Mapping the Mid-Ocean Ridge• The mid-ocean ridge is thelongest chain of mountains in theworld.

• In the 1950s scientist mappedthe mid-ocean ridge using sonar.

•Sonar is an instrument that usessound waves to measuredistance.

• It bounces sound waves offunderwater objects and recordsthe echoes of these sounds.

•The time that it takes the echoindicates the distance to theobject.

• The scientists found out thatthe ocean floor was not flat.

•This discovery peaked theircuriosity to discover what theridge was and how it got there.

Ex: Mid-AtlanticRidge

There are huge mountain rangescalled ridges.

The Global Mid-Oceanic Ridge System

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,which forms 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 olderrock to both sides of the ridge.

Evidence for Sea-Floor Spreading

• Hess called this processSea-Floor Spreading.

• Molten material, magneticstripes, and drilling samplessupported Hess’s theory.

• In the 1960s, scientist used a small submarine called Alvin toexplore the ocean floor.

• Alvin’s crew found rocks shaped like pillows or toothpaste squeezed from a tube.

• These rocks showed that molten materialhad erupted many different times from cracksalong the mid-ocean ridge.

Evidence from Molten Material

• The Earth is like a giant magnet witha north and south pole.

• The Earth’s magnetic poles reversedthemselves 780,000 years ago.

•Rocks on the ocean floor are in apattern of magnetized stripes.

•These stripes show when the Earthreversed its magnetic field.

Evidence from Magnetic Stripes

Reversalshappen on

average onlyabout once every250,000 years,and they take

hundreds if notthousands of

years tocomplete. Themagnetic field

does not vanishduring this time.

• Molten material contains iron.

•As it cooled, the iron bits lined upin the direction of Earth’s magneticpoles.

•When the rock hardened, the ironwas locked in place, giving therocks a permanent “magneticmemory”.

More Evidence from Magnetic Stripes

•Scientist recorded this “magneticmemory” on both sides of the mid-ocean ridge.

• They found a stripe of when themagnetic field pointed north and aparallel stripe that pointed south.

•Rock that hardens at the same timewould have the same magneticmemory.

More Evidence from Magnetic Stripes

The Glomar Challenger is adrilling ship that recovereddrilling samples from theocean floor.

• They studied the age of therocks sampled.

• They found that the farther fromthe ridge, the older the rock.

• The youngest rocks were at thecenter of the ridge.

Evidence from Drilling Samples

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, oldercrust down beneath the trench.

• Subduction allowsthe ocean floor tosink back into themantle that takestens of millionsof years to recycle.

•Subduction and sea-floorspreading change the sizeand shape of the oceans.

• The ocean floor is renewedevery 200 million years.

Subduction and Earth’s Ocean’s

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.