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SGES 1302
INTRODUCTION
TO EARTH SYSTEM
LECTURE 4: Continental Drift & Seafloor Spreading
Lecture 4: Continental Drift & Seafloor Spreading
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Plate Tectonics
Plate tectonics is a theory of geology that has been developed to explain the observed evidence for large
scale motions of the Earth's lithosphere. The theory encompassed and superseded the older theory of
continental drift from the first half of the 20th century and the concept of seafloor spreading developed during
the 1960s.
The outermost part of the Earth's interior is made up of two layers: the lithosphere, & asthenosphere.
Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow on geological
time scales. The deeper mantle below the asthenosphere is more rigid again.
The lithosphere is broken up into what are called tectonic plates—in the case of Earth, there are seven major
and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move in relation to
one another. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate
boundaries. The lateral movement of the plates is typically at speeds of 0.66 to 8.50 cm per year.
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Continental Drift

Plate tectonic theory arose out of the hypothesis of continental drift first proposed by
Alfred Wegener in 1912 that said in part that all the continents once been together in
a single land mass that had drifted apart.

Many geologists had noted earlier that the shapes of continents on either side of the
Atlantic Ocean (most notably, Africa and South America) seem to fit together.

The similarity of southern continent fossil and some geological formations had led a
small number of geologists as early as 1900, believe that all the continents had once
been joined into a supercontinent (now known as Pangaea). Taylor suggested that
the continents were dragged towards the equator by increased lunar gravity during
the Cretaceous, thus forming the Himalaya and Alps on the southern faces.

Alfred Wegener was unable to provide a convincing explanation for the physical
processes which might have caused this drift. His suggestion that the continents had
been pulled apart by the centrifugal force of the Earth's rotation was considered
unrealistic by the scientific community.

The idea of continental drift did not become widely accepted even as theory until the
late 1950s and the 1960s. The hypothesis of continental drift became part of the
larger theory of plate tectonics.
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Continental Drift

The hypothesis of continental drift introduced by German meteorologist Alfred
Wegener suggested a supercontinent, called Pangaea (meaning "all land"), that
broke up millions of years ago, slowly moved to their current positions, and continue
to move today.
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Continental Drift

When Wegener first noticed the
similarities in the shoreline of continent
on both sides of the Atlantic Ocean, he
then began fitting them together. He
particularly liked the way in which
South America and Africa fit. It was not
until the 1960's that scientists
proposed a new map using the edges
of the continental shelfs as the true
boundaries of the continents.
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Continental Drift

Wegener used documented cases of fossil organisms that had been found on
different landmass that could not have crossed the current oceans. Fossils of
Mesosarus have been found only in South America and Africa. Remains of other
organisms appear to link these two continents during the late Paleozoic and early
Mesozoic eras.
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Continental Drift

This picture shows matching mountain
ranges on each side of the Atlantic
Ocean. The Appalachian Mountains
that disappear off the coast of
Newfoundland match mountains in the
British Isles and Scandinavia which
are comparable in age and structure.
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Continental Drift

The last form of evidence
that Wegener use to
support his continental drift
hypothesis was ancient
climates. When the
continents are put together
to form Pangaea the
remains of glacial materials
fit together to form a pattern
like the large ice sheets that
cover our poles today.
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Seafloor Spreading

Seafloor spreading occurs at mid-ocean ridges, where new oceanic crust is formed
through volcanic activity and then gradually moves away from the ridge. Seafloor
spreading helps explain continental drift in the theory of plate tectonics.

The idea that the seafloor itself moves (and carries the continents with it) as it
expands from a central axis was proposed by Harry Hess from Princeton University in
the 1960s. The theory is well-accepted now, and the phenomenon is known to be
caused by convection currents in the plastic and weak asthenosphere.
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Seafloor Spreading

In the general case, sea floor spreading starts as a rift in a
continental land mass, similar to the Red Sea-East Africa Rift
System today.

The process starts with heating at the base of the continental crust
which causes it to become more plastic and less dense. Because
less dense objects rise in relation to more dense objects, the area
being heated becomes a broad dome.

As the crust bows upward, fractures occur that gradually grow into
rifts. The typical rift system consists of three rift arms at
approximately 120 degree angles.
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Seafloor Spreading
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If spreading continues past the initial stage described above, two of
the rift arms will open while the third arm stops opening and
becomes a 'failed rift'.
As the two active rifts continue to open, eventually the continental
crust is thined as far as it will stretch. At this point basaltic oceanic
crust begins to form between the separating continental fragments.
When one of the rifts opens into the existing ocean, the rift system is
flooded with seawater and becomes a new sea.
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Next Lecture
TECTONIC PLATE BOUNDARIES