Chapter 7 Earth’s Moving Crust Lesson 1 Moving Plates.

Chapter 7 Earth’s Moving Crust

Lesson 1 Moving Plates

Movement of Earth’s Crust

• Crust – Earth’s solid surface• Volcanoes and Earthquakes are sudden motions where you can actually see the crust moving.• The crust is also moving so slowly you can’t

see it

• Evidence supporting the movement of Earth’s crust– Original Horizontality – Rocks that form in flat,

horizontal layers– Many layers have been twisted or tilted, a sign

that the crust is moving slowly

Continental Drift• Alfred Wegener – Listed evidence in his book that the continents

had once fit together like pieces of a jigsaw puzzle• Today’s shapes of the continents match

• Matching rock types and structures, such as deposits from glaciers

• Matching remains of ancient life and living organisms

• Matching past climates

– Called the huge supercontinent “Pangaea” meaning “all lands”

– 200 million years ago, he believed Pangaea split into pieces that are today’s continents

– After the split, the continents drifted apart over the years.

– His hypothesis became known as “continental drift”

– He did not explain why or how the continents drifted

Sea-Floor Spreading• New crustal material is forming at the ridges, spreading apart the old sea floor on both sides. http://app.discoveryeducation.com/search?Ntt=mid+atlantic+ridge

• In 1947 the research ship Atlantis mapped the floor of the ocean and discovered a series of mountains separated by huge valleys and canyons.

• This became known as the Mid-Atlantic Ridge

http://app.discoveryeducation.com/search?Ntt=mid+atlantic+ridge

• Rock samples that were taken from the mountains were formed from volcanic activity.

• In later years, scientists found other mid-ocean ridges in other oceans.

• Together, the ridges form a chain of mountains along Earth’s sea floors.

• Huge cracks split the tops of the ridges• In parts of the sea, the floor plunges down into deep valleys or trenches

• How it works:– The mid ocean-ridges are pushed up by hot rock

material from deep beneath the crust– The crust cracks where the sea floor is spreading apart– Just below the ridges of the sea floor there is magma– Magma flows up through the cracks– The magma cools and hardens into new solid rock

along the ridges– This process keeps making new rock material along

the ridges and pushing older rock material farther away

• http://www.youtube.com/watch?v=uSKzdbEVsI8&feature=endscreen&NR=1

Evidence of sea-floor spreading– Rocks that make up the continents are much older

than rocks that make up the ocean floor.– Most ocean floor rocks are volcanic (the cooling

and hardening of magma)– The youngest ocean floor rocks are found at the

mid-ocean ridges– On either side of the ridge, the ocean floor rocks

get older towards the continents

• The rocks of the sea floor show evidence of the back and forth switch of Earth’s magnetic field.

(Earth has a magnetic field around it that a compass aligns with. The arrow points to the north pole of Earth’s magnetic field. Earth’s magnetic field has reversed itself several times (the north and south poles switched back and forth).)

• Magma contains magnetic particles such as iron.• As the magma flows, the particles line up with Earth’s

magnetic field.• As the magma cools and hardens, the magnetic particles

are locked in place.• Scientists have found that the magnetism in the rocks

alternates from one direction to the other in a simple pattern of narrow strips.

• The pattern matches on either side of the ocean ridges.

Plate tectonics• Describes Earth’s crust as broken into pieces,

or plates.• Each plate includes material from the mantle,

a layer below the crust.• Plates slide on the lower part of the mantle.

http://app.discoveryeducation.com/search?Ntt=mid+atlantic+ridge

• Plates can move away from each other, collide, or slide past each other. – The Mid-Atlantic ridge is located on plates that

move away from each other.

• Each continent is part of a plate and moves with the plate.

How plates move

• Hot melted rock from deep in the mantle rises upward and squeezes between the edges of two plates.

• The two plates are forced apart. • The melted rock touching the plates cools,

hardens, and becomes part of the plate. • Melted rock continues to flow up, pushing the

plates farther apart.

Plate boundaries• Divergent boundaries– Where plates move apart– Mid-Atlantic Ridge• Sea-floor spreading

– The Great Rift Valley of Africa • Located at a place where a divergent boundary splits a

part of a continent, rather than the sea floor.

• Convergent boundaries– Where plates are colliding– When there is a continent on both of the colliding

plates, it can cause rocks to crumple.• The crumpling can build up mountains• The Himalaya Mountains

– When an ocean plate and continental plate collide the ocean plate will slide beneath the continental plate. • Off the coast of Chile, an ocean plate is sliding beneath the

plate carrying South America. • Where volcanoes are found

– As one plate slides under another, hot rock material in the upper mantle is able to melt and become magma.

– It flows toward the surface, where it can erupt through cracks, producing volcanic mountains.

– When two ocean plates collide the denser of the two slides beneath the other. • Undersea volcanoes or deep undersea valleys can form.

• Transform boundaries– When plates slide past each other.– As the plates smash and grind past each other

there are many earthquakes.– The rocks along here get broken and shattered and

pile up in long, narrow ridges and valleys.• The San Andreas Fault in California.

• Recycled Earth• Subduction– The process when two plates collide, a denser

ocean plate slides under another plate.• As the process continues, part of the crust

sinks into the mantle and becomes part of the mantle

• Convection current– Older rock in destroyed by subduction in the

trenches – New rock is forming in the mid-ocean ridges.

• Effects– As two ocean plates collide, a trench forms. – Melted rock from beneath the sea floor can rise

up to produce a string of volcanic mountains. – They can rise up above the sea floor and result in

a string of islands called an island arc. • Japan are a volcanic island arc.