Transcript Geologic Time

Geologic Time
and dating rock sequences
Geologic Time Scale
The Geologic Time
Scale shows different
eons, eras, periods
and epochs
 These were originally
divided by geologic
features and fossils
found in different
locations.
 The actual ages were
not then known
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How Much Time is Precambrian?
Geologic Eras
Cenozoic, 65,000,000
Mesozoic,
180,000,000
Precambrian
Paleozoic
Mesozoic
Cenozoic
Paleozoic,
300,000,000
Precambrian,
4,000,000,000
Your reference page
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Use your reference page
to answer the following
questions
When did life first show
up on the land?
When did the first
reptiles evolve?
When did dinosaurs
become extinct?
When did North America
and Africa collide to form
Pangaea?
Geologic Time
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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?
Evolution
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
 These were passed on to other
generations until -- Competition again caused them to evolve
into a different species or become extinct
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Examples
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.
 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
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Origins of our atmosphere
The early 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.
 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
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Relative Age Dating
Geologists are very interested in figuring out
how conditions on earth have changed.
 This enables them to predict some future
events.
 How do they interpret the past?
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– There are no written records back more than a few
thousand years.
– They depend on the records left in rocks.
They use the
concept of
Uniformitarianism
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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.
He also explained that
geologic time must be
immense in order for these
processes to create the
geologic features we see.
Sample cross section
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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.
Any cross section will be a combination of
the features shown below
Combine them and figure out -how did this occur?
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Sedimentary rocks are
deposited in horizontal
layers. This is called the
principle of original
horizontality.
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?
Did you figure it out?
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First the lower layers were
deposited (probably under
water).
Then they were tilted
(possibly by tectonic
activity), the layers were
lifted out of the water and
the top was eroded.
Then the area was again
covered by water and new
sediments were deposited
on top of the old ones.
This feature is an
unconformity.
Here’s another one to try
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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.
Other samples
Worksheet activity
Absolute Dating
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.
 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.
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Activity setup
We will be doing an activity on this.
 Let’s make sure we know what we are
doing
 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)
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Simple half life problems
Half lives of: carbon 14 – 5730 years, U235 700 million years
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If a fossil has ½ of its C14 left, how old
is it?
If a rock has 1/4th of its U 235 left, how
old is it?
If a fossil has 200 grams of C14 and 600
grams of N14, how old is it?
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.
 There are different types of fossils.
 Fossils also show us about the environments
which formed the rocks.
 Fossils helped geologists to reconstruct
geologic columns.
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