Discovering Earth’s History
 In the 18th and 19th
centuries scientists knew
the Earth had a very long
history.
 They developed a
Geologic Time Scale that
showed the sequence, or
order of events based on
several principles of
relative dating.
Rocks record geological events and changing life
forms of the past. Erosion has removed much of
the Earth’s rock record, but enough still remains
to allow a lot of Earth’s history to be interpreted.
The geologic time scale revolutionized the way
people think about time and how they perceive
our planet.
Rocks Record Earth History
 We have learned that Earth is much older
than anyone had previously imagined and
that its surface and interior have been
changed by the same geological processes
that continue today.
 The primary goal of geologists is to interpret
Earth’s history.
 By studying rocks, especially sedimentary
rocks, the geologists can begin to put the
pieces together.
In the mid-1600/s Archbishop James Ussher
constructed a timeline for both Earth and human
history. He determined that the Earth was
created in 4004 B.C. In the 1700’s James Hutton
published the Theory of the Earth.
Ussher
Hutton
A Brief History of Geology
In Hutton’s work he introduced this fundamental
principle:
 Uniformitarianism: the forces and processes
that we observe today have been at work for a
very long time. (plates moving, rain, wind, etc)
Today, Scientists understand that these same
processes may not always have had the same
relative importance or operated at the same rate.
Some important geologic processes aren’t
currently observable, but evidence that they occur
is well established, for example meteorites.
Relative Dating—Key Principles
 Relative dating tells us the sequence in which
events occurred, not how long ago they
occurred.
There are 3 observations in which Nicolaus Steno, a
Danish scientist and priest, is credited with describing a
set of geologic observations that are the basis of
Relative dating.
1st Law
Law of Superposition
-in an undeformed
sequence of
sedimentary rocks,
each bed is older
than the one above
it and younger than
the one below it.
Grand Canyon
Relative Dating—2nd Law
 Principle of Original Horizontality
• layers of sediment are generally deposited in a
horizontal position.
Relative Dating—3rd Law
 Principle of Cross-Cutting Relationships
• when a fault cuts through rock layers, or when magma
intrudes other rocks and crystallizes, the fault or intrusion
is younger than the rocks affected.
Applying Cross-Cutting Relationships
Youngest Event
Layer 4
Layer 3
Layer 2
Dike B
Layer 1
Oldest Event
Fault A
Relative Dating—Key Principles
 Inclusions
• Inclusions are rocks contained within other rocks.
• Rocks containing inclusions are younger than the
inclusions they contain.
Formation of Inclusions
Relative Dating—Key Principles
 Unconformities
• represents a long period during which deposition
stopped, erosion removed previously formed rocks,
and then deposition resumed.
•There recognition helps us identify what intervals of
time are not represented in the rock record.
3 different kinds of unconformities
An angular unconformity indicates that during the
pause in deposition, a period of deformation (folding
or tilting) and erosion occurred.
Formation of an Angular Conformity
Relative Dating—Key Principles
 Unconformities
• A disconformity is
when two sedimentary
rock layers are separated
by an erosional surface.
A nonconformity is when
the erosional surface
separates older
metamorphic or intrusive
igneous rocks from
younger sedimentary
rocks.
A Record of Uplift, Erosion, and
Deposition
Correlation of Rock Layers
 Correlation is establishing the equivalence of rocks of
similar age in different areas.