The Age of Earth
and its Rocks
...and how we know!
Grade 8 Science
Mr. Banino & Mr. Carozza
Learning Target
I Can:
● ...appreciate and comprehend the
scale of Earth's history
● ...see causal relationships between
global events in Earth’s history
Earth History: Main Concepts
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●
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Formation of the Earth
Measuring time
Important events
Dating of rocks
○ Relative vs. Absolute ages
Activity: Making a Timeline of
Important Events in Earth's History
● placing mini-posters
● walking the timeline
When you come back:
● we’ll be in Mr. Carozza’s room
● Sit with your mini-poster collaborator
● bring your journals if you haven’t already got ‘em
Age of
the Earth
(cont’d)
...but before we start:
● hand in remaining books
● check printed mini-posters
Learning Target
I Can:
● ...understand the difference between
correlation and causation
Bring your journals & get on board!
at each “stop” you will have to:
● ask at least 1 question
● share at least 1 thought/response
Causation
(noun)
1. The action of causing or producing.
2. The relation of cause to effect;
causality.
Correlation
(noun)
The mutual relation of two or more
things, parts, etc.:
Studies find a positive correlation between severity
of illness andnutritional status of the patients.
Synonyms: similarity, correspondence, matching;
parallelism, equivalence
Correlation is NOT Causation
For any two correlated events, A and B, the
following relationships are possible:
● A causes B
● B causes A
● A and B are consequences of a common
cause, but do not cause each other
● There is no connection between A and B;
the correlation is coincidental
Correlation is NOT Causation
...but it is human nature to look for patterns,
even false ones!
Spurious correlations
Principle of Uniformitarianism
Formally:
The assumption that the same natural laws and processes
that operate in the universe now have always operated in
the universe in the past and apply everywhere in the
universe.
Informally:
The present is the key to the past!
Law of Superposition
Formally:
Sedimentary layers are deposited in a time sequence, with
the oldest on the bottom and the youngest on the top.
Therefore, in undisturbed strata, the
older material is always beneath the
younger
Video Interlude: Calculating Earth’s Age
Relative Dating
As we've seen, dating rocks (finding their
absolute ages) isn't easy!
But it's usually pretty easy to determine relative
(qualitative) ages--if it is older or younger than
another nearby rock.
Learning Target
I Can:
● ...estimate the ages of fossils and events
based on knowledge of Earth’s history
● ...estimate the ages of rocks and other
geological features using contextual
clues
There are 3 Rock Types or
"Families"
Igneous rocks
● formed from crystallization of a molten mixture of
minerals and dissolved gases
Sedimentary rocks
● formed from sediments (solid fragments of material that
have been transported and then deposited by air, water,
or ice)
Metamorphic rocks
● formed from other rocks that have been changed by
exposure to extreme heat, pressure, or chemical activity
Sedimentary Rock Characteristics
Sedimentary Rock: Characteristics
1. They form layers
○ Because sediments are deposited horizontally one
upon the other
○ This layering of strata or beds (called stratification or
bedding) is preserved during lithification
Sedimentary Rock: Characteristics
2. Their surfaces can show
distinctive features similar to
those on present-day sediments:
clues to past environments!
○
○
○
ripple marks from waves
in shallow water
cracks from drying mud
footprints/tracks
example at right:
lithified impressions of dried mud cracks with
a trekking pole for scale
Glacier National Park, MT
Sedimentary Rocks
Characteristic Surface Features
Example: Ripples formed in shallow water
Modern ripples
Lithified ripples from ancient tidal flats
(tidal flats in South Carolina)
(Queensland, Australia)
Sedimentary Rock: Characteristics
3. They can contain fossils
○ fossils are the preserved remains or traces of animals,
plants, and other organisms from the remote past.
■ mineralized pieces (bones, teeth, shells, wood etc.)
■ casts or moulds (includes tracks/footprints)
Fossils
● Fossils can make it easier to determine
when a sedimentary rock was formed
○ some ancient organisms were widespread for a
○
○
relatively short period of geologic time
therefore when we find different rocks from distant
locations containing their fossils, we can tell that
those rocks formed around the same time
These are called index fossils
Relative Dating: Stratigraphy
Stratigraphy is the study of layered rocks.
Remember these principles/laws?
1. The Principle of Uniformitarianism
2. The Law of Superposition
3. The Principle of Original Horizontality:
Layers of sediment are originally deposited
horizontally under the action of gravity.
Relative Dating: Stratigraphy
Stratigraphy is the study of layered rocks.
Remember these principles/laws?
1. The Principle of Uniformitarianism
2. The Law of Superposition
3. The Principle of Original Horizontality
4. The Principle of Original Lateral Continuity:
Sediments are deposited over a large area in a continuous
sheet. Rock layers extend continuously in all directions, until
they thin out at the edge of the depositional basin, or grade
into a different type of sediment.
Relative Dating:
The Principle of Original Lateral Continuity
Relative Dating:
Cross-Cutting Relationships
Where rocks of different ages are found adjacent
to one another
We'll discuss:
● Faults
● Intrusions
● Contact metamorphism
● Unconformities
● Inclusions/Xenoliths
Relative Dating: Faults
Faults are planar fractures (breaks) in rock
A fault is always younger than the rock layers it
cuts through.
Duh.
Relative Dating: Fault Types
Normal fault
(tensional stress)
Thrust fault
(compressional stress)
Transform fault
(shear stress)
Relative Dating: Faults
Relative Dating: Intrusions
Intrusions are cause when molten magma
intrudes into cracks in existing rock.
An intrusion is always younger than the rock
layers it cuts through.
...DUH
Relative Dating: Intrusions
...can cause Contact Metamorphism
Remember that
the rock nearest
the intrusion will
have undergone
some contact
metamorphism
due to its heat!
...so the rock that has changed is always older
that the intruding rock.
DUH.
Relative Dating: Unconformities
An unconformity is a buried erosional or nondepositional surface separating two rock masses or
strata of different ages.
● It indicates that sediment deposition was not
continuous.
That means
Earth's "history
book" in this
location is
missing some
pages!
Relative Dating: Unconformities
An unconformity is always younger than
the rocks that have been eroded.
...Duh.
Relative Dating: Unconformities
Relative Dating: Unconformities
Relative Dating: Unconformities
Relative Dating: Practice
Order the layers from oldest to youngest
(assume the unlabeled folded gray rock is
oldest)
Relative Dating:
Clasts/ Inclusions/ Xenoliths
When pieces of rock are imbedded in a larger
matrix, they are older than the rock that contains
them.
Duh.
Relative Dating:
Clasts/ Inclusions/ Xenoliths
Homework
See Blog!!!!
(answering several questions about relative dating)
Test Practice
Test ANSWERS
Test Practice
Test ANSWERS
Test Practice
Test ANSWERS
Relative Dating Review: Key Terms
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Principle of Superposition
Fault
Intrusion
Erosional surface
Inclusion
Xenolith
Unconformity
Contact Metamorphism
index fossil
Relative Dating
One more thing... remember Index fossils?
...let's find out.
Test Practice
Test ANSWERS
Test Practice
Test ANSWERS
Test ANSWERS
Absolute Dating
An absolute age is quantitative.
Geologists usually express it in:
years before present (B.P.)
or
Millions of Years Ago (MYA or ma)
Absolute Dating: Radiometric Dating
One of the great discoveries of science is
Radiometric Dating, which allows us to
determine how long ago things formed.
Video: 100 Greatest Discoveries - Radiometric Dating
Absolute Dating: Radiometric Dating
1.
As igneous rocks cool and crystals form, some natural
radioactive atoms are trapped inside.
2.
Over time, these isotopes decay into other elements at a
known rate. (the half-life)
3.
The decay products are trapped along with the remaining
original isotope.
4.
We can determine how much time has elapsed by
comparing the ratio of original isotopes to decay products
in the sample.
Half-Life
Let's say that Baninium-137 has a half life of 1 year.
If I have a 32 g sample of 137Bn, how many grams will be left:
...after 1 year?
...after 2 years?
...after 3 years?
...after 4 years?
...after 5 years?
...after 6 years?
_____
_____
_____
_____
_____
_____
Radiometric Dating: Half-Lives
Source: Your Earth Science Reference Tables! Front Page!
Test
Practice
Test
ANSWERS
Test Practice
Test ANSWERS