Geologic Tim e
2014
Geologic time is fundamental to understanding both the physical and biologic
history of our planet. Geologists use two different frames of reference when
speaking of geologic time:
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R elative Dating : placing geologic events in sequential order as
determined from their position in geologic record but do not tell us how long
ago a particular event occurred.
Absolute Dating: results in specific dates for rock units or events
expressed in years before the present. e.g. Radiometric dating
Today geologic time scale is really a dual scale: a relative scale based on rock
sequences with radiometric dates expressed as years before the present added
to it.
Relative Dating
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Placing rocks and events in sequence: Before the development of radiometric
dating techniques, geologist had no reliable means of absolute age dating and
depended solely on the relative dating methods which allow events to be placed in
sequential order.
Principle of Uniformitarianism – present is key to the past.
assumes that present-day processes have operated through out geologic time
Principles and rules: Six fundamental geologic principles are used in relative
dating
1) Law of superposition – oldest rocks are on the bottom
2) Principle of original horizontality – sediment is deposited
horizontally
3) Principle of lateral continuity – sediments extend laterally in all
directions
4) Principle of cross-cutting relationships – younger feature cuts
through an older feature.
5) Principle of Inclusions – one rock contained within another
(rock containing the inclusions is younger)
6) Principles of Fossil Succession – fossils at the bottom of a
sequence are older than those at the top of the sequence (principle of
faunal and floral succession)
Principles and rules: Six fundamental geologic principles are used in relative
dating
1) Law of superposition – oldest rocks are on the bottom:
“in an undisturbed successions of sedimentary rock layers, the oldest
layer is at the bottom and the youngest layer is at the top. It is the basis
for relative age determination of strata and their contained fossils.
2. Principle of original horizontality – sediment is deposited horizontally
sedimentary particles settle from water under the influence of gravity, sediment
is deposited in essentially horizontal layers. Therefore, a sequence of
sedimentary rock layers that is inclined form horizontal suggests tilting after
deposition and lithification.
3. Principle of lateral continuity – sediments extend laterally in all directions
until it thins and pinches out or terminates against the edge of the
depositional basin.
Superposition, Original Horizontality and Lateral Continuity
Cross-cutting relationships
4. Principle of cross-cutting relationships – younger feature cuts through
an older feature.
* an igneous intrusion or fault must be younger than the rock it intrudes or cut.
DYKE
FAULT
Cross-cutting relationships

Cross-cutting relationships
Buried lava flows and sills look very similar in a sequence of strata. A buried
lava flow is older than the rocks above but younger than the rocks below
(principle of superposition) whereas sill is younger than all the beds below it
and younger than the bed immediately above it.
CASE: DIFFERENTIATING INTRUSION FROM UNCONFORMITY
A BURIED LAVA FLOW: THERE ARE INCLUSIONS
AND NO BAKING AT THE TOP UNITS
A SILL: BAKING AT THE TOP AND BOTTOM, NO
INCLUSIONS AT THE OVERLYING UNITS
5. Principle of Inclusions:
one rock contained within another
(rock containing the inclusions is younger)
CASE: DIFFERENTIATING INTRUSION FROM UNCONFORMITY
SANDSTONE IS OLDER THAN GRANITE
SANDSTONE IS YOUNGER THAN GRANITE
6. Principles of Fossil Succession:
fossils at the bottom of a
sequence are older than
those at the top of the
sequence (principle of faunal
and floral succession)
Fossils: evidence of past life

Fossils and correlation
• Principle of fossil succession
• Fossils succeed one another in a definite and determinable
order.
Determ ining the ages of
rocks using fossils
Fossils: evidence of past life

Fossils and correlation
• Index fossils
• Widespread geographically.
• Existed for a short range of geologic time.
Six fundamental geologic principles are used in relative dating:
1) Law of superposition
2) Principle of original horizontality
3) Principle of lateral continuity
4) Principle of cross-cutting relationships
5) Principles of Inclusions
6) Principles of Fossil Succession
Unconform ities

Unconformity is a surface of non-deposition or erosion that separates
younger strata from older ones. It represents a break in geologic
record/time. So, any interval of geologic time is not represented by strata is
called Hiatus.
Nonconformity: if the erosional surface cut into metamorphic or igneous rocks, is
covered by sedimentary rocks.
Angular unconformity: is an erosional surface on tilted or folded strata over which
younger strata have been deposited. There are some case where both the older
and the younger strata may dip, there unconformity can be defined if angles are
different (generally older strata dips more steeply).
Disconformity: if a well defined erosional surface separates the older from the
younger parallel beds.
Paraconformity: is a surface of erosion and non-deposition between younger and
older beds that are parallel with one another. There is no apperent erosion and
the unconformity surface look like a simple bedding plane.
Unconform ities
Form ation of
Disconform ity
Form ation of
Angular
Unconform ity
Form ation of
Nonconform ity
Several unconform ities (Grand Canyon)
Paraconformity
Absolute Dating:
R adioactivity and R adiom etric Dating

There are 92 naturally occurring isotopes in the earth. Among them, most
are stable but some are radioactive and spontaneously decay
to other more stable isotopes of elements, releasing energy in the
process.

It is the decay rate of unstable isotopes which is used to determine
the absolute age of rocks.
R adioactivity and radiom etric dating

Radiometric dating
• Half-life – the time for one-half of the radioactive nuclei to decay.
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Requires a closed system.
Cross-checks are used for accuracy.
Complex procedure.
Yields numerical dates.
C14 Dating
DISCREPANCY BETWEEN C14 AND TREE-RING DATING
CARBON CYCLE AND SOURCE OF C14
Tree-ring Dating
Cross dating using different trees from different woods and establishing ring-width
chronology backward in time.
Geologic tim e scale
The Geologic Time Scale
* Divides geologic history into units.
* Originally created using relative dates.
Subdivisions
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Eon (largest time span)
Era
Eras of the Phanerozoic eon
Cenozoic ("recent life")
Mesozoic ("middle life")
Paleozoic ("ancient life")
Eras are subdivided into periods.
Periods are subdivided into epochs.
A hypothetical diagram in
w hich various relative dating
m ethods can be applied