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Sea floor spreading

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Jenny Dixon
Jenny Dixon

Scientists in the 1950s used sonar to map the mid-ocean ridge and discovered it was not flat but contained underwater mountains. This discovery led them to research what the ridge was and how it formed. Evidence from molten rock samples, magnetic stripes in the ocean crust, and the ages of rocks drilled from the ocean floor supported Harry Hess' theory from 1960 of sea-floor spreading, where new crust forms at mid-ocean ridges and spreads outward over time.

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Sea floor spreading
Mapping the Mid-Ocean Ridge • The mid-ocean ridge is the longest chain of mountains in the world. • In the 1950’s scientist mapped the mid-ocean ridge using sonar. •Sonar is an instrument that uses sound waves to measure distance. • It bounces sound waves off underwater objects and records the echoes of these sounds.
•The time that it takes the echo indicates the distance to the object. • The scientists found out that the ocean floor was not flat. •This discovery peaked their curiosity to discover what the ridge was and how it got there.
There are huge mountain ranges called ridges. Mid-Atlantic Ridge
Evidence for Sea-Floor Spreading • In 1960, Harry Hess studied Wegener’s theory. • Hess proposed the radical idea that the ocean floors move like a conveyer belt, which in turn move the continents. • This movement begins at the mid-ocean ridge, which forms along in a crack in the oceanic crust. • At the mid-ocean ridge, molten materials rise from the mantle and erupts. •The molten material spreads out, pushing older rock to both sides of the ridge.
• Hess called this process Sea-Floor Spreading. • Molten material, magnetic stripes and drilling samples supported Hess’s theory.
Evidence from Molten Material •In the ’60’s, scientist used a small submarine called Alvin to explore the ocean floor. • Alvin’s crew found rocks shaped like pillows or toothpaste squeezed from a tube. • These rocks showed that molten material had erupted many different times from cracks along the mid-ocean ridge.
Evidence from Magnetic Stripes • The Earth is like a giant magnet with a north and south pole. • The Earth’s magnetic poles reversed themselves 780,000 years ago. •Rocks on the ocean floor are in a pattern of magnetized stripes. •These stripes show when the Earth reversed it’s magnetic field.
Sea floor spreading
More Evidence from Magnetic Stripes • Molten material contains iron. •As it cooled, the iron bits lined up in the direction of Earth’s magnetic poles. •When the rock hardened, the iron was locked in place, giving the rocks a permanent “magnetic memory”.
More Evidence from Magnetic Stripes • Scientist recorded this “magnetic memory” on both sides of the mid-ocean ridge. • They found a stripe of when the magnetic field pointed north and a parallel stripe that pointed south. •Rock that hardens at the same time would have the same magnetic memory.
Evidence from Drilling Samples • The Glomar Challenger is a drilling ship that recovered drilling samples from the ocean floor. • They studied the age of the rocks sampled. • They found that the farther from the ridge, the older the rock. • The youngest rocks were at the center of the ridge.
Sea floor spreading
Subduction at Deep-Ocean Trenches • The ocean floor plunges into deep underwater canyons called deep-ocean trenches. • Subduction takes place where there are deep-ocean trenches. • New oceanic crust is hot. • It moves away from the mid-ocean ridge and cools, making it more dense.
• Gravity pulls the denser, older crust down beneath the trench. • Subduction allows the ocean floor to sink back into the mantle that takes tens of millions of years.
Subduction and Earth’s Ocean’s • Subduction and sea-floor spreading change the size and shape of the oceans. • The ocean floor is renewed every 200 million years.
Subduction in the Pacific Ocean • The Pacific Ocean covers 1/3 of the planet, but it is shrinking. • There is a ring of trenches that surrounds the Pacific Ocean. • This occurs because a deep ocean trench swallows more oceanic crust than the mid- ocean ridge can produce. • If new crust is not added fast enough, the width of the ocean shrinks.
Subduction in the Atlantic Ocean • The Atlantic Ocean is expanding. • The Atlantic Ocean has only a few trenches. • The Atlantic Ocean floor is attached to the continental crust of the continents. • As the sea-floor spreads, the continents along that edge also move.

More Related Content

Sea floor spreading

  • 2. Mapping the Mid-Ocean Ridge • The mid-ocean ridge is the longest chain of mountains in the world. • In the 1950’s scientist mapped the mid-ocean ridge using sonar. •Sonar is an instrument that uses sound waves to measure distance. • It bounces sound waves off underwater objects and records the echoes of these sounds.
  • 3. •The time that it takes the echo indicates the distance to the object. • The scientists found out that the ocean floor was not flat. •This discovery peaked their curiosity to discover what the ridge was and how it got there.
  • 4. There are huge mountain ranges called ridges. Mid-Atlantic Ridge
  • 5. Evidence for Sea-Floor Spreading • In 1960, Harry Hess studied Wegener’s theory. • Hess proposed the radical idea that the ocean floors move like a conveyer belt, which in turn move the continents. • This movement begins at the mid-ocean ridge, which forms along in a crack in the oceanic crust. • At the mid-ocean ridge, molten materials rise from the mantle and erupts. •The molten material spreads out, pushing older rock to both sides of the ridge.
  • 6. • Hess called this process Sea-Floor Spreading. • Molten material, magnetic stripes and drilling samples supported Hess’s theory.
  • 7. Evidence from Molten Material •In the ’60’s, scientist used a small submarine called Alvin to explore the ocean floor. • Alvin’s crew found rocks shaped like pillows or toothpaste squeezed from a tube. • These rocks showed that molten material had erupted many different times from cracks along the mid-ocean ridge.
  • 8. Evidence from Magnetic Stripes • The Earth is like a giant magnet with a north and south pole. • The Earth’s magnetic poles reversed themselves 780,000 years ago. •Rocks on the ocean floor are in a pattern of magnetized stripes. •These stripes show when the Earth reversed it’s magnetic field.
  • 10. More Evidence from Magnetic Stripes • Molten material contains iron. •As it cooled, the iron bits lined up in the direction of Earth’s magnetic poles. •When the rock hardened, the iron was locked in place, giving the rocks a permanent “magnetic memory”.
  • 11. More Evidence from Magnetic Stripes • Scientist recorded this “magnetic memory” on both sides of the mid-ocean ridge. • They found a stripe of when the magnetic field pointed north and a parallel stripe that pointed south. •Rock that hardens at the same time would have the same magnetic memory.
  • 12. Evidence from Drilling Samples • The Glomar Challenger is a drilling ship that recovered drilling samples from the ocean floor. • They studied the age of the rocks sampled. • They found that the farther from the ridge, the older the rock. • The youngest rocks were at the center of the ridge.
  • 14. Subduction at Deep-Ocean Trenches • The ocean floor plunges into deep underwater canyons called deep-ocean trenches. • Subduction takes place where there are deep-ocean trenches. • New oceanic crust is hot. • It moves away from the mid-ocean ridge and cools, making it more dense.
  • 15. • Gravity pulls the denser, older crust down beneath the trench. • Subduction allows the ocean floor to sink back into the mantle that takes tens of millions of years.
  • 16. Subduction and Earth’s Ocean’s • Subduction and sea-floor spreading change the size and shape of the oceans. • The ocean floor is renewed every 200 million years.
  • 17. Subduction in the Pacific Ocean • The Pacific Ocean covers 1/3 of the planet, but it is shrinking. • There is a ring of trenches that surrounds the Pacific Ocean. • This occurs because a deep ocean trench swallows more oceanic crust than the mid- ocean ridge can produce. • If new crust is not added fast enough, the width of the ocean shrinks.
  • 18. Subduction in the Atlantic Ocean • The Atlantic Ocean is expanding. • The Atlantic Ocean has only a few trenches. • The Atlantic Ocean floor is attached to the continental crust of the continents. • As the sea-floor spreads, the continents along that edge also move.