Geologic History
Relative Dating
Birth of Modern Geology
• Began in the late 1700’s
when James Hutton
published his Theory of the
Earth.
• He was the first scientist to
argue effectively that
geologic processes proceed
over long spans of time.
Siccar Point, Scotland
Hadrian’s Wall, Scottland
Built c. 1500, Hutton worked in 1700s.
Relative Dating
• Prior to the discovery of radioactivity,
geologists had no reliable way of giving
specific dates to geologic events and had
to rely on relative dating techniques.
• Relative dating means placing rocks or
events in their proper order, based on a
comparison to other rocks
Relative Dating Laws
1) The Principle of Uniformitarianism states that
the physical, chemical, and biological processes
that operate today have also operated in the past.
“The present is the key to the past.”
Relative Dating Laws
2) Principle of Original Horizontality
states that sediments are deposited in
horizontal layers
Layer 3 – Sandstone
Layer 2 – Limestone
Layer 1 – Siltstone
What happened to these rock strata?
What happened to this rock strata?
Relative Dating Laws
3) Principle of Superposition states that in
an undeformed sequence of strata, each
bed is older than the one above it and
younger than the one below it.
*The layers on top are youngest.
* The layers on the bottom are oldest.
• Relative dating (nonconformity
animation); (Angular Unconformity)
Which layers is youngest? Oldest?
Possible Exceptions to the Law of Superposition
Folding of Strata over upon itself.
Possible exceptions to the law of
superposition
Strata is displaced by a Thrust Fault
Relative Dating Laws
4) Principle of Cross-Cutting Relationships states
that an intrusion or fault is younger than the layers
it cuts through.
Cross-Cutting Relationships
Fault
Cross-Cutting Relationships
Relative Dating Laws
5) Principle of Inclusions states that an inclusion is
older than the rock layer it is in.
Unconformities
• Buried erosional surfaces that are
preserved in the rock record
• Unconformities represent gaps in the
rock record.
• Relative dating (nonconformity
animation); (Angular Unconformity)
unconformity
animation
Angular Unconformity
Put the rock layers in order from
oldest to youngest.
1) Rock layers A-E are deposited.
2) Rock layer D (a sill) intrudes.
3) Rock layer F (a dike) intrudes.
Volcanic Column with radiating dikes, exposed after
erosion.
4) Layers are tilted and eroded (an
angular unconformity).
5) Deposition of rock layers G-K
.
6) Upper surface is eroded.