Transcript Relative Dating Notes Powerpoint

Relative Dating
Earth Science 2015
What is it?
Relative dating is the process of identifying the relative ages of the layers of the
Earth. The goal of relative dating is to identify which layers of Earth are older or
younger compared to the layers around them. However, scientists do NOT use
this process to provide an absolute date. It is only used to figure out the order in
which the geologic events occurred.
How can we do this?
The layers of rock in the crust are thousands to hundreds of thousands to million
years old. How are scientists able to figure out which are older or younger?
There are some rules and relationships we’ve figured out, but the most important
one is this:
“Processes we observe happening today happened the same way in the past.”
We call this idea uniformitarianism. Without this idea, nothing else we discuss
today would be valid.
The Laws: Law of Superposition
Sedimentary rock forms when sediment collects on
the surface, gets covered up by more sediment on
top and then slowly gets compacted and cemented.
So that means if it’s close to the surface, it probably
only got there recently. The stuff way down near the
bottom has been there for a long time. This is the
Law of Superposition. In a cross-section (or sample
of layers), layers of rock farther away from the
surface are the oldest. Layers near the top are the
youngest.
The Laws: Principle of Original Horizontality
Layers of the Earth form in horizontal layers. That’s how
gravity works. So if the layers of rock being analyzed aren’t
parallel with the surface, then the event that made them not be
horizontal happened after they were deposited. We can still
figure out which are older by looking at how they’re angled
and the farthest from the surface is the oldest.
The Laws: Principle of Cross-Cutting Relationships
Start with the same idea: sedimentary rock gets
deposited horizontally and the rock near the top is
young, while rock near the bottom is old. What if
some magma comes up through those layers and
leaves an intrusion of igneous rock behind. That
lava tube is younger, because the rock had to be
there for it to intrude through it! Same principle is
applied if the rock has a fault (crack) running
through it. If the rock is cracked, it had to be
there for it to crack!
Unconformities
Rocks are constantly changing. When this happens, it leaves a gap in the relative
dating record. We call these unconformities. If rock formation in an area stops,
the top layers start getting weathered and eroded. If rock formation starts back up
again afterwards, new horizontal layers will be deposited, but now there’s a lot of
information missing!
Types of unconformities
Disconformity: Everything is still horizontal, but
there is an uneven layer in between two of the
layers. The uneven layer is the disconformity.
Angular unconformity: Layers become angled, then
the exposed rock gets eroded away, and then
horizontal layers are deposited on top.
Nonconformity: An igneous or metamorphic
intrusion reaches the surface, becomes eroded
and a layer of newer sedimentary is deposited on
top.