Continental drift
[14] This appeared to be confirmed by the exploration of the deep sea beds conducted by the Challenger expedition, 1872–1876, which showed that contrary to expectation, land debris brought down by rivers to the ocean is deposited comparatively close to the shore on what is now known as the continental shelf.
[17] Apart from the earlier speculations mentioned above, the idea that the American continents had once formed a single landmass with Eurasia and Africa was postulated by several scientists before Alfred Wegener's 1912 paper.
He suggested that the continents had been pulled apart by the centrifugal pseudoforce (Polflucht) of the Earth's rotation or by a small component of astronomical precession, but calculations showed that the force was not sufficient.
Although now accepted, and even with a minority of scientific proponents over the decades, the theory of continental drift was largely rejected for many years, with evidence in its favor considered insufficient.
He proposed in 1931 that the Earth's mantle contained convection cells which dissipated heat produced by radioactive decay and moved the crust at the surface.
[38] Hans Stille and Leopold Kober opposed the idea of continental drift and worked on a "fixist"[39] geosyncline model with Earth contraction playing a key role in the formation of orogens.
[47] David Attenborough, who attended university in the second half of the 1940s, recounted an incident illustrating its lack of acceptance then: "I once asked one of my lecturers why he was not talking to us about continental drift and I was told, sneeringly, that if I could prove there was a force that could move continents, then he might think about it.
[50] From the 1930s to the late 1950s, works by Vening-Meinesz, Holmes, Umbgrove, and numerous others outlined concepts that were close or nearly identical to modern plate tectonics theory.
In particular, the English geologist Arthur Holmes proposed in 1920 that plate junctions might lie beneath the sea, and in 1928 that convection currents within the mantle might be the driving force.
[51] Holmes's views were particularly influential: in his bestselling textbook, Principles of Physical Geology, he included a chapter on continental drift, proposing that Earth's mantle contained convection cells which dissipated radioactive heat and moved the crust at the surface.
However, scientific communication in the 1930s and 1940s was inhibited by World War II, and the theory still required work to avoid foundering on the orogeny and isostasy objections.
[citation needed] In 1947, a team of scientists led by Maurice Ewing confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the sediments was chemically and physically different from continental crust.
[56] Meanwhile, scientists began recognizing odd magnetic variations across the ocean floor using devices developed during World War II to detect submarines.
New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust.
[61] Without workable alternatives to explain the stripes, geophysicists were forced to conclude that Holmes had been right: ocean rifts were sites of perpetual orogeny at the boundaries of convection cells.
Oceanic crust is created at spreading centers, and this, along with subduction, drives the system of plates in a chaotic manner, resulting in continuous orogeny and areas of isostatic imbalance.
The widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence for the theory of continental drift.
The continuity of glaciers, inferred from oriented glacial striations and deposits called tillites, suggested the existence of the supercontinent of Gondwana, which became a central element of the concept of continental drift.
