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Grade 8
Science
Unit: 07 Lesson: 01
Theory Development
Abraham Ortelius
Lived 1527–1598
Dutch Map Maker and Geographer
1596 – Ortelius suggested that North and South
America were connected to Eurasia and Africa,
but have drifted apart by natural events such as
flooding and earthquakes. He believed that the
continents fit together like pieces of a jigsaw
puzzle. The continental shelves, located
underneath the surface of the ocean, fit together
especially close.
http://commons.wikimedia.org/wiki/File:Abraham_Ortelius.jpg
Alfred Lothar Wegener
Lived 1880–1930
German Meteorologist and Geophysist
Pangaea Theory – Wegener claimed that the
continents had formed a single landmass, which
split apart. He was not the first to suggest that the
continents had been one, but was the first to
present evidence from several fields.
http://commons.wikimedia.org/wiki/File:Alfred_Wegener.jpg
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1912 - Continental Drift Theory – Wegener studied
South America and Africa, which are separated by
a large ocean and found that each had similar
glacial deposits, which would indicate that the
continents once had a very different climate. He
also found that fossils of tropical plants were
located on an Arctic island. Additionally, fossil
remains of a reptile, similar to a crocodile, were
found on South America and Africa. This indicated
that the two continents must have touched at
some point because the reptile would have been
unable to swim across the ocean.
Wegener’s theory was initially rejected because
he did not have geological proof that would
explain how the continents had been separated.
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Grade 8
Science
Unit: 07 Lesson: 01
Arthur Holmes
Lived 1890–1965
British Geologist
http://en.wikipedia.org/wiki/File:Arthurholmesin1912.jpg
1929 - Convection Currents Theory – As
Wegener’s theory was being dismissed because
he did not have sufficient geological proof to
explain why the continents moved, Holmes
introduced his theory on thermal convection in
the mantle. His theory explained that the molten
material in the mantle moved in a circular current.
As the magma heated, it would rise up to the
crust, then cool off, and sink again. Much like the
material in a lava lamp. When the magma reaches
the Earth’s crust, sometimes it breaks through
and pushes the continents in opposite directions.
This theory was not widely accepted at the time.
Harry Hess
Lived 1906–1969
American Geologist
1959 – The theory of Sea Floor Spreading explains
that the sea floor is moving away from a central
point, which causes the continents to move with
it. The theory is well-accepted now. It is caused
by convection currents in the molten, very weak
upper mantle, or asthenosphere.
http://en.wikipedia.org/wiki/File:Hess.gif
©2012, TESCCC
This geological mechanism was the missing piece
of Alfred Wegener’s Continental Drift Theory.
Hess explained that magma could push up
between two plates, then cool and expand,
pushing the plates beside it. This explained how
North and South America could have been pushed
west and Eurasia and Africa could have been
pushed east, away from the Americas. It is called
Sea Floor Spreading because the ocean is getting
wider, but the coastlines of the land masses do
not change dramatically.
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Grade 8
Science
Unit: 07 Lesson: 01
Frederick John Vine
Lives 1939–
British Marine Geologist and
Geophysicist
1962 – Vine-Morley-Matthews’ hypothesis was
further evidence that built on Harry Hess’ sea floor
spreading theory. Vine, along with Lawrence W.
Morley and Drummond Matthews, developed the
theory that the sea floor was spreading equally
away from the mid-ocean ridge. Their idea was
that the magnetic poles of the Earth have shifted
from North to South and back again several times.
As magma pushes through the Earth’s surface,
before it cools, the magnetic minerals, such as
iron, would turn in the direction of the poles. If the
Earth’s polarity changed, then the minerals would
change direction. This would create magnetic
stripes in the rocks of the sea floor. If the sea floor
was spreading, as Hess described, then the
magnetic stripes would be symmetrical on both
sides of the mid-ocean ridge.
The Glomar Challenger
1968
http://en.wikipedia.org/wiki/File:GlomarChallengerBW.JPG
©2012, TESCCC
The Glomar Challenger was a scientific drilling
ship that set out across the Atlantic Ocean from
South America towards Africa. Its job was to drill
into the ocean floor and take samples of rock so
that scientists could determine the age of the
rocks. What they found helped to solidify the
theory of sea floor spreading. The rock samples
closest to South America and Africa were the
same age and much older than the age of the sea
floor closer to the mid-ocean ridge. This meant
that new crust forms in the middle of the ocean
and the older crust was at one time close to the
mid-ocean ridge, placing South America and
Africa together.
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Grade 8
Science
Unit: 07 Lesson: 01
Dan McKenzie
Lives 1942–
English Geophysicist
1968 - Current Theory of Plate Tectonics – A
combination of two earlier ideas: continental drift
and sea floor spreading. This theory was formed
as new information was learned about the nature
of the ocean floor, Earth's ancient magnetism
patterns, the location of volcanoes and
earthquakes, the flow of heat from Earth's interior,
and the worldwide distribution of plant and animal
fossils. McKenzie wrote a paper summing up the
currently known information.
Earth's outermost layer, the lithosphere, is broken
into seven large, rigid pieces called plates: the
African, North American, South American,
Eurasian, Australian, Antarctic, and Pacific plates.
Several minor plates also exist. The plates are all
moving in different directions and at different
speeds, from 2 cm to 10 cm per year in
relationship to each other.
The main features of plate tectonics are:
• The Earth's surface is covered by a series of
crustal plates.
• The ocean floors are continually moving,
spreading from the center, sinking at the
edges, and being regenerated.
©2012, TESCCC
•
Convection currents beneath the plates
move the crustal plates in different
directions.
•
The source of heat driving the convection
currents is from radioactivity deep in the
Earth’s mantle.
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