Geologic Time
MMSA Earth-Space Science
-Time scale based upon the
fossil record
-The fossil record indicates a wide
variety of life has existed on
Earth. Most has become extinct.
-Humans have been around for 0.2
millions of years.
Geologic Principles
Law of Superposition: Oldest rock layers are at the
bottom of a sequence of layers
Geologic Principles
Law of Original Horizontality: Rock layers were deposited in a
horizontal orientation. If units are not horizontal, an event occurred
subsequent to the deposition which caused the layers to fold or tilt
Geologic Principles
Law of Lateral Continuity: Assumes that deposits
originally extended in all directions
Geologic Principles
Cross-Cutting Relationship: An intrusion or fault is
younger than the rock it cuts across
Geologic Principles
Law of Superposition: Oldest rock layers are at the
bottom of a sequence of layers
Law of Original Horizontality: Rock layers were deposited in
a horizontal orientation. If units are not horizontal, an event
occurred subsequent to the deposition which caused the
layers to fold or tilt
Law of Lateral Continuity: Assumes that deposits originally
extended in all directions
Cross-Cutting Relationship: An intrusion or fault is younger
than the rock it cuts across
Unconformities: an erosional surface becomes buried by
younger rocks. Results in a gap in the rock record.
Inclusions: pieces of rock from the layer below. The inclusions
are older than the rock they are in.
Absolute Age Dating of Rocks
-Relative age dating: Comparing one event with another over time
-Absolute Age Dating: determining an object’s actual age by means of
radiometric dating, tree rings, etc…
Absolute Age Dating of Rocks
Based on Radioactive Decay: the emission of radioactive particles
resulting in the change of one element into another.
Radioactive “Parent element”
More stable “Daughter element”
Half-Life
Half-life: length of time it takes for one-half of the original amount of an element to decay.
DO THIS NOW: Assume you have a 1600 gram radioactive sample in your hand.
Every year, half of the radioactive material decays into a more stable element. Plot time
on the x-axis, and radioactive mass on the y-axis. Your first coordinate is (0, 1600), your
second coordinate is (1, 800). Complete this graph for up to 6 years. Connect your points.
Decay Rate
1800
1600
Mass of Uranium
1400
Exponential
Decay! Its not
linear!
1200
1000
800
600
400
200
0
0
1
2
3
4
Millions of Years
5
6
7