Transcript What is relative dating?

Chapter 21
Section 4Geologic Time
Chapter 21 Section 4 – What You’ll Learn - page
Before you read - write the reading’s objectives in this space:
1 – _____________________________________________
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2 -______________________________________________
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3 -______________________________________________
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4 - ______________________________________________
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5 - ______________________________________________
Chapter 21 Section 4 – What You’ll Learn - page
Before you read - write the reading’s objectives in this space:
1 – Distinguish among geologic eras, periods, and epochs.
2 – Explain how scientists decide when a geologic time period ends
and another begins.
3 – Explain the methods and logic used in relative dating.
4 – Interpret a geologic cross section.
5 – Describe the methods and logic used in absolute dating.
What is absolute dating?
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What is relative dating?
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NOTE - relative dating can only determine the sequential
order in which a series of events occurred, not when they
occur.
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What is absolute dating?
The process of assigning a precise numerical age to an
organism, object, or event.
What is relative dating?
The process of placing objects or events in their proper
sequence in time.
NOTE - relative dating can only determine the sequential
order in which a series of events occurred, not when they
occur.
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What does Uniformitarianism state?
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NOTE - The only changes might be in their rate or scale.
State the Principle of superposition:
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What does Uniformitarianism state?
The laws of nature operate today as they
have in the past.
“The present is the key to the past.”
NOTE - The only changes might be in their rate or scale.
State the Principle of superposition:
The oldest rocks in an undisturbed
sequence of rock layers will be at the
bottom of the sequence.
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If I have sedimentary layers that are not horizontal what
information can I say has happened?
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Sideling Hill, Md.
Reveals a 350 million year old section
of rock which was folded by a massive
compression of the earth's crust some
245 million years ago (during
formation of Pangaea), leading to the
cut's distinctive "U" shaped fold
which is called a syncline.
If I have sedimentary layers that are not horizontal what
information can I say has happened?
Some type of disturbance.
Sideling Hill, Md.
Reveals a 350 million year old section
of rock which was folded by a massive
compression of the earth's crust some
245 million years ago (during
formation of Pangaea), leading to the
cut's distinctive "U" shaped fold
which is called a syncline.
The principle of cross cutting
relationships states what? _________
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The principle of cross cutting
relationships states what? Any rock
formation or fault is younger than the
rock or feature that it cuts through. You
can't cut across something that isn't
already there!
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What do Unconformities represent? _______________
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Angular unconformity
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What do Unconformities represent? Gaps in the rock
record during which erosion occurred or deposition was
absent.
Angular unconformity
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What are fossils? _____________________________
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The process of matching distinctive rock units from
different regions is called correlation. What makes
correlation useful? ____________________________
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What is an index fossil?
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Tim and Moby
Ammonites lived about 240 million years ago.
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What are fossils? The remains or traces of organisms found
in the geologic rock record
The process of matching distinctive rock units from
different regions is called correlation. What makes
correlation useful? Scientists can use fossils to match the
ages of rock units around the world.
What is an index fossil? Fossils
that lived for only a short time
period but were widely spread
throughout the world.
Tim and Moby
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Ammonites lived about 240 million years ago.
Paleontologists observe
changes in the fossil
record and associate
them with boundaries
of time units.
At these boundaries,
fossils of certain life
forms are no longer
present and new life
forms begin to appear.
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Radioactive dating is based on
the fact that a radioactive
isotope (form) of an element
changes (breaks down) into an
isotope of another element at a
fixed rate. Each radioactive
isotope, or radioisotope, has its
own rate, expressed in terms of
its half-life.
Half-life is the time taken for a
substance or collection of
particles to decay by half of its
original amount.
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What is a geologic map?
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What does a geologic map
show?
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What is a geologic map?
Geologic cross-sections
represent a vertical slice taken
through Earth.
What does a geologic map
show? Geologic maps show the
horizontal surface distribution
of various rock formations.
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Explain how basins and domes are
formed:
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**In basins, the youngest rocks are
exposed in the center and tilt or dip
inward.
**Domes are essentially the
opposite of basins.
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Explain how basins and domes are
formed:
First - The principle of original
horizontality states that sediments
are laid down in horizontal layers.
Next- Compressional forces cause
the layers to fold or to dip
downward.
**In basins, the youngest rocks are
exposed in the center and tilt or dip
inward.
**Domes are essentially the
opposite of basins.
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Rocks subjected to the compression might wrinkle or fold,
what is an upward wrinkle of rock formation called?
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What is a downward wrinkle of a rock formation called?
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Anticline and syncline in layered Precambrian gneiss along NJ Route 23
near the rest area exit ramp (west of Butler, NJ).
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Rocks subjected to the compression might wrinkle or fold,
what is an upward wrinkle of rock formation called?
An anticline
What is a downward wrinkle of a rock formation called?
A syncline.
Anticline and syncline in layered Precambrian gneiss along NJ Route 23
near the rest area exit ramp (west of Butler, NJ).
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What are Plunging Folds?
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What are Plunging Folds?
If the forces are applied unevenly
the axis of the fold might plunge
back into the Earth, producing a
surface pattern of rock like the one
shown in the figure.
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Section Check
4
Question 1
_______ is the process of placing objects or
events in their proper sequence in time.
A. Relative dating
B. Absolute dating
C. Carbon dating
D. Fossil dating
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Section Check
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Answer
The answer is A. When you date events
relatively, you know which came first,
second, and so on.
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Section Check
4
Question 2
Why is Fossil C
considered to be
between 2.1 and 2.4
million years old?
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Section Check
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Answer
You could conclude
from the rock layers at
Location A that Fossil
C is between 2.1 and
2.4 million years old,
Fossil A is older than
2.4 million years, and
Fossil E is younger
than 2.1 million years.
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Section Check
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Question 3
Which time unit of the geological time scale is
the shortest?
A. eon
B. epoch
C. period
D. era
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Section Check
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Answer
The answer is B. An epoch is the shortest time
unit of the geological time scale.
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