Warm Up – Page 78Warm Up – Page 781. Explain Alfred Wegener’s three pieces

of evidence for continental drift.2. Write today’s date and EQ on page 79.

He could not find the force that was causing the continents to drift.Because of this, he could not convince anyone that continents could move.He died in Greenland on an expedition. At the time of his death, no one believed his hypothesis!

Technology developed during the 1940’s changed all that!

Discovery of Sea-Floor Spreading

Sea floor spreading video clip

Sea-Floor Spreading

m.y. means million years ago

Notice this compass. It is important for the next slide

As the sea floor spreads, the lava cools according to the magnetic poles at the time. The rocks on the ocean floor have proved that the earth’s magnetic field sometimes reverses. The inner core flips and so the north pole moves to the southern hemisphere! The earth itself does not flip.

http://platetectonics.pwnet.org/img/blocks.gif

What causes sea floor spreading? Convection currents!

Click here for a hyperlink to an animation of convection

Be sure to click the play button!

Animation of sea floor spreading

Can you explain this diagram!?

The place where two plates move apart or diverge is called a divergent boundary.

This is a model of sea floor spreading at a divergent boundary called a mid ocean ridge.

Did you know that the Earth’s longest mountain range is underwater and is called the mid-ocean ridge?

: www.ocean.udel.edu

The Mid-Ocean Ridge system, shown above snaking its way between the continents, is more than 56,000 kilometers (35,000 mi) long. It circles the earth like the stitching on a baseball!

On page 78:On page 78:Create a magazine cover for a

National Geographic issue about Continental Drift and Sea-Floor Spreading.

Be sure to include info on:◦Alfred Wegener and Continental Drift

3 pieces of evidence◦Harry Hess & sea floor spreading

SONAR Features of ocean floor

◦MUST BE IN COLOR