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Why don't continents subduct? What happens when the collide?

Benjamin Franklin and Ralph Waldo Emerson were amongst the earliest pre-supposers of what we now accept as Continental Drift or Tectonic Plates theorising.

One argument advanced for plate tectonics is seafloor spreading. In ocean basins, along mid-ocean ridges (e.g., the Mid-Atlantic Ridge and East Pacific Rise), observations are interpreted to indicate that plates are diverging, with molten material from the mantle rising up in the gap between the plates and cooling to form new crust under the ocean. The youngest crust is at the ridge axis, with progressively older rocks away from the axis. Worldwide, it is estimated that currently about 20 cubic kilometers of molten magma rises each year to create new oceanic crust.

The continental drift hypothesis was first introduced by Alfred Wegener in 1914.

The red arrows show the direction of drift at the plate boundaries.

Another observation that contributed to the hypothesis is the fact that similar flora and fauna can found on completely different continents, where there was no obvious way for them to travel in between.

By the 1960s science had become generally accepting of a theory of Plate Tectonics where the "solid plates" of a so-called Lithosphere - be they continents or ocean floor - effectively "floated" on a so-called Asthenosphere, of underlying rock that is under such high pressure and temperature conditions as to permit slight flows of movement that over geological timescales have had a cumulative effect that has distinctly shaped our planet.

These are purported to be the "trails" of the moving continents.

Why should the Earth behave in such a strange manner? The other planetsin our solar system don't appear to have plate tectonics; why should theEarth? Perhaps Prof. Sandwell and other geologists have created some weird,convoluted theory because they don't really understand how the Earth works.

Some of the geographical evidence collected in the last part of the 20th century to support the theory of the continental drift is the discovery of plate tectonics Hess and Deitz modified the theory called "Sea-floor Spreading".

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Concerned with the continents - not the oceans

cause the continental land masses to "drift" relative to each other

It is accepted that the Earth's crust is comprise of 'plates' which move, and as a result can drift away from, collide with, or shift laterally 'sliding against', other plates.

Drift has affected all aspects of the surface of the earth

Some sources continue to show the Indian and Australian plates featured here as a single Indo-Australian plate.)

The fact that large portions of the ocean floor, as well as continental expanses, seem to be "solid plates" has tended to discredit the term - continental drift - and to establish the alternative term - plate tectonics.

"The Origin of Continents and Oceans"

As you read the book and listen to the lectures always think to yourself;Is plate tectonics really true? Are these observations consistent withplate tectonic theory? Is there an alternate theory that can explain allof the observations?

Hard to argue that the continents were fixed in place

Middle: Just before the ice age, these same continents had been home to a very uniform assemblage of land plants and animals. Geologists had explained such groupings by hypothetical "land bridges."

Butwhy did the continental crust expand suddenly about 3 eons ago?

Inthe southern hemisphere, where geologists were familiar with the rocks thatWegener used to support his hypothesis, continental drift was generallyaccepted.

The weight of the continent equals the displaced weightof the mantle.

The tendency for volcanoes to erupt and for earthquakes to impact in various parts of the world seems to relate directly to the existence of underlying tectonic plates and the type of plate movements:-

Generally speaking Divergent / Spreading plate boundaries do not seem to dramatically feature earthquakes but there is continuous volcanic activity along a ridge marking the - constructive - divergence of the plates.
Such divergences take places at various long ridges on the Earth's crust where there is an upswelling of materials from deep in the Earth.

In the process of continentstipping, some places are moving down.

In the following two lectures I'll provide for the theory. Recent exploration of the ocean basins aswell as geodetic and seismological data provide very strong confirmingevidence for plate tectonics. By the end of the course, I hope that youbelieve that the plates really do slide thousands of kilometers acrossthe top of the slippery mantle and then fall back onto the Earth when theyare old, cold, and heavy. This plate tectonic activity provides and efficientmechanism for releasing the radiogenic heat that forms in the interiorof the Earth.

When continents collide, they form folded mountain ranges.

This upwelling represents the engine behind tectonic plate movements as newly-forming crust displaces pre-existing crust such that existing plates - despite their unimaginable weight are 'pushed' away from the upswelling ridge.

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