Relative Age Relative Age of Sedimentary Rocksof Sedimentary Rocks

By looking at different rock layers it’s possible to see which ones are older because they were the layers that were deposited first.

Youngest layers were last and are on top.

Oldest on bottom.

This is called Sedimentary Superposition

Sedimentary Superposition Sedimentary Superposition

•Where are the oldest rock layers?

•Why?

•Can Radioactive dating can be used to prove this.

Geologic ErasGeologic Eras

Radioactive DecayRadioactive Decay• The half life of potassium-

40, a radioactive element, is 1.3 billion years. This means that half of the potassium-40 in a sample will break down into argon-40 every 1.3 billion years. The graph shows the breakdown of a 1-gram sample of potassium-40 into argon-40

over billions of years.

- The Fossil Record

Radioactive DecayRadioactive Decay

– The red line represents the amount of potassium-40. The blue line represents the amount of argon-40.

• Reading Graphs:

– What does the red line represent? What does the blue line represent?

- The Fossil Record

Radioactive DecayRadioactive Decay

– Potassium-40–100%; argon-40–0%

• Reading Graphs:

– At 2.6 billion years ago, how much of the sample consisted of potassium-40? How much of the sample consisted of argon-40?

- The Fossil Record

Radioactive DecayRadioactive Decay

– About 1.3 billion years

• Reading Graphs:

– At what point in time do the two graph lines cross?

- The Fossil Record

Radioactive DecayRadioactive Decay

– 50% of each; the half-life of potassium-40 is 1.3 billion years, which means that half will break down into argon-40 every 1.3 billion years.

• Interpreting Data:

– At the point where the graph lines cross, how much of the sample consisted of potassium-40? How much consisted of argon-40? Explain why this is the case.

- The Fossil Record

Earth’s History as a ClockEarth’s History as a Clock• Fossils found in rock layers tell the history

of life on Earth. The history of life can be compared to 12 hours on a clock.

- The Fossil Record

Clair Patterson u 238

Absolute Ages Absolute Ages from from Measuring Measuring Radioactive DecayRadioactive Decay

• All radioactive elements break down over time• The time that it takes for HALF of the original material to

turn into the final product is called the Half-Life• Using this we can look at the amounts of material left in

a sample to get its actual age

Geologic Time ChartGeologic Time Chart

• Cenozoic Era- 65 MYBP to NOWMammals dominate every niche Age of Mammals

Whales, bats, primates, rodents, horses, elephants, rhinos appear

• Mesozoic Era -250 to 65 MYBP• Early Mammals and Dinosaurs to Birds AGE of REPTILES

• Paleozoic Era -550 to 250 MYBPMulti-cellular life takes off and in the end moves onto land

• Archaean -4.5 BYBP to 550 MYBP• Single celled life emerges, photosynthesis invented • Jellyfish and Worms show up at the end.

Absolute AgesAbsolute Ages

• How do we know the Earth is 4.5 billion years old?

• We use radioactive half life's of different Isotopes.

• Each element has a different Half-life and can be used to date materials of different ages

Radioactive Dating Radioactive Dating

• Different elements decay at different rates C-14 has a half life of 5,730 years

• U 235 has a half life of 704 million years

• When we look at the oldest rocks on Earth and asteroids we get the same 4.5 billion year date for the solar systems formation.

Half Life'sHalf Life's

• ½ of a Carbon 14 sample will turn into Carbon 12 in 5,730 years

• ½ of U (uranium) 238 to Pb (lead) 206 in 4.7 billion years

• For each time scale we want to look at; there is a radioactive element with the right half-life to show the absolute age of a sample.

Index FossilsIndex Fossils• Certain

fossils are found only in particular rock layers or strata.

• This helps us date rock layers and find

• Mass Extinctions

What the Geologic/Fossil Record What the Geologic/Fossil Record ShowsShows

• Life appears over 3.5 billion years ago and has been gaining in complexity ever since

• Development of biomes happens in a logical sequence (The story of life makes since with land plants coming before land animals)

• Most (99.99%) of all species that have ever lived are now extinct

Cosmic Calendar Cosmic Calendar

• Big Bang January 1• Origin of Milky Way Galaxy May 1 • Origin of the solar system September 9 Formation of the

Earth September 14• Origin of life on Earth ~ September 25 • Formation of the oldest rocks known on Earth October 2• Date of oldest fossils (bacteria and blue-green algae)

October 9 • Invention of sex (by microorganisms) November 1• Oldest fossil photosynthetic plants November 12• Eukaryotes (first cells with nuclei) flourish November 15

Cosmic Calendar- December