Plate Tectonics

•Relative Dating and Rock Layers

http://www.youtube.com/watch?v=_mcC8kFacrk

Earth’s History

How old is our Universe?

13.7 billion years old

How old is Earth?

4.6 billion years old

Earth was once made of molten lava and magma, eventually cooling to form igneous rock

Weathering, Erosion,

Compaction, Cementation,

Heat, and Pressure

eventually formed the rocks that

cover our Earth’s surface.

What we know about Earth’s history comes from studying its rocks, rock

formations, rock layers, and even the rocks from outer space

Meteorites5. All meteorites formed

about the same time that Earth formed, so studying them helps us study Earth.

6. Meteorites have not been affected by erosion, weathering or other forces the way Earth’s rocks have.

Studying Earth’s Rocks

Rock Layers• Which book was

placed here first?

• The bottom one!

7. When rocks form, the oldest ones are usually found beneath younger rocks

http://www.youtube.com/watch?v=9Df4CtlZ0AQ

Fossils

8. Most fossils are found in Sedimentary rock layers

• Why?– Heat, pressure, and

melting would probably destroy fossils during the formation of Igneous and Metamorphic rocks

9. Sedimentary rocks, and the fossils they trap, are often found in layers, oldest rocks are found on the bottom.

http://www.prehistoricplanet.com/news/index.php?id=48

11. Digging through the layers of rocks, helps us understand more about the history of Earth.

12. The Grand Canyon (Arizona) was carved by the Colorado river, exposing millions of years worth of rock layers.

13. Sometimes, the rock layers aren’t always horizontal…

14. And sometimes ocean fossils are found high in the

mountains…

Why?

15. Turns out, our Earth’s surface is constantly shifting, changing, and eroding.

• And the layers get pushed around.

16. Only in the last 200 years, have we begun to understand the structure, formation and history of our Earth.

I wonder what’s inside

Earth?

Relative Age of Rocks

• To figure out the ages of rocks and their fossils, geologists rely on a few rules…

Law of Superposition

• In any undisturbed rock, the oldest layer is at the bottom and the youngest layer is at the top.

The Cross-Cutting Law

• Any feature that cuts across a body of rock is younger than the body of sediment or rock it cuts across.

The Law of Inclusions

• If one rock layer contains fragments (inclusions) of another rock layer, it must be younger than the fragments of rocks it contains.

Oldest

Younger

Youngest

Oldest? Youngest?

Oldest? Youngest?

Oldest? Youngest? D – oldest – at the bottom – superposition law

A – next to bottom – superposition law

B Tilt – plate tectonics – Law of Horizontality

E – intrusion – cross-cutting lawErosion – not horizontal/straight

line - Law of HorizontalityC – intrusion – Law of cross-

cuttingErosion – Horizontality

 DABEC (old to young)

 E and C are igneous (magma

rising)A, B, D are sedimentary (layers)

Oldest? Youngest?

Oldest? Youngest?