Mapping the Mid-Ocean Ridge• The mid-ocean ridge is the longest chain ofmountains in the world.• In the 1950’s scientist mapped the mid-oceanridge using sonar.•Sonar is an instrument that uses sound wavesto measure distance.• It bounces sound waves off underwater objectsand records the echoes of these sounds.
•The time that it takes the echo indicates thedistance to the object.• The scientists found out that the ocean floor wasnot flat.•This discovery peaked their curiosity to discoverwhat the ridge was and how it got there.
There are huge mountain rangescalled 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 floorsmove like a conveyer belt, which in turn move thecontinents.• This movement begins at the mid-ocean ridge, whichforms along in a crack in the oceanic crust.• At the mid-ocean ridge, molten materials rise fromthe mantle and erupts.•The molten material spreads out, pushing older rockto both sides of the ridge.
• Hess called this processSea-Floor Spreading.• Molten material, magneticstripes and drilling samplessupported Hess’s theory.
Evidence from Molten Material•In the ’60’s, scientist used a small submarinecalled Alvin to explore the ocean floor.• Alvin’s crew found rocks shaped like pillows ortoothpaste squeezed from a tube.• These rocks showed that molten material haderupted many different times from cracks alongthe mid-ocean ridge.
Evidence from Magnetic Stripes• The Earth is like a giant magnet with a north andsouth pole.• The Earth’s magnetic poles reversed themselves780,000 years ago.•Rocks on the ocean floor are in a pattern ofmagnetized stripes.•These stripes show when the Earth reversed it’smagnetic field.
More Evidence from Magnetic Stripes• Molten material contains iron.•As it cooled, the iron bits lined up inthe direction of Earth’s magnetic poles.•When the rock hardened, the iron waslocked in place, giving the rocks apermanent “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 magneticfield pointed north and a parallel stripe thatpointed south.•Rock that hardens at the same time wouldhave the same magnetic memory.
Evidence from Drilling Samples• The Glomar Challenger is a drilling ship thatrecovered drilling samples from the oceanfloor.• They studied the age of the rockssampled.• They found that the farther from the ridge,the older the rock.• The youngest rocks were at the center ofthe ridge.
Subduction at Deep-Ocean Trenches• The ocean floor plunges into deepunderwater canyons called deep-oceantrenches.• Subduction takes place where there aredeep-ocean trenches.• New oceanic crust is hot.• It moves away from the mid-ocean ridgeand cools, making it more dense.
• Gravity pulls the denser, older crustdown 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 spreadingchange the size and shape of theoceans.• The ocean floor is renewed every 200million 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 thePacific Ocean.• This occurs because a deep ocean trenchswallows more oceanic crust than the mid-ocean ridge can produce.• If new crust is not added fast enough, thewidth of the ocean shrinks.
Subduction in the Atlantic Ocean• The Atlantic Ocean is expanding.• The Atlantic Ocean has only a fewtrenches.• The Atlantic Ocean floor is attached to thecontinental crust of the continents.• As the sea-floor spreads, the continentsalong that edge also move.