3.4 EARTH’S GEOLOGIC HISTORY
Uniformitarianism
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Uniformitarianism means
the forces and processes we
observe today have been at
work for a very long time.
“The present is the key to
the past”
Relative Dating – Key Principles


Relative dating tells us the
sequence in which events
occurred, not how long ago they
occurred.
Law of superposition

States that in an undeformed
sequence of sedimentary rocks,
each bed is older than the one
above it and younger than the one
below it
Law of Superposition
Relative Dating – Key Principles

Principle of Original
Horizontality

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Layers of sediment are initially
deposited in a horizontal position
Principle of cross-cutting
relationships

when a fault cuts through rock
layers or magma intrudes other
rocks, we can assume the fault or
intrusion is younger than the rocks it
affects
Relative Dating – Key Principles

Inclusions
 Inclusions
are rocks contained within other rocks
 Rocks containing inclusions are younger than the inclusions they have
Relative Dating – Key Principles

Unconformities
A
long period which deposition stopped, erosion removed previously
formed rocks, and then deposition resumed
 3 types of unconformities:
 Angular
unconformity– during the pause in deposition, a period of deformation
(folding or tilting) and erosion occurred
 Nonconformity – when sedimentary rock lies above and was deposited on preexisting and eroded metamorphic or igneous rock.
 Disconformity - when two sedimentary rock layers are separated by an
erosional surface.
Dating with Radioactivity


Radioactivity is the
spontaneous decay of certain
unstable atoms
Half-life is the amount of time
necessary for one-half of the
atoms in a sample to decay to
a stable isotope (version)
 Isotopes
are different versions
of the same element, just have a
different amount of neutrons
Dating with Radioactivity

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Radioactive isotopes decay at a
constant rate since the formation
of the rock they occur in
Radiometric dating is the
procedure of calculating absolute
ages of rocks/minerals containing
radioactive isotopes
As radioactive (parent) isotope
decays, atoms of the stable
(daughter) product form
Radioactive Isotopes Frequently Used in Radiometric
Dating
Dating with Radioactivity

Example - Dating with Carbon-14
 Radiocarbon
dating is the method for finding age by comparing amount of
carbon-14 (parent) to amount of carbon-12 (daughter) in a sample
 When an organism dies, the amount of C-14 decreases as it decays. By
looking at ratio of C-14 to C-12 in a sample, age can be determined

Importance of Radiometric dating
 It
has supported ideas of James Hutton, Charles Darwin, and others who
inferred that geologic time must be immense.
Fossil Formation


Fossils are the remains or traces of prehistoric life. They are important
parts of sediment/sedimentary rocks
The type of fossil that is formed is determined by the conditions which
an organism died and how it was buried.
Fossil Formation

Unaltered Remains
 Some
remains of organisms—such as teeth, bones, and shells—
may not have been altered (hardly changed at all over time)
Fossil Formation

Altered Remains
 The
remains of an organism are likely to be changed over time
 Fossils often become petrified (organic parts replaced with minerals) or
turned to stone
 Molds – organism dies, covered by sediments, its flesh and bone decays,
and leaves a cavity of what it looks like
 Casts – created if the hollow spaces of a mold are filled with mineral matter
 Carbon films – organic material is compressed, leaving only a carbon
residue
Fossil Formation

Indirect Evidence
 Trace
fossils are indirect evidence of prehistoric life (footprints, burrows,
coprolite)

Conditions Favoring Preservation
 Two
conditions are important for preservation: Rapid burial, and possession
of hard parts.
Fossils and Correlation

Principle of fossil succession:
 Fossil
organisms succeed (come after) one another in a definite order.
Therefore any time period can be figured out by its fossil content

Index fossils are widespread on Earth, are limited to a short span of
time, and occur in large numbers
 Tool
for determining age of rock layers
Fossils and Correlation

Interpreting Environments
 Fossils
can also be used to interpret and describe ancient environments
 For example, suppose geologists found fossil clam shells in limestone. They
can infer that the region was once covered by a shallow sea.
Structure of the Time Scale

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Geologic time scale is a timeline that divides Earth’s history into units
representing specific intervals of time
Eons represent the greatest expanses of time.
Eons are divided into eras.
Each era is subdivided into periods
Periods are divided into smaller units called epochs
Structure of the Time Scale

There are three eras in the Phanerozoic eon:
 Paleozoic
– means “ancient life”
 Mesozoic – means “middle life”
 Cenozoic – means “recent life


Each period within an era is defined by less significant changes in life
forms compared to changes that occur in an era
The periods of the Cenozoic era are divided into smaller units called
epochs, which have even less significant changes in life forms
Precambrian Time – Vast and Puzzling

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Time frame: 4.56 BYA to 540 MYA
Rocks were large masses of metamorphic rock (Shields)
Atmosphere was water vapor, CO2, NO OXYGEN
Oxygen began to accumulate 2.5 BYA due to evolution of
photosynthetic plants
Most common Precambrian fossils are stromatolites - layered mounds
of CaCO3 made from algae
Paleozoic Era – Life Explodes

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Time frame: 540 MYA to about 250 MYA
By end of Paleozoic, all continents had fused into supercontinent
Pangaea
Life in early Paleozoic was restricted to seas
Late Paleozoic
 Some
400 MYA, plants had adapted to live at water’s edge, began to move
inland, becoming land plants
 Amphibians rapidly diversified because they had minimal competition from
other land dwellers
Mesozoic Era: Age of Reptiles

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Time frame: About 250 MYA to about 65 MYA
Major tectonic event broke up Pangaea
Seed-producing plants (gymnosperms) quickly became dominant
With perfection of shelled egg, reptiles quickly became dominant land
animals
At end of Mesozoic, many reptile groups became extinct
 Bye-
bye dinosaurs!
Cenozoic Era – Age of Mammals

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Time frame: about 65 MYA to present
Plate tectonics caused many events of mountain building, volcanism,
and earthquakes
Mammals replaced reptiles as dominant land animals
Angiosperms – flowering plants with covered seeds – replaced
gymnosperms as dominant land plants
With extinction of dinosaurs, mammals diversified forming many new
groups including bats and whales