Law of Superposition

RELATIVE DATING-

GEOLOGIC SEQUENCES

GEOLOGIC SEQUENCES

⦿ Geologists study layers of

rock exposed at the surface
and infer the order in which
the layers formed.

⦿ Relative dating is the

method that geologists use
to place rocks in
chronological order.

⦿ Geologic Sequence =

profile (side) view of rock
layers

LAW OF SUPERPOSITION

⦿ Superpostition explains that

undisturbed older rocks are
on the bottom and younger
on top.

⦿ States that in an undeformed

sequence of sedimentary
rocks, each layer (strata) is
older than the one above it
and younger than the one
below it.

⦿ Shows what events happen

in time (1st, 2nd, & so on)

LAW OF SUPERPOSITION

In a series of sedimentary rocks,
the bottom layer is the oldest and
the top layer is the youngest.

⦿ Lower layers must be placed

before younger rocks can be
deposited on top of them.

⦿ Exception: when something

occurs to overturn layers (such
as inclusion or cross-cutting)

5

Grand Canyon

7

Law of Superposition

■Fossils are generally the same age as the rock
layers in which they are found

■Animal remains are deposited along with the
sediments that will turn into sedimentary rocks

8

Finish sentence

Draw a picture

Explain how this helps
geologists determine age
of rocks

Superposition states
that …

undisturbed
older rocks are
on the bottom
and younger on
top

B

C younger

A older

shows what
events happen
in time 1st, 2nd,
& so on…

Multiple Choice

Which layer is the youngest?

• A

• B

• C

• D

Multiple Choice

Which statement is true?

• A is older than B

• C is younger than D

• D is the youngest

• B is older than C

PRINCIPLE OF ORIGINAL HORIZONTALITY

⦿ States that most rock materials are deposited in horizontal

layers. (Grand Canyon)

⦿ Rocks form in flat layers. If they are bent and/or tilted, the

layers must have done so after the rock layers had formed.

⦿ Shows which came 1st, 2nd, etc.

Older
Younger

12

1
3

Finish sentence

Draw a picture

Explain how this helps
geologists determine age
of rocks

Original Horizontality
states that…

most rock
materials are
deposited in
horizontal layers

shows which
came 1st, 2nd,
etc

UNCONFORMITIES

Sometimes rock layers are
missing from a geologic
sequence as the result of
erosion.

An unconformity is where
rock has eroded away
and has been buried by
newer rocks.

Shows a gap in time in
the geologic sequence
because some rock layers
have been lost to
erosion.

*Unconformities

There are three types of unconformities.

1. An unconformity in which stratified (layers) of

rock rests upon unstratified rock is called a
nonconformity.

2. The boundary between a set of tilted layers

and a set of horizontal layers is called an
angular unconformity.

3. The boundary between horizontal layers of old

sedimentary rock and younger, overlying
layers that are deposited on an eroded

surface is called a disconformity.

According to the Law of Superposition, all rocks beneath an unconformity are older than the rocks above the
unconformity.

1
6

1
7

Finish sentence

Draw a picture

Explain how this helps
geologists determine age
of rocks

Unconformity is…

where rock has
eroded away and
has been buried
by newer rocks.

shows a gap in the
geologic
sequence because
some rock layers
have been lost to
erosion.

EROSIONAL SURFACE

⦿ If the erosion is not buried and is exposed at the

surface of the Earth, it is not called an
unconformity, it is simply an erosional surface.

Multiple Choice

All unconformities include ____.

• uplift

• superposition

• tilting

• erosion

CROSS-CUTTING RELATIONSHIPS

⦿ When rock layers have been disturbed by faults

or intrusions, determining age may be difficult.
The Principle of Cross-Cutting Relationships can
be used.

⦿ Cross-cutting relationships are if one geologic

feature cuts across another feature, the cutter
is younger.

⦿ Helps geologists determine age of intrusion or

fault in relation to surrounding rock.

CROSS-CUTTING RELATIONSHIPS

⦿ Fold – the rocks have to be there first in order to

be folded, so a fold is always YOUNGER than the
rocks that were folded. Sometimes a fold is called
a tilt if the layers are just tilted off the horizontal

⦿ In the example below, layers 1-4 were deposited

first, then they were all folded.

CROSS-CUTTING RELATIONSHIPS

⦿ Fault – the rocks have to be there first in order

to be faulted, so a fault is always YOUNGER
than the rocks that were faulted

⦿ In the example to the right, the limestone,

sandstone, and shale were deposited first, then
they were all faulted.

PRINCIPLE OF CROSS-CUTTING RELATIONSHIPS

⦿ States that a fault or

intrusion must be
younger than any
geologic formation
through which it cuts.

(A geologic dike is a

flat body of rock that
cuts through another
type of rock. Dikes
cut across the other
type of rock at a
different angle than
the rest of the
structure)

CROSS-CUTTING RELATIONSHIPS

⦿ Igneous Intrusion/Extrusion=magma coming

up through rock layers. An intrusion/extrusion
is always YOUNGER than the rocks that it cuts
through.

◼ It is an intrusion if magma stays

underground (cools slowly and makes large
crystals).

◼ It is an extrusion if it becomes lava at

the surface (cools quickly and makes
small crystals).

25

https://www.youtube.com/watch?v=
fmJx5xKu2vI

https://www.youtube.com/watch?v=fmJx5xKu2vI

2
6

Finish sentence

Draw a picture

Explain how this helps
geologists determine age
of rocks

Cross-cutting
relationships are…

if one geologic
feature cuts
across another
feature, the cutter
is younger

determine age of
intrusion or fault
in relation to
surrounding
rock

younger than
rock layers

younger than
rock layers

27

Igneous Intrusion - Cross Cutting

2
8

Law of Superposition

■Igneous intrusions are younger than the
rock that they cut through or flow out of

2
9

Finish sentence

Draw a picture

Explain how this helps
geologists determine age
of rocks

Intrusions are…

younger than
the rock that
they cut through
or flow out of

determine age of
intrusion
compared to
surrounding
rock

Multiple Choice

Which geologic feature is the youngest?

• 1

• 2

• 3

• 4

Multiple Choice

Which statement is true?

• 1 is younger than 4

• 5 is the youngest feature

• 3 is younger than 5

• 4 is older than 2

3
1

Correlation

■Matching similar
rock layers in
different locations
to see if they
formed at the
same time

3
3

Finish sentence

Draw a picture

Explain how this helps
geologists determine age
of rocks

Correlations are…

matching rocks
& fossils from
separate
locations

used to build the
geologic time in
each area

Fossils

■ Preserved remains or traces of

animals, plants, and other organisms
from the remote past

■ Generally only hard parts get preserved

■ Bones, teeth, shells

■ Fossil Record – History of life as

documented by all fossils, preserved in
sedimentary rock strata

■ Provides a snapshot of past events in

Earth’s geological history

Geologic Time Scale

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*Index Fossils

Index Fossils are fossils that occur in
only in rock layers of a particular
geological age.

Requirements:

1. Abundant
2. Wide geographic distribution
3. Lived a short time (a few million years)
4. Easily recognizable (ex. trilobites &

ammonites)

Helps geologists infer layers of similar
age

■Easily recognizable

■Abundant

■Wide geographic distribution

■Live a short time

Fossil Record and
Rock Strata

■ Index fossils are used to

correlate the age of the rock
strata

■ If two different rock strata in

different areas on Earth
contain the same index fossils,
then the strata are probably
the same age.

Geologic Time Scale

Which organism would

make the best index

fossil?

3
7

Finish sentence

Draw a picture

Explain how this helps
geologists determine age
of rocks

Index Fossils are…

fossils that
occur only in
rock layers of a
particular
geological age

help tell the
relative ages of
the rock layers
in which they
occur

X X X index

fossils

o V v O
o V v O

o V v O

3
8

DEFINE

EXAMPLE

HOW GEOLOGISTS
USE

Relative Dating is
used to…

indicate that
one layer is
older or
younger than
another layer

arrange
geological
events in
sequence

the rock is
younger than
layer below it

3
9

DEFINE

EXAMPLE

HOW GEOLOGISTS
USE

Absolute Dating

give age of rock
as the # of years
since the rock
formed

determine how
much time has
passed since
rocks formed

the rock is 55
million years old

4
0

Compare/Contrast

Similarities

Differences

both determine
age of rock
layers & used to
determine
history of Earth

relative -
compares ages
to other layers;
not specific
absolute-
specific # of
years since
formed

Multiple Choice

____ is used to indicate that one layer is older or younger than another layer.

• relative dating

• absolute dating

• index fossils

• superposition fossils

Multiple Choice

____ is used to give age of rock as the # of years since the rock formed.

• relative dating

• absolute dating

• index fossils

• superposition fossils