Topic: Chapter 9: Plate Tectonics
Sec 2: Drifting Continents and Sec 3:
Sea-Floor Spreading
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What was Wegener’s Theory of
Continental Drift?
Early 1900’s, Alfred Wegener formed hypothesis that all continents were
once joined together in a single landmass. He named his supercontinent,
Pangaea “all lands.”
1. Landforms were similar on different continents (Mountain ranges running
east and west in South Africa lined up exactly with mountain ranges in
Argentina. European coal fields matched up with coal fields in North
America - fit together like puzzles). 2. Fossils (trace of an organism
preserved in rock) that could never have crossed oceans by swimming or
being carried by wind were found on both continents. 3. Climate Change:
Fossils of tropical plants and animals were found on extremely cold climate
landmasses and vice-versa.
Wegener lsp attempted to explain how drift took place and how mountains
were formed when continents collided. What he could not do was provide a
satisfactory explanation of the forces (a push or a pull) that moved the
continents. Without a cause for continental drift, most scientists rejected his
idea.
At the bottom of the ocean where temperatures are freezing but life
proliferates around cracks or hot-water vents in the Earth’s crust where water
is heated by contact with magma spurting from the mantle. The geological
features of the environment provided scientists with evidence to strongly
support Wegener’s hypothesis of continental drift.
Scientists discovered the longest mountain range on Earth at the very middle
of the ocean. It curves like the seams on a baseball along the sea floor into all
of the Earth’s oceans. Most of the mountains are completely covered by
ocean but Iceland is one island where the mid-ocean ridge extends above the
surface of the North Atlantic Ocean. A steep-sided valley splits the top of the
range.
Sonar was used to map the ocean floor. The discovery of the mid-ocean ridge
made scientists curious about HOW it got there!
What (3) types of evidence did
Wegener gather to convince him to
develop this theory that all
landmasses where once connected as
a supercontinent that he named
Pangaea and that the continents had
since drifted apart?
How was Wegener’s hypothesis of
continental drift received by his
peers?
Where did scientists finally find
support for Wegener’s Theory?
What is the Mid-Ocean Ridge?
What did scientists use to map the
ocean floor?
What did Harry Hess Discover?
What is sea-floor spreading?
Where did the evidence for sea-floor
spreading come from?
How can the sea floor keep getting
wider and wider?
What happens at deep-ocean
trenches?
Harry Hess discovered that Alfred Wegener was right! The continents did
drift. In 1960, he suggested that ocean floors move like conveyor belts
carrying continents with them and that the movement starts at the mid-ocean
ridge where molten material rises from the mantle and erupts. The molten
material spreads out, pushing older rock to both sides of the ridge.
Hess called this process of continually adding new materials (constructive
process) to the ocean floor “sea-floor spreading.” NEW CRUST IS ADDED.
1. Molten Material: Scientists went down in “Alvin” and saw the evidence of
molten material spewing out of the cracks and hardening into new crust. 2.
Magnetic Stripes: Evidence shows that Earth’s magnetic poles reverse
themselves. The Iron that came from the outer core as liquid had hardened on
the ocean floor as the sea-floor spread. It was in stripes facing magnetic
north. However, 780,000 years ago, magnetic poles changed. Iron that
spewed up and hardened at that time was facing magnetic south. Scientists
see these alternating stripes on both sides of the mid-ocean ridge – proof of
sea-floor spreading.
3. Drilling Samples: Drilling samples of the crust at the mid-ocean ridge
indicated that the farther away from the ridge, the older the rock. Youngest,
newly formed rock was always at the center of the ridges. Combine this
evidence and have strong support for sea-floor spreading.
It doesn’t. Eventually, it plunges into deep-ocean trenches where the crust
dives back down into the mantle.
“Subduction” (diving down of the denser, older crust underneath the newer,
less dense crust) allows part of the ocean floor to sink back into the mantle.
Sea-floor spreading occurs, pushes older crust to the sides as though on a
conveyor belt and then it dives down into the mantle at a deep-ocean trench
and melts to become magma again. Therefore, the process keeps the sea floor
from getting wider and wider.
Why is the Pacific Ocean shrinking?
Why is the Atlantic Ocean
expanding?
Summary
The deep ocean trench there is swallowing more oceanic rust back into the
mantle than the mid-ocean trench is creating causing the width of the ocean to
shrink.
It has only a few short trenches. The spreading ocean floor has almost
nowhere to go. Here, the oceanic crust is attached to the continental crust.
As the oceanic crust spreads, it pushes the continental crust causing the
continents around the Atlantic Ocean to move. The ocean gets wider.