Download How old is that rock?

How old is that rock?
Relative dating of geologic material.
This is NOT
what is meant
in geology
when we use
the term,
“relative
dating.”
Uniformitarianism
One of the most fundamental aspects of science is
uniformitarianism. This is the idea that the
processes that we can observe around us are
likely the way the processes have happened in
the past as well as the same processes that are
happening in places that we cannot observe.
For example...
Uniformitarianism
If I observe that
a sandy beach
in Virginia adds
an average of
20 tons of sand
per year per
mile of beach...
Uniformitarianism
I can predict
that a sandy
beach in Africa
probably adds
close to 20 tons
of sand per mile
per year.
Uniformitarianism
I can also
predict that 65
million years
ago, sandy
beaches
probably grew
at a similar rate.
Uniformitarianism
If the world
spins at a rate
causing a ~24
hour day
today...
Uniformitarianism
A million years
ago, the Earth
probably had a
day of ~24
hours.
Uniformitarianism
If light on Earth
travels at
~3x108 m/s
today...
Uniformitarianism
It probably
moves at the
same speed in
the rest of the
universe.
Uniformitarianism
If hydrogen has
all of the
properties of
hydrogen every
time we test it...
Uniformitarianism
Then all
hydrogen in the
universe
probably
acts/acted the
same way.
Original Horizontality
Since we observe
that sediments today
are deposited
horizontally, we infer
that all sediments on
the Earth probably
started off
horizontally.
Superposition
Since we observe
that new sediments
are deposited from
deposition onto the
top layers of the
Earth, we infer that
the lower the layer in
the geological
record, the older it is.
Superposition
Superposition
doesn’t tell us
how old things
are, but it does
tell us which
layers are older
than the others.
Intrusions and Inclusions
Intrusions and Inclusions
As we have discussed, the
Earth is not a static thing.
Once crust is laid down, lots
of things can change it.
When magma pushes its
way through the layers, it’s
called intrusion.
Intrusions and Inclusions
As we have discussed, the
Earth is not a static thing.
Once crust is laid down, lots
of things can change it.
When magma pushes its
way through the layers, it’s
called intrusion.
Intrusions are YOUNGER
than the rock they are
intruding upon.
Intrusions and
Inclusions
Due to weathering
and erosion,
sometimes old rock
can break off and
mix into new rock.
The rock inside the
new rock is older and
called an inclusion.
Unconformities
Erosion, weathering, deposition, and other movement of rocks on the surface of
the Earth can make reading the geological record confusing. These big changes
are different types of unconformities.
Unconformities
So in short, an unconformity is a mix up in the geologic record due to changes in
the Earth.