Section 4 Notes Plate Tectonics and Seafloor Spreading

Plate Tectonics

1. Chapter One: Plate Chapter One: Plate
TectonicsTectonics
Section 4: Sea-Floor Spreading Section 4: Sea-Floor Spreading
Pages 33-39Pages 33-39

2. Do mountains exist under water? Do mountains exist under water?

3. Mid-Ocean Ridge Mid-Ocean Ridge

Longest chain of Longest chain of mountainsmountains
in the worldin the world

It extends into all of Earth’s It extends into all of Earth’s oceansoceans

Most of the Most of the mountainsmountains are hidden are hidden
under waterunder water

A steep-sided valley splits the top for A steep-sided valley splits the top for
most of its length most of its length

Mid Atlantic Ridge https://www.youtube.com/watch?v=bGye6vlOpbYMid Atlantic Ridge https://www.youtube.com/watch?v=bGye6vlOpbY

https://www.youtube.com/watch?v=sgDM6m0lUGYhttps://www.youtube.com/watch?v=sgDM6m0lUGY

4. SonarSonar

Scientists use Scientists use sonarsonar to map to map
the the oceanocean floor. floor.

SonarSonar = = a device that bounces a device that bounces
sound waves off underwater sound waves off underwater
objects and then records theobjects and then records the
echoes of these sound waves.echoes of these sound waves.

The The timetime it takes for the echo it takes for the echo
to arrive indicates the to arrive indicates the distancedistance
to the object. to the object.

5. Scientists wondered…Scientists wondered…

What is this ridge? What is this ridge?

How did it get here?How did it get here?

6. Harry HessHarry Hess

Harry Hess, an American Harry Hess, an American geologistgeologist, was , was
one of the scientists who studied the mid-one of the scientists who studied the mid-
ocean ridge. ocean ridge.

In 1960, he suggested that the In 1960, he suggested that the ocean ocean
floorsfloors move like conveyor belts, carrying move like conveyor belts, carrying
the continents along with them. the continents along with them.

Maybe Wegener was right!!!Maybe Wegener was right!!!

7. Sea-Floor SpreadingSea-Floor Spreading

At the mid-ocean ridge, molten material At the mid-ocean ridge, molten material risesrises
from the from the mantlemantle and erupts. The molten and erupts. The molten
material then spreads out, material then spreads out, pushingpushing older rock older rock
to both sides of the ridge. to both sides of the ridge.

Hess called this process that continually Hess called this process that continually
adds newadds new material to the ocean floor material to the ocean floor sea-sea-
floor spreadingfloor spreading. .

Bill Nye www.youtube.com/watch?v=GyMLlLxbfa4Bill Nye www.youtube.com/watch?v=GyMLlLxbfa4

8. evidence
supporting
sea-floor
spreading
molten
material
magnetic
stripes
drilling
samples

9. Evidence from Molten MaterialEvidence from Molten Material

Scientists traveled to the ocean floor in Scientists traveled to the ocean floor in
AlvinAlvin, a submersible built to handle the , a submersible built to handle the
crushing crushing pressurepressure. .

They discovered strange rocks shaped like They discovered strange rocks shaped like
pillowspillows. .

These rocks can only form when These rocks can only form when moltenmolten
material hardens quickly after erupting material hardens quickly after erupting
under waterunder water. .

10. Evidence from Magnetic StripesEvidence from Magnetic Stripes

Earth’s magnetic poles have Earth’s magnetic poles have reversedreversed
themselves. themselves.

If the poles suddenly reversed themselves If the poles suddenly reversed themselves
today, your compass needle would point today, your compass needle would point
southsouth..

Rock that makes up the ocean floor lies in Rock that makes up the ocean floor lies in
a pattern of magnetized “stripes”. a pattern of magnetized “stripes”.

11. http://www.sciencechannel.com/tv-shows/greatest-discoveries/videos/100-greatest-http://www.sciencechannel.com/tv-shows/greatest-discoveries/videos/100-greatest-
discoveries-magnetic-field-reversal.htmdiscoveries-magnetic-field-reversal.htm

12. Evidence from Drilling SamplesEvidence from Drilling Samples
The The Glomar ChallengerGlomar Challenger, a drilling ship built in , a drilling ship built in
1968, gathered rock samples by 1968, gathered rock samples by drillingdrilling into into
the ocean floor. the ocean floor.
Scientists determined the Scientists determined the ageage of the rock. of the rock.
The The fartherfarther away from the ridge the samples away from the ridge the samples
were taken, the were taken, the olderolder the rocks were. the rocks were.
The The youngestyoungest rocks were always in the rocks were always in the
centercenter of the ridges. of the ridges.

13. Subduction Subduction

OceanicOceanic crust created along the mid-ocean crust created along the mid-ocean
ridge is destroyed at a deep-ocean trench. ridge is destroyed at a deep-ocean trench.

In the process of In the process of subductionsubduction, oceanic crust , oceanic crust
sinks downsinks down beneath the trench into the beneath the trench into the
mantle. mantle.

14. Are the oceans shrinking?Are the oceans shrinking?

Subduction and sea floor spreading can Subduction and sea floor spreading can
change the change the sizesize and and shapeshape of the oceans. of the oceans.

The ocean floor is renewed about every The ocean floor is renewed about every 200200
million years. million years.

Pacific Ocean is Pacific Ocean is shrinkingshrinking while the Atlantic while the Atlantic
Ocean is Ocean is expandingexpanding. .

15. Words Worth Knowing (ISN #44)Words Worth Knowing (ISN #44)

mid-ocean ridgemid-ocean ridge

sonarsonar

sea-floor spreadingsea-floor spreading

deep-ocean trenchdeep-ocean trench

subductionsubduction