Download Relative Dating: Which Came First? Chapter 6 Section 2

The Relative Age Of Rocks
Chapter 8 Section 2
Absolute Age
Absolute Age of a rock is the number of
years since that rock formed, its EXACT age.
Ex. The rock layer is 45 million years old.
Ex. This rock is 1.4 million years old.
***Uses actual numbers, example would be
450 million years ago (or 450 mya)
Relative Age
Relative Age is the age of the rock
COMPARED to other rocks around it.
Ex. That rock layer is OLDER than that
fossil.
Ex. This rock is YOUNGER than that rock.
***Not exact, used more frequently in science
The Law of Superposition
• When layers of sedimentary rock are
undisturbed the oldest rock layer is on the
bottom.
• As you move from the bottom up to the top,
the layers get younger.
• This is the Law Of Superposition—oldest
rock layers are on the bottom, newer ones
are on top.
Disturbances to Rock Layers
• When trying to determine the relative ages of
things—what’s older or younger, scientists
don’t just look at where the layers are in the
“stack”, they also look at disturbances in the
rock layers.
• Intrusions, Extrusions, Faults and
Unconformities disturb the rock layers, and
are all YOUNGER than the rock layers…the
rocks had to be there FIRST in order to be
“disturbed.”
Cross-cutting Relationships
• Faults are a break in the
Earth’s crust, a place where
rock moves on either side of
it. They are always YOUNGER
than the rock it breaks
• Unconformities are where
through.
an old rock surface meets a
new rock surface, rock
layers are missing (gaps).
This is often because of
erosion. They are always
YOUNGER than the rocks
below it.
Extrusions and Intrusions
• Extrusions are lava flows that covered the rock
surface and hardened into igneous rock. They are
always YOUNGER than the rock below it.
• Intrusions are pockets of magma that push
through layers of rock, stops, then hardens into
rock. They are always YOUNGER than the rock it
cuts through.
Index Fossils
• Index Fossils are fossils that
were widely distributed and
only lived on Earth for a short
period of time.
• They are helpful in determining
how old rock layers are
because when an index fossil is
found, the rock layer its found
in is the same age.
Try this one!
How about this one?
3-2-1
1) Name 3 ways that rock layers can be
“disturbed.”
2) Compare and contrast extrusions and
intrusions.
3) What one process causes rock layers to “go
missing” over time and create unconformities?