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Sea-Floor Spreading

Section 3 in Chapter 1

Christopher Landry

on 27 November 2013

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Transcript of Sea-Floor Spreading

Inside the Earth
Sea-Floor Spreading
Mid-Ocean Ridges
The longest chain of mountains in the world is the system of
mid-ocean ridges
. In the mid-1900s, scientists mapped the mid-ocean ridges using sonar.
is a device that that bounces sound waves off underwater objects and then records the echoes of these sound waves. The time it takes for the echo to arrive indicates the distance to the object.
The mid-ocean ridges curve along the sea floor, extending into all of Earth's oceans. Most of the mountains in the mid-ocean ridges lie hidden under hundreds of meters of water.
In a few places the ridge pokes above the surface. For example, the island of Iceland is a part of the mid-ocean ridge that rises above the surface in the North Atlantic Ocean. A steep-sided valley splits the top of some mid-ocean ridges.
Sea-Floor Spreading
Harry Hess
, an American geologist, proposed a radical idea. He suggested that a process called sea-floor spreading continually adds new material to the ocean floor.
sea-floor spreading
, the sea floor spreads apart along both sides of a mid-ocean ridge as new crust is added. As a result, the ocean floors move like conveyor belts, carrying the continents along with them.
Earth's ocean floors move like conveyor belts, carrying the continents along with them. This movement begins at a mid-ocean ridge. A ridge forms along a crack in the oceanic crust.
At a mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge.
As the molten material cools, it forms a strip of solid rock in the center of the ridge. Then more molten material splits apart the strip of solid rock that formed before, pushing it aside.
What is the process of sea-floor spreading?
Evidence for Sea-Floor Spreading
Evidence from Molten Material
Evidence from Magnetic Stripes
Evidence from Drilling Samples
Several types of evidence supported Hess's theory of sea-floor spreading: Eruptions of molten material, magnetic stripes in the rock of the ocean floor, and the ages of the rocks themselves.
This led scientists to look again at Wegener's hypothesis of continental drift.
Scientists have found strange rocks shaped like pillows in the central valley of mid-ocean ridges.
Such rocks can form only if molten material hardens quickly after erupting under water.
The presence of these rocks supports the theory of sea-floor spreading.
More support came when scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized 'stripes'.
Surprisingly, Earth's magnetic poles have reversed themselves many times during Earth's history. The last reversal happened 780,000 years ago.
The rock on the ocean floor contains iron. The rock began as molten material that cooled and hardened.
The final proof of sea-floor spreading came from rock samples obtained by drilling into the ocean floor.
Scientists found that the farther from a ridge the rocks were taken, the older they were.
What evidence is there for sea-floor spreading?
Subduction at Trenches
The ocean floor does not just keep spreading. Instead, it sinks beneath deep underwater canyons called
deep-ocean trenches
At a deep-ocean trench, the oceanic crust bends downward. In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches.
The Process of Subduction
Where there are trenches, subduction takes place.
is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.
As subduction occurs, crust closer to a mid-ocean ridge moves away from the ridge and toward a deep-ocean trench. Sea-floor spreading and subduction work together. They move the ocean floor as if it were on a giant conveyor belt.
What happens at deep-ocean trenches?
Subduction and Earth's Oceans
The processes of subduction and sea-floor spreading can change the size and shape of the oceans. Because of these processes, the ocean floor is renewed about every 200 million years.
The Pacific Ocean covers almost one third of the planet, and yet it is shrinking Its many trenches are swallowing more ocean crust than the mid-ocean ridge is producing.
The Atlantic Ocean is expanding. In most places, the oceanic crust of the Atlantic Ocean is attached to continental crust. As the Atlantic's floor spreads, the continents along its edges also move. Over time, the whole ocean gets wider.
The pattern is the same on both sides of the ridge. These stripes hold a record of reversals in Earth's magnetic field.
If the magnetic poles suddenly reversed themselves today, you would find that your compass needle would point south.
As the rock cooled, the iron bits inside lined up in the direction of Earth's magnetic poles. This locked the iron bits in place, giving the rocks a permanent 'magnetic memory'.
At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle.
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