Transcript Sept 19_ Principles of Geology (Relative Dating)

Important Principles in Geology
*Relative Age Dating*
James Hutton
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Scottish geologist during the 18th century
Considered the father of modern geology
He tried to calculate soil accumulating in
the ocean to figure out how much time
had passed since the formation of the
Earth
Uniformitarianism
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What happened in the past is still the
processes occurring today, and will
continue to happen in the future.
• Ex: Rocks were formed from magma cooling in
the past, this is still happening today and will
continue to happen in the future the same
way.
Stratigraphy
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Geologists can make predictions by
looking at sedimentary processes
going on today
They realize that you can identify the
age of certain layers by comparing
the materials and fossils that it
contains
This has become an important tool to
unravel the history of the Earth
Relative Age Dating
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Before absolute dating of rocks was
developed in the 20th century,
geologists had to rely on relative age
dating
Relative dating places rock in order
from oldest to youngest
Geologists use specific principles to
determine the ‘age’ of rocks
1. Principle of Superposition
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In a sequence of unformed
sedimentary rock, the oldest beds
are on the bottom and the youngest
on the top
2. Principle of Original Horizontality
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Sediment layers are nearly horizontal
when originally deposited.
Strata that are not horizontal have
been deformed by movements of the
Earth’s crust.
Oldest layer is still the bottom.
3. Principle of Lateral Continuity
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Layer of sedimentary rock extends
laterally in all directions, until it thins out.
4. Principle of Crosscutting Relations
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Geological features, such as faults
and fractures are younger than the
rocks that they cut.
5. Principle of Inclusion
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Inclusions (fragments of rock) found in
another rock are always OLDER than the
rocks they are found in.
List the rocks from oldest to youngest.
Explain how you know.
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Answer:
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A–B–C
A is the bottom layer (oldest), C cuts
through A and B, therefore A and B had to
exist first. Also, B rock fragments are found
in C, therefore, B is older (principle of
inclusion)
6. Principle of Faunal Succession
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Groups of fossil
plants and animals
occur in the
geological record
in a definite order.
A period of
geological time can
be recognized by
its specific fossils.
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List oldest to youngest and explain
your answer.
Hint ---these are excellent test questions
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Answer
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A- C–B
C cuts through A, therefore, A had to exist
first. C fragments are in B therefore C is the
oldest (principle of inclusion)