Transcript Rock and fossil record

The Rock and Fossil
Record
Chapter 6
Relative Dating
• What is it?
– Figuring out whether an object or event is older or
younger than other objects or events is called
relative dating.
– Comparing the age of a rock to the rocks around it.
– In other words, you are estimating the age of a rock
in relation to other rocks.
The Principle of Superposition
• Younger rocks on top and
Older rocks on bottom!
C
 Youngest
B
A
 Oldest
It is not just that easy….
• Not all rock sequences are arranges with the oldest
layers on the bottom and the youngest layers on
the top…
• There are disturbances!
• The forces with in Earth can push other rocks into
sequences
– Tilt or fold rock layers!
– Break sequences into parts that move!
– Some sequences are even upside down!
• There is a valuable tool to help us
with this confusion…
… The Geologic Column!
• What is it?
– An arrangement of rock layers in which the oldest
rocks are at the bottom
– Geologists rely on the geologic column to
interpret the order of rocks
So, is there a place on Earth that has a
continuous sequence of all the rocks formed
throughout history?
NO!
NO… ???
• There is no single location on Earth
that has a continuous sequence of all
the rocks in order
• Scientists have put different rock
layers in order to form the Geologic
Column to determine the relative age
of rocks and other objects!
Let’s Talk about the Disturbances!
• Read P. 158 – 159
Missing Evidence
• Missing rock layers create breaks in rock-layer sequences =
unconformities!
• Unconformity is a break in the geologic record created when rock layers
are eroded or when sediment is not deposited for a long period of time
– In other words, a rock layer that is partially missing due to erosion
and is always younger than the rock layers beneath it!
You just look for
what is missing!
Try this one!
 Did you get it? 
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6
Absolute dating
• The process of establishing the age of an object by
determining the number of years it has existed is
called absolute dating
• You can figure out the absolute age of a rock if you
know the rate of decay for one of the elements in the
rock
• Determining that age is called radiometric dating
• A half-life is the time it takes one-half of a radioactive
sample to decay
Petrified Remains
• Hard and rock like
• Original material replaced by minerals
• Ex. bones, wood
Carbonaceous Films
• Burial forces gases and
liquid out
• Forms thin film of carbon
residue
• Outline of organism
• Ex. plants
Mold & Cast
• Organism is buried
• Outside decays leaves a
cavity = mold
• Sediment fills in mold and
hardens to rock = cast
The cast looks like the organis
Original Remains
•
•
•
•
actual or part of organism
trapped in Amber – insects
frozen – woolly mammoth
tar pits
• What is a tar pit???
Los Angeles
La Brea Tar Pits
This is a
recreation of a
tar pit that
illustrates how
animals would get
trapped in tar pits.
Trace Fossils
• evidence of animal activity
• tells how organism lives
• Ex. tracks, worm holes
Worm tracks!
↓
These are dinosaur foot prints!!
Science
Do Now: 10-20
HW: COMPLETE THE FOSSIL RECORD ACTIVITY
W/QUESTIONS ON THE BACK
1.Clear your table
TODAY
~ Fossil Record activity
~ candy update
Index Fossils
• Used to date other fossils and rock layers.
• Fossils of animals that
• Existed for a short period of time
• Were abundant, there were many of them
• Lived all over the world
EUOMPHALUS
But wait…. There’s more….
TRILOBITE