Evidence of Plate Motion

1. Geology Rocks! Evidence of Plate Motion 6th Science Std: 1a Chapter 6, Section 3

2. When German meteorologist Alfred Wegner proposed his hypothesis about continental drift….. It wasn’t widely accepted. Why wasn’t it?

3. The reason why it wasn’t accepted Mostly was because he didn’t have proof of movement

4. The reason why it wasn’t accepted And why didn’t he have the proof….. The tools and technology to get the proof hadn’t been invented yet.

5. Let’s look at the time line Continental Drift Theory was published in 1915 What was around in 1915 You flew in a biplane Only 40% of houses had electricity Around 5% of houses had a phone Movies didn’t have sound

6. Let’s look at the time line First Technology Breakthrough – Mid 1930’s Accurate measurement of magnetic fields It showed that minerals in the ground that were natural magnets didn’t always point exactly at north So either the ground moved or north moved Evidence that the continents have moved….

7. Let’s look at the time line Next – Invention of Sonar in 1940’s Made to find submarines during WWII It showed that there was a large mountain chain in the middle of the Atlantic Ocean They were called Mid Ocean Ridges New ocean floor was being created here Evidence that the something was going on there

8. Let’s look at the time line Next – Improvements in monitoring for earthquakes in the 1950’s Seismometers were used to watch for atomic bomb testing It showed that earthquakes don’t happen randomly scattered everywhere Most of them line up in patterns Evidence that helped define the plate edges

9. Let’s look at the time line Next – Improvements in monitoring for changes in magnetic fields in 1960’s Used to search for very quiet submarines It works like a metal detector Showed a unique magnetic pattern in the oceans It showed areas of reversed north pole Evidence that showed magnetic north has changed

10. Let’s look at the time line Next – Improvements in monitoring for changes in magnetic fields in 1960’s The detectors showed the same pattern on both sides of the mid ocean ridges So what was happening on one side of the ridge… …Was happening on the other side This was called zebra striping Evidence that showed similar changes

11. Let’s look at the time line Next – Improvement in finding the age of rocks in 1960’s Used radioactive rocks Radioactive elements change to non radioactive Compare amount of radioactive element to non radioactive If you know how long it takes to change – you can figure out how old the rock is Evidence that showed age of rocks in ocean floor

12. Let’s look at the time line Put Magnetic Striping and Age of Rocks together… The age of the rocks across the ocean floor were checked Young rocks were close to the Mid Ocean Ridge The rocks got older the farther away you went from the ridge And the age on one side of the ridge matched the age on the other side Evidence that showed age of rocks in ocean floor

13. Let’s look at the time line Put all of this information together…. New crust was being made at ocean ridges The crust got older the farther away you got The aging was symmetrical (even) on both sides of ridges So it was like the crust was being made and moving away (spreading) from ridges Evidence that showed the crust was moving

14. But this caused another problem If new crust was made at ridges…. Why wasn’t the Earth growing bigger? Like a balloon blowing up… …And maybe pop? Since the Earth wasn’t growing bigger – the extra crust had to be going somewhere It didn’t just vanish or become invisible…..

15. But this caused another problem Then, a new theory….. What if the crust went back underground Back down into the hot magma of the mantle Crust is made from magma and eventually gets recycled back into magma Making a complete cycle

16. But this caused another problem So where did it go back underground? Scientists checked the ages of the rocks on the sea floor They got older until they reached an ocean trench When you get past the trench the rocks get younger again So the crust was heading for the trenches where it went underground

17. Changing a Theory…. So from the original idea of “Continental Drift” Came the new theory of Plate Tectonics Here, continents drifted around the surface of the earth Here, continents are on a “conveyor belt” of crust moving from one point to another

18. So now the theory says… New crust is created from magma The crust gets pushed (or pulled) away from where it is made…. …And get moved by currents created by the moving magma underground… …until it get pulled back underground where it starts the cycle all over again

19. Classwork: We have covered a lot of new information…. Take some time and work on your Plate Tectonics vocabulary words Work together in your lab groups to fill in the definitions for as many words as you can for what we have covered.

20. For next time: We have the theory that the plates are moving But just how fast are they moving? Question: What could you use to measure just how fast the plates are moving?