Download Geologic Time

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.
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.
How many layers of rock are
there? Which is the oldest?
B. Law of Original Horizontality
- folds/faults disturb (mess-up) layers
after they are Lithified .
How did the
sequence change in
picture below? What
caused this?
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
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 that
shows which rocks have been affected by the
intruding magma.
- indicates the rocks have been
affected by the intrusion when it was magma.
Analyze these two geologic cross sections, describe
how they are different?
III. Important Features
• Ash Layers –
Created by erupting
volcano, occurs at one
specific time and indicates
the time eruption.
What else indicates one
specific time?
Do not copy
This layer is 65 million years old. Below this layer, scientists
find fossils of dinosaurs. Above this layer, there are no
dinosaur fossils. What does this layer tell us?
K-T ash Boundary which marks the end of the Cretaceous
Period and the beginning of the Paleogene Period.
• Unconformity
(UNC)- VIDEO
- a gap in the
sedimentary rock
record.
- caused by erosion
Closure 1: Place the letters in relative
order from oldest to youngest.