How Do Earth’s Plates Move?

Chapter 1, Section 2, pD14-19

The Movement of Earth’s Plates• Lithospheric plates float on asthenosphere– Interact with other plates as they move

• Divergent boundary: plates move away from each other• Convergent boundary: plates collide• Transform-fault boundary: plates grind past each other

– Plate movements have produced most of the world’s:• Volcanoes• Earthquakes• High mountain ranges• Deep-ocean trenches

– Can occur along plate boundaries or old plate boundaries

Divergent Boundary

• Hypothesis: divergent boundaries caused by: – Magma pushed upwards under a plate– Magma melts, stretches, and cracks the plate– Magma oozes into the gap– Magma cools forming new crust• If pressure is great enough – magma erupts as volcano

Divergent Boundary

• Most divergent boundaries:– Mid-ocean ridge: chain of mountains that run

through all the world’s oceans• Rift: a deep valley

– Forms where plates move apart

• Sea-floor spreading: process where magma is slowly pushed up through cracks in a rift and cools to form new sea floor– Rocks on each side of the rift are slowly pushed away– Atlantic Ocean formed this way over last 165 million years

Divergent Boundary

• Some divergent boundaries occur within continents– East African Rift (Great Rift Valley)• Small plates moving away from African plate• One day east Africa will split from the rest of Africa• Started millions of years ago

– Crust stretched thin by spreading plates– Two cracks opened up in crust– Floor of the rift valley dropped hundreds of feet– Cliffs and volcanoes on both sides of the valley today

Convergent Boundary

• Old crust melts at convergent boundaries where plates collide– Balance the new crust being formed at divergent

boundaries– Earth’s size is not changing

Convergent Boundaries

• Two oceanic plates collide– One plate is pushed under the other (subduction)• Subducting plate melts in asthenosphere• Volcanic islands form above the melting plate• Collision causes deep-ocean trench to form

– Deepest trench on Earth – Mariana trench

Convergent Boundaries

• Two continental plates collide– Plates crumple and fold forming mountains• Produced some of world’s highest mountain ranges

– Himalayas» India crashing into Asia» Himalayan peaks are still rising 5 cm a year

Convergent Boundaries

• Oceanic plate collides with continental plate– Oceanic plate subducts under continental plate• Oceanic plate more dense• Oceanic plate melts in asthenosphere

– Mountains and volcanoes form along boundary• Andes mountains

Convergent Boundaries

• Destruction of plates causes a lot of activity along convergent boundaries– 80% world’s volcanoes– 90% world’s earthquakes• Mt. St. Helens volcano in Washington state

– Sometimes violent activity continues at old convergent boundaries

Transform Fault Boundaries

• Two plates move past each other in opposite directions at transform boundary– Most occur in oceanic crust– A few are within continents• San Andres fault

– Pacific plate moving past North American plate– Part of California may be an island if movement continues for

millions of years

Transform Boundaries

• Earthquakes are common along transform fault boundary– San Andres fault has many earthquakes• Volcanoes & mountains usually don’t form here

Earth’s Surface Changes Over Time

• Earth’s plates move– Carry continents and oceans with them• Continents and oceans were in different places in past• They will continue to move to other places in the future

– Rocks, volcanoes, and earthquake patterns• Help form hypothesis of how Earth’s surface changed

over past 500 million years

Earth’s Surface Changes Over Time

• Ancient continents separated & then collided again in different locations– North American & African plates collided 225 mya• Formed supercontinent “Pangaea”• Collision formed southern Appalachian mountains

– Might have been taller than Himalayas

Earth’s Surface Changes Over Time

• Movement of continents cause oceans to disappear and new oceans to open– South American & African plates split apart • Formed South Atlantic ocean• 135 mya

Summary

• Earth’s plates interact in different ways:– Plates move away at divergent boundaries– Plates collide at convergent boundaries– Plates grind past others at transform boundaries

• The movement of plates cause Earth’s continents to:– Move apart– Move together– Slide past one another

HomeworK

• Read Section 2, pD14-19• Answer ?s on Google form or WS