1. Geologic Time and dating rock sequences

2. Geologic Time Scale The Geologic Time Scale shows different eons, eras, periods and epochs The Geologic Time Scale shows different eons, eras, periods and epochs These were originally divided by geologic features and fossils found in different locations. These were originally divided by geologic features and fossils found in different locations. The actual ages were not then known The actual ages were not then known

3. How Much Time is Precambrian?

4. Your reference page Use your reference page to answer the following questions Use your reference page to answer the following questions 1. When did life first show up on the land? 2. When did the first reptiles evolve? 3. When did dinosaurs become extinct? 4. When did North America and Africa collide to form Pangaea?

5. Geologic Time You should understand however, that during these time periods many organisms evolved and that most of the species that have ever lived are now extinct. Why? You should understand however, that during these time periods many organisms evolved and that most of the species that have ever lived are now extinct. Why?

6. Evolution Because as conditions on earth changed competition enabled the organisms most fit to survive to live in the surroundings Because as conditions on earth changed competition enabled the organisms most fit to survive to live in the surroundings They had offspring who carried on their genetic characteristics They had offspring who carried on their genetic characteristics These were passed on to other generations until --- These were passed on to other generations until --- Competition again caused them to evolve into a different species or become extinct Competition again caused them to evolve into a different species or become extinct

7. Examples At the end of the Paleozoic Era, over 90% of all species that existed became extinct. At the end of the Paleozoic Era, over 90% of all species that existed became extinct. Scientists are unsure about the reason for this extinction, although the formation of Pangaea is probably to blame. Scientists are unsure about the reason for this extinction, although the formation of Pangaea is probably to blame. Dinosaurs became extinct at the end of the Mesozoic, and small mammals survived, ultimately leading to the reign of mammals. Dinosaurs became extinct at the end of the Mesozoic, and small mammals survived, ultimately leading to the reign of mammals. This may have been caused by a large asteroid or comet impact This may have been caused by a large asteroid or comet impact

8. Origins of our atmosphere The early atmosphere The early atmosphereearly atmosphereearly atmosphere We owe our oxygen-rich atmosphere to small organisms - algae, who changed the carbon dioxide in the early atmosphere to the oxygen that we breathe. Also, this oxygen formed the ozone layer which protects us from cosmic radiation. We owe our oxygen-rich atmosphere to small organisms - algae, who changed the carbon dioxide in the early atmosphere to the oxygen that we breathe. Also, this oxygen formed the ozone layer which protects us from cosmic radiation. Ironically, as the atmosphere became rich in oxygen, many organisms died out, since the oxygen was poison to them Ironically, as the atmosphere became rich in oxygen, many organisms died out, since the oxygen was poison to them So, again, changes and competition enabled our ancestors to survive and reproduce, thus passing on their genetic traits So, again, changes and competition enabled our ancestors to survive and reproduce, thus passing on their genetic traits

9. Relative Age Dating Geologists are very interested in figuring out how conditions on earth have changed. Geologists are very interested in figuring out how conditions on earth have changed. This enables them to predict some future events. This enables them to predict some future events. How do they interpret the past? How do they interpret the past? –There are no written records back more than a few thousand years. –They depend on the records left in rocks.

10. They use the concept of Uniformitarianism That we can interpret the past by looking at processes currently working in the earth. That we can interpret the past by looking at processes currently working in the earth. This idea was the brainchild of James Hutton, known as the father of Modern Geology. This idea was the brainchild of James Hutton, known as the father of Modern Geology. He also explained that geologic time must be immense in order for these processes to create the geologic features we see. He also explained that geologic time must be immense in order for these processes to create the geologic features we see.

11. Sample cross section This sketch to the right is a cross section of a rock sequence. In other words, if a road was cut through a particular location, you might see rocks in this sequence. This one is particularly difficult to interpret. Let’s start with an easier one. This sketch to the right is a cross section of a rock sequence. In other words, if a road was cut through a particular location, you might see rocks in this sequence. This one is particularly difficult to interpret. Let’s start with an easier one.

12. Any cross section will be a combination of the features shown below

13. Combine them and figure out -- how did this occur? 1. Sedimentary rocks are deposited in horizontal layers. This is called the principle of original horizontality. 2. And older sedimentary rocks are deposited first, with younger ones on top. This is known as the principle of superposition. So therefore, what happened here?

14. Did you figure it out? 1. First the lower layers were deposited (probably under water). 2. Then they were tilted (possibly by tectonic activity), the layers were lifted out of the water and the top was eroded. 3. Then the area was again covered by water and new sediments were deposited on top of the old ones. 4. This feature is an unconformity.

15. Here’s another one to try What is the order of the rocks and what happened? What is the order of the rocks and what happened? First the sedimentary layers were deposited under water, the oldest on the bottom. Next the area was faulted. Probably this was due to some type of tectonic activity Then the intrusion of gabbro occurred. This intrusion probably happened deep under the earth’s surface.

16. Other samples

17. Worksheet activity

18. Absolute Dating Geologists now use isotopes with different half lives to get actual dates for rocks. Geologists now use isotopes with different half lives to get actual dates for rocks. What is a half life? The amount of time it takes for half of an unstable element to change into another element. What is a half life? The amount of time it takes for half of an unstable element to change into another element. For example, you probably know that Carbon 14 is used to date fossils. This can be done because C14 changes to N14 over time. It takes about 5730 years for half of the C14 in a fossil to change to N14. For example, you probably know that Carbon 14 is used to date fossils. This can be done because C14 changes to N14 over time. It takes about 5730 years for half of the C14 in a fossil to change to N14.

19. Activity setup We will be doing an activity on this. We will be doing an activity on this. Let’s make sure we know what we are doing Let’s make sure we know what we are doing First, let’s look at our simple activity (Dots of Time) First, let’s look at our simple activity (Dots of Time) Then we will do the lab – please read this so that it will go smoothly. (Flipping Pennies) Then we will do the lab – please read this so that it will go smoothly. (Flipping Pennies)Flipping PenniesFlipping Pennies

20. Simple half life problems 1. If a fossil has ½ of its C14 left, how old is it? 2. If a rock has 1/4 th of its U 235 left, how old is it? 3. If a fossil has 200 grams of C14 and 600 grams of N14, how old is it? Half lives of: carbon 14 – 5730 years, U235 700 million years

21. Fossils We discover past species through finding fossils We discover past species through finding fossils Fossils are also important in determining the age of rocks. We can compare fossils in rocks to known fossils and use this to determine the rock’s age containing that fossil. Fossils are also important in determining the age of rocks. We can compare fossils in rocks to known fossils and use this to determine the rock’s age containing that fossil. There are different types of fossils. There are different types of fossils. Fossils also show us about the environments which formed the rocks. Fossils also show us about the environments which formed the rocks. Fossils helped geologists to reconstruct geologic columns. Fossils helped geologists to reconstruct geologic columns.