Sea Floor Spreading

Sea-Floor SpreadingSea-Floor Spreading

StandardStandard•Explain how constructive Explain how constructive

and destructive forces (e.g. and destructive forces (e.g. continental drift, continental drift, earthquakes, volcanoes, earthquakes, volcanoes, weathering, erosion) weathering, erosion) change the Earth’s surface.change the Earth’s surface.

Essential Essential QuestionQuestion•Is the Earth Is the Earth getting bigger or getting bigger or smaller?smaller?

What caused Wegener’s theory What caused Wegener’s theory to be accepted?to be accepted?

• Evidence of sea-floor spreadingEvidence of sea-floor spreading

• New technology called sonar New technology called sonar

• Sonar allowed sea floor mapping Sonar allowed sea floor mapping

• Huge crack discovered in the middle Huge crack discovered in the middle of the Atlantic Oceanof the Atlantic Ocean

There is a chain of There is a chain of submerged mountains submerged mountains running through the running through the center of the Atlantic center of the Atlantic

Ocean.Ocean.• This chain is called the Mid-This chain is called the Mid-

Atlantic Ridge.Atlantic Ridge.• This ridge is part of a system of This ridge is part of a system of

ocean ridges.ocean ridges.• Mid-ocean ridges are underwater Mid-ocean ridges are underwater

mountain chains that run through mountain chains that run through Earth’s ocean basins.Earth’s ocean basins.

The Earth’s longest The Earth’s longest mountain range is underwater mountain range is underwater

Mid-Ocean RidgeMid-Ocean Ridgehttps://www.ocean.udel.edu/extreme2004/geology/mission/divelocation/ridge.htmlhttps://www.ocean.udel.edu/extreme2004/geology/mission/divelocation/ridge.html

• The mid-ocean ridge (shown in red) winds The mid-ocean ridge (shown in red) winds its way between the continents much like its way between the continents much like the seam on a baseball.the seam on a baseball.

Mid Ocean Ridges are Mid Ocean Ridges are where sea floor where sea floor spreading occurs.spreading occurs.• Sea floor spreading is the process by Sea floor spreading is the process by

which new oceanic lithosphere is which new oceanic lithosphere is created as older materials are created as older materials are pushed away.pushed away.

• As tectonic plates move away from As tectonic plates move away from each other, the sea floor spreads each other, the sea floor spreads apart and magma rises to fill the gap.apart and magma rises to fill the gap.

Example of sea floor Example of sea floor spreadingspreading

http://www.uwsp.edu/geo/faculty/ritter/glossary/s_u/sea_flr_spread.htmlhttp://www.uwsp.edu/geo/faculty/ritter/glossary/s_u/sea_flr_spread.html

Evidence of sea-floor Evidence of sea-floor spreadingspreading• New Molten MaterialNew Molten Material

Evidence of sea-floor Evidence of sea-floor spreadingspreading

• Magnetic stripesMagnetic stripes

• Drilling SamplesDrilling Samples– The The Glomar ChallengerGlomar Challenger drilled for core samples. drilled for core samples.– Samples revealed that the rocks get older the Samples revealed that the rocks get older the

further away from the ridge the samples are further away from the ridge the samples are taken.taken.

The crust increases in The crust increases in age the farther it moves age the farther it moves away from the mid-ocean away from the mid-ocean ridge.ridge.

• What does this mean exactly?What does this mean exactly?

• It means that newer rock is closest to It means that newer rock is closest to the ridge and older rock is farther away.the ridge and older rock is farther away.

• The oldest crust in the Atlantic Ocean is The oldest crust in the Atlantic Ocean is found near the edges of the continents.found near the edges of the continents.

Why doesn’t the Earth get Why doesn’t the Earth get bigger?bigger?

• SubductionSubduction

• http://ve.ou.edu/weaver/plates/subduct.htmhttp://ve.ou.edu/weaver/plates/subduct.htm

• Oceanic crust is denser than continental Oceanic crust is denser than continental crust causing it to subduct under the crust causing it to subduct under the continental crust when plates converge.continental crust when plates converge.

• When oceanic plates converge, the When oceanic plates converge, the older and denser plate subducts under older and denser plate subducts under the younger and less dense plate.the younger and less dense plate.

• Continental plates converging create Continental plates converging create mountains. mountains.

• Subduction is causing the Pacific Subduction is causing the Pacific ocean to get smaller.ocean to get smaller.

• The Atlantic ocean has few The Atlantic ocean has few subduction zones and is therefore subduction zones and is therefore getting larger.getting larger.

The theory of Plate The theory of Plate TectonicsTectonics• The proof of sea-floor spreading The proof of sea-floor spreading

supported Wegener’s idea that supported Wegener’s idea that continents move.continents move.

• Both oceanic and continental crust Both oceanic and continental crust appear to move, so a new theory was appear to move, so a new theory was devised to explain both continental devised to explain both continental drift and sea-floor spreading: The drift and sea-floor spreading: The Theory of Plate Tectonics.Theory of Plate Tectonics.

Major Tectonic Plates Major Tectonic Plates

• Plate tectonics is the theory that Plate tectonics is the theory that Earth’s lithosphere is divided into Earth’s lithosphere is divided into tectonic plates that move around tectonic plates that move around on top of the asthenosphere.on top of the asthenosphere.

• Scientists have theorized that Scientists have theorized that there are three different ways there are three different ways plates are able to move: plates are able to move: convection, slab pull, and ridge convection, slab pull, and ridge push.push.

ConvectionConvection• In the process of convection, hot material In the process of convection, hot material

from deep within the Earth rises while from deep within the Earth rises while cooler material near the surface sinks.cooler material near the surface sinks.

• When the warmer material cools, it When the warmer material cools, it becomes denser and begins to sink back becomes denser and begins to sink back down.down.

• The motion of convecting mantle material The motion of convecting mantle material drags tectonic plates sideways.drags tectonic plates sideways.

Ridge PushRidge Push• At mid-ocean At mid-ocean

ridges, the oceanic ridges, the oceanic lithosphere is lithosphere is higher than it is higher than it is where it sinks where it sinks beneath continental beneath continental lithosphere.lithosphere.

• Ridge push is the Ridge push is the process by which process by which an oceanic plate an oceanic plate slides down the slides down the lithosphere-lithosphere-asthenosphere asthenosphere boundary.boundary.

Slab PullSlab Pull• Because oceanic lithosphere is Because oceanic lithosphere is

denser than the asthenosphere, the denser than the asthenosphere, the edge of the oceanic plate sinks and edge of the oceanic plate sinks and pulls the rest of the tectonic plate pulls the rest of the tectonic plate with it in a process called slab pull.with it in a process called slab pull.