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WHO'S ON FIRST?
A RELATIVE DATING ACTIVITY
[Modified
from MARSHA BARBER and DIANA SCHEIDLE BARTOS]
INTRODUCTION The
study and comparison of exposed rock layers or strata in various parts
of the earth led scientists in the early 19th century to propose that the
rock layers could be correlated from place to place. Locally, physical
characteristics of rocks can be compared and correlated. On a larger scale,
even between continents, fossil evidence can help in correlating rock layers.
The Law of Superposition, which states that in an undisturbed
horizontal sequence of rocks, the oldest rock layers will be on the bottom,
with successively younger rocks on top of these, helps geologists correlate
rock layers around the world. This also means that fossils found in the
lowest levels in a sequence of layered rocks represent the oldest record of
life there. By matching partial sequences, the truly oldest layers with
fossils can be worked out. By
correlating fossils from various parts of the world, scientists are able to
give relative ages to particular strata. This is called relative dating.
Relative dating tells scientists if a rock layer is "older" or
"younger" than another. This would also mean that fossils found in
the deepest layer of rocks in an area would represent the oldest forms of
life in that particular rock formation. In reading earth history, these
layers would be "read" from bottom to top or oldest to most recent.
If certain fossils are typically found only in a particular rock unit and are
found in many places worldwide, they may be useful as index or guide fossils
in determining the age of undated strata. By using this information from rock
formations in various parts of the world and correlating the studies,
scientists have been able to establish the geologic time scale. This relative time
scale divides the vast amount of earth history into various sections based on
geological events (sea encroachments, mountain-building, and depositional
events), and notable biological events (appearance, relative abundance, or
extinction of certain life forms). Objectives: When
you complete this activity, you will be able to: (1)
sequence information using items which overlap
specific sets (2)
relate sequencing to the Law of Superposition (3)
show how fossils can be used to give relative dates
to rock layers. Materials: 2 sets of sequence cards in random order (set
A: nonsense syllables; set
B: sketches of fossils), pencil, paper Procedure for Set A: 1) Spread the cards with the nonsense syllables on the table
and determine the correct sequence of the eight cards by comparing letters
that are common to individual cards and, therefore, overlap. The first card
in the sequence has "Card 1, Set A" in the lower left-hand corner
and represents the bottom of the sequence. If the letters "T" and
"C" represent fossils in the oldest rock layer, they are the oldest
fossils, or the first fossils formed in the past for this sequence of rock
layers. 2) Now, look for a card that has either a "T" or
"C" written on it. Since this card has a common letter with the
first card, it must go on top of the "TC" card. The fossils
represented by the letters on this card are "younger" than the
"T" or "C" fossils on the "TC" card which
represents fossils in the oldest rock layer. Sequence the remaining cards by
using the same process. When you finish, you should have a vertical stack of
cards with the top card representing the youngest fossils of this rock
sequence and the "TC" card at the bottom of the stack representing
the oldest fossils. Interpretation Questions: 1) How do
you know that "X" is older than "M"?
_____________________________________________________ ____________________________________________________________________________________________ ____________________________________________________________________________________________
____________________________________________________________________________________________ Procedure Set B: 2) The
oldest rock layer is marked with the letter "M" in the lower
left-hand corner. The letters on the other cards have no significance to the
sequencing procedure and should be ignored at this time. Find a rock layer
that has at least one of the fossils you found in the oldest rock layer. This
rock layer would be younger as indicated by the appearance of new fossils in the
rock stratum. Keep in mind that extinction is forever. Once an organism
disappears from the sequence it cannot reappear later. Use this information
to sequence the cards in a vertical stack of fossils in rock strata. Arrange
them from oldest to youngest with the oldest layer on the bottom and the
youngest on top. Interpretation Questions:
_____________________________________________________________________________________ 2) Name
three organisms represented that probably could not be used as index fossils
and explain why. 3) In what
kinds of rocks might you find the fossils from this activity?
___________________________ 4) State
the Law of Superposition and explain how this activity illustrates this law.
_________________ ________________________________________________________________________________________ Set A
Set B
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