JOURNAL 24
 Which
rock is oldest?
 Which one is youngest?
THE ROCK
RECORD
TODAY’S LEARNING GOAL
We will determine the relative age of rock layers.
 We will explain the law of superposition.
 We will state the principle of uniformitariansm.

DETERMINING RELATIVE AGE
 James
Hutton (18th Century Scottish
physician) was a keen observer of geologic
changes.
 He helped determine that the Earth is
about 4.6 billion years old.
 He believed that studying the present is
the key to understanding the past.
UNIFORMITARIANISM
 Uniformitarianism-
a principle that
geologic processes that occurred in the
past can ben explained by geological
processes.
 Examples: Vocanism, erosion
 This principle of one of the basic
foundations of geology
 Geologists realize that although the
processes of the past and present are the
same, the rates of the processes vary
over time
EARTH’S AGE
 Before
Hutton, many people believed the
Earth was 6,000 years old and all geologic
features had formed at the same time.
 Hutton observed that forces that changed
land operated very slowly.
 He reasoned that millions of years must
be needed for those same forces to create
the complicated rock structures observed
in Earth’s crust.
 One way to learn about Earth’s past is to
determine the order in which rock layers
and other rock structures formed.
LET’S REVIEW WHAT WE’VE LEARNED
 What
evidence did Hutton propose
to who that the Earth is very old?
He observed that the forces that
changed on the land operated
very slowly. He reasoned that
millions of years must be needed
for those same forces to create
the complicated rock structures
in the Earth’s crust
RELATIVE AGE
 Layers
of rock are called strata
 Strata show the sequence of events that
took place in the past and who the order
in which rock layers formed.
 Once scientists know the order in which
rock layers formed, they can determine
the relative age of each rock layer
 Relative Age- indicates that one layer is
older or younger than another layer but
does not indicate the rock’s age in years.
LAW OF SUPERPOSITION
LAW OF SUPERPOSITION
 Sedimentary
rocks form when new
sediments are deposited on top of old
layers of sediment.
 As the sediments accumulate they are
compressed and hardened into
sedimentary rock layers which are called
beds.
 The boundary between 2 beds is called the
bedding plane.
LAW OF SUPERPOSITION
 Scientists
use a basic principle called the
law of superposition to determined the
relative age of a layer of sedimentary rock.
 This law states that an undeformed
sedimentary rock layer is older than the
layers above it and younger than the
layers below it.
PRINCIPLE OF ORIGINAL
HORIZONTALITY
 Sedimentary
rock generally forms in
horizontal layers.
 The principle of original horizontality is
that sedimentary rocks left undisturbed
will remain in horizontal layers.
 Therefore, sedimentary rock layers that
are not horizontal have been tilted or
deformed by crustal movements that
happened after the layers formed.
PRINCIPLE OF ORIGINAL
HORIZONTALITY
\
PRINCIPLE OF ORIGINAL
HORIZONTALITY
 In
some cases, tectonic forces push older
layers on top of younger ones or overturn
a group of rock layers.
 In such cases, the law of superposition
cannot be easily applied.
Grading beds
 Cross beds
 Ripple marks

GRADING BEDDING
 One
clue to the original position of rock
layers in the size of the particles in the
layers.
 In some environments, the largest
particles of sediment are deposited in the
bottom layers.
 The arrangement of layers in which
coarse and heavy particles are located in
the bottom layers is called graded
bedding. If larger particles are located in
the top layers, the layers may have been
overturned by tectonic forces.
CROSS BEDS
 Another
clue to the original position of
rock layers is in the shape of the bedding
planes.
 When sand is deposited, sandy sediment
forms curved beds at an angle to the
bedding plane.
 These beds are called cross-beds.
 The tops of these layers commonly erode
before new layers are deposited.
 So, the sediment appears to be curved at
the bottom of the layer and to be cut off at
the top.
CROSS BEDS
RIPPLE MARKS
 Ripple
marks are small waves that form
on the surface of sand because of the
action of water or wind.
 When the sand becomes sandstone, the
ripple marks may be preserved.
 In undisturbed sedimentary rock layers,
the crests of the ripple marks point
upward.
RIPPLE MARKS
CROSSCUTTING RELATIONSHIPS
 When
layers have been disturbed by
faults or intrusions, determining relative
age may be difficult.
 A fault is a break or crack in the Earth’s
crust along which rocks shift their
position.
 An intrusion is a mass of igneous rock
that forms when magma is injected into
rock and then cools and solidifies.
CROSSCUTTING RELATIONSHIPS
 The
law of crosscutting relationships
is that a fault or igneous intrusion is
always younger than the rock layers it
cuts through.
 If a fault or intrusion cuts through an
unconformity, the fault or intrusion is
younger than all the rocks it cuts
through above and below the
unconformity.
CROSSCUTTING RELATIONSHIPS
VOCABULARY SQUARES
Law of superposition
 Graded bedding
 Cross-beds
 Ripple marks
 Law of crosscutting relationships

TODAY’S LEARNING GOAL
We will determine the relative age of rock layers.
 We will explain the law of superposition.
 We will state the principle of uniformitariansm.

REVIEW
 Which
rock is oldest?
 Which one is youngest?