Transcript U6-Geologic History Notes

Earth’ s History
Unit 6
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Vocabulary List
1.
Relative Dating
2.
Absolute Dating
3.
Superposition
4.
Cross-cutting relationships
5.
Uniformitarianism
6.
Original Horizontality
7.
Correlation
8.
Unconformity
9.
Index Fossils
10.
Radioactive Decay
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List of Focus Questions
Date
Focus
____
What is relative dating? What principles do
we follow so we can put events into order?
____
What is correlation? What two things do we
use to correlate rock layers?
____
What is absolute dating? What is C14 used to
date?
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A. Order of Geologic Events
When studying the history of Earth, we must be able
to place items or events in a particular order so we
can determine which event happened first, which
happened next, and so on.
We order events in two ways:
1. Relative Dating:____________________________
___________________________________________
___________________________________________
2. Absolute Dating: __________________________
___________________________________________
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To determine an order of events using relative ages,
we use the following four principles:
1. __________________ Layers of rock on the
bottom are older than the rock layers above them,
provided they have not been overturned.
Youngest Rock
Layer
Oldest Rock
Layer
2. __________________________ Rock layers are
older than any event that cuts across them.
Example: _________________________________
_________________________________________.
The sedimentary rocks
are older than the
igneous intrusion.
Note the contact
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metamorphism.
3. ____________________________________:
Processes that occur now (erosion, deposition,
plate movement, etc.) occur the same way now
as they always have throughout geologic history.
3. ____________________________Sediments that
form sedimentary rocks are usually deposited in flat
layers. If we see rock layers that are not flat and level,
we can assume that they were changed (tilted, folded
or faulted) after they formed.
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B. Matching Rock Layers
When studying the history of Earth, we must also be
able to compare rock layers found in one place to
those found in another.
Correlation:__________________________________
___________________________________________
___________________________________________
Correlation of a single rock layer can often be
difficult. Often, it is better to try to correlate a
sequence of rock strata. This is often done by a
technique called “walking the outcrop”: ___________
___________________________________________
___________________________________________
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Walking the outcrop can be limited because the rock
strata may be covered by sediment or other rock.
Also, part of the rock strata may be missing from a
location.
Unconformity: _____________________________
___________________________________________
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There are two other methods used to correlate rock
layers; index fossils and volcanic eruptions.
Index Fossils: _______________________________
__________________________________________
To be a good index fossil, a fossil must:
a. __________________________________
b. __________________________________
Volcanic Eruptions: _________________________
___________________________________________
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C. Absolute Age of Rocks
How can the absolute age of rock strata be
determined? Some rocks contain elements that are
unstable.
Radioactive Decay: __________________________
___________________________________________
Uranium 238
Radioactive
Decay
Lead 206
+ Energy
As the uranium decays, it does so in a very predictable
manner and at a predictable rate.
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Half-life: ___________________________________
___________________________________________
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As each half-life is reached, __________ of the
remaining radioactive element has decayed. Notice
that you will never reach a point where all of the
radioactive element has decayed. However, there will
come a point where: ___________________________
__________________________.
Half-lives may be as short as ___________________
or on the order of _________________________.
The front cover of your Earth Science Reference Table
has radioactive decay data for several radioactive
elements or isotopes.
A radioactive isotope with a appropriate half life must
be used to correctly measure age. If an item that is
only several thousand years old was dated using an
isotope with a half life of 5 billion years, not enough
radioactive decay would have occurred to be
measurable. If a very old item was dated using an
isotope with a short half life, then not enough
radioactive element would be remaining to measure.
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Radioactive Dating Example
QUESTION: A sample is found to contain 100 grams
total of the radioactive element Carbon-14 and its
daughter element Nitrogen-14. If 25 grams of that
total is C14 and 75 grams is N14, how old is the
sample?
ANSWER: First, determine the number of half lives
this sample has undergone.
•After one half life, there is _____grams of C14 &
_____ grams of N14.
•After two half lives, there is ____grams of C14 &
_____ grams of N14.
So, if the half life of C14 is _______ years (see your
ESRT) and there have been ____ half lives, then
_________ X _______ = __________ years old!
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D. Absolute Age of Rocks
Geologic time is divided into units based upon:
___________________________________________
___________________________________________
___________________________________________
The units are not based directly on _______!
When dividing Earth history, begin with the largest
units _______. They are divided into _____; they are
divided into _______; and they are divided into
_______.
NOTE: Compared to the overall geologic time scale
(about 4.6 billion years), human existence is
considered ________________
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Evidence found in rock strata and in their fossils allow
scientists to infer what life and the environment were
like at that particular time and in that particular
location. For example, coral fossils found in
limestone would indicate what type of environment?
_____________________________
Of all the species on Earth, __________ of them have
become extinct since life was first recorded in the rock
record.
Most scientists believe the fossil record indicates life
forms have ______ (changed) through geologic time,
probably to adapt to changes in their environment.
This theory, know as _________________________,
is not a change from simple life like one-celled
organisms to complex like humans. It is an increase
in the variety of life forms.
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