Geologic time : The span of time since the formation of the Earth.
The Earth is about 4.6 billion years old; however, no rocks of that age still exist.
The oldest rock on Earth are about 4 billion years old.
~4 billion
year old
gneiss from
Canada
Within a sedimentary rock located in Jack Hills, Western Australia, the worlds
oldest detrital mineral grain was found. It is a mineral called zircon that was dated
at ~4.4 billion years old (pictured below using Cathodoluminescence to give the
mineral a false color)
http://www.geology.wisc.edu/zircon/Earliest%20Piece/Earliest.html
http://www.geology.wisc.edu/zircon/Earliest%20Piece/Earliest.html
Moon rocks have yielded ages as old a 4.57 billion years old, and it is thought that
the moon formed from the Earth, so the Earth must be older than 4.57 billions
years old.
Geologist like to use two terms to describe time:
Relative age: The age of one geologic feature with respect to another.
Numerical age: The absolute or numerical age of a geologic feature.
There are several PHYSICAL PRINCIPALS for defining relative age:
The principal of uniformitarianism: The physical processes we observe today also
operated in the past in the same way, and at comparable rates.
Mud cracks that are forming today
Mud cracks that formed 100’s of
millions of years ago.
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Principle of superposition : In a sequence of sedimentary rock layers, each layer
must be younger than the one below, for a layer of sediment cannot accumulate
unless there is already a substrate on which it can collect.
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principle of original horizontality: Layers of sediment, when originally deposited,
are fairly horizontal.
principle of original continuity: Sedimentary layers, before erosion, formed fairly
continuous sheets over a region.
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The Grand Canyon is an example of superposition, original horizontality and
original continuity.
principle of cross-cutting relationships: If one geologic feature cuts across
another, the feature that has been cut is older.
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principle of inclusions : If a rock contains fragments of another rock, the
fragments must be older than the rock containing them.
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principle of baked contacts : When an igneous intrusion “bakes” (metamorphoses)
surrounding rock, the rock that has been baked must be older than the intrusion.
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Geologic History
A cross-section through the earth reveals the variety of
geologic features. View 1 of this animation identifies a
variety of geologic features; View 2 animates the
sequence of events that produced these features, and
demonstrates how geologists apply established
principles to deduce geologic history.
8PC version
principle of fossil succession : In a stratigraphic
sequence, different species of fossil organisms
appear in a definite order; once a fossil species
disappears in a sequence of strata, it never
reappears higher in the sequence.
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fossil: The remnant, or trace, of an ancient living organism that has been
preserved in rock or sediment.
fossil assemblage : A group of fossil species found in a specific sequence of
sedimentary rock.
Range: The interval of a sequence of strata in which a specific fossil species
appears.
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fossil correlation: A determination of the stratigraphic relation between two
sedimentary rock units, reached by studying fossils.
fossil correlation: A determination of the stratigraphic relation between two
sedimentary rock units, reached by studying fossils.
Cherts in Washington State correlate
to cherts in Oregon due to the same
fossils in both rocks.
Cherts in Oregon State correlate to
cherts in Washington due to the
same fossils in both rocks.
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Lithologic correlation: A correlation based on similarities in rock type.
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Lithologic correlation
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Lithologic correlation
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Commonly there are gaps in geologic
Commonly there are gaps in geologic
time because rocks of a certain age
could be missing. We call these...
unconformities: A boundary between
two different rock sequences
representing an interval of time
during which new strata were not
deposited and/or were eroded.
There are 3 types of unconformities:
1) Angular unconformity: An
unconformity in which the strata
below were tilted or folded before the
unconformity developed; strata below
the unconformity therefore have a
different tilt than strata above.
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Nonconformity : A type of unconformity
at which sedimentary rocks overlie
basement (older intrusive igneous
rocks and/or metamorphic rocks).
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disconformity : An unconformity parallel to the two sedimentary sequences it
separates.
Types of Unconformity
This animation shows the stages in the development of
three main types of unconformity in cross-section, and
explains how an incomplete succession of strata
provides a record of Earth history. View 1 shows a
disconformity, View 2 shows a nonconformity and
View 3 shows an angular unconformity.
8PC version
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Angular unconformity
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Disconformities and nonconformities are
common in the grand canyon
The succession of rocks in the Grand Canyon can be
divided into formations based on notable changes in
rock type and changes in fossil assemblages
8PC version
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geologic column : A composite stratigraphic chart that represents the entirety of
the Earth’s history.
Eon: The largest subdivision of geologic time.
Era : An interval of geologic time representing the largest subdivision of the
Phanerozoic Eon.
Period: An interval of geologic time representing a subdivision of a geologic era.
Epoch: An interval of geologic time representing the largest subdivision of a
period.
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hadean
archean
proterzoic
carbonifperm- erous
ian
devonian
silurain
triassic
jurassic
cretaceous
holocene
Cambrian
ordovician
paleoeocene cene
pliocene
pleistocene
mioceneoligocene
Life evolution in the
context of the geologic
column.
before the
Cambrian, there
was little life on
Earth, then there
was an explosion of
life starting at the
Cambrian.
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A number of mass extinctions has also occurred during geologic time. These
extinctions helped define the periods.
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Radiometric dating techniques have lead to the assignment of ages to the
geologic time scale.
Radiometric dating works by the radioactive decay (The process by which a
radioactive atom undergoes fission or releases particles thereby transforming
into a new element) of a mineral. The time it takes for a parent isotope (A
radioactive isotope that undergoes decay) to decay into a daughter isotopes (The
decay product of radioactive decay) is known.
radioactive decay: The process by which a radioactive atom undergoes fission or
releases particles thereby transforming into a new element.
radioactive isotope : An unstable isotope of a given element.
isotope: Different versions of a given element that have the same atomic number
but different atomic weights.
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geologic time scale: A scale that describes the
intervals of geologic time.
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