CHAPTER 8 DETERMINING EARTH’S AGE RELATIVE AND ABSOLUTE ROCK AGES Loulousis
CHAPTER 8
DETERMINING EARTH’S AGE
RELATIVE AND ABSOLUTE
ROCK AGES
Loulousis
Bellringer
What are 5 visual clues that help you determine if
someone is older or younger than you?
Color of hair
Wrinkles in skin
Height
General demeanor
Style of dress
8.1 Determining Relative Age
Objectives Explain the principle of uniformitarianism and compare
the three types Explain how the law of superposition can be used to
determine relative age of rocks. Apply the law of crosscutting relationships to determine
the relative age of rocks. Explain the importance of the law of original horizontality
Uniformitarianism
a principle that states geologic processes that
occurred in the past can be explained by current
geologic processes
James Hutton (18th century) theorized that the same
forces that change Earth’s surface now, such as
volcanism and erosion, are the same forces that
were at work in the past.
Earths Age
Until Hutton scientists thought Earth was only 6,000 years old, and that all Earth’s geologic features had formed at the same time. (its actually 4.6 billion yrs. old)
Hutton argued that the changes on his farm operated very slowly and must have taken millions of years
Hutton’s ideas about uniformitarianism encouraged
other scientists to learn more about Earth’s history.
Stratigraphy
Stratigraphy- The branch of geology that seeks to
understand the geometric relationships between
different rock layers (called strata), and to interpret
the history represented by these rock layers.
Stratigraphy
Geologists try to determine the order in which
events have happened during Earth’s history They rely on rocks and fossils to help them in their
investigation
Relative dating Process of determining whether an event or object is
older or younger than other events or objects
Relative Age
the age of an object in relation to the ages of other
objects
indicates one layer is older or younger than another but
does not indicate the rock’s actual age in years
Stop and Think
How does sedimentary rock form?
Sedimentary rocks form when new sediments are
deposited on top of old layers of sediment
Scientists study the layers in sedimentary rocks to
determine relative age
1. Law of Superposition
Layers of sedimentary rock, such as the ones shown
below, are stacked like pancakes
As you move from top to bottom in layers of
sedimentary rock, the lower layers are older
Law of Superposition -states that younger rocks lie
above older rocks, if layers have not been disturbed
Stratum (strata) -A horizontal
layer of material, especially
one of several parallel layers
arranged one on top of
another.
Example of Relative Age Dating and Correlation
2. Law of Original Horizontality
Scientist know that sedimentary rock generally forms in horizontal layers
Original Horizontality Law states that sedimentary rocks left undisturbed will remain in horizontal layers
Not all rock sequences are arranged with oldest
layers on the bottom and youngest layers on top Some rock sequences have been disturbed by forces
within Earth These forces can: Push other rocks into a sequence Tilt or fold rock layers Break sequences into moveable parts
The Geologic Column
Geologic column- An ideal
sequence of rock layers that
contains all the known fossils
and rock formations on
Earth, arranged from oldest
to youngest
Use column to:
Interpret rock sequences
Identify the layers in
puzzling rock sequences
Disturbed Rock Layers
Geologists often find features that cut across
existing layers of rock
They assign relative ages to the features and layers
The features must be younger than the rock layers
because the rock layers had to be present before
the features could cut across them
Four Types of Rock Disturbances
1. A fault
Break in Earth’s crust along
which blocks of crust slide
relative to one another
2. An intrusion
Molten rock from Earth’s
interior that squeezes into
existing rock and cools
3.Folding
Occurs when rock layers bend
and buckle from Earth’s internal
forces
4. Tilting
Occurs when internal forces in
Earth slant rock layers
Clues for Original Horizontality
1.Graded-bedding: size of particles in the layers 2. Bedding plane shape: Scientists can study the
shape of the cross-beds to determine the original position of the layers (beds are called cross-beds)
3. ripple marks: small waves that form on the surface of sand due to the action of water or wind If undisturbed crests of the ripple marks point upward
Unconformities
a break in the geologic record created when rock layers are eroded or when sediment is not deposited (nondeposition) for a long period of time
The missing layers create a break in the geologic
record, called an unconformity. Like missing pages in book
Types of Unconformities Disconformities Nonconformities Angular unconformities
Types of Unconformities
1.Disconformities exist where part of a sequence of parallel rock layer is missing
Types of Unconformities
2. Nonconformities exist where sedimentary rock
layers lie on top of an eroded surface of nonlayered
igneous or metamorphic rock
3. Angular unconformities exist between horizontal
rock layers and rock layers that tilted or folded
Law of Crosscutting Relationships
a fault or body of rock is younger than any other body of rock that it cuts through
List strata, the intrusion, and fault in order from
oldest to youngest based on relative ages.
Applying Law of Crosscutting and
Superposition
1. Place the geologic features in order from oldest
to youngest
A – conglomerate
B – Shale
C – Basalt
D – Limestone
E- Sandstone
F -fault
L- Limestone
A – Sandstone
B –Sandstone
C- Shale
F – Faulting
G – Magma instrusion forms Granite
Bellringer
1. Explain why it is important for scientists to be
able to determine the relative age of rocks.
2. State the principle of uniformitarianism in your
own words.
8.2 Determining Absolute Age
Objectives Summarize the limitations of using the rates of erosion
and deposition to determine the absolute age of rock formations.
Explain how varves are used to determine absolute
age Explain how the process of radioactive decay can be
used to determine the absolute age of rocks. Identify four types of radioactive decay
Absolute Age
Absolute dating- any method of measuring the age of an event or object in years
absolute age -the numeric age of an object or event, often
stated in years Scientists use a variety of ways to determine absolute age Varves Radioactive decay
Erosion rate and deposition rate are limited to short time frame (10,000 years) and not always accurate
because rates can change over time
Varves
Varve- banded layer of sand and silt
deposited annually in a lake
Like counting tree rings
Usually in glacier lakes
Summer snow/ice melts quickly and in
winter freezes quickly making a
permanent layer of sediment
Radioactive Decay
Radiometric dating - determining the absolute age of a
sample, based on the ratio of parent to daughter
material
If you know rate of decay for element in a rock you can
figure out the absolute age of rock
To determine the absolute ages of fossils and rocks
scientists analyze decay of radioactive element isotopes
Isotopes- atoms of the same element that have the same
number of protons but different number of neutrons
Radioactive Decay
Most isotopes are
stable, meaning they
stay in their original
form
Other isotopes are unstable and are called
radioactive
Radioactive decay- radioactive isotopes tend to
break down into stable isotopes of the same or
other elements
How does it work? decay
Radioactive decay occurs at a steady rate so scientists
can use the relative amounts of stable and unstable
isotopes present in an object to determine the objects
age
Parent isotope – the unstable radioactive isotope
Daughter isotope – stable isotope produced by the
radioactive decay of the parent isotope
Rate of radioactive decay is constant
So scientists can compare the amount of parent material with the amount of daughter material to date rock
The more daughter material there is the older the rock
Half-life – the time needed for half of a sample of
radioactive substance to undergo radioactive decay
After every half-life, the amount of parent material
decreases by one-half
Four Radiometric-dating techniques
1. Potassium-Argon Method
Potassium-40 has a half-life of 1.3 billion years,
argon is daughter material
Used to date rock older than 100,000 years
2. Uranium-Lead Method
Uranium-238 has half-life of 4.5 billion years,
daughter material is lead- 206
Used to date rocks more than 10 million years old
3. Rubidium-strontium Method
Rubidium-87 forms daughter material of strontium-87
Half-life is 49 billion years, used on rocks older than
10million years
Carbon-14 method
Plants and animals contain constant carbon-14 which
decreases when dead
Half-life is 5,730 years
Used on things that lived within the last 50,000 years
8.3 Fossils
Objectives
Identify 5 major types of fossils and explain how
they form
Describe what an index fossil is and why geologist
can use them in absolute dating
8.3 Fossil Record
Fossil- preserved remains of a once-living organism
(usually in sedimentary rock)
Fossil information
Clues about organisms (body, lifestyle, evolution..)
Evidence about how Earth’s surface has changed
Understand past environments were like
Types of Fossils
5 Main Types
Molds and Casts
Petrified Fossils
Carbon Films
Trace Fossils
Preserved Remains
Other Types
Mummification
Coprolites- fossilized dung or waste
Molds and Casts
Mold forms when hard parts of an
organism are buried in sediment
Hard parts completely dissolve and
leave behind a hollow area with
organisms shape
Cast forms as the result of a mold
Water with dissolved minerals and
sediment fills the mold’s empty
space making a cast
A cast is the opposite of its mold
Petrified Fossils
Petrified means “turning
into stone”
Form when minerals
replace all or part of an
organism
Water (full of dissolved
minerals) seeps through
sediment to dead organism
and evaporates leaving
hardened minerals behind
Carbon Films
All living things have carbon
Eventually after decay all
that’s left of an organism is
carbon
The thin layer of carbon left
behind can show an
organism’s delicate parts like
leaves on a plant
Trace Fossils
Show the activities of organisms
When animal makes footprint in sand or must and
over time sediment buries it becoming solid rock
Index Fossils
Fossil used to establish the age of rock layers because it is distinct, abundant, and widespread
Only existed for a short span of geologic time
Can help determine relative age of rock and estimate absolute age
Because lived short geologic time, the rock layer in which an index fossil is found can be dated accurately
Trilobite-lived from lower Cambrian
Period to Permian Period
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