sea-floor spreading key concepts what is the process of sea-floor spreading? what is the evidence...
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Sea-Floor SpreadingSea-Floor SpreadingKey ConceptsKey ConceptsWhat is the process of sea-floor What is the process of sea-floor spreading?spreading?What is the evidence for sea-floor What is the evidence for sea-floor spreading?spreading?What happens at deep-ocean trenches?What happens at deep-ocean trenches?
Key TermsKey Terms
Mid-ocean ridgeMid-ocean ridge
SonarSonar
Sea-floor spreadingSea-floor spreading
Deep-ocean trenchDeep-ocean trench
subductionsubduction
Mapping the Ocean FloorMapping the Ocean Floor
SonarSonar was used in the mid 1900’s to was used in the mid 1900’s to map the ocean floormap the ocean floor
Sonar Sonar bouncesbounces sound waves off sound waves off objects and objects and records the echoes records the echoes of of these waves.these waves.
Time it takes for echo to come back Time it takes for echo to come back indicates the indicates the distance from the distance from the objectobject
Sonar discovered the Sonar discovered the mid-ocean mid-ocean ridgesridges
Mid-Ocean RidgesMid-Ocean Ridges
Mid-ocean ridge system is a Mid-ocean ridge system is a chain of chain of underwater mountains underwater mountains that extend that extend into all of Earth’s oceans. into all of Earth’s oceans. They are more than They are more than 50,000 km 50,000 km longlongMost are under Most are under hundreds of meters hundreds of meters of waterof waterThey reach the surface in a few They reach the surface in a few places such as places such as IcelandIceland..A A steep sided valley steep sided valley splits the top of splits the top of some mid-ocean ridges.some mid-ocean ridges.
Sea Floor SpreadingSea Floor Spreading
In 1960 In 1960 Harry HessHarry Hess, an American , an American geologist, suggested that the sea floor geologist, suggested that the sea floor spread apart along spread apart along both sides of a mid-both sides of a mid-ocean ridge ocean ridge as new crust is added.as new crust is added.
As a result the ocean floors move like As a result the ocean floors move like conveyor belts conveyor belts carrying continents with carrying continents with them.them.
He called this He called this sea-floor spreadingsea-floor spreading
Molten material Molten material erupts through the valley erupts through the valley in the center of the ridge. Hardens and in the center of the ridge. Hardens and cools to form new sea floor.cools to form new sea floor.
Evidence for sea floor spreadingEvidence for sea floor spreading
From Molten MaterialFrom Molten Material– Alvin,Alvin, The world’s first deep ocean The world’s first deep ocean
submersible used for deep sea submersible used for deep sea exploration, found strange rocks shaped exploration, found strange rocks shaped like pillows in the mid-ocean ridge. This like pillows in the mid-ocean ridge. This was evidence that was evidence that molten material came molten material came out of the ridge and hardened quicklyout of the ridge and hardened quickly
Evidence from Magnetic Evidence from Magnetic StripesStripes
Scientists studied the Scientists studied the rock patterns rock patterns in the in the ocean floor .ocean floor .
They found that the rock on the ocean They found that the rock on the ocean floor lies in a pattern of floor lies in a pattern of magnetized magnetized stripesstripes
Magnetic stripes in ocean floor rocks are Magnetic stripes in ocean floor rocks are formed by the formed by the reversal of the Earth’s reversal of the Earth’s magnetic polesmagnetic poles. .
The last reversal happened 780,000 years The last reversal happened 780,000 years ago.ago.
The pattern of stripes The pattern of stripes matchesmatches on each on each side of the ocean ridge.side of the ocean ridge.
Evidence from drilling samplesEvidence from drilling samples
Glomar Challenger Glomar Challenger gathered samples of gathered samples of rocks by drilling into the rocks by drilling into the ocean floorocean floor
Drilled through Drilled through 6 km of water 6 km of water into ocean into ocean floorfloor
Scientists then determined the Scientists then determined the age of the age of the rocksrocks
They found that the rocks next to ocean They found that the rocks next to ocean ridges are ridges are youngeryounger than rocks farther than rocks farther away.away.
As the ocean floor spreads the As the ocean floor spreads the older rocks older rocks move farther away.move farther away.
Is the Earth’s surface Is the Earth’s surface getting larger from sea-floor getting larger from sea-floor
spreading?spreading?
Subduction at TrenchesSubduction at Trenches
Deep ocean trenches are Deep ocean trenches are deep deep underwater canyons underwater canyons
In a process that takes In a process that takes tens of tens of millions of years,millions of years, the ocean floor the ocean floor sinks back into the sinks back into the mantle mantle at deep at deep ocean trenches.ocean trenches.
Subduction Subduction is the process where is the process where the ocean floor sinks beneath a the ocean floor sinks beneath a deep ocean trench.deep ocean trench.
The Process of SubductionThe Process of Subduction
Sea-floor spreading and subduction Sea-floor spreading and subduction work together like a conveyor belt.work together like a conveyor belt.
New oceanic crust is hot.New oceanic crust is hot.
As it moves away from the ridge it As it moves away from the ridge it cools, and becomes more densecools, and becomes more dense
At a trench, the older, more dense At a trench, the older, more dense crust is pulled by gravity and sinks crust is pulled by gravity and sinks down beneath the trenchdown beneath the trench
Subduction and the Earth’s Subduction and the Earth’s OceansOceans
Sea-Floor spreading and subduction can Sea-Floor spreading and subduction can change the size and shape of the oceans.change the size and shape of the oceans.
Ocean floor is renewed every 200 million Ocean floor is renewed every 200 million years – the time it takes for the floor to years – the time it takes for the floor to travel from ridge to trench.travel from ridge to trench.
The Pacific Ocean is shrinking. More crust The Pacific Ocean is shrinking. More crust is being subducted than is being formed.is being subducted than is being formed.
The Atlantic Ocean is expanding. More The Atlantic Ocean is expanding. More crust is being formed than is being crust is being formed than is being subducted.subducted.
ReferencesReferences
http://www.whoi.edu/page.do?pid=8422http://www.whoi.edu/page.do?pid=8422
http://ocean-ridge.ldeo.columbia.edu/http://ocean-ridge.ldeo.columbia.edu/courses/subgeol/courses/subgeol/mid_ocean_landscape.htmlmid_ocean_landscape.html
http://oceansjsu.com/105/http://oceansjsu.com/105/exped_commotion/8.htmlexped_commotion/8.html