Chapter 5 Plate Tectonics Section 4 sea-floor Spreading

How do rocks along the centreal valley of the mid-ocean ridge provide evidence of sea-floor spreading?

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What are the steps in the process of sea-floor spreading?

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deep-ocean trenche

A deep valley along the ocean floor beneath which oceanic crust slowly sinks toward the mantle.

What happens as subduction occurs?

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.

New ocenaic crust is hot. But as it moves away from the mid-ocean ridge, it cools and becomes more dense. Eventually, gravity pulls this older, denser oceanice crust down beneath the trench.

As the sinking crust floating on the water gets wet, its density increases and begins to sink.

What happens to oceanic crust at a deep-ocean trench?

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.

Magnetic Stripes

Figure 17....____________________ in the rock of the ocean floor show the direction of Earth's magnetic field at the time the rock hardened.

around these hot-water vents in the ocean floor the most bizarre creatures live

Giant, red tipped tube worms, giant clams and strange spider like crabs.

What scientist helped to discover the processs of sea-floor spreading?

Harry Hess

What is unusual about Iceland?

Iceland is a portion of the mid-atlantic ridge that rose above sea level. it has frequent volcanic activity

What happens at deep-ocean trenches?

In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches.

Wht occurs at trenches?

In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches.

In 1960 Hess suggested that a process called sea-floor spreading contiually adds new material to the ocean floor.

In sea-floor spreading, the sea floor spreads apart along both sides of a mid-ocean ridge as new crst is added. As a result, the ocean floors move like conveyor belts, carrying the continents along with them.

What is the process of sea-floor spreading?

In 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.

Why is the Atlantic Ocean expanding?

In the Pacific Ocean, subduction through the many trenches that ring the ocean is occurring faster than new crust can be added. Unlike the Pacific Ocean, the Atlantic Ocean is expanding. It has only a few short trenches. As a result, the spreading ocean floor has nowhere to go. As the Atlantic's ocean floor spreads, the continents along its edges also move and the ocean gets wider.

Along what feature of the ocean floor does sea-floor spreading begin?

Mid-Ocean ridge

Where would you expect to find the oldest rock on the ocean floor?

Nearer to deep ocean trenches

What is the evidence for sea-floor spreading?

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.

The Deep-Ocean Floor

Shrimp, crabs, and other organisms cluster near hot water vents in the ocean floor.

Why is the Pacific Ocean shrinking?

Sometimes a deep-ocean trench swallows more oceanic crust then a mid-ocean ridge can produce. Then, if the ridge does not add new crust fast enough, the width of the ocean will shrink. In the pacific ocean, subduction through the many trenches that ring the ocean occur-ing faster then the new crust can be added.

What device is used to map the ocean floor?

Sonar

How are these matching stripes evidnece of sea-floor spreading? (figure 17 pg. 144)

Stripes of rock formed when Earth's magnetic field pointed north alternate with stripes of rock that formed when the magnetic field pointed south. The alternating pattern is the same on both sides of a mid-ocean ridge.

Sea-Floor Drilling

The Glomar Challenger was the first research ship designed to drill samples of rock from the deep ocean floor.

What happens to the rock along the ridge when new molten material erupts?

The older rock moves outward on both sides of the ridge as new rock forms in the center of the ridge from cooling of molten material. This process is sea floor spreading.

Subduction

The process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary.

Subduction and Earth's oceans

The process 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. That is the time it takes for new rock to form at the mid- ocean ridge, move atoss the ocean, and sink into a trench.

Why does the rock of the ocean floor have a pattern of magnetic stripes?

The rock of the ocean floor contains iron. As molten material cooled and hardenend, the iron bits inside lined up in the direction of Earth's magnetic poles, creating a pattern of magnetized stripes.

The mid-ocean ridge system is more than 50,000 kilometers long.

They extend into all of Earth's oceans. Most of the mountains in the mid-ocean ridge system lie hidden under hundreds of meters of water. But in a few place the ridge pokes above the surface. ex 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.

How does new oceanic crust form?

When molten material erupts through the mid ocean ridge called sea-floor spreading

Deep in the ocean, the temperature is near freezing.There is no light, and living things are generally scarce.

Yet some areas of the deep-ocean floor are teeming with life. One of these areas is the East Pacific Rise. This area forms part of the Pacific Ocean floor off the coasts of Mexico and South America. Here Ocean water sinks through cracks, or vents in the crust. The water is heated by contact with hot material from the mantle. The hot water then spurts back into the ocean.

What is a deep-ocean trench?

a deep valley along the ocean floor where oceanic crust slowly sinks toward the mantle; a convergent boundary

sonar

a device 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.

Harry Hess, an American geologist was one of the scientists who studied mid-ocean ridges.

after examining maps of mid-ocean ridge system he had a startling conclusion...Maybe wegener was right! Perhaps the continents do move.

Sea-Floor Spreading begins at a mid-ocean ridge, which forms along a crack in the oceanic crust.

along the ridge, molten material that forms several kilometers beneath the surface rises and erupts. at the same time, older rock moves outward on both sides of the ridge. As the molten material cools, it forms a strip of solid rock in the center of the ridge. When more molten material flows into the crack it forms a new strip of rock.

Mid-ocean ridges

an undersea mountain chain where new ocean floor is produced.

Several types of evidence supported Hess's theory of sea floor spreading:

erruptions of molten material, magnetic strips in the rock of the ocean floor, and the ages of the rocks themselves.

What three types of evidence provided support for the 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.

Surprisingly, the geological features of the strange environment on the ocean floor provide some of the best evidence

for Wegener's hypothesis of contintal drift.

Because of sea-floor spreading, the distance between Europe and Noth America

is increasing by a few centimeters per year.

Sea- Floor Spreading

molten material erupts throught the valley that runs along the center of some mid-ocean ridges. This material hardens to form the rock of the ocean floor.

The Pacific Ocean covers almost

one third of the plantet.

How can the ocean floor keep getting wider and wider?

the ocean floor generally does not just keep spreading. Instead, the ocean floor plunges into deep underwater canyons called deep-ocean trenches. At a deep-ocean trench, the oceanic crust bends downward.

sea-floor spreading

the process by which molten material adds new oceanic crust to the ocean floor

Where would the densest oceanic crust be found?

under the deep ocean trenches. New oceanic crust is hot. As it moves away from the mid-ocean ridge toward a deep ocean trench it cools and becomes more dense.