7. Law of Superposition
EQ: How do fossils provide a record of Earth’s
geologic history?
 Learning Target: Students will be able to describe
the Law of Superposition.
 Learning Task: Task: I will practice using Law of
Superposition to determine the relative age of
rocks layers. I will identify index fossils within rock
layers.

Warm-up: Tuesday 4/8/14
1. What parts of
an animal
become fossils?
2. How are molds
and cast fossils
alike?
3. How are mold
and cast fossils
different?
Record your responses at
www.goo.gl/vta7d1
Intro to Stratigraphy


Stratigraphy is a branch of geology which studies
rock layers and layering
A key idea in Stratigraphy is the Law of
Superposition: the idea that lower rock layers are
older than the upper rock layers
Intrusion and
extrusion
Clues From
Igneous Rock
Lava that cools at
the surface is called
an extrusion. Rock
below an extrusion
is always older.
Magma that cools
beneath the surface
is called an
intrusion. An
intrusion is always
younger than the
rock layers around
an beneath it.
Which layer would be the oldest?



A
B
C
A
B
C
Which rock layer would be the
youngest?



A
B
C
A
B
C
4th Quarter
Table of Contents
1
Title
Assignment #
Ice Cores
2
Ice Core Webquest
3
Fossil Notes
4
Getting into the Fossil Record
5
Fossil Vocab
6
Fossils over Time (Molds vs Casts) 7
Law of Superposition Vocab
8
Who’s on First? (Wkst)
9
Law of Superposition VOCAB


Law of superposition – the idea that rocks and fossils found in lower layers
are older than the rocks and fossils found in the top layers
relative dating – a method of dating rocks which can’t give us an actual age,
but can tell us whether a rock layer is older or younger than another

strata – a certain rock layer

unconformity - gaps in rock layers and/or the fossil record




8
intrusion - a rock layer that forms when magma hardens beneath Earth’s
surface
extrusion - a rock layer that forms when lava flows onto Earth’s surface and
hardens
radiometric dating – using radioactive elements found naturally in substances
in order to determine how old they are.
absolute age – the actual age of a rock or fossil
Who’s on First?




Today you will start an assignment to help you learn
how to determine the relative age of rocks and
fossils.
You need a partner, and each person needs their
own paper.
You will share a set of cards.
You must finish with the pink cards to get the green
cards!
Fossils and Superposition

What is a fossil?
The trace or remains of an organism
that lived long ago, most commonly
preserved in sedimentary rock

What is a superposition?
Younger rocks lie above older rocks if
the layers have not been disturbed
Relative Dating and Index Fossils

What is relative dating?
◦ Any method of determining whether an event or
object is older or younger than other events or
objects.

What is an index fossil?
◦ A fossil that is found in the rock layers of only
one geologic age and is used to establish the age
of the rock layers.
◦ Is found in rock layers
around the world, ex Trilobites
EX: GUM, ANE, TZR, RID, MAN, DOG, BYS, NEB

START WITH TZR
Set A – Pink Cards
On your desk, you have 8 pink cards with
nonsense letters placed on them.
 Your task is to determine what the
correct sequence of the letters are.
 You have two clues:

1. The card with the letters “C” and “T” is on
the bottom, or the oldest layer
2. Look for a card that has either a “T” or “C”
written on it for the second layer
This is one possible way to arrange the cards.
Questions:
DM
1.What letter is the oldest?
2.What letter is the youngest?
OXD
3.What letter showed up the most?
4.Which letters only showed up once?
ON
5.Which letters could be index fossils?
6.How did you know which was older: “M”
BN or “X”?
UBN
AU
CGA
CT
Set B – Green Cards
Obtain a set of Green Fossil Cards
 Clues:

1. The oldest layer is labeled SET B
2. Find a rock layer that has at least one of the
fossils you found in the oldest rock layer.
This rock layer will be younger because of
the presence of new fossils.
3. Extinction is forever! Once an organism
disappears from the sequence it cannot
reappear later
To think about…
What problems did you run into when
trying to arrange the fossils into the
correct sequence?
 Would this have been more difficult if you
did not know which layer was the oldest
to start the activity?
 Which organism is the most complex of
all the fossils and why?

Extension:


If you finish, you can complete the Skills Lab in our
book on page 330, and answer Questions 1-9.
Turn this in on Google DRIVE instead of paper.
EXIT