Seafloor SpreadingSeafloor Spreading

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p. 141-147p. 141-147

Seafloor SpreadingSeafloor Spreading

•Until the 1950’s little was known about Until the 1950’s little was known about the ocean floor, but after sonar was the ocean floor, but after sonar was invented, a complex ocean floor with invented, a complex ocean floor with mountains, valleys, trenches, and mountains, valleys, trenches, and ridges. ridges.

•The most complex mountain range The most complex mountain range was found in the center of the Atlantic. was found in the center of the Atlantic. Scientists wondered what could have Scientists wondered what could have caused these mountain ranges.caused these mountain ranges.

Marianas TrenchMarianas Trench

• Deepest part of the Deepest part of the earthearth

• Has barely been Has barely been exploredexplored

• 36,000 feet deep at 36,000 feet deep at the “Challenger the “Challenger Deep”Deep”

• Can fit Mt. Everest Can fit Mt. Everest in the holein the hole

Mid Ocean RidgesMid Ocean Ridges

•A ridge of mountains that are A ridge of mountains that are very deep under the oceans and very deep under the oceans and wind along the Earth’s like seams wind along the Earth’s like seams on a baseball. on a baseball.

•At these ridges new crust is being At these ridges new crust is being made from molten material that made from molten material that reaches the surface.reaches the surface.

Sea-floor spreadingSea-floor spreading•The process of continually adding new The process of continually adding new

material to the sea floor while other material to the sea floor while other material was being recycled back into material was being recycled back into the mantle.the mantle.

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

Seafloor Spreading at Seafloor Spreading at MarianaMariana

Harry HessHarry Hess• In 1960 scientist Harry Hess suggested an explanation. In 1960 scientist Harry Hess suggested an explanation. • His theory called seafloor spreading stated that hot molten His theory called seafloor spreading stated that hot molten

material is forced upward to the surface at the mid-ocean material is forced upward to the surface at the mid-ocean ridge. It flows sideways carrying the seafloor away from the ridge. It flows sideways carrying the seafloor away from the ridge. ridge.

• New magma then fills in the empty spots, then solidifies and New magma then fills in the empty spots, then solidifies and forms the new seafloor. forms the new seafloor.

• Three pieces of evidence showed this theory to be correct.Three pieces of evidence showed this theory to be correct.

Evidence from Molten Evidence from Molten MaterialMaterial

• Scientists dived 4 km to the ocean Scientists dived 4 km to the ocean floor aboard the submarine floor aboard the submarine ALVIN ALVIN where at the mid ocean ridge they where at the mid ocean ridge they found strange rocks shaped like found strange rocks shaped like pillows or toothpaste squeezed from pillows or toothpaste squeezed from a tube.a tube.

• These rocks form only from rapidly These rocks form only from rapidly cooling magma under water.cooling magma under water.

Underwater lavaUnderwater lava

Evidence from Magnetic Evidence from Magnetic StripesStripes

•The magnetic field on the Earth has The magnetic field on the Earth has reversed itself many times over the course reversed itself many times over the course of history of history

•When that occurs the atoms within the When that occurs the atoms within the rocks that have magnetic properties align rocks that have magnetic properties align themselves with whichever way is North at themselves with whichever way is North at the time.the time.

•Over time there are stripes that align with Over time there are stripes that align with the magnetic field.the magnetic field.

Evidence from Drilling Evidence from Drilling samplessamples

• In 1968 the In 1968 the Glomar Challenger Glomar Challenger drilled drilled 6 km down in the ocean floor. 6 km down in the ocean floor. Samples were brought up and they Samples were brought up and they determined the ages of the rocks. determined the ages of the rocks.

• The younger rocks were closest to The younger rocks were closest to the mid ocean ridge and the oldest the mid ocean ridge and the oldest were closest to the continental were closest to the continental shores.shores.

Subduction at TrenchesSubduction at Trenches

• If the seafloor is spreading- dose it keep If the seafloor is spreading- dose it keep getting wider and wider??????getting wider and wider??????

• NO- As rock reaches the continents the NO- As rock reaches the continents the ocean floor plunges under the continents ocean floor plunges under the continents at sites called deep ocean trenches. This at sites called deep ocean trenches. This is where the ocean crust bends downward is where the ocean crust bends downward and the ocean crust sinks back into the and the ocean crust sinks back into the mantle.mantle.

The Process of SubductionThe Process of Subduction

• Subduction is the process of ocean Subduction is the process of ocean crust returning to the mantle by crust returning to the mantle by sinking over millions of years.sinking over millions of years.

• The reason the oceanic crust can The reason the oceanic crust can sink is because it is more dense that sink is because it is more dense that the continental crust.the continental crust.

Subduction and the Earth’s Subduction and the Earth’s OceansOceans

• The ocean floor is renewed about every The ocean floor is renewed about every 200 million years200 million years

• The Pacific ocean is shrinking in size The Pacific ocean is shrinking in size because the mid-ocean ridge is because the mid-ocean ridge is producing less new rock than is producing less new rock than is returning to the mantle while the returning to the mantle while the Atlantic Ocean is expanding because it Atlantic Ocean is expanding because it only has a few trenches but many only has a few trenches but many ridges.ridges.

The Theory of Plate The Theory of Plate TectonicsTectonics

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p. 150-154p. 150-154

Lithosphere’s Eggshell Lithosphere’s Eggshell texturetexture• In 1965, Wilson observed cracks in the In 1965, Wilson observed cracks in the

continents similar to those on the ocean continents similar to those on the ocean floor and proposed that the lithosphere is floor and proposed that the lithosphere is broken into separate plates.broken into separate plates.

• These plates carry pieces of continents These plates carry pieces of continents and ocean floor, or both.and ocean floor, or both.

• This combined the theory of continental This combined the theory of continental drift and seafloor spreading into one drift and seafloor spreading into one theory called plate tectonics.theory called plate tectonics.

How plates MOVEHow plates MOVE

• The plates are in constant motion The plates are in constant motion moved by convection currents deep moved by convection currents deep in the mantle.in the mantle.

• As the plates move they collide, pull As the plates move they collide, pull apart, and grind into one another apart, and grind into one another which causes volcanoes, mountain which causes volcanoes, mountain building, and trenches.building, and trenches.

Plate BoundariesPlate Boundaries

• Three types of boundariesThree types of boundaries– ConvergentConvergent– DivergentDivergent– TransformTransform

Divergent BoundariesDivergent Boundaries

• The boundary between two plates The boundary between two plates that are pulling apart is called a that are pulling apart is called a divergent boundary. Two examples divergent boundary. Two examples of divergent boundaries are the Mid-of divergent boundaries are the Mid-Atlantic Ridge and the Great Rift Atlantic Ridge and the Great Rift Valley in Africa.Valley in Africa.

Convergent BoundariesConvergent Boundaries

• When the seafloor cools, becomes When the seafloor cools, becomes dense, and sinks, or where two dense, and sinks, or where two plates collide it is called a convergent plates collide it is called a convergent boundary. boundary.

• There are three types of convergent There are three types of convergent boundariesboundaries

Ocean-Continent CollisionOcean-Continent Collision

• When an ocean floor plate collides with a When an ocean floor plate collides with a continental plate, the ocean floor plate is continental plate, the ocean floor plate is denser and goes under the continental plate. denser and goes under the continental plate.

• This is called a subduction zone. This is called a subduction zone.

• Volcanoes and trenches occur at subduction Volcanoes and trenches occur at subduction zones; because the ocean floor plate is melting zones; because the ocean floor plate is melting as it descends under the continental plate. as it descends under the continental plate.

• The Andes Mountains in South America is an The Andes Mountains in South America is an example.example.

Ocean-Ocean CollisionsOcean-Ocean Collisions

• Ocean-Ocean Collisions occur when Ocean-Ocean Collisions occur when two ocean plates collide, or when one two ocean plates collide, or when one plate cools and begins to sink. plate cools and begins to sink.

• A deep sea trench is formed as one A deep sea trench is formed as one plate goes under the other and islands plate goes under the other and islands and volcanoes are also formed as new and volcanoes are also formed as new magma is forced to the surface. magma is forced to the surface.

• The islands of Japan were formed this The islands of Japan were formed this wayway

Di-Continental Plate CollideDi-Continental Plate Collide

• When 2 continental plates collide When 2 continental plates collide neither is less dense than the other neither is less dense than the other so no subduction occurs, therefore so no subduction occurs, therefore the two plates crumple upward the two plates crumple upward forming mountain ranges.forming mountain ranges.

Appalachian MountainsAppalachian Mountains• A look at rocks exposed in today's Appalachian mountains A look at rocks exposed in today's Appalachian mountains

reveals elongated belts of folded and thrust faulted marine reveals elongated belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient sedimentary rocks, volcanic rocks and slivers of ancient ocean floor, which provides strong evidence that these rocks ocean floor, which provides strong evidence that these rocks were deformed during plate collision. were deformed during plate collision.

• The birth of the Appalachian ranges, some 480 million years The birth of the Appalachian ranges, some 480 million years ago, marks the first of several mountain building plate ago, marks the first of several mountain building plate collisions that culminated in the construction of the collisions that culminated in the construction of the

supercontinesupercontinentnt Pangaea with the Appalachians near the center. Pangaea with the Appalachians near the center.

• Because North America and Africa were connected, the Because North America and Africa were connected, the Appalachians formed part of the same mountain chain as the Appalachians formed part of the same mountain chain as the Anti-Atlas in Morocco. To the northeast, the same mountain Anti-Atlas in Morocco. To the northeast, the same mountain chain continued into Scotland, from the North chain continued into Scotland, from the North America/Europe collision. America/Europe collision.

Transform Fault BoundariesTransform Fault Boundaries

• Transform fault boundaries occur when Transform fault boundaries occur when two plates slide past each other and are two plates slide past each other and are moving either in opposite directions or moving either in opposite directions or in the same direction at different rates. in the same direction at different rates.

• When 2 plate slides past each other, an When 2 plate slides past each other, an earthquake will occur. The San Andreas earthquake will occur. The San Andreas fault in California is the most famous fault in California is the most famous transform fault boundary. transform fault boundary.

Effects of Plate TectonicsEffects of Plate Tectonics

• Two of the main effects caused by Two of the main effects caused by plate tectonics are volcanoes and plate tectonics are volcanoes and earthquakes. Plate tectonics also form earthquakes. Plate tectonics also form all of the rifts and mountains to form. all of the rifts and mountains to form.

• Tension forces cause the plates to Tension forces cause the plates to stretch and tilt blocks forming a fault-stretch and tilt blocks forming a fault-block mountain. block mountain.

• If two blocks separate a rift valley can If two blocks separate a rift valley can occur.occur.

Compression ForcesCompression Forces

Compression causes massive forces Compression causes massive forces creating mountains as they smash creating mountains as they smash together the plates.together the plates.