WHO’S ON FIRST?
How do we know the age of fossils
and rock layers?
How do we know the age of fossils
and rock layers?
Law of Superposition – in undisturbed rock layers
the youngest layer is on top and each successive
downward layer is older, so the oldest is on the
bottom.
• Relative dating – the oldest fossils are in the
oldest rock layers.
• Index fossils – are found only in a particular
rock layer worldwide, and can be used to
determine the age of rocks or newly
discovered fossils found with them.
Who’s on 1st?
A RELATIVE DATING MODEL
ACTIVITY OBJECTIVES
1)Sequence events based on overlapping
information
2)Relate sequencing to the Law of
Superposition
3) Show how fossils can be used to give
relative dates to rock layers
Who’s on 1st?
A RELATIVE DATING MODEL
BACKGROUND INFORMATION
A) Fossils are found in sedimentary rock.
– the type indicates the particular environment
• limestone – marine invertebrates (hard parts,
such as shells, are made of CaCO3).
• sandstone – sediment from beaches or riverbeds
– How to use Index fossils: - a fossil found in
one layer only can indicate the age of the
other fossils found with it.
Card Set A
Sequence layers using the Law of Superposition.
DIRECTIONS:
• The cards will be put in sequence, bottom to top.
• The first card has “Card 1, Set A” in the lower-left
hand corner. It represents the bottom and oldest
of the “rock” sequence.
• The letters T and C on this card represent two
fossils in this “rock layer.”
• Now look for a card with a T or a C on it. This is
the next layer of rock. Put it over the first card. It
is younger than the first layer.
Questions Card Set A
Answer the following questions:
1) After the cards are in order write the
sequence of letters, starting with the top
(youngest layer) card.
2) How do you know that X is older than M?
3) Explain why D in the rock layer DM is the
same age as M?
4) Explain why D in the rock layer OXD is
older than the D in the layer DM.
Card Set B
These fossils are from sedimentary rock of marine origin.
DIRECTIONS:
• Each layer represents a layer of rock and
the fossils found in them.
• The oldest layer of rock is marked with an
“M” in the lower left hand corner.
• Find a rock layer that has at least one of
the fossils from the bottom (M) layer. Place
this “younger” rock layer above the bottom
card.
• Complete the sequencing.
Keep in mind extinction is forever.
Questions Set B
1) Using the letters in the lower left corner of
each layer, write the sequence of letters
from the youngest to the oldest.
2) Which fossil organisms could possibly be
used as index fossils?
3) Name three fossil organisms represented
that could not be used as index fossils.
4) In what kind of rock might you find these
fossils?
5) State the Law of Superposition.
6) Explain how this activity illustrates the law.
NOTES
The fossil record provides
evidence of Earth’s history.
• Many organisms that lived long ago are
extinct.
• Extinction of species is common: most of
the species that have lived on the Earth no
longer exist.
• Fossils provide evidence of how life and
environmental conditions have changed.
The Geologic Time Scale
Made by scientists using the information
from fossils and rock formations from
various parts of the world.
This relative time scale divides the vast
history of earth into sections based on
geologic events (such as ice ages, sea
enchroachments, mountain building and
erosion)and notable biological events
(appearance, relative abundance, and
extinctions).
One simple depiction →
New Ideas in Darwin’s Time
Earth is very old and
undergoes continual
change.
Charles Lyell
Wrote “The Principles of Geology”
which promoted James Hutton’s
theory of Uniformitarianism.
1) The Earth is dynamic. It is very old and
undergoes constant change.
2) Uniformitarianism – the same processes that
shaped the Earth are affecting it today. They
are continual.
3) Law of Superposition – in undisturbed rock
layers the youngest layer is on top and each
layer is older with each downward layer, so
the oldest is on the bottom.
Radiometric dating – is the use of the
natural radioactivity of certain elements to
determine the age of rock. It involves
measurement of the half-life of a
radioactive isotope.
Ex.s) Uranium and Carbon-14