Pangaea Activity 1) Once I say go, your group should open the folder and do what you think is being asked of you. 2) Once you have it, have one person in your group shout out your color group and done. (Make sure you are finished because once you say done you can not make any more changes. 3) Your group should keep your answer on your desk until all groups are finished and then I will check answers.

Theory of Sea-Floor Spreading Standard Explain how the theory of plate tectonics accounts for the motion of the lithospheric plates, the geologic activities at the plate boundaries, and the changes in landforms over geologic time.

How the Ocean Floor was Mapped Before WW1, the ocean floor was mapped by throwing down a weight on a long chain or cable was lowered from a ship until the weight hit bottom. The depth of water at that point was determined. Then the weight was dragged slowly to show how the depths changed from place to place. What are the limitations of this method?

Development of Sonar Right after the Titanic sank in 1912, scientist worked on finding a way to use sound to detect what was ahead of the ship. Scientist developed SONAR which uses sound waves sent out from a source. Once the wave hits an object, the wave is bounced off and returned to the source which is also a receiver. In this way an image of the ocean floor can be “seen”

Harry Hess Harry Hess was a geologist professor who joined the U.S. Navy during WW2. he was assigned to a ship with the new technology called SONAR.

What did Harry Find? Harry Hess began to map the ocean floor. Continuous mountains in the Atlantic and Pacific.

A Theory Begins Harry used the information to come up with a theory that the sea-floor was spreading. Harry noticed that the “mountains” were ridges and in between the ridges were valleys Harry thought these might be volcanoes because new magma was rising out of the valley. As it came up, the new magma pushes up forming the ridges and then as new material comes up the old ridges are pushed aside and new ones are made.

Sea-floor Spreading: Hess’s theory that the new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge.

Evidence for Sea-Floor Spreading In 1968 Glomar Challenger (science research ship) Gathers info on rocks on the ocean floor around the ridges. Drills down for core samples

They found that no rock samples were older than 200 my!!! The Earth is 4.5 by. 1. The rock samples closest to the ridge were the youngest. As you went out from the ridge, the rocks became older and older. These rock samples are stored at Columbia University in the city of New York

Do you think the Earth is Growing? 2)They also found: the Earth’s magnetic pole have been reversed!!! More than once! As the molten magma comes out of the ocean floor it is still moveable. The iron sediments in the different rocks line up in the direction of magnetic north. Sometimes that was directional south.

What is a Magnetometer? A magnetometer is an instrument used to detect magnetic fields in rocks.

nes/SeafloorMagnet.swf Example of Sea-Floor Spreading

The Earth is not growing As new earth is made and moves out, it becomes more dense. Eventually it becomes so dense it sinks down into a trench which returns the material to the mantle. This area is called a subduction zone.

We proved HOW it happens with the discovery of the asthenosphere & lithosphere. Added Evidence for the Theory Under the hard brittle lithosphere, the asthenosphere rises because of convection currents from the mantle, and breaks through the lithosphere.

By plotting volcanic & earthquake data scientists noticed something interesting. They outline the plate boundaries Also: