Rocks & Fossils:
 Geologists study rock layers, fossils,
and ice cores to learn about Earth’s
history.
 Using these findings, they have been
able to develop the Geologic Time
Scale.
Determining the Age of Rocks
 Rocks and fossils are used to date events
and determine when events on Earth took
place in history.
 Absolute Age tells the actual age of a
rock or fossil. Example: Many of you
are 14, or the Earth is 4.7 billion years
old – these are absolute ages.
 Absolute age is determined by using
radioactive dating.
 Radioactive Dating measures the age of
rock or fossil by comparing the amount
of a radioactive form of an element in a
rock or fossil with the amount of its
decay.
 Over time, things break down by
releasing energy that is called
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radioactive decay (this is what gets
measured).
While absolute age gives a measurement
of how long ago a rock or fossil formed,
it is not always possible to date them…so
relative age is used.
Relative Age is the age of a rock or
fossil compared to another rock or fossil.
It determines which events occur earlier
or later than others without giving a
definite date. Example: Saying you are
older than your brother but younger than
your sister, you are describing your
relative age.
Relative age can be determined with the
Law of Superposition, which says that
older layers of rock are below younger
layers because they formed first.
So you could determine the relative age
of the rock layers in the Grand Canyon
by knowing that the oldest ones are at the
bottom and the youngest ones are at the
top.
 Beneath the surface of the Earth there is
magma that may push into bodies of
rock.
 This is called an intrusion and it always
younger than the rock layers beneath (it
will always be the youngest rock layer!).
Fossils Provide Clues to Earth’s Past
 Fossil records show how species have
changed over time and evolved into new
species.
 Shared traits of dinosaurs and birds
suggest that birds evolved from
dinosaurs.
 Fossils can provide information about the
size, shape, and structure of extinct
organisms. Example: the shape of a
tooth can tell what an animal ate.
 An organism that is extinct is no longer
found living on Earth.
 Tracks can tell about the size, weight,
and stride of the animal that made them.
Aging Rocks Using Fossils
 Suppose two similar rocks in different
locations contain the same kinds of
fossils.
 The fossil may suggest that the rocks
were formed in a similar place and time
and later became separated.
 When the absolute age of a fossil is
known, the fossil can be used to identify
the relative age of the rock layer where it
was found.
Review Questions:
1. If you found many tracks of the same
species of a prehistoric animal in an
area, what might you conclude about
that animal?
- It lived in groups (packs). There are a lot
of that species in that area.
2. Scientists found the abundance of a
certain fossil in similar-aged rock layers
around Earth suddenly decreased in the
rocks above those layers. How can you
explain the sudden decrease?
- The fossil species went extinct.
3. Compare and contrast relative and
absolute age.
- Both are used to date rocks and fossils.
Relative age states whether something is
older or younger than something else,
while absolute age will tell you the age in
exact years.
4. A geologist finds a cliff where the edges
of several rock layers are exposed.
Which layer is oldest and why?
- The bottom is the oldest. The Law of
Superposition says that the bottom rocks
are the first to form, thus the oldest.