Geologic time and dating
Most figures and tables contained here are from course text:
Understanding Earth Fourth Edition by Frank Press, Raymond Siever,
John Grotzinger, and Thomas H. Jordan
Geologic time re fers to the ages relevant to Earth’s history
This shows major events of Earth’s history on a logarithmic scale
In a logarithmic scale numbers a re plotted by the exponent used
to raise ten to in order to get to the respective number.
(i.e. 10 plots a 1, 0.1 plots as -1, 1000 plots as 3)
Earth changes on scales
much larger than human
experience
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The following slides discuss basic principles used in relative
age dating.
Relative age dating is where geologist analyze relationships
in the rock to describe a sequence of events as to how rocks
were formed and deposited through time without regard to
absolute age.
Absolute dating is where a specific age (in years or millions
or of years old are assigned to the rock)
Absolute dating was not possible until radiometric age
dating became available.
Sedimentary rock re cords clues as to Earth’s history
Later t ectonic uplift and erosion uncovers these rock.
Geologist study rock to understand Earth’s history
Original horizontality – the concept that rock units were originally
deposited in relatively horizontal planes.
New deposits
Older folded and eroded rocks
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Superposition – the concept that in layered rock the oldest units are
at the bottom and as we move vertically through the rock column
we are looking at successively younger rock.
Faunal succession is the observation that fossils (and therefore the
life forms that left the m behind) change throughout geologic time.
As a result fossils can be used to correlate and calibrate rocks of
different ages
Index fossils are fossils used to identify rocks of similar ages.
Index fossils have been calibrated using radiometric age
dating and can also be used to identify the age of the rock that
contains them.
A good index fossil must:
1) Be short lived in geologic time (as a species)
2) Geographically wide spread
3) Have parts that are easily prese rved (hard parts like teeth or
shells)
4) Easily recognized (by an expert)
3
Trilobites are a good index fossil for much of the Paleozoic
These are one species but the trilobite changed much throughout
geologic time.
Ammonites are also
important index fossils.
Their species ca me in
many sizes and shapes
and the shells are easily
identified by the suture
patterns.
sutures
Fossil assemblages (groups of fossils from a time period can be used
to narrow ages more than one fossil alone)
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Cross cutting relationships: a geological unit that cuts across and
other unit will be the younger of the two.
In this example:
dike
Young
E
Igneous intrusion
D
Dep E
C
Dep D
Dep C
B
A
Dep B
Dep A
batholith
Old
Cross cutting relationships:
In this example:
Young
D
D
Dep D
C
B
A
C
Fault
Dep C
B
Dep B
A
Dep A
Old
fault
5
Sill vs. lava flow
D
Basalt
Vesicular texture in the basalt
would indicate this is a lava and
therefore older than unit D.
Contact metamo rphism in unit D
would indicate this is a sill and
therefore younger than unit D
C
B
A
Contact metamo rphism
Inclusions:
If fragments of one rock unit (A) are contained in an other (B)
then the unit providing the rock fragments (A ) must be older.
Unconformities – periods of erosion where the rock sequence has
been lost. This must usually be preceded by uplift. Uplift will
change a location from a depositional to an erosional environment
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7
8
Example of the development of a region and how to interpret
the relative ages of events and units
9
10
Seismic data is an important to to look at subsurface geology
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12
13
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The geologic time scale is a description of rock units ordered in
time. When these units were first describe radiometric age
dating was not possible but using the geological principles
discussed above there was a recognition that these rock units
represented a progression of Earth history.
15
This time scale w as produced by correlating rocks from distant
locations from one another and then continuing the description as
a progression through time in younger rocks at the new locations
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Absolute age dating by radiometric methods
Types of nuclear decay:
Beta decay: An electron escapes fro m a neutron and the neutron
becomes a proton (e.g. Rb87 -Sr87 ). Atomic weight does not change
atomic number does.
Electron capture: a proton captures an electron and becomes a
neutron (e.g. K40 – Ar40 ). Atomic weight does not change atomic
number does.
Alpha- An alpha particle is emitted. An alpha particle is
equivalent to a helium nucleus with an atomic weight of 4
(2 protons and 2 neutrons). This is part of the uranium lead decay
sequence.
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Beta decay
Half life is the time it takes for ½ the parent product to decay to
the daughter product
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For example:
At t=0 we will only have parent product.
At t= 1 half-life there will be an equal amount of parent and
daughter atoms.
Hypothetically speaking if we start with 64 parent atoms of an
element with 5,000 year half life, the following will be true:
At t=
0
5,000
10,000
15,000
20,000
25,000
parent atom
64
32
16
8
4
2
daughter atoms
0
32
48
56
60
62
number
of half-lifes
0
1
2
3
4
5
Assumptions in the above plan for radiometric age dating are:
1) There was no daughter when the rock formed. There are in
fact methods using many samples that enable us to estimate and
correct for the amount of daughter product when the rock
formed.
2) The system re mained closed: did not loose daughter or parent
product with time. There are methods to identify samples with
that leaked. If a system leaks it is unlikely to leak in a regular
way. If many mine rals fro m the same rock are dated it is
unlikely a leaky system will yield many mineral with the same
age. Therefore this can be overcome by repeated
measure ments. There are other methods using multiple dating
techniques that can also overcome this problem
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