Geologic Time

• Rocks record geologic events and the changing life forms of the past.

• Uniformitarianism: The forces and processes that we observe today have been at work for a very long time.

Relative Dating

• Tells us the sequence (order) in which events occurred, but NOT HOW LONG AGO they occurred.

Relative Dating

*3 Laws/Principles: 1. Law of Superposition:

In an undisturbed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it.

Stratigraphy

Relative Dating

2. Principle of Original Horizontality: Layers of sediment are generally deposited in a horizontal position.

Relative Dating

3. Principle of Cross-Cutting Relationships: States that when a fault cuts through or when magma intrudes into other rocks and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected.

Relative Dating

• Inclusion: Pieces of rock units that are contained within another (are younger than the rock they are in).

Relative Dating

• Unconformity: a gap in the geologic record that occurs when rock is eroded, exposing older rock and then new rock forms on the much older exposed rock.

Absolute Dating

• Three Types:1. Radiometric Dating2. Tree Ring Dating3. Ice Core Dating

Radiometric Dating

• Radiometric Dating: Calculating the age of rocks and minerals that contain certain radioactive isotopes.

• Uses the half-lives of those isotopes to calculate the age of rocks.

Radiometric Dating

• Half-Life: The time it takes for 50% (1/2) of the nuclei in a radioactive sample to decay to its stable isotope.

• Multiply the number of half-lives by the half-life time to get the age of a fossil.

Half-Life Practice

• A bone of a T-Rex contains 1/16 the original parent material. Haw many half-lives have occurred in the T-Rex bone?

• If 2 half lives have passed, what fraction of the original parent material remains?

• If the half-life time of an isotope is 10,000 years and 3 half lives have passed, what is the age of the fossil?

Radiometric Dating

• This method is possible because each radioactive isotope decays at a CONSTANT rate.

• Cannot be used for sedimentary rocks because they are formed from many pieces of older rocks, so there would be many ages of rocks.

Tree Ring Dating

• Tree rings are a glimpse into the past.

• Width of bands indicate climate changes and number of rings indicate age.

• Must use a sample to be sure the results are accurate.

Ice Core Dating

• Each layer of ice in a core corresponds to a single year, or sometimes even a single season and most everything that fell in the snow that year remains behind, including wind-blown dust, ash, atmospheric gases and even radioactivity.

Ice Core Dating