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IT IS ESTIMATED THAT THE EARTH FORMED
ALONG WITH THE SOLAR SYSTEM 4.6 BILLION
YEARS AGO
GEOLOGIC TIME SCALE
A SUMMARY OF THE MAJOR EVENTS IN EARTH’S HISTORY
EON – largest segment of geologic time
ERA
PERIOD
EPOCH – smallest segment of geologic time
Dating Techniques
Either
relative or
absolute
Time is: a measured period during
which an event, process, or
condition exists or continues.
One way in which geologists look at
time: Relative Age
Relative Age of a rock…
Example:
My niece,
Amber, is
older than my
other niece,
Jordan.
What happened first?
What happened next?
…is its age
compared to
the ages of
other rocks.
Absolute Age of a rock…
Example:
My niece,
Amber, is 3
years old and
my niece,
Jordan, is 3
months old.
…is the
number of
years since
the rock
formed.
RELATIVE TIME
VS.
PLACES EVENTS IN A
SEQUENCE BUT DOES
NOT IDENTIFY THEIR
ACTUAL DATE OF
OCCURRENCE
ABSOLUTE TIME
IDENTIFIES THE ACTUAL
DATES OF GEOLOGIC
EVENTS
EXAMPLE
A LIST IN
CHRONOLOGICAL
ORDER OF WHAT YOU
HAVE DONE TODAY UP
UNTIL THIS TIME
THE EXACT TIMES AT
WHICH YOU DID
THESE THINGS
Law of Superposition - IN
UNDISTURBED
SEDIMENTARY ROCKS The oldest rocks are on the
bottom and the youngest rocks
are on the top.
Q#1
Principle of Superposition
Younger Rocks
Older Rocks
• Which
rocks are
older?
• Which
rocks are
younger?
LAW OF CROSS-CUTTING -- AN IGNEOUS
INTRUSION IS YOUNGER THAN THE ROCK IT
HAS INTRUDED INTO.
Principle of Cross-Cutting
• You can't cut
something until that
something exists,
therefore the cut has
to be younger.
• So faults are younger
than the rock it cuts
through.
Volcanic vents cutting across
sedimentary rock layers Which is youngest?
Q#2
Principle of Cross-Cutting
• What is
younger?
• The rocks
or the
fault line?
Principle of Inclusions
The inclusion is always younger
than the rock layers around and
beneath it.
Q#3
Principle of Inclusions
Which is older? The rock layer or
the rocks included in the layer?
LAW OF INCLUDED FRAGMENTS - IF FRAGMENTS
OF ONE TYPE OF ROCK ARE FOUND IN ANOTHER
ROCK LAYER THE ROCK FRAGMENTS MUST BE
OLDER THAN THE ROCK LAYER IN WHICH THEY
ARE FOUND
FAULTED AND FOLDED LAYERS - LAYERS OF
ROCK THAT HAVE BEEN FAULTED OR FOLDED
MUST HAVE BEEN PRESENT BEFORE THE ACTIONS
OF FAULTING OR FOLDING TOOK PLACE
FOLDING
FAULTING
Folding and Faulting
Folding
Faulting
UNCONFORMITY- A PLACE IN THE ROCK RECORD WHERE
LAYERS OF ROCK ARE MISSING BECAUSE OF UPLIFT AND
EROSION. THE RESULT CAN BE A LARGE AGE
DIFFERENCE BETWEEN THE ROCKS ABOVE AND THOSE
BELOW THE EROSIONAL SURFACE (IT APPEARS LIKE A
SQUIGGLY LINE IN A CROSS-SECTION)
Where is the youngest rock?
How
do
you
know?
What rock layer is the oldest?
How
do
you
know?
Which is younger?
The fault or the rock layers?
FAULT
How
do
you
know?
PLACE THE FOLLOWING EVENTS IN ORDER STARTING
WITH THE OLDEST.
E
L
B
I
G
K
M
F
A
C
D
H
J
J H C A F D K G I M B E L
Using Fossils to Date Rocks
•
•
•
•
Give relative age to a layer of rock
Match layers of rock at other locations
Use index fossils to match rock layers
Index fossils are distributed in
different areas & only occur in one area
A FOSSIL IS ANY EVIDENCE OF EARLIER LIFE
PRESERVED IN THE ROCK
ORIGINAL REMAINS (RARE) – THE ACTUAL
UNCHANGED REMAINS OF THE PLANT OR
ANIMAL ARE PRESERVED.
REPLACED REMAINS – THE SOFT PARTS OF THE
ORIGINAL ANIMAL HAVE DISAPPEARED AND THE
HARD PARTS HAVE BEEN REPLACED BY MINERAL
MATERIAL. (PETRIFIED WOOD)
MOLDS AND CASTS – FOSSIL SHELLS OR BONES ARE
DISSOLVED COMPLETELY OUT OF THE ROCK LEAVING
A HOLLOW DEPRESSION IN THE ROCK. NEW MINERAL
MATERIAL FILLS THE MOLD IT FORMS A CAST OF THE
ORIGINAL FOSSIL.
TRACE FOSSILS – EVIDENCE OF
LIFE OTHER THEN REMAINS,
WHICH INCLUDES ANY
IMPRESSIONS LEFT IN THE ROCK.
(TRAILS, FOOTPRINTS, TRACKS,
BURROWS)
INDEX FOSSIL
EASILY IDENTIFIABLE
SHORT-LIVED
WIDESPREAD OCCURRENCE
MEASURING ABSOLUTE TIME
TREE RINGS
EACH RING REPRESENTS A SINGLE
YEAR (SPRING/FALL) THE WIDTH OF THE
RING DEPENDS UPON THE
TEMPERATURE AND RAINFALL
RADIOACTIVE DATING
USED TO DATE FAR BACK IN TIME. CERTAIN ROCKS CONTAIN
RADIOACTIVE ISOTOPES
RADIOACTIVE ISOTOPES ARE ATOMS OF
ELEMENTS THAT GIVE OFF RADIATION FROM
THEIR NUCLEI
RADIOACTIVE DECAY IS THE PROCESS BY
WHICH A RADIOACTIVE ISOTOPE CHANGES
INTO A NEW STABLE ELEMENT
RADIOCARBON DATING USES THE RADIOACTIVE
ISOTOPE CARBON-14 FOUND IN ALL LIVING THINGS.
BECAUSE CARBON-14 IS CONTINUALLY ABSORBED BY FOOD
AND WATER IT STAYS CONSTANT IN LIVING THINGS. WHEN
THE LIVING THING DIES THE PERCENTAGE OF CARBON-14
DECREASES AT THE RATE OF ITS HALF-LIFE. CAN BE USED
TO DATE BACK ABOUT 100,000 YEARS