How Geologists Determined The Way That Mountains Formed

Mount Everest, view from Kala Patthar (5.700 m), with most important geological formations added. Image by Uwe Gille (2005), used under creative commons license.

Asked why he wanted to climb Mount Everest, George Mallory famously responded ?because it?s there.? But there was also a more practical reason ? ?for the stone from the top for geologists.?

This wasn?t a jest ? geologists were intensely interested in Everest. When Mallory, together with Sandy Irvine, attempted the ascent June 6, 1924, mountaineer and geologist Noel Ewart Odel ? the last person to see the two alive ? was collecting rocks at the base of the summit era of the 8,848 metres (29,029 ft) a.s.l. high mountain.

Other geological evidence, especially fossils of organisms, collected by Noeal Ewart demonstrated that ?the summit of Mt. Everest is marine limestone.? Geologist and author John McPhee pointed out this geological contradiction ? the highest point on earth was once an ancient seafloor.

The discovery of shells and remains of marine organism high on mountains was not completely new for the time. Renaissance artist and naturalist Leonardo da Vinci had recognized shells embedded in the rocks of the ?mountains of Parma and Piacenza,? arguing correctly that the flood described in the Bible could not transport and deposit undamaged shells so high on the mountains. Da Vinci speculated that parts of earth?s crust had instead collapsed into large subterranean cavities filled with water. The displaced water would itself push other parts of the crust up. This mechanism, thought da Vinci, could explain the distribution of mountains and valleys.

Unfortunately, da Vinci never published these observations and speculations. Thus, many European naturalists continued to consider the Biblical Flood as a good explanation for the observed geological oddities found in mountains. The catastrophic flood and its strong currents, they thought, would redeposit sediments and fold and disrupt the newly formed layers of rocks.

Physician Johann Jakob Scheuchzer, one of the first naturalists to publish about the Alps, used examples of large-scale folds observed in the Swiss Alps as evidence for the veracity of the Biblical account of the Flood. Sedimentary rocks are widespread in plains and valleys and it?s easy to observe how rivers and occasionally floods erode, transport and deposit new sediments. However, in mountains the situation is more difficult to explain. Naturalists, like Scheuchzer, couldn?t really determine from where the water needed to cover the highest peaks, came from or disappeared to after the Flood.
 

Helvetic Nappes of Switzerland. (Credit: Kurt St?we, via imaggeo.egu.eu)

In the 18th and 19th centuries, geologists started to accurately map the distribution of rocks and disposition of their layers. Additionally, volcanoes were better understood and were now considered important geological forces. Soon, geologists recognized that in mountain ranges ? or at least in the Alps ? layered sedimentary rocks surrounded a central core of undifferentiated rocks, which were formed by slow cooling of intruded lava-like rocks.

German geologist Leopold von Buch was convinced that mountains formed like a bubble on earth?s crust: magma from earth?s mantle pushes up, displaces and folds the Earth?s crust, and finally forms a mountain. Von Buch?s ?crater of elevation? theory became very popular at the time and was shared by most European geologists. French geologist Elie de Beaumont, adopting von Buch?s theory, could seemingly use it to explain even the complex geology of the Alps.

Beaumont explained that different tilted layers of sediments, as found in the Alps, were formed by periodic ?magmatic? pulses. In a first phase, the horizontally deposited sediments were tilted by the intrusion of a large magmatic core. In a later phase, the already tilted layers become even steeper and then new layers ? partly formed by the erosion of older layers and partly as the mountain-forming process increasingly involves a larger area ? start to tilt. However, British geologists later showed that this theory couldn?t work as proposed. If a mountain formed around a single bubble of magma, the all the layers should be disposed like in an onion. But ther the strata in the Alps were tilted chaotically and often even folded.

A new theory ? the Contracting Earth theory ? was later formulated by the American geologist James Dwigth Dana. This theory explained mountains and continents as products of a cooling and subsequently shrinking earth. Like an old and dry apple, the theory explained, the shrinking surface of Earth would develop fissures (basins) and wrinkles (mountains).

Following on this work, Austrian Geologist Eduard Suess published in his multi-volume work Das Antlitz der Erde this hand-colored map, the different colors of which show the preserved old ?cores of crust? surrounded by the younger basins today filled with oceans. Curiously, he suggested that the deep-sea trenches found along the borders of the Pacific are zones where the seafloor is pushed under the continents.

The Contracting Earth theory could explain the immense forces needed to crack and fold rocks. But it failed to explain the irregular distribution of mountains on Earth. According to the theory, features of the Earth?s crust should be distributed randomly on the surface of the cooling and uniformly shrinking planet. However, even a short glimpse on a map or globe shows that mountain ranges are not randomly distributed, but rather form long chains, like the Alp, Caucasus, or Himalayas; or are instead found along one side of a continent, like the Rocky Mountains or the Andes, but not on the other side.

SEARLE, M. (2013): Colliding Continents: A geological exploration of the Himalaya. Oxford University Press: 438