1. Geologic Time (History as told by Rocks)

3. The Rock Record Geological Time Scale
Dividing the Earth’s history into sections based on the fossils and rock evidence.

4. How do we know?
Principle of UNIFORMITARIANISM means that the natural processes in place today have been occurring since Earth formed.
Processes in the past happened according to the same laws as processes do today.

5. Dividing It Up Eon Era Period Epoch
Longest time unit measured in BILLIONS of years.
Era
next longest measured in HUNDREDS of MILLIONS of years
Period
measured in TENS of MILLIONS of years
Epoch
measured in MILLIONS of years

6. Historical Events Bacteria Form
Trilobites are abundant, First fish appear, Appalachians begin to form
First Land Plants and Insects appear
Reptiles appear
Appalachians finish forming, glaciers retreat

7. Important Events Pangaea breaks up, Mammals appear
Dinosaurs dominate, mountain building in Western U.S.
Flowering plants appear, Rockies form, Dinosaurs become extinct
Alps and Himalayan Mountains form, mammals dominate
0.01 million years ago – Homo Sapiens evolves.

8. Fish
Trilobite
Insects
Land Plants
Reptiles

9. Geological Principles
Original horizontality
sedimentary rocks were formed in HORIZONTAL LAYERS
Superposition
oldest rock layers are on the BOTTOM
Cross-cutting relationships
Intrusions or Faults that cut across a layer are YOUNGER than the layer they cut through.
Unconformity
missing layer of eroded rock

10. Original Horizontality Cross-cutting Relationships
Superposition
Cross-cutting Relationships
Unconformity

11. Bending and Breaking Layers
Rock layers are disturbed by geological processes
FOLDING: Compression forces BEND the layers
TILTING: Lifting forces lift one end of a layer of rock
FAULTING: Shear forces shift some layers of the rock

13. Identifying the RELATIVE AGE of Rock
We can use the patterns in the rock layers to determine RELATIVE AGE of the layers
Place the layers in order from
Oldest  Youngest
Bottom Top

18. Absolute-Age Dating
In addition to learning about the relative age, scientists also have methods of determining the absolute-age.
Absolute age = The rock is 2 million years old
Relative age = The rock is older than the other rock.

19. Radiometric Dating
Scientists can use UNSTABLE atoms to determine the age of some rocks and fossils.
Unstable atoms decay (release energy) at a predictable rate over time.
The atoms change as they decay.
Comparing the number of unchanged atoms (parent) and changed atoms (daughter), the age can be determined
The rate these atoms decay is called the half-life.

20. Radioactive Atoms and Their Use
Carbon-14 is used for organic material less than 75,000 years old.
The half-life of Carbon-14 is 5,730 years
Carbon-14 has 6 protons and 8 neutrons and it decays to Nitrogen-14 with 7 protons and 7 neutrons
Organisms continually take in new carbon while they are alive but stop when they die, so the carbon-14 “clock” doesn’t start until after death.

22. Other Radiometric Dating
Uranium-235 – half-life of 0.7 billion years
Decays to Lead-207
Uranium-238 – half-life of 4.6 billion years
Decays to Lead-208
Potassium-40 – half-life of 1.3 billion years
Decays to Argon-40
These decay processes also have intermediate steps so we can use them to date rocks as young as 50,000 years to the oldest rocks in our solar system.

23. Dendrochronology
We can examine the annual growth of trees (tree rings) to determine an absolute age of tree.
Using tree rings to determine age is called dendrochronology.
The width of the rings also can tell us if the climate was wet or dry and if any major forest fires occurred during the life of the tree.

25. Other Age Indicators
We can also determine age by looking for indicators of climate change
Varves are bands of deposited sediments in sedimentary rocks. Thick bands mean more sediment was carried – so more water.
Key beds are layers of distinct sediments formed by unique events – such as asteroid impacts

26. Fossil Record Fossil – evidence or remains of once living organisms
If you know the type of organism, it can tell you about the environment
If you find a fossil shell in the desert, it was probably a marine environment when the fossil formed.
If you have actual genetic material, you can use the DNA record and carbon dating to determine age.

27. How do fossils form? Fossils usually only form in sedimentary rock.
Sediments have to accumulate over the organism in order to preserve it.
volcanic eruption, mudslide, tree sap or slow sediment accumulation on the bottom of a lake
Most fossils are marine in origin, or are land animals that fell into a water environment.
This is because sediments easily accumulate in water environments (lakes, streams, oceans).

28. Fossil Record
Trace fossils – fossils left by animal or plant ACTIVITY (footprints, worm burrows)
Mold – Rock formed around an organism that later decays out, leaving a empty space in the shape of the organism
Cast – Molds that get filled up with later rock
Permineralization – minerals from ground water seep into the organism and slowly fill in the pores in the animal
Original Preservation – remains that have not changed since the organisms death (often frozen!)

29. CAST FOSSIL This ammonite cast was discovered in the United Kingdom.
Petrified
Fossils
MOLD FOSSIL
This mold, or imprint, is of an extinct mollusk called an ammonite.
CAST FOSSIL This ammonite cast was discovered in the United Kingdom.
Trace
Fossils
Preserved
Remains
Carbon Films

30. Index Fossil
Index fossils are remains of organisms that can be used to relate rock layers that spread over a large geographic area.
Index fossils must be
Abundant
Spread out geographically
Lived only for a limited time frame
If you know when that organism was alive, you can determine when the rock formed.

31. Which two rock layers formed at the same time?