Dating_Worksheet_Week6-2
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Oregon State University, Corvallis *
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240
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Geology
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Apr 3, 2024
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Anthro 240
How Old Was This
Fossil?
Dating Techniques
When we talk about geological time, sedimentation, and fossilization, we must have a
temporal frame of reference for the past. That frame of reference can be either an actual
point in time or a comparison with another sedimentary layer or fossil. These describe two main
categories of dating techniques: absolute dating and relative dating. It is beyond the scope of this
lab manual to discuss individual methods, but the following exercises reinforce the general
principles of absolute (radiometric, chronometric) methods and relative (comparative) methods.
Relative Dating: Stratigraphy
The diagram below represents an "outcrop," a rocky exposure such as those seen on the
sides of cliffs or along road cuts made to create freeways. In outcrops we often can see layers,
called strata (singular= stratum), also called beds. Each stratum is the result of some previous
natural geological event or process, such as a flood or erosion. Over time, the accumulation of
these processes causes layers of sediment to pile up and form sedimentary sequences. According
to the well-established Principle of Superposition, the older strata are deeper in the earth and the
younger ones closer to the surface. Not shown in the diagram here (or dealt with here) are the
many complex geological events that can occur to interfere with neat, clear layers. These include
folding events, faulting, erosion, intrusions by other sediment types, and so on.
When fossils have been deposited in sedimentary rocks, the relative ages of the fossils can be
determined through comparison of the strata in which they are contained. Thus, relative dating
will answer questions about "what's older" or "what's younger" rather than "when did it live?"
The following diagram is a key for interpreting the fossil beds, as well as some other useful
information. Examine the diagram, the key, and the hints below, then answer the questions that
follow.
A Few Hints:
•
Ash is sediment resulting from volcanic activity.
•
A mix of silt, pebbles, and other unsorted sediments are the result of flooding.
•
Sandy sediment (including resulting sandstone) often reflects an extended dry period.
•
From absolute dating techniques, we know that Bed 2 is about 3 million years old.
•
Bed 4 is about 4.5 million years old.
•
Shales are usually deposited in slow-moving water such as in former lakes, lagoons, river deltas,
and floodplains.
Questions:
1.
List the fossils ABCD in order from the most recent to the oldest.
2.
Which bed number
contains Fossil C?
3.
A volcano 100 miles from this fossil locality has erupted several times, each time spewing ash for hundreds of miles. Which stratum or strata formed during times of volcanic activity? List the number.
4.
Which fossil likely represented a species that lived in or near a lake? List the letter. 5.
Which fossil (or fossils) was (were) deposited around the time of flooding? List by letter.
6.
What do you know about the age of Fossil A?
7.
Which strata probably formed during arid (dry) periods of time?
Absolute Dating: Radiometry
Only after the discovery of radioactivity, around A.D. 1900, were we successful in obtaining
absolute dates for geologic events. As a result of techniques that depend upon radioactivity, we
know that the Earth formed about 4.6 billion years ago, and that dinosaurs became extinct 66
million years ago. To use absolute dating on rock, it must contain radioactive isotopes. Radioactive isotopes
(parent atoms) decay into more stable “daughter” atoms and, in the process, release energy
(radiation). Because the decay occurs at a constant rate, it can be used to measure time
elapsed. The relative amount of parent and daughter atoms can be used to determine the age of
the rock, within a certain margin of error. The decay rate is measured in terms of half-life, the
time it takes for ½ of the parent atoms to decay to form the daughter atoms.
One radiometric technique is called Potassium-Argon dating. Potassium 40 (K
40
) decays to
Argon 40 (Ar
40
) at a half-life of 1.3 billion years. At the time of the rock's formation (time
zero), there is only K
40
and no Ar
40
. After 1.3 billion years, half of the original K
40
atoms
have been transformed into Ar
40
atoms.
1.
A rock specimen contains 30 mmol (millimoles) of K
40
and 30 mmol of Ar
40
. How old is the rock?
2.
Would a specimen containing 30 mmol of K
40
and only 5 mmol of Ar
40
be younger or older than the rock in the previous question?
3.
Approximately how old is a rock specimen that has 15 mmol of K
40
and 45 mmol of Ar
40
?
Show your work.
a.
2.6 billion years old
b.
650,000,000 years old
c.
1.3 billion years old
d.
Cannot answer from the information given
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