Transcript Geologic Time

Geologic Time
An introduction
Which elephant is older????
Oldest
Youngest
These two questions are related to relative
time versus absolute ages.
 Relative time: Which elephant is oldest? Youngest?
 Absolute age: How old is the oldest elephant? Youngest?
oldest
youngest
Born in 1970
Born in 1999
Rocks Record Earth’s History
 Rocks record geological events and changing life forms
of the past
 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 same two questions are frequently asked
in the geosciences.
Relative time: Which rock
unit is oldest? Youngest
Absolute age: How old is the
oldest rock unit? Youngest?
younger
375 Ma
older
975 Ma
I made these dates up!
Three terms
 Strata-layers of rock
 Stratigraphy- science of studying, describing,
and correlating strata
 Stratigraphic Column- An ordered sequence
of strata.
Discovering Earth’s History
 Uniformitarianism means that the forces and
processes that we observe today have been at
work for a very long time.
Law of Superposition
 In an undisturbed rock sequence, younger
rocks are found on top of older rocks.
Unconformity
 An unconformity represents a long period
during which deposition stopped, erosion
removed previously formed rocks, and then
deposition resumed.
Principle of Cross-Cutting
Relationships
 The principle of cross-cutting
relationships states that when a fault cuts
through rock layers, or when magma intrudes
other rocks and crystallizes, we can assume that
the fault or intrusion is younger than the rocks
affected.
Principle of Cross-Cutting
Relationships
Another Example
Oldest to Youngest 1,2,3,4,5