Notes: Relative Ages of Rock
Chapter 7, Lesson 1
Modern Geology
• James Hutton was the first person to realize that
one process formed rock and another process tore
it down.
Uniformitarianism
• Scientists can observe the processes that are active
today, and interpret what happened in the past.
Uniformitarianism
• Uniformitarianism states that the same Earth
processes have been at work for a very long time.
• Geological processes that are at work today were also
at work in the past.
• Geological processes are so slow that direct observation
is not possible.
The Rock Cycle
• The rock cycle is a series of processes that make and
change rocks through:
•
•
•
•
•
•
•
heating
melting
cooling
uplift
weathering
burial
increasing pressure
The Rock Cycle
The Rock Cycle
Three Major Types of Rocks
• Igneous Rocks - produced when magma solidifies
Three Major Types of Rocks
• Metamorphic Rocks - any rock that is put under
extreme pressure or heat
Three Major Types of Rocks
• Sedimentary Rocks - form from compacted and
cemented sediments
Sediment Formation and Layering
• Sedimentary rocks form from preexisting rocks.
• Four steps in the formation process:
1.
2.
3.
4.
Weathering
Transportation
Deposition
Lithification
Sediment Formation and Layering
1. Weathering
• Weathering is the physical or chemical breakdown
of rocks into smaller pieces.
1. Weathering
• Physical weathering breaks down rocks without
changing the mineral composition.
1. Weathering
• Chemical weathering changes the mineral
composition of rocks.
1. Weathering
2. Transportation
• Transportation occurs when sediments move
downhill to lower areas and come to rest.
2. Transportation
• Clasts are different-sized
sediments (from large
boulders to microscopic
bits of rock) that require
different amounts of force
to move them.
3. Deposition
• Deposition occurs when sediment being
transported by water, wind, or a glacier slows
down or stops.
3. Deposition
• This usually happens in
low areas called
depositional
environments.
• Two characteristics are
parallel horizontal layers
and sorting.
4. Lithification
• Lithification occurs when older sediment layers
become compacted beneath younger layers.
• Mineral-rich liquids seep into the pore spaces between
the sediment grains.
• The water evaporates and the minerals are left behind
to cement the grains together.
Superposition and the Fossil Record
• Layers of rocks are called
strata.
Superposition and the Fossil Record
• 4 principles help geologists study strata and
interpret the rocks’ history:
1.
2.
3.
4.
Superposition
Original horizontality
Original lateral continuity
Cross-cutting relationships
1. Principle of Superposition
• In a stack of undisturbed sedimentary rock layers,
the layers on the bottom were deposited before the
layers on top.
1. Principle of Superposition
• Relative age tells how old something is when
compared to something else.
1. Principle of
Superposition
2. Principle of Original Horizontality
• Rock layers are originally deposited in horizontal,
or nearly horizontal, layers.
3. Principle of Original Lateral Continuity
• Sedimentary rocks form layers that cover large
areas.
4. Principle of Cross-Cutting Relationships
• A layer or feature that cuts
across other rock layers is
younger than the layer(s)
being cut.
Principles
Fossils and Relative Age
• Geologists keep track of which fossils came from
which strata and apply the principle of
superposition.
• Fossil occurrences in layers are used to confirm or
assign relative ages to rock strata.
Fossils and Relative Age
Fossils and Relative Age
• With relative age, we compare the age of one rock
to the age of other rocks.
Example: “This rock is older than that rock.”
Fossils and Relative Age
• With absolute age, we know the number of years
since a rock formed.
Example, “This rock is 1 million years old.”
Fossils and Relative Age
•
•
Certain fossils, called index fossils, help
geologists match rock layers.
Index fossils are useful because:
1. They are widely distributed.
2. They tell the relative ages of rock layers in which
they occur.
3. They represent a type of organism that existed only
briefly.
Index Fossils
7.1 Relative Ages of Rocks
1.
2.
3.
4.
A
B
C
D
What principle states that processes
at work today are the same
processes that occurred in Earth’s
past?
A superposition
B relative age
C original lateral continuity
D uniformitarianism
7.1 Relative Ages of Rocks
1.
2.
3.
4.
A
B
C
D
What type of rock is formed when put
under extreme pressure or heat?
A igneous
B metamorphic
C strata
D sedimentary
7.1 Relative Ages of Rocks
1.
2.
3.
4.
A
B
C
D
What process slows or stops
sediments in low areas of the
landscape?
A deposition
B lithification
C weathering
D transportation
1.
2.
3.
4.
A
B
C
D
What term describes the physical or
chemical breakdown of rocks into
smaller pieces?
A deposition
B erosion
C lithification
D weathering
1.
2.
3.
4.
A
B
C
D
What principle states that the bottom
layers of sedimentary rocks were
deposited before the top layers?
A lithification
B uniformitarianism
C superposition
D original horizontality
1.
2.
3.
4.
A
B
C
D
What process includes heating,
melting, cooling, uplift, weathering,
and increasing pressure?
A sediment formation
B metamorphic rock formation
C igneous rock formation
D the rock cycle
SCI 4.c
1.
2.
3.
4.
A
B
C
D
What does the principle of original lateral
continuity state?
A layers on the bottom are deposited
before layers on the top
B sediments are deposited horizontally
C sedimentary rocks form layers that cover
large areas
D sediments always remain horizontal
1.
2.
3.
4.
A
B
C
D
Which type of rock is the most
useful for relative dating?
A igneous
B sedimentary
C magma
D metamorphic
1.
2.
3.
4.
A
B
C
D
The relative age of something is
_____.
A its age in relation to Earth
B its age in comparison to other things
C its age since it was discovered
D its age since it died
Igneous
Sedimentary
Metamorphic