Determining the Age of Rocks and Fossils
_____________
Dating and
_______ Dating
● Scientists use ________and
__________ to try and date
events, or determine when in
history events took place on
earth
● Scientists use different
techniques of dating:
○ Relative Dating
○ Radioactive Dating
Radioactive (Radiometric) Dating
How Radioactive (radiometric) dating works:
● Geologists use ________________dating to estimate
how long ago rocks formed
● Knowing how old a ____________layer is helps to
determine the ___________ of fossils contained
within the rocks
● Gives more of an ____________age (can give an
estimate of about how old something is)
● Measured in age in ___________
● The universe is full of naturally occurring radioactive
_______________
● Over time radioactive “parent atoms” ___________into
stable “daughter atoms.”
● When molten rock cools, it forms igneous rock
● ______________rock contains radioactive atoms
How Radioactive (radiometric) dating works
● Radioactive atoms decay at a ________________rate
● By comparing the quantity of unstable _________left in
a rock and comparing it to the quantity of stable
daughter atoms in the rocks, scientists can estimate the
amount of time that has passed since the rock formed.
● We can get more of an absolute date based on
radiometric dating (more accurate than relative dating)
● ______________dating is an example of absolute
dating
Relative Dating (order of events)
● When absolute age (radioactive dating) is
not possible, scientists use relative age
● ______________age describes the age of
an object or even in comparison to another
object or event
● It determines which events occurred
earlier or later than others
______________giving a definite date
● Use words like “___________ or
______________” to describe the ages
Index Fossils (used in relative dating of rocks)
● Not all fossils are considered index fossils
● In order for a fossil to be considered an index fossil, it must:
1. Be ________________distributed (not just found in a
small area)
2. Many fossils of the organism must be found in rock layers
3. Represent a type of organism that existed only
________________
● Index fossils are useful because they tell the relative ages of
the rock layers in which they occur
Law of Superposition (a type of relative dating)
● States that in undisturbed layers
of ___________________rock,
______________layers of rock
lie beneath
______________rock layers
● Older sediments must be laid
down first before the younger
ones pile on top
Index Fossil Example: Trilobites
● Hard-shelled animals whose body
had three sections
● Lived in shallow seas and became
extinct about 245 million years ago
● Therefore, if a trilobite is found in a
particular rock layer, it can be
compared with trilobites from other
layers to _________________the
age of the layer of
____________that it was found in
Disturbing Rock Layers
Unconformity
● Forces within Earth can disturb
rock layers
● Examples of features that
disturb these layers:
○ ______________- a break in
the rocks due to movement
of on either side of the fault
○ ________________- magma
(molten rock) pushes through
existing rock layers
● A gap (missing part) in the geological
record
● Sometimes a __________can be
_______________in one area of rock
bed
● Usually the result of
_____________and nondeposition
(the sediments did not get deposited)
● Unconformities make it more difficult
to understand how earth changed at
a specific time
Intrusions are younger than the
rock layers that they cut through
Fossils Provide Clues to Earth’s Past
Throughout History
● Fossils can be used to determine the
geological history, or age, of the rock that
contains them
● Younger fossils may be found where the
original boundary was an
ocean/continent
● _____________fossils are found where
plates collided, formed mountains, and
are now eroding
● Throughout history numerous
species of organisms have gone
_______________
● Some scientists estimate that
___________of all species that
have ever lived on earth are now
extinct
● Average species live for 2 to 10
million years
Mass Extinctions
Extinction - Causes
● Occurs when ________numbers
species die out in a
____________amount of time
● Mass extinctions have occurred many
times in Earth’s history
● Each event has wiped out more than
50% of species
● Have _________________in rock
layers
● Abundant fossils suddenly disappear
● Increase in ____________
● Increase or decrease in
rainfall
● _____________ eruption
● Flooding or drought
● Shifting land or sea
● A ____________in food
supply
● A new predator or disease