Transcript File

Geologic Time – Unlocking the Mystery
Relative Dating
(is not kissing cousins!)
• Placing rock layers and processes in their
proper sequence of order.
• Makes use of:
– The principle of superposition
– The principle of original horizontality
– Cross-cutting
– Inclusions
– Unconformities
Principle of Superposition:
• In a sequence of rocks,the youngest
rocks are on top and the oldest are on the
bottom.
Superposition in the Grand Canyon
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Principle of Original Horizontality
•Layers of sediment are generally deposited
in a horizontal position
•Rock layers that are flat have not been
disturbed
Principle of Original Horizontality
• After deposition and cementation, this
rock was subjected to intense folding.
Not in text.
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Principle of cross-cutting:
– Younger features cut across
older features.
Cross-Cutting Relationships
Faults and dikes are younger than the rock they cut across.
Fault is younger than
rock layers.
Not in text.
Because
Dike C has
been cut by
Dike B, Dike
C is older.
Because
Dike B has
been cut by
Dike A, Dike
B is older.
• Inclusions
•An inclusion is a piece of
rock that is enclosed within
another rock.
•Rock containing the
inclusion is younger.
Inclusions
Granite is an intrusion that is younger
than the layer above it!
Sedimentary rock is
younger than granite,
because it contains
inclusions.
Unconformity
•An unconformity is a break in the
rock record produced by erosion
and/or nondeposition of rock units
•Layers of rock are said to be
conformable when they are found to
have been deposited essentially
without interruption
Angular Unconformity – tilted or
folded sedimentary rock layers that
are overlain by younger layers.
(Conformable layers: deposited without interruption)
Not in text.
Three Types of Unconformities
• Disconformity
• Angular unconformity
• Noncomformity
An angular Unconformity occurs when tilted or folded
sedimentary rocks are overlain by younger, more flatlying layers.
Recipe for an angular unconformity
1. Deposition
2. Folding/Uplift
3. Erosion
4. Subsidence/more erosion
Angular Unconformity
Not in text.
A disconformity occurs when layers on either
side of the break in the rock record
(unconformity) are parallel.
Disconformity
Nonconformity – a break separates
older metamorphic or intrusive
igneous rock from younger
sedimentary layers.
Nonconformity
Disconformity
Not in text.
Angular Unconformities and
Nonconformities
Relative Dating Example
• Sequence of geologic units/events
from oldest (1) to youngest (6)
Not in text.
• 1) Limestone Deposited
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• 2) Magma intrudes limestone, cools
to form granite.
Not in text.
• 3) Limestone and granite are
eroded.
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• 4) Sandstone deposited
• 5) Lava flows over sandstone
• 6) Shale deposited.
Not in text.
Relative Dating Example
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Relative Geologic Dating
Sequencing Quiz -- order from oldest to youngest:
Correlation
• Matching up of rocks of similar age in
different regions to provide a more
comprehensive view of the geologic
history of a region.
• Over short distances correlation can be
done using rock samples.
• Over large distances correlation must be
done using faunal succession (animal
fossils).
Faunal Succession
• Fossil organisms succeed one
another in a definite and
determinable order.
• As a result, any time period can be
recognized by its fossil content.
• Of most use are index fossils. These
fossils are widespread and limited to
a short span of geologic time.
Principle of Faunal Succession
• Fossil species succeed each other in
definite, recognizable order.
This diagram illustrates how not all fossils are found in each
location, but by using correlation we can determine the history.
Not in text.
Though
these three
areas are
separated
by large
distances,
correlation
has
allowed
geologists
to piece
together
their
history.
Not in text.
“Index fossils”
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Dating Rocks Using
Overlapping Fossil Ranges
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2
Indiana
correlated sequence
1
Kansas
Correlating Age of Coal Deposits
3
Ohio
4
Pennsylvania
Mostly
nonmarine
limestone
Not in text.
Fossils: Evidence of Past Life
• Fossil : traces or remains of
prehistoric life now preserved in rock
• Fossils are generally found in
sediment or sedimentary rock (rarely
in metamorphic and never in
igneous rock) WHY???
• Paleontology: is the study of fossils
• Geologically fossils are important
because they
•Aid in interpretation of the geologic
past
•Serve as important time indicators
•Allow for correlation of rocks from
different places
• Conditions favoring preservation include:
•Rapid burial
•Possession of hard parts (skeleton,
shell, etc.)
• Therefore, fossil record is biased – why?
Fossils
Not in text.
Fossilized Trilobites
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Carbonized Leaf
The oils in the dead organisms body are sucked out and
the remaining matter is a carbon film.
Not in text.
Midge Fly Caught in Amber
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Petrified Wood
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Dinosaur Footprint
in Limestone
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Geologic Time Scale
• Divided into 3 eras.
•Paleozoic (ancient life)
•Mesozoic (middle life)
•Cenozoic (recent life)
• Eras are subdivided into periods.
• Each of the periods are further
divided into epochs.
Comparing relative length
of Precambrian Era with all
others
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Comparing Geologic Time with a Movie
• Assume the film speed is 16 frames
per second
• Assume that one frame represented
100 years.
• Then 100 years flashes every 1/16 th
of a second.
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• Your life would – in its entirety – be
represented in only one frame.
• Then the birth of Christ until the present
would be viewed in 1 ¼ seconds!
• To show from the present to the last major
ice age would require 11 ¼ seconds.
• To return to witness the last dinosaur die
would require more than 11 hours of tape
being rewound.
• To return to the formation of Earth 4.5
billion years ago would require rewinding
32 ½ days of film.
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