Topic VII
Geologic History
How do you determine how old a rock is?

Uniformitarianism: “the present is the key
to the past” (what is happening today—
uplifting, erosion… has always happened)

Relative Age of Rock Layers: comparing
rock layers in an outcrop—exposed rock
(does not tell you an exact age—but can
tell you which rocks are older)
“Relatives”

Absolute Age: finding the actual age of
the rock (radioactive dating)
Six ways to determine the relative age of an
outcrop—“Sandwich Rules”
1.
Law of Original Horizontality: all
sedimentary rocks formed in horizontal
layers
2.
Law of Superposition: oldest rock is on
the bottom (in a sedimentary layer that
has not been overturned when folded)
3.
Igneous Intrusions (inside) /
Extrusions (outside): are always
younger than the rocks they are in or on
(toothpick)

Look for contact metamorphism (burn)
marks (if you see them, the rock layer is
older than the metamorphism)
4.
Mineral Veins are younger than the
rocks they are in
5.
Sediments that make up sedimentary
rocks are older than the rocks
themselves
6.
Folds, faults, and tilts are younger than
the rocks they are in (cut)
Sequence of Geologic Events:
Events animation
But there is a problem….

Unconformities: buried erosional surfaces
(rock layers are destroyed—usually indicated
by a horizontal wavy line in the cross
section)—creates a “Gap in Time”
More unconformities….
More unconformities….
1 More unconformity….
What is correlation?

Correlation: matching up of rock and
geologic events in two separate areas
How can rocks be correlated?
1.
Similarity of rock layers (color,
composition)
2.
Fossils (remains of prehistoric life):
seeing if fossils match up in separate
rock layers—can tell the organisms
environment (does not work for all
fossils)
Fossil Correlation

Index Fossils: fossils that have a wide
geographic distribution and a short period
of existence (1 rock layer)
What is needed to make a fossil:
Hard parts (soft parts decay quickly)
 Quick burial (slows decay)

3.
Volcanic Ash: can be spread throughout
the world and settle as a thin layer very
quickly (very good time marker)
**Index Fossils are the best way to correlate
rock layers
How can scientists determine the
absolute age of rock layers?

Radioactive Dating: determining the age
of something by comparing the amount of
radioactive isotope to the stable decay
product
meaning…

The INSTANT a rock is created, it contains
100% of some radioactive isotope. As a
rock gets older, the radioactive isotope
breaks down into a decay
product….scientists can measure this
breakdown.
But this can also be used to date the
remains of once LIVING things…

All living things on planet Earth are part of the
CARBON CYCLE… (think Carbon
Dioxide)…meaning over the course of our
life, we build-up Carbon in our bodies…the
instant we die, we have 100% Carbon
So, how do you do this…

Half Life: amount of time it takes for a
radioactive isotope to decay to ½ its
original mass (occurs at a predictable rate)
ESRT Cover

Carbon-14: isotope used to date the
remains of once living things—people,
shirts, mammoths—less than 50,000 years
old (this is because its half life is only 5700
years—very short)
The box method…

If 100 grams of pure carbon-14 (C-14) starts to
decay, how much C-14 will be left after 11,400
years?
•
If 100 grams of pure carbon-14 starts to
decay, how much nitrogen-14 (N-14) will
be created in 11,400 years?
**half life never changes for a particular
isotope no matter what happens!
Fire
Crushing
Geologic Time Scale

Geologic Time Scale: time-line that
shows the history of the Earth (created by
correlating fossil evidence throughout the
world)
Problems with the geologic time scale:

Unconformities—time scale may be too
short
What does the Geologic Time Scale
show about life on Earth?
1.
Simple life forms began first
2.
Evolution: species have changed to
increase their chance of survival (this
leads to diversity of life)
3.
Most life forms have become extinct