Geologic Time

How can geologists date rocks?

R

O

C

K

D

A

T

I

N

G

How can geologists date rocks?

Absolute Dating

Called Radiometric dating

Use radioactive decay to determine the actual time the rocks were deposited.

Relative Dating

Using the natural processes of weathering, erosion and deposition of sediments to determine the age rocks relative to eachother.

Rock Joke!!

• What does a rock want to be when it grows up?

• A Rock Star!!

How does the Law of Superposition help to age rocks?

• Rocks layers below another rock layer are OLDER than the rock layer ABOVE it.

• Geologist can assume rocks are older or younger than other rocks in the same rock formation.

What is the Principle of Original Horizontality?

• Sedimentary rocks are deposited in horizontal layers.

• Any other geologic forces such as faults or intrusions MUST have occurred after the rocks were deposited.

How do these Principles help to determine Earth’s History?

How do Index Fossils help to age rocks?

Fossils provide physical evidence of what the environment and life was like when the rocks were deposited.

NOT all fossils are index fossils. They must have:• Lived for a short period of time.• Been unique and easy to identify.• Been found all over the WORLD.

Index fossils found in two different rock layers are the SAME age.

How can geologists determine the actual age of rocks and fossils?

Rocks and fossils contain certain amounts of radioactive elements.

By measuring the ratio of decayed and undecayed elements can provide an absolute date to when the rocks were deposited or fossils died.

This is based on the radioactive elements half –life. (The time it takes half the amount of radioactive element to decay)

http://app.discoveryeducation.com/player/view/assetGuid/832A7297-C0ED-4F28-9226-D8293A67FB89

How does radioactive decay work?

• Carbon 14- halflife of 5730 years

• Carbon 14 decays to Nitrogen 14

• How old is a fossil with 25 % Nitrogen and 75 % Carbon 12?

3-2-1 Summarize!!!

Write 3 facts you wrote in your notes in complete sentences.

Write 2 facts given to you verbally that you did NOT write down in complete sentences.

Write 1 question that presented itself to you while taking notes.