Plate TectonicsPlate Tectonics

The Theory of Seafloor SpreadingThe Theory of Seafloor Spreading

1. On the ocean floor there are areas where the seafloor is getting bigger.

2. Researchers used sound waves to discover a system of underwater ridges or mountains that can be found all over the world.

The scientistThe scientist

Mid Ocean RidgesMid Ocean Ridges

• This underwater chain of mountains and volcanoes. – Most known ones are Mid Atlantic Ridge and

East Pacific Rise

• AKA oceanic spreading centers http://www.youtube.com/watch?v=DWKWdfJtNl0

• http://www.youtube.com/watch?v=6CsTTmvX6mc

Why does the Seafloor Spread?Why does the Seafloor Spread?

1. Hot, Less dense material below Earth’s crust rises upward to the surfaces at the mid ocean ridges.

2. Then, it, flows sideways, carrying the seafloor away from the ridge.

3. As the seafloor spreads apart, magma moves up and flows from the cracks, cools, and forms new seafloor.

Cross section view

Oldest youngest

2 Types2 Types

• Slow moving

• Fast moving

Slow MovingSlow Moving

• Mid Atlantic Ridge

• About an (1) inch a year

• Really wide gap between each side

• “cooler “

Fast movingFast moving

• “hotter” (there is a constant presence of hot asthenosphere)

• East pacific rise

• Like taffy being pulled

• Growth is varied

Evidence for Seafloor SpreadingEvidence for Seafloor Spreading

• Youngest rocks are located at mid-ocean ridges.• Oldest is near the continent

Oldest youngest

Evidence Evidence

Evidence for Seafloor SpreadingEvidence for Seafloor Spreading

1. Reversal of Earth’s magnetic field are recorded by rocks in strips parallel to ridges.

2. These iron rich rocks point towards magnetic North wherever it is at the time they cool.

• Paper activity

Normal Reverse

Reverse polarity

Normal polarity

Discussion QuestionDiscussion Question

How could seafloor spreading be related to continental drift?

AnswerAnswer

• If the Seafloor is constantly spreading apart and moving, it will effect and move the continents as well.

Theory of Plate TectonicsTheory of Plate Tectonics

• Earth’s crust and upper mantle are broken into sections called plates, they move on a plastic-like layer of the mantle.

• Crust and rigid upper mantle form the lithosphere.

• Plastic-like layer below is called asthenosphere.

Plate BoundariesPlate Boundaries

• Divergent Plate boundaries– Plates moving apart.

• Convergent Boundaries– Plates moving together.– Denser plates sink under less dense plates.– Newly formed hot magma forced upward

forms volcanic mountains

Plates collidePlates collide

• Plates crumple up to form mountain ranges.

• Earthquakes are common

• Transform boundary– Plates slide past one another.– Sudden movement can cause earthquakes

• Convection inside the Earth is thought to be the force behind plate tectonics.

• Features caused by plate tectonics– Faults and rift valleys– Mountains and volcanoes– Strike- slip faults – cause of earthquakes.