caused the sedimentary layers to be deformed.

J.D. Dana (1813-1895), an influential American geologist, claimed (1873) that these geosynclines couldn't be a result of subsidence due to sediment loading because sediments are not dense enough to depress the crust. Downwarping must have occurred first, leaving a trough which collected sediments.

Dana accepted isostacy. He believed the continents were less dense granitic and sedimentary materials that "floated" high in the mantle while the relatively more dense basaltic crust of the oceans did not float as high in the mantle. He also believed, following Kelvin and others, that the Earth cooled from an initially molten mass. As the Earth's surface solidified some areas formed less dense granitic crust that formed the highstanding continents and some areas formed somewhat more dense basaltic crust that became the ocean basins. As the Earth continued to cool it contracted. The boundaries between the oceans and continents were prime zones of focused stresses. The contraction caused downwarped troughs to form near the edges of the continents. These troughs were filled with sediments produced by weathering on the continent. The deposited sedimentary layers were deformed and folded and uplifted as a result of continued cooling and contraction.

Dana's geosynclinal theory was expanded and modified by others such as the Austrian geologist , Edward Suess (1831-1914). This became a primary theory to explain the large scale dynamics of the Earth and was taught up until the 1960's.

The geosynclinal concept and deformation derived from a contracting Earth, although very appealing, could not explain all of the observations. As mountain belts were studied it became obvious that the deformation implied very large contractions. But physical models of the cooling Earth allowed only a small amount of contraction. How could you account for the extra contraction or horizontal motion? Another problem was the fact that the crust is not everywhere in compression. There are areas today and geologic terranes formed in the geologic past that are now or once were being stretched and extended. A world that was simply contracting should have no areas of stretching. Dana's theory seemed very well suited for mountainous regions like the Appalachians, Rockies, and Andes that parallel the coast, but not for landlocked mountain ranges like the Urals. At the turn of the twentieth century the discovery of radioactivity cast doubt on Kelvin's cooling Earth model.

The geosynclinal theory envisioned primarily vertical motions (downwarp, upwarp) with only minor horizontal motions due to contraction. Almost no serious scientist through the early twentieth century considered horizontal motions of the Earth's crust. The continents were formed and remained in place and for endless eons they were the site of the continuous struggle between erosion (destruction) and upheaval (construction).

Moving Continents?

As early as the fifteenth century, European scholars studying new maps of the world produced as a result of the early explorations of the new world and Africa noticed the complementary fit of the continents on opposite sides of the Atlantic Ocean. By the mid-nineteenth through early twentieth century a small number of people had proposed some form of continental motion either in terms of continental separation to explain this complementary fit or in terms of collisions to explain mountain belts. The most coherent hypothesis was that of American geographer and geologist F.B. Taylor (1910). But even Taylor's proposal had little supporting evidence and no real mechanism.

The Rest of the Story

This story continues with the Theory of Continental Drift proposed by Alfred Wegener in 1912. While Wegener's theory was not accepted because of the difficlutly of understanding a mechanism for it, evidence continued to mount through the 20th century. Research in paleomagnetism in the 1950s and 1960s confirmed Wegener's Pangea reconstruction and continental motions in general. The study of marine magnetic anomalies in the 1960s gave rise to the discovery of seafloor spreading, which provided the mechanism for crustal motion.

Continental Drift to Plate Tectonics