Sea Floor Spreading The Mid-ocean Ridge Vocabulary Mid-ocean ridgeMid-ocean ridge SonarSonar...

Sea Floor Spreading

The Mid-ocean Ridge

VocabularyVocabulary

•Mid-ocean ridgeMid-ocean ridge

•SonarSonar

•Sea-floor spreadingSea-floor spreading

•Deep-ocean trenchDeep-ocean trench

•SubductionSubduction

60 years ago, in the 60 years ago, in the 1950’s, 1950’s, SonarSonar was was

invented.invented.

Bats use it!

So do dolphinsSo do dolphins!

So Does So Does Aquaman!Aquaman!

How did sonar bring Wegner’s How did sonar bring Wegner’s theory back to life?theory back to life?

SonarSonar made it possible made it possible for us to observe the for us to observe the

ocean floor.ocean floor.

Scientists observed a large Scientists observed a large mountain chain mountain chain running running down the center of the down the center of the

Atlantic ocean.Atlantic ocean.

Scientists found that the Scientists found that the mountain range mountain range connected with connected with others and encircled the globeothers and encircled the globe!

Mapping the Mid-ocean Mapping the Mid-ocean RidgesRidges

• Ridges connect and curve around Ridges connect and curve around the globe like a baseballthe globe like a baseball

• Ridges lie hundreds of meters below Ridges lie hundreds of meters below the surface of the oceanthe surface of the ocean

• Most ridges are underwaterMost ridges are underwater

• Section of ridge that is on land is Section of ridge that is on land is East African Rift ValleyEast African Rift Valley

The discovery and study of the mid-The discovery and study of the mid-ocean ridges led to the theoryocean ridges led to the theory

of sea floor spreadingof sea floor spreading

Sea-Floor Spreading

•Harry Hess

No! Harry HessHarry Hess, not Harry Potter!

• An American geologist who studied mid-ocean ridges.

• He suggested that the ocean floors move like conveyor belts, carrying the continents along with them.

• Seafloor spreading- new ocean crust is formed at Seafloor spreading- new ocean crust is formed at ocean ridges and destroyed at deep sea trenchesocean ridges and destroyed at deep sea trenches– Magma forced toward crustMagma forced toward crust

– Fills gaps and hardensFills gaps and hardens

– Forms new ocean floorForms new ocean floor

• Divergent boundary-Divergent boundary-– Moving apartMoving apart

– Most are found on seafloorMost are found on seafloor

– Form ocean ridgesForm ocean ridges

– When on continent, form rift valleyWhen on continent, form rift valley

Pillow Lava forms when magma cools in water. This is evidence of volcanic activity!

• Magnetic reversal- change in earth’s Magnetic reversal- change in earth’s magnetic fieldmagnetic field– Same as present field- normal polaritySame as present field- normal polarity

– Opposite to present- reversedOpposite to present- reversed

This shows you the age of the sea floor. The This shows you the age of the sea floor. The red areas are new rocks and the blue areas red areas are new rocks and the blue areas are the oldest oceanic rock.are the oldest oceanic rock.

Iceland is spreading Iceland is spreading open on the Mid-open on the Mid-Atlantic Ridge!Atlantic Ridge!

Can you see where the Mid-Atlantic Ridge bisects Iceland? This is a rift valley!

Sea Floor SpreadingSea Floor Spreading• Sonar • Mid-Ocean Ridge• Ocean is spreading open!!!• Pillow Lava• Symmetrical Magnetic Stripes• Symmetrical Age Patterns• Newest Rock at the Ridge

If new crust is being added to If new crust is being added to Earth’s surface, is Earth Earth’s surface, is Earth

getting bigger?getting bigger?

• Convergent boundary-Convergent boundary-– Moving toward each otherMoving toward each other– Classified according to type of crust involvedClassified according to type of crust involved– Oceanic crust mostly basaltOceanic crust mostly basalt– Continental crust granite and sedimentary rockContinental crust granite and sedimentary rock– Oceanic-oceanic- subduction occurs (one plate Oceanic-oceanic- subduction occurs (one plate

descends) creates deep sea trench, forms arc of descends) creates deep sea trench, forms arc of volcanic islands: Mariana trench and islandsvolcanic islands: Mariana trench and islands

Subduction is a process is a process where the ocean floor sinks back where the ocean floor sinks back into the into the mantlemantle at a at a deep ocean deep ocean

trench.trench.

• Ridge push- forces in mantle cause Ridge push- forces in mantle cause asthenoshere to riseasthenoshere to rise– Pushes oceanic plate toward trenchPushes oceanic plate toward trench– Could create drag on lithosphereCould create drag on lithosphere

• Slab pull- sinking region of mantle Slab pull- sinking region of mantle convection sucks oceanic plate downwardconvection sucks oceanic plate downward