Dating Fossils

Relative age- No specific age is given. Ex: “I’m older than my sister.” Absolute age- A specific age is given. Ex: “That fossil is 65 million years old.”

Rock Layers & Relative age

The Law of Superposition states that the oldest rock layers are normally on the bottom.

Cross cutting relationships says that an intrusion or fault is younger than the layers it cuts through.

An unconformity is when erosion results in a gap in the rock record. unconformity.jpg There should be a layer here, but the rock was eroded (water/wind broke down the rock and it was carried away). This gap is known as an unconformity.

Index fossil- Lived only during a short period of time so we can use it to date common fossils found in the same area. Trilobites lived only from 530 to million years ago. If you find a rock with a trilobite fossil in it, you know that the rock’s age is somewhere between million years ago.

Human Skull Early Hominid Skull Nautilus Shell Dinosaur Skull Trilobites Archaeopteryx (Earliest known bird, 150 million years ago, first fossil evidence linking dinosaurs and birds )

Radioactive Decay & Absolute Age

Radioactive decay is when one element decays (breaks down) into another element and releases radioactive particles.

Radioactive elements have an unstable nucleus that breaks down over time. We can use this information to date things. This decay rate always occurs at the same rate for the same element BUT the decay rate varies from element to element. The decay rate can be a few years, thousands or billions of years.

Example elements are: Rubidium -87 Half life of 48.6 billion years Decays to Strontium-87 Uranium-238 Half life of 4.5 billion years Decays to Lead-206 Carbon-14 Decays to Nitrogen-14 In 5730 years.

Uranium-238 Half life of 4.5 billion years Decays to Lead-206

13.) A half life is the time it takes for half of a radioactive substance to decay. Ex: 1600g  800g  400g  200g  100g  50g  25g  12.5g  6.25g 1st half life 2nd 3rd 4th 5th 6th 7th 8th

Other ways of dating include using tree rings, seasonal climate changes (varve counting), and distinctive sediment layers arman_Pamela/tree%20rings.JPG

Earth’s Age and Fossils

The oldest rocks have been found in CANADA. They are over 4 Billion years old! (4.28 Billion years old)

The oldest rocks on the moon are billion years old. -We think that the earth and the moon formed at about the same time. -Why are earth’s younger? SUBDUCTION! Also, rocks are “recycled” in the rock cycle

END!

Rock Layers and Fossils n.jpg

Warm-Up 1. What does the principle of cross cutting relationships say? 2. What is an index fossil? 3. What is an unconformity? 4. TURN IN ANY MISSING WORKSHEETS!!!