SEAFLOOR SPREADING AND MAGNETIC REVERSAL
The idea of continental drift circulated in scientific circles until World War II, when sounding gear called SONAR
produced new evidence of what the seafloor looked like. The gear, developed in the 1930s, bounced sound waves off the seafloor
to determine its depth and features.
It happened that the command of one attack transport ship, the USS Cape Johnson, was given to Harry Hammond
Hess, a geologist from Princeton University. Hess, then in his late thirties, wanted to continue his scientific investigations even
while at war. So, he left his ship's sounding gear all of the time, not just when approaching port or navigating a difficult landing.
What Hess discovered was a big surprise.
What did Harry Hess and his men accidentally discover when they explored the oceanic floor? Were they able to locate
the start of all the movements on the Earth's surface? Moreover, did the Harry Hess team gather much strong evidence to
support the claim that continents are drifting?
Ocean floor exploration continued, and by the 1950s, other researchers had found that a huge rift ran along the top of
the Mid-Atlantic Ridge. That enabled Hess to understand his ocean floor profiles in the Pacific. He discovered that the bottom of
the sea was not as smooth as expected, but full of canyons, trenches, and volcanic sea mountains. He realized that the Earth's
crust had been moving away on each side of oceanic ridges, down the Atlantic and Pacific oceans, long and volcanically active.
Harry Hess observed that the rate of formation of new seafloor at the mid-ocean ridge is not always as fast as the
destruction of the old seafloor at the subduction zone. This explains why the Pacific Ocean is getting smaller and why the
Atlantic Ocean is getting wider. If the subduction zone is faster than the seafloor spreading, the ocean shrinks. He published his
theory in History of Ocean Basins (1962), and it came to be called "seafloor spreading."
In the early 1960s, dating of ocean-core samples showed that the ocean floor was younger at the Mid-Atlantic
Ridge but progressively older in either direction, confirming the reality of seafloor spreading.
ACTIVITY:
: ACTIVITY 1: LET’S EXPLAIN!
Let's find out what you have learned from our lesson on Sea Floor Spreading. Can you answer the following
questions?
1. Explain the theory of seafloor spreading?
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2. What geologic feature was discovered by Harry Hess and his team in the oceanic crust that stretches from
Northern to Southern region?
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3. What can you say about the ages of the oceanic rocks near the Mid-Atlantic Ridge? Do the ages of oceanic rocks
get older as you move away from the ridge?
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4. What are the reasons why the shape of the Earth remains constant?
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5. Explain the two reasons why the shape of the Earth remains constant.
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6. What SONAR stand for?
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7. What is the result of seafloor spreading and subduction?
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