Chapter 2 notes

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Published on 18 Jun 2011
Chapter 2
What is plate tectonics?
Tectonics is the study of the origin and arrangement of the broad structural features
of the Earths surface which includes folds, faults, mountain belts, continents and
earthquake belts.
Plate Tectonic is a theory that the Earths surface is divided into large few thick
plates that change position and size.
Intense geologic activity occurs at plate boundaries where plates move away from
one another, past one another or toward one another.
Plate tectonic theory helps explain earthquake distribution, the origin of mountain
belts, the origin of sea-floor topography, the distribution and composition of
volcanoes and etc.
This concept was developed in late 1960s by combining two-pre-existing ideas of
continental drift and sea-floor spreading.
Continental drift is the idea that continents move freely over the Earths surface,
changing their positions relative to one another.
Sea-floor spreading is a hypothesis that the sea floor forms at the crest of mid-
oceanic ridges, then moves horizontally away from the ridge crest toward an oceanic
How did the Plate tectonics theory develop?
The early case for Continental drift
Continents can be made to fit together like pieces of jigsaw puzzle.
In the early 1900s Alfred Wegner, a German meteorologist made a strong case to
support continental drift.
He noted that South America, Africa, India, Antarctica and Australia had almost
identical late Palaeozoic rocks and fossils.
The plant Glossopteris is found in Pennsylvanian and Permian-age rock on all five
continents, and fossil remains if Mesosaurus, a freshwater reptile, is found in
Permian-age rocks only in Brazil and South Africa.
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Also, fossil remains of land-dwelling reptiles Lysotrosaurus and Cynognathus are
found in Triassic-age rocks on all 5 continents.
Wegner proposed that all continents were once assembled together as a super
continent called Pangaea.
Pangaea initially separated into two parts; Laurasia was the northern
supercontinent, containing the present day North America and Euraisa (excluding
India). Gondwanaland was the southern supercontinent, composed of all the present
day southern hemisphere continents and India.
Wegner also began studying ancient climates (Paleoclimatology).
He found out that glacial features indicate a cold climate near the North Pole or
South Pole. Coral reefs indicate warm water near the equator. Crossbedded
sandstones can indicate ancient deserts near 30 degrees North and 30 degrees South
Skepticism about Continental drift
Wegners idea was not clear-cut since he had no mechanism to account for
continental drift
Wegner proposed that continents ploughed through the oceanic crust.
Most geologists disagreed and thought it violated what was known about the
strength of rocks at the time.
The driving mechanism proposed by Wegner for continental drift was a combination
of centrifugal force from the Earths rotation and the gravitational forces that cause
tides. However, calculations showed that these forces were too small to move
continents. Therefore, because of this little attention was given to Wegners ideas in
the northern Hemisphere including North America.
Work completed in 190s and 1950s set the stage for the revival of the idea of
continental drift along with new ideas about sea-floor spreading, into the concept of
plate tectonics.
Investigations were conducted in the two areas: (1) study of the sea floor and (2)
geophysical research, especially in relation to rock magnetism.
Study of the Sea floor
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Although studying the sea-floor rocks can be challenging, few techniques were used
to obtain samples of rocks and sediments from the sea floor.
Rocks can be broken from the sea floor using Rock dredge, which is an open
container dragged over the ocean bottom at the end of a cable.
Sediments can be sampled with a corer, a weighted steel pipe dropped vertically into
the mud and sand of the ocean floor.
Rocks and sediments can also be sampled using sea floor drilling. A ship with a
drilling derrick on its deck that drill holes in the deep sea floor far from the land.
The drill cuts long, rodlike rock cores from the ocean floor.
Furthermore, research submarines known as submersibles can take geologists to
many parts of the sea floor to observe, photograph, and sample rock and sediment.
A tool for indirectly studying the sea floor is the single-beam echo sounder, which
measures water depth and draws profiles of submarine topography. A sound signal
sent downward from a ship bounces off the sea floor and returns to the ship. The
water depth is determined from the time it takes the sound to make the round trip.
Sidescan solar- Measures the intensity of sound reflected back to the tow vehicle
from the ocean floor and provides detailed images of the sea floor and information
about sediments and bedforms.
A seismic reflection profler works the on the same principles of echo sounders but
uses a louder noise at lower frequencies. This sound penetrates the sea floor and
reflects from layers within the underlying sediment and rock.
Geophysical Research
Evidence about polar wandering, apparent movement of the Earths poles, came
from the study of rock magnetism.
Historical measurements show that the position of the magnetics poles moves from
year to year, but that the magnetic poles stay close to the geographic poles as they
The study of magnetic fields is called paleomagnetism.
Old rocks reveal very different magnetic pole positions to those at present and it was
once thought that this indicated the movement of the poles (polar wandering).
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