1. Geologic Time
Earth Science Ch.12

2. 12.1 Discovering Earth’s History
Laws and Principles of Geology
Uniformitarianism
Geologic processes that exist today are the same processes that existed in the past.
The processes may operate at different rates and some processes that occurred in the past may not be active today.
The present is the key to the past

3. Original horizontality
Superposition
In an undeformed or unchanged sequence of sedimentary rock, a rock layer is older than the layers above and younger than the layers below
Original horizontality
Sediment is deposited in flat, horizontal layers.
Any folding, fracturing, or faulting must have occurred after the layer was deposited.

4. Crosscutting Relations
A fault or intrusion must be younger than the rock layers they cut through.

5. Absolute dating provides an age for the rock or feature.
Relative vs. Absolute Dating
Relative dating provides a chronological order as to when rocks formed.
The order of formation can be determined but the definite age of the rock cannot.
Absolute dating provides an age for the rock or feature.

6. Unconformities: a surface that represents a break in the rock record.
Unconformities occur when deposition stops, erosion removes rocks, and deposition occurs again.

7. 2 sedimentary rock layers are separated by an erosional surface
3 Types of Unconformities
Angular
Layers of rock form over older sedimentary rock layers that are tilted or folded
Disconformity
2 sedimentary rock layers are separated by an erosional surface
Nonconformity
Erosional surface separates a older metamorphic or igneous rocks from younger sedimentary rocks

10. Inclusions
Pieces of one rock unit that are contained within another rock unit.
The inclusions are older than the rock layers they are found in.

12. Correlation
Process of matching rock layers at different locations that formed at the same time and by the same processes.
Provides a complete interpretation of the rock record

15. 12.2 Fossils Petrified Turned into stone due to minerals in water
Fossils are preserved remains or traces of an organism
Types of fossils
Petrified
Turned into stone due to minerals in water

16. Molds and Casts
A mold is created when an organic structure is buried in sediment and then dissolved.
The mold reflects the shape and surface features of the organism
A cast is formed if the hollow space of the mold is filled with minerals

17. Compression fossils organic remains
Form when pressure squeezes out liquids and gases from a buried organism leaving behind a thin film of carbon.
Most are of plants.

18. Impression fossils Do not contain any organic matter
Show detail of the external structure of an organism
Commonly made in soft muds

19. Unaltered remains
All or part of the organism is preserved with very little change
Organisms that are fossilized in this manner are preserved in permafrost (ice), tar, or amber (sap)

20. Trace fossils
No remains of the organism but there is evidence for the presence or activity of the organism
Tracks
Burrows
Coprolites are fossils of dung
Gastroliths are polished stomach stones

21. Rapid burial to prevent being eaten or decomposed
The Fossil Record
2 conditions that favor preservation of an organism in order to make a fossil
Rapid burial to prevent being eaten or decomposed
Hard parts such as bones, shells, and teeth
The only rock type that fossils are likely to be found in is sedimentary.
Most common rock is limestone
The processes that form metamorphic and igneous rocks are more likely to destroy the fossil.

22. Used to determine the age of a rock layer 3 Characteristics
Index Fossils
Used to determine the age of a rock layer
3 Characteristics
Geographically widespread
Abundant
Existed for a short amount of time

23. Changes in fossils over time provides evidence for natural selection.
This idea was made popular by Charles Darwin as he tried to find a mechanism for evolution.
Natural selection means that organisms with the most favorable traits will survive.
Sometimes called survival of the fittest

24. 12.3 Dating with Radioactivity
Different forms of an element are called isotopes.
Isotopes occur when there is a different amount of neutrons in the nucleus of an atom. The number of protons does not change.
In some isotopes there is an excess of energy in the nucleus. These are called unstable nuclei.
Unstable nuclei break down and release energy until they become stable.

25. The term half-life is used to express the rate of radioactive decay.
The unstable or radioactive isotope is called the parent isotope and the stable isotope is called the daughter isotope.
U-238 (parent) → Pb-206 (daughter)
The term half-life is used to express the rate of radioactive decay.
A half life is the amount of time necessary for one half of the nuclei to break down into the stable isotope.

27. Radiometric Dating Method of calculating the absolute ages
The absolute age is the approximate number of years since a sample formed
In radiometric dating the number of parent to daughter isotopes are compared. The older the sample the more daughter isotopes.
Rates of decay do not change
Based on this information, geologists believe the earth is about 4.5 billion years old.

28. Carbon-14 Dating
Carbon-14 dating is used to date organic material.
Organic material is any material that contains carbon and comes from living things
The stable form of carbon-14 is carbon-12.
By comparing the amount of C-14 to C-12 geologists can determine the age of a sample.
Because the half life of carbon-14 is small (5730 yrs) C-14 dating is only useful for relatively young samples.