Please get out your notebook!!
Draw a chemical equation and label both
sides in your Daily Question section
Relative and Absolute Ages

What is
relative
age?
The relative age of a rock is its
age compared to the ages of other
rocks.
– Example: Comparing your age
with someone else’s age.
– When you say that you are older
than your brother but younger
than your sister, you are using
relative age.
Relative and Absolute Ages

What is
absolute
age?
The Absolute age is the
number of years since the rock
formed.
Paleontology

The study of fossils
The Position of Rock Layer
How do
scientist
determine
the age of
sedimentary
rocks?
 Geologist
use the Law of
Superposition to determine
the relative ages of
sedimentary rocks.
The Law or Superposition
 According
to the Law of
Superposition, in horizontal
sedimentary rock layers the
What is the
oldest layer is at the
law of
bottom.
Superposition?
 Each higher layer is
younger than the layers
below it.
The Law or Superposition
walls of the Grand Canyon in
Arizona illustrate the law of
superposition.
younger
 The
Other clues to Relative Age
How else
can you
find the
age of
rocks and
fossils?
Geologists
use clues
from extrusions and
intrusions for
igneous rock and
faults.
Other clues to Relative Age
 Clues
rocks:
from igneous
–Extrusions – Lava
that hardens on the
surface.
–The rock layers
below an extrusion
are always older
than the extrusion.
Other clues to Relative Age

Clues from igneous rocks:
– Intrusions – Beneath the surface, magma
may push into bodies of rock, cool then
harden into a mass of igneous rock.
– An intrusion is always younger than the
rock layers around and beneath it.
Intrusion:
Newer Rock
Other clues to Relative Age
 Clues
From Faults:
– Faults – is a break
in Earth’s crust.
– Forces inside Earth
cause movement
of the rock on
opposite sided of
the fault.
– A fault is always
younger than the
rock it cuts
through.
Using Fossils to Date Rocks
How can
fossils be
used to
date
rocks?
 To
date rocks geologists
first give a relative age to a
layer of rock at one
location.
 Then they can give the
same age to matching
layers of rock at other
locations.
 Certain fossils, called index
fossils, help geologists
match rock to layers.
Trace Fossil
Imprint: soft part such as a leaf or
feather.
 Trace: such as a footprint
 Found in sediment, made up of small
loose pieces of rock or sand.
 Sedimentary rock: rock formed from
layers of compacted sediment.

Using Fossils to Date Rocks
What are
index
fossils?
 Index
Fossils - are fossils
that must be widely
distributed and represent a
type of organism that
existed only briefly.
 Index fossils are useful
because they tell the
relative ages of the rock
layers in which they occur.
Using Fossils to Date Rocks
 Geologists
use particular types of
organisms as index fossils.

For example:
– Trilobites were a group of hard – shelled animals whose
bodies had their distinct parts.
– Evolved in shallow seas more than 500 million years
ago.
– Over time, many different types of trilobites appeared.
– They became extinct about 245 million years ago.
– Trilobite fossils have been found in many places.
Using Fossils to Date Rocks

You can use index fossils to match rock
layers.