Privileged Planet: continental drift (Introduction)

by David Turell @, Monday, February 18, 2019, 15:22 (587 days ago) @ dhw
edited by David Turell, Monday, February 18, 2019, 15:29

The movement of Earth's plates, with roving continents is a concept that is less than 55 years old! The following is a long history of the geologist's garbled thinking, but we wouldn't be here without continental drift and subduction:

https://aeon.co/essays/when-geology-left-solid-ground-how-mountains-came-to-be?utm_sour...

"A lone exception was the British geologist Arthur Holmes (1890-1965) who as early as 1929 proposed that convection cells in the mantle – like rolling wheels beneath the crust – could be the motor for both Wegener’s continents and the explanation for mountain-building. But even he declined to push the idea too hard, opting to present the most fully developed version of his theory in his geology textbook.

"...in the guise of newly declassified naval data that revealed for the first time the strange world of the abyssal ocean floor. Bathymetric charts and corresponding maps of the magnetic properties of deep-sea rocks provided the critical clues to the mechanism for moving the continents. That is, seafloor spreading, in which fresh volcanic ocean crust is continuously produced at submarine ridges, and then forced outward as successive batches of basaltic magma rise. But it took several years before the complementary process, subduction, was understood, and this, at last, provided an explanation for how mountains grow.


"When seafloor basalt is around 150 million years old, and hundreds of miles from its natal ridge, it has become about as dense as the underlying mantle, and sinks back into Earth’s interior at a slant, pulling the rest of the plate behind it like a blanket sliding off a bed. Holmes had been right – mantle convection was the key, but the convection cells were not deep subterranean rotors – they included the crust. (And to be fair to disciples of geosynclinal theory, the vertical pull of gravity is, in the end, responsible for horizontal plate motions.) The process of subduction recycles ocean crust and generates devastating earthquakes, but over longer timescales is a constructive phenomenon, since it sets the stage for mountain-building. Many subduction zones occur close to coastlines, as in the case of western South America, where part of the floor of the Pacific Ocean plunges eastward beneath the continent. In such a setting, if a subducting slab of ocean crust is towing a continent, an eventual head-on collision between landmasses is inevitable. That’s why Peach and Horne’s Scottish Caledonides, Willis’s Appalachians and Mount Everest itself are there.

***

"Many of the great mountain belts in the world, like the Andes and McConnell’s Rockies, haven’t involved actual continent-on-continent pile-ups. In the case of the Canadian Rockies, it seems that a series of smaller unsubductable factors such as Hawaiian-type islands or orphaned continental fragments might have careened into western North America and raised the mountains. The Colorado Rockies, somewhat younger, are strange because they formed far inland from the plate boundary and don’t seem to have involved a collision at all. Instead, the cause of crustal shortening might have been subduction of a still-buoyant ocean plate that refused to go quietly into the mantle, raising welts in the continent above.

***

"In the past decade or so, we have gained an appreciation for how mountain-building is not in fact driven purely by tectonics but by the subtle interplay between tectonic processes and the external agents of erosion even as mountains are forming. Some of these insights have come from a new generation of scale models that, together with computational ones, allow us to watch mountains growing in real time. Among the most elegant ideas to emerge is ‘critical taper theory’, which posits that the outer part of a growing mountain belt – where shelf sediments are shoved up onto the adjacent continent – can be approximated by a wedge of sand being pushed by a bulldozer blade.

***

"Fortunately, nature will always be messier, subtler, richer – and far more interesting than our cartoonish depictions of it. How wonderful to know that we live on a planet that is not shrivelling in its old age, but is vibrantly alive and self-rejuvenating, stirring its interior, reworking its surface, recycling its crust, and raising new mountains."

Comment: We now know Life needs a vibrant living changing planet. Old theories die hard (Thomas Kuhn).


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