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Chapter 8
The Rock Record
Section 1 Determining Relative Age
The age of the Earth is approximately _____________________. This idea
came from James Hutton, an 18th century Scottish Physician and Farmer.
He is the “Father of Modern Geology”. He came up with his ideas from
observing changes taking place on his farm. He also stated “The Present is
the Key to the Past”.
Uniformitarianism –
1.
a.
b.
c.
d.
2.
Geologic processes now operating were also active in the past.
__________________________
__________________________
__________________________
__________________________
Present day geologic features were formed by these processes over
long periods of time.
Before Hutton’s time people (science) thought all of the world’s features
were created by catastrophic events.
I Earths Age
Before Hutton most people thought the age of the Earth was about 6000
years old. They also thought that all geologic features were created the
same way and over a long period of time. He concluded that the Earth had
to be older for the changes that he found, to occur.
A. Relative Age
Relative DatingUsing rock layers to place events in the sequence in which they occurred.
This does not identify the actual date, but a date close to when the event
occurred.
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B. LAW OF SUPERPOSITION_________________________________________________, the
newest rocks are on the top. Sedimentary rocks forms in horizontal
layers. If the rock layers are overturned (flipped over) this is not true.
C. Principals of Original Horizontality
Sedimentary rocks generally form in horizontal layers. The principal states
that left undisturbed, the layers will remain horizontal. If the layers are not
horizontal, some tectonic force (action) had to occur. Scientist’s would
have to retrace what happen first in order to apply the Law of Super
Position.
1.
2.
3.
Graded bedding
Large particles usually on the bottom are smaller are on top.
Cross-beds
When sand is deposited, it usually forms beds at an angle
because the particles slide down.
Ripple Marks
Small waves formed on the surface of sand that later becomes a
rock. Can be used to show which layer was on top.
Unconformity
Is a gap in the geologic rock record sequence; it is a layer of rock
that is missing in the strata sequence. Shows that either the deposition
stopped or part of the layer was removed by erosion.
Ex. Erosion of rock layers
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1.
2.
3.
4.
______________________
When sedimentary layers are deposited on igneous and
metamorphic rock layers that have been uplifted and
parts eroded away.
______________________
Rocks that have tilted during uplift, or folded, then
eroded. When erosion stops, deposition once again
occurs.
_______________________
Layers that have been uplifted (not tilted still horizontal)
and then eroded away. Then new layers are deposited.
This makes gaps in the layering.
____________________________
They are igneous layers that are intruded into
sedimentary rocks must be younger than the rocks they
flowed through.
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Section 2 Determining Absolute Age
Absolute Age (time)
The actual age is the actual time an event happened like
when school get out at 2:45pm.
I.
Absolute Dating Methods
If we know the absolute time between two separate events we can
determine the relative time of all the events that occurred between them.
Also we can determine how long it took for the processes to form the
Earth’s features.
A.
______________________________
An estimate of absolute age based on known erosional rates of
geologic features. This can only be used for features that were
formed within the past 10-20 thousand years. It can be used for
Niagara Falls, but not the Grand Canyon.
B.
_________________________________
This can be used to estimate absolute age based on the deposition
rates of common rocks such as limestone, shale and sandstone.
Generally the rate is about 30cm in 1000years. If flooding occurs, this
formula can not be used.
C.
__________________________________
This is like counting the rings on a tree. Certain layers of sediment
occur annually and can be counted. In areas with glacial lakes, fine
sediments are deposited in the winter where larger more course
sediments are deposited in the summers. Geologists look for the color
variation and count the layers.
II.
Radiometric Dating
Radioactive decay is the spontaneous emission of energy from an atom.
It measures the amount of a parent and daughter isotope within a rock or mineral. By
using a known decay rate they can determine the age of something.
Each time a particle is emitted or captured from a parent isotope, the isotopes atomic
number changes and becomes a different element (daughter isotope). Radiation is
released until a stable isotope is formed.
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1. __________________________
At the time a rock is formed it starts to decay at a constant rate. Half
life is the rate at which a radioactive element decays. It is the time it takes
for half of the radioactive atoms in a sample to decay to a stable product.
The ratio of the amount of radioactive element left in the rock to the
amount of stable product can be used to determine the absolute age of the
rock.
It will always be half, of a half, of a half
2. Radioactive Isotopes
a.
__________________________
- Potassium 40 decays to Argon 40
- It has a half life of 1.3 billion years
- Potassium is very common, found in feldspar, micas and
amphiboles.
- It is common in all rock families
- It can date rocks a young as 50,000 years and as old as 4.6 billion.
b.
______________________________
- Rubidium 87 decays to Strontium 87
- Its half life is 47 billion years
- Best for dating extremely old rocks from 10 million to 4.6 billion
- It is common in feldspar & micas (most all igneous rocks)
c.
______________________________
- Uranium 238 decays to Lead 206
- Its half life is 4.5 billion years
- It occurs only in trace amounts in Zircon (igneous)
- It rarely in Sedimentary or Metamorphic Rocks
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- It does not give reliable results for rocks younger than
10 million years old
3. Radio Carbon Dating:
- It measures the ratio of Carbon 14 to Carbon 12.
- C12 Normal Carbon while C14 Radioactive Carbon
Limitations:
It can only be used on organic objects
Its half life is 5730 so it can only be used for objects70,000 years old
or younger.
Section 3 Fossils
Fossils
_____________________________________________
Paleontology
_____________________________________________
Paleontologist
_____________________________________________
I. Interpreting the Fossil Records
By looking at fossils we can see how an organism lived and how they have
either changed or gone extinct. This process allows us to glimpse their
past and how Earth has changed by climate, land mass, temp etc.
II. Fossilization
1.
2.
3.
4.
_______________________
Taking internal organs out and replacing it with a substance
usually herbs, then wrapping them in cloth strips prepared with
a solution to preserve a body.
_______________________
a. Organism that is preserved in its entirety
b. Frozen animals and humans (wooly mammoth)
c. Bone and teeth
d. Any organism trapped in amber
_______________________
Tar fro underground comes to the surface and traps an animal
in it. La Brea Tar Pits, Cal.
________________________
This occurs when an animal or organism is frozen quickly,
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5..
bacteria did not decay the soft tissue. These objects are frozen
whole. The wooly mammoth is an example.
__________________________
It is when soft organic material is decayed away to form a mold
and the minerals such as calcite, silica and pyrite are deposited
to form the original shape.
Types of Fossils
1.
2.
3.
4.
.__________________________
It is when carbon compounds making up the tissue undergo a chemical
change and form a thin film on the object.
__________________________
1. Molds are a hollow depression in the rock that shows the original shape
and surface of the fossil.
2. Casts form as hard parts (and soft sediments) sediment hardens.
__________________________
Fossilized dung or waste material
.__________________________
Use to help dinosaurs digest. They have a distinct shape, smooth and
round.
_________________________
They are preserved indirect evidence of prehistoric life. Includes
impressions left in rocks like footprints, tracks, burrows and bite marks.
III. Index Fossils
Using Index Fossils
They are the remains of animals that lived for a short period of time and
then became extinct. Therefore they would be found only in one layer.
There are 4 characteristics:
1. __________________________________
2. __________________________________
3. __________________________________
4. __________________________________
IV. Index Fossils and Absolute Age
Since they are only around for a short period scientists can use them to
absolute age an area. This can be done in separate locations to prove
they have something in common or even to find oil and gas reserves.