Geologic TimeGeologic Time

and dating rock sequencesand dating rock sequences

Geologic Time ScaleGeologic Time Scale The Geologic Time The Geologic Time

Scale shows Scale shows different eons, eras, different eons, eras, periods and epochsperiods and epochs

These were These were originally divided by originally divided by geologic features geologic features and fossils found in and fossils found in different locations.different locations.

The actual ages The actual ages were not then were not then knownknown

How Much Time is How Much Time is Precambrian?Precambrian?

Geologic Eras

Precambrian, 4,000,000,000

Paleozoic, 300,000,000

Mesozoic, 180,000,000

Cenozoic, 65,000,000

Precambrian

Paleozoic

Mesozoic

Cenozoic

Your reference pageYour reference page Use your reference Use your reference

page to answer the page to answer the following questionsfollowing questions

1.1. When did life first show When did life first show up on the land?up on the land?

2.2. When did the first When did the first reptiles evolve?reptiles evolve?

3.3. When did dinosaurs When did dinosaurs become extinct?become extinct?

4.4. When did North When did North America and Africa America and Africa collide to form collide to form Pangaea?Pangaea?

Geologic TimeGeologic Time

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

EvolutionEvolution

Because as conditions on earth changed Because as conditions on earth changed competition enabled the organisms most competition enabled the organisms most fit to survive to live in the surroundings fit to survive to live in the surroundings

They had offspring who carried on their They had offspring who carried on their genetic characteristicsgenetic characteristics

These were passed on to other These were passed on to other generations until ---generations until ---

Competition again caused them to Competition again caused them to evolve into a different species or become evolve into a different species or become extinctextinct

ExamplesExamples

At the end of the Paleozoic Era, over 90% At the end of the Paleozoic Era, over 90% of all species that existed became extinct.of all species that existed became extinct.

Scientists are unsure about the reason for Scientists are unsure about the reason for this extinction, although the formation of this extinction, although the formation of Pangaea is probably to blame.Pangaea is probably to blame.

Dinosaurs became extinct at the end of Dinosaurs became extinct at the end of the Mesozoic, and small mammals the Mesozoic, and small mammals survived, ultimately leading to the reign of survived, ultimately leading to the reign of mammals. mammals.

This may have been caused by a large This may have been caused by a large asteroid or comet impactasteroid or comet impact

Origins of our atmosphereOrigins of our atmosphere The The early atmosphere We owe our oxygen-rich atmosphere to small We owe our oxygen-rich atmosphere to small

organisms - algae, who changed the carbon organisms - algae, who changed the carbon dioxide in the early atmosphere to the oxygen dioxide in the early atmosphere to the oxygen that we breathe. Also, this oxygen formed the that we breathe. Also, this oxygen formed the ozone layer which protects us from cosmic ozone layer which protects us from cosmic radiation. radiation.

Ironically, as the atmosphere became rich in Ironically, as the atmosphere became rich in oxygen, many organisms died out, since the oxygen, many organisms died out, since the oxygen was poison to themoxygen was poison to them

So, again, changes and competition enabled our So, again, changes and competition enabled our ancestors to survive and reproduce, thus passing ancestors to survive and reproduce, thus passing on their genetic traitson their genetic traits

Relative Age DatingRelative Age Dating

Geologists are very interested in figuring Geologists are very interested in figuring out how conditions on earth have out how conditions on earth have changed.changed.

This enables them to predict some future This enables them to predict some future events.events.

How do they interpret the past?How do they interpret the past?– There are no written records back more than There are no written records back more than

a few thousand years.a few thousand years.– They depend on the records left in rocks.They depend on the records left in rocks.

They use the They use the concept of concept of

UniformitarianiUniformitarianismsm

• That we can interpret the That we can interpret the past by looking at past by looking at processes currently processes currently working in the earth.working in the earth.

• This idea was the This idea was the brainchild of James brainchild of James Hutton, known as the Hutton, known as the father of Modern Geology.father of Modern Geology.

• He also explained that He also explained that geologic time must be geologic time must be immense in order for immense in order for these processes to create these processes to create the geologic features we the geologic features we see.see.

Sample cross sectionSample cross section This sketch to the This sketch to the

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

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

Combine them and figure out -- Combine them and figure out -- how did this occur?how did this occur?

1.1. Sedimentary rocks are Sedimentary rocks are deposited in horizontal deposited in horizontal layers. This is called the layers. This is called the principle of original principle of original horizontalityhorizontality..

2.2. And older sedimentary And older sedimentary rocks are deposited rocks are deposited first, with younger ones first, with younger ones on top. This is known as on top. This is known as the the principle of principle of superpositionsuperposition. So . So therefore, what therefore, what happened here?happened here?

Did you figure it out?Did you figure it out?

1.1. First the lower layers First the lower layers were deposited were deposited (probably under water).(probably under water).

2.2. Then they were tilted Then they were tilted (possibly by tectonic (possibly by tectonic activity), the layers were activity), the layers were lifted out of the water lifted out of the water and the top was eroded.and the top was eroded.

3.3. Then the area was again Then the area was again covered by water and covered by water and new sediments were new sediments were deposited on top of the deposited on top of the old ones.old ones.

4.4. This feature is an This feature is an unconformity.unconformity.

Here’s another one to tryHere’s another one to try What is the order What is the order

of the rocks and of the rocks and what happened?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.

Other samplesOther samples

Worksheet Worksheet activityactivity

Absolute DatingAbsolute Dating

Geologists now use isotopes with different Geologists now use isotopes with different half lives to get actual dates for rocks.half lives to get actual dates for rocks.

What is a half life? The amount of time it What is a half life? The amount of time it takes for half of an unstable element to takes for half of an unstable element to change into another element.change into another element.

For example, you probably know that For example, you probably know that Carbon 14 is used to date fossils. This can Carbon 14 is used to date fossils. This can be done because C14 changes to N14 over be done because C14 changes to N14 over time. It takes about 5730 years for half of time. It takes about 5730 years for half of the C14 in a fossil to change to N14.the C14 in a fossil to change to N14.

Activity setupActivity 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 Let’s make sure we know what we

are doingare doing First, let’s look at our simple activity First, let’s look at our simple activity

(Dots of Time)(Dots of Time) Then we will do the lab – please read Then we will do the lab – please read

this so that it will go smoothly. (this so that it will go smoothly. (Flipping Pennies))

Simple half life problemsSimple half life problems

1.1. If a fossil has ½ of its C14 left, how If a fossil has ½ of its C14 left, how old is it?old is it?

2.2. If a rock has 1/4If a rock has 1/4thth of its U 235 left, of its U 235 left, how old is it?how old is it?

3.3. If a fossil has 200 grams of C14 and If a fossil has 200 grams of C14 and 600 grams of N14, how old is it?600 grams of N14, how old is it?

Half lives of: carbon 14 – 5730 years, U235 700 million years

FossilsFossils We discover past species through finding We discover past species through finding

fossilsfossils Fossils are also important in determining Fossils are also important in determining

the age of rocks. We can compare fossils in the age of rocks. We can compare fossils in rocks to known fossils and use this to rocks to known fossils and use this to determine the rock’s age containing that determine the rock’s age containing that fossil.fossil.

There are different types of fossils.There are different types of fossils. Fossils also show us about the Fossils also show us about the

environments which formed the rocks.environments which formed the rocks. Fossils helped geologists to reconstruct Fossils helped geologists to reconstruct

geologic columns.geologic columns.