# Geologic Time.

## Presentation on theme: "Geologic Time."— Presentation transcript:

Geologic Time

never lived at the same time
Why is this statement false: “The caveman had dinosaur for breakfast.”? man and dinosaurs never lived at the same time

Laws of geologic history
Superposition—Oldest rocks on the bottom of an undisturbed sequence of rock Law of original horizontality—All rocks are initially deposited in flat (horizontal) rows Cross-cutting relationships—Objects which cross-cut rocks are younger than the rocks themselves (ex: fault or intrusion)

Superposition

Using superposition, label the strata below from oldest to youngest.

Horizontality All rock layers are originally deposited in flat (horizontal) layers

Faults are always (older, younger) than the rocks they cut through.
Cross-cutting relationships Faults are always (older, younger) than the rocks they cut through.

there first (it's older)
Which is older: F or S How do you know? F cuts through S S must have been there first (it's older)

Uniformitarianism These laws are based on the assumption that the forces at work today are the same forces at work in history = uniformitarianism.

If a geologist finds an igneous sill, how can she determine if the sill is an intrusion or an extrusion?

Is “H” an intrusion or extrusion? How can you tell?

H is an intrusion. Contact metamorphism on top

On to the next question...

a buried erosional surface
What is an unconformity? How does it complicate the relative dating of rock layers? a buried erosional surface a part of the rock record is missing

Using the diagram to the left, identify where the unconformity is located by drawing an arrow and writing the word “unconformity” next to it.

What are Index fossils?

What characteristics must fossils have in order to be good index fossils?
lived over a large geographic area (large horizontal distribution) lived for a short period of time (small vertical distribution)

In geology, this is referred to as CORRELATION
In the diagram below, a geologist has matched up rock layers based on index fossils.

On to the next question...

their half-lives are constant
Absolute Age Why are radioactive isotopes useful in determining the absolute age of a rock? their half-lives are constant

NOTHING! it is reliable to calculate age
Absolute Age What can be done to change the half-life of a radioactive isotope? Why is this important? NOTHING! it is reliable to calculate age

stable atoms unstable atoms Draw the generic graph for the
half-life of a radioactive isotope. stable atoms unstable atoms

carbon-14 What radioactive isotope could be
Absolute Age What radioactive isotope could be used to determine the absolute age of material that was recently living? carbon-14

What is the half life of uranium-238?
Please answer the two questions on page 7 of your Notes What is the half life of uranium-238? 4.5 x 109 4,500,000,000 4.5 billion years choice (c)

3 half-lives x (5.7 x 103) =1.71 x 104 = 17,100 years
If there is a 100g sample of C14, how many grams of C14 would remain after three half-lives? How long would this take? Show all work. 100g 50g 25g 12.5g 3 half-lives x (5.7 x 103) =1.71 x 104 = 17,100 years