HOW OLD IS THE EARTH?
• The Earth is about 4.6 billion years old
• Much of its history is recorded in the rocks
• Observations of fossils, rock types, evidence of
faulting, uplift, and folding as well as igneous
activity are all clues about the earth’s history
THE KEY TO THE PAST
Relative Time- “this rock is older than that”
Principles Used to Determine Relative Age
Unconformities
Correlation
The Standard Geologic Time Scale
Index Fossils
Absolute Time- “this rock is 28 million
years old”
Principles of radioactive decay
Instruments
The age of the Earth
JAMES HUTTON 1ST GEOLOGIST
“from what has actually been, we
have data for concluding with
regard to that which is to
happen thereafter.”
“All inferences from experience
suppose…that the future will
resemble the past”
Because of many of his ideas, he
was thought to be atheistic.
IMPORTANT RELATIVE AGE DATING
PRINCIPLES
Brothers, sisters, first cousins are off limits!
It may
lead to
this!
GEOLOGIC PRINCIPLES
There are five major geologic principles:
1. Principle of Uniformitarianism
2. Principle of Lateral Continuity
3. Principle of Original Horizontality
4. Principle of Superposition
5. Principle of Cross-Cutting Relationships

These principles help geologists make predictions about
sequences of events and thus the earth’s history.
PRINCIPLE OF UNIFORMITARIANISM
This principle states that all the geological processes
(Weathering, Erosion, Volcanism, Earthquakes, etc.)
that occur today also occurred in the past in the same
ways.
CHARLES LYELL
“The present is the key to
the past”
Much of our knowledge of
geologic materials,
features, and past events
is based on observation
of currently active
processes.
Lateral Continuity: original sedimentary
layers extend laterally until it thins out at
edges
rocks that are otherwise similar, but are now
separated by a valley or other erosional feature,
can be assumed to be originally continuous.
PRINCIPLES OF ORIGINAL HORIZONTALITY
• States that sediments are deposited in flat lying or
horizontal layers that are parallel to the surface on
which they were deposited.
• This means that if rocks are tilted or folded, they have
been deformed after deposition.
IMPORTANT RELATIVE AGE DATING
PRINCIPLES
Original Horizontality: all
beds originally deposited
in water formed close to
horizontal
PRINCIPLE OF SUPERPOSITION
• States that the rock layers on the bottom are oldest,
the rock layers on the top are youngest.
• Unless the rock layers are overturned or folded
• Basically, if you pile a stack of papers, what ever paper
was put down first, must be the oldest.
PRINCIPLE OF SUPERPOSITION
YOUNGEST ROCK
MIDDLE ROCK
OLDEST ROCK
PRINCIPLE OF CROSS CUTTING RELATIONS
States that if something (such as faults, cracks, veins, or
intrusions - magma cooling underground, extrusions lava cooling on the surface) cuts through rock layers,
the rock layers themselves must be older than the
thing cutting through them.
Cross-cutting Relationships: disruptions in any rock sequence
occurred after the youngest established event in the
undisturbed sequence
Ie. A rock or fault is younger than any rock (or fault) through
which it cuts
SEDIMENTARY DEPOSITION
INTRUSION
TILTING & EROSION
SUBSIDENCE AND NEW
MARINE DEPOSITION
MISSING FORMATION
DIKE EVENT
EROSION AND EXPOSURE
SUBSIDENCE &
DEPOSITION
FLUVIAL DEPOSITION
SUBSURFACE
GEOLOGY
CONTACT RELATIONS