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Laryngitis Materials:

• Chapter 11
• Online assignment

All work is due on April 12!

Class Schedule:

• Tues 4/5: class
• Thurs 4/7: no class

How do we know the age of the Earth?

Outline

• Introduction to today's lecture
• How do we know the Earth's age?
• The Earth's internal structure
• Rocks and Minerals
• Three types of rock
• More about igneous rocks
• Powerpoint lecture notes
  • Outline
  • Basic Concepts
  • Geologic Time1
  • Geologic Time2
  • An Illustration of Geologic Time
  • Geologic Time Simplified -We covered up to this point in class-
  • How do we know the Earth's age?
  • The Earth's internal structure
  • Rocks and Minerals
  • Three types of Rock
  • Igneous Rock

How do we know the Earth's age?

We do it using radioactive decay. Perhaps you've heard of the "Carbon 14 method"... we use something similar to determine the age of the Earth. Instead of radioactive isotopes of Carbon, we use an isotope of Uranium, U-238. (There are other methods too, but this is the one that I will tell you about)

Like other radioactive isotopes, U-238 'falls apart' because of its unusual size. It is just like other Uranium atoms, except that is has a few extra neutrons that give it some extra mass. When the atom falls apart, we call it 'radioactive decay'. U-238 turns into Lead 206 (Pb-206) in this process (the numbers signify the mass of the atoms, or the number of protons and neutrons that the atom has).

U-238 and other radioactive elements, like C-14, decay at constant rate that scientists can measure, or at least estimate. Here's how we use it to determine the age of the Earth:

1. Look for Zircon crystals. These are fairly common, and they are tough. When they form they also contain a known amount of U-238.
2. Count how many U-238 atoms have turned into Pb-206 atoms.
3. Use this to calculate the age: the more Pb-206 in the sample, the older it is.

We've used this to find the age of several things:

• Oldest Earth rock: 4.03 Ga ('Ga' stands for Giga- annum, or 'Billions of years')
• Oldest Earth mineral crystal: 4.3 Ga
• Oldest Moon rock: 4.5 Ga (we collected this sample during our Apollo missions to the Moon)
• Oldest Meteorite: 4.54 Ga

After looking at all of these samples, we started seeing a pattern: nothing is older than about 4.5 Billion years. So, we round this number up to 4.6 Billion years, since the Earth should be a little older than the oldest solid rock. That, more or less, is how we know the age of the Earth.

To put this in perspective, here are some other age estimates:

• Age of our solar system, the Milky Way: 11-13 Ga
• Age of the Universe: 10-15 Ga

Next up: The Earth's Internal Structure