Transcript Relative Dating

Aim: How do Scientists Read Rocks?
Do Now:
In your notes, answer the following
question. Of the three types of rocks we have
discussed (Igneous, Sedimentary and
Metamorphic), which one do you think would
be able to hold clues to Earth’s past? Why?
Outcrop – section of rock exposed at the
surface that shows a geologic sequence
(layers).
I. Reading Earth’s History
• Uniformitarianism - processes we observe
today are most likely the same processes
that occurred millions of years ago.
When in
Earth’s
history does
this painting
show?
Triassic ~200 mya
Today (2015)
Devonian ~360 mya
• Relative dating - tells us the sequence
in which events occurred.
*Does not tell us the actual age or use
numbers to express time.
II. Rules for Relative Dating:
A. Law of Superposition
- sedimentary layers are created flat and even.
- the oldest layers are at the bottom since
they were deposited
first.
How many layers of rock are
there? Which is the oldest?
B. Law of Original Horizontality
- folds/faults disturb layers after they are
Lithified. (caused by plate tectonics).
How did the sequence
change in picture
below? What caused
this change?
C. Law of Cross-Cutting Relationships
- layers can be cut after they form by
intrusions and faults.
- Intrusions are layers of magma that melt
through pre-existing rock and then solidify.
How did the
sequence change in
the picture below?
Igneous Intrusion cuts through the pre-existing rock.
- Which came first, the intrusion or the rock it is in?
- Which principle lets us know this?
- Contact Metamorphism – a symbol geologist
use to help show which rocks have been affected
by the intruding magma (intrusions).
- indicates the rocks have been
affected by the intrusion when it was magma (hot!).
Analyze these two geologic cross sections, describe
how they are different?
III. Important Features
• Ash Layers –
Created by an erupting
volcano, occurs at one
specific time in Earth’s
history.
Do not copy
This layer is 65 million years old and is called the KT ash
boundary. Below this layer, scientists find fossils of
dinosaurs. Above this layer, there are no dinosaur fossils.
What does this layer tell us?
This layer marks the extinction of the Dinosaurs
• Fossils
- Remains of ancient life preserved in
sedimentary rock.
Why are fossils found only in Sedimentary rock and not
Igneous or Metamorphic?
- Knowing when the organisms lived, helps
geologist put the rocks containing them in
order.
Turtle fossil, China
Dragonfly, North America
• Unconformity
(UNC)- VIDEO
- a gap in the
sedimentary rock
record.
- caused by erosion
Unconformity (UNC)
Closure 1: Place the letters in relative
order from oldest to youngest.
Closure 2: Place the letters in relative
order from oldest to youngest.
Closure 3: Place the letters in relative
order from oldest to youngest.
Closure 4:
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