Rock/Mineral Exam – Next Week
Take advantage of the help sessions.
Samples on the exam will not be the same
samples you saw in lab, so memorizing them
is not useful.
The rock exam is an important component of
your grade in lab, however, overall it is worth
less than half of a single lecture exam.
Geologic Time
Our understanding started with simple observations in the field.
Siccar Point, Scotland - James Hutton in the late 1700’s.
The archetypical example of relative dating, and the first
realization of the great depth of geologic time.
Now the determination of geologic time is a quantitative science called Geochronology.
Argon Geochronology Laboratory (NIGL) at UNLV
Important Questions – Geologic Time
• How do we determine the order of geologic
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events using the relative ages of rocks?
How was the geologic time scale constructed?
How do we recognize gaps in the rock record,
i.e. missing rock record?
How are the absolute ages of rocks
determined?
How have we determined the age of Earth?
How do we reconstruct the geologic history of
Earth with rocks?
Understanding Geologic Time
• What do we need to know?
• The sequence of events in Earth’s history.
• The time required for each step along the way.
• Two ways to determine geologic time.
• Relative Dating - Observe rocks in the field and
determine the order of events that produced them.
• Absolute Dating - To actually know how long ago an
event occurred, or when a rock formed in the past.
(This requires laboratory analysis using naturally
occurring radioactive elements in rocks and mass
spectrometers to measure them.)
Understanding Geologic Time
Relative ages establish a sequence of events
without knowing exactly how long ago they
occurred. Events are put in order: what happened
first, what happened next, and what happened most
recently. This is all early geologists had.
Absolute ages establish when an event took place
in the past. Absolute ages are numerical,
quantitative, ages of geologic events, and have
analytical uncertainties. This is the realm of mass
spectrometry measurements in the past ~50-60
years.
How Do We Determine Relative Ages of Rocks?
• We use a set of geologic “rules”…. there are 6.
• The most simple one is the Principle of Superposition.
This states that sedimentary (or volcanic) rocks are
created in succession, with the oldest rocks at the
bottom, and progressively younger rocks above.
In this sequence the oldest
sedimentary rock deposited is
A, whereas B, C and D were
deposited in order and are
progressively younger.
How Do We Determine Relative Ages of Rocks?
• We use a set of 6 geologic “rules”.
• The most simple one is the Principle of Superposition.
This states that sedimentary (or volcanic) rocks are
created in succession, with the oldest rocks at the
bottom, and progressively younger rocks above.
Photo of the Grand Canyon.
Older rocks are down by the
river, those forming the cliffs in
the background are the
youngest.
How Do We Determine Relative Ages of Rocks?
• Principle of Original Horizontality – sedimentary rock
layers are deposited horizontally when they form.
Flat layers of
sedimentary rocks
that are no longer
horizontal. Some
tectonic event - which
occurred after they
were deposited - has
tilted these up to the
angle they are now
found at.
How Do We Determine Relative Ages of Rocks?
• Principle of Cross-Cutting Relationships – geologic
features such as dikes and faults that cut across rock
must be younger than the rock they cut through.
How Do We Determine Relative Ages of Rocks?
• Principle of Inclusions – objects enclosed in a rock
must be older than the rock itself.
Inclusions of granite in overlying
sedimentary rock – the granite is
older.
Inclusions of sedimentary rock in
underlying granite – the granite is
younger.
How Do We Determine Relative Ages of Rocks?
What if the rocks are in different areas???
• Principle of Lateral Continuity – rock layers are
continuous until encountering an obstruction
The Grand Canyon – The same rock layers are exposed for 100’s of km. We can infer
the underground layers of rock from those exposed at the surface, or in drill holes.
How Do We Determine Relative Ages of Rocks?
What if the rocks are in different areas???
• Principle of Faunal Succession
• Fossils of different organisms first appear at
different times in the rock record.
• Fossils of related organisms exhibit regular
changes in progressively younger rocks
everywhere they are found.
• When they become extinct fossil organisms
disappear from the rock record everywhere at the
same time and do not reappear in younger rocks.
How Do We Determine Relative Ages of Rocks?
What if the rocks are in different areas???
What about rocks that formed 100’s of km apart, perhaps even on different continents?
How can we correlate (relate) them to each other?
Using Fossils For Determining Relative Ages.
Index fossil – exists only for a brief interval of time.
Up
Younger
So, rocks found anywhere which contain the assemblage of
fossils in A or B above must be the same age.
How Do We Determine Relative Ages of Rocks?
What if the rocks are in different areas???
Correlation is the process of
matching up the ages of rocks
found in different places, i.e.
finding rocks of equivalent age.
The Geologic Time Scale
The Geologic Time Scale
The Geologic Time Scale
• Originally created based on fossils.
• Was thus a relative time scale.
• Has now been quantified by isotopic
dating – absolute time.
• Structure of the Geologic Time Scale
• Names of the eons – the largest division
• Phanerozoic (“visible life”) – the most recent
eon, began about 540 million years ago
• Proterozoic (together these are the Precambrian)
• Archean
The Geologic Time Scale
• Precambrian time
• Nearly 4 billion years prior to the
Cambrian period (beginning of the
Phanerozoic), ~88% of Earth’s history.
• Not divided into smaller time units
(periods, epochs) because the events of
Precambrian history are not know in
great enough detail.
• First abundant fossil evidence does not
appear until the beginning of the Cambrian.
The Geologic Time Scale
• Structure of the geologic time scale
• Eon – Largest subdivision
• Era – subdivision of an eon
• Eras of the Phanerozoic eon
• Cenozoic (“recent life”)
• Mesozoic (“middle life”)
• Paleozoic (“ancient life”)
• Eras are subdivided into periods
• Jurassic – part of the Mesozoic
• Periods are subdivided into epochs
The Geologic Time Scale
The Geologic Time Scale
Relative ages of fossils defined intervals of geologic time. We use the geologic principles
discussed earlier to correlate these rock layers on the Earth.
The Geologic Time Scale
Once a time scale was constructed it is possible to determine the age of a rock anywhere simply
by noting the types of fossils contained in it….
How do we recognize gaps in the rock record?
Unconformities – gaps in the rock
record when erosion occurred
rather than deposition, 3 types.
An angular unconformity is where two layers
of rock meet that are inclined at different
angles to one another.
angular unconformity
How do we recognize gaps in the rock record?
A disconformity is a gap between
two sedimentary layers that are
parallel. Erosion, but no tilting.
disconformity
How do we recognize gaps in the rock record?
A nonconformity is where sedimentary
or volcanic rocks lie directly on
igneous or metamorphic rocks.
nonconformity
“Practice” Understanding Relative Time Here
There May Be Exam Questions On This Diagram!
1) Layers of sedimentary rock
are deposited, with the
oldest at the bottom.
2) Fault A cuts across these
rocks.
3) Erosion occurs.
4) Renewed deposition of
sedimentary rocks, starting
with conglomerate.
5) Fault B cuts across all of the
sedimentary rock layers.
6) Magma forms an igneous intrusion which cuts across sedimentary rocks and fault B.
7) Dike B intrudes, cutting across the igneous intrusion, fault B, and forming a sill.
8) Dike A intrudes, cutting across sedimentary rocks and the sill. This may have fed
volcanoes at the surface.
9) Erosion produced the current landscape.
Early Thoughts on the Age of the Earth
• Zoroaster, Persia, ~3,600 years ago: Earth is
12,000 years old.
• Ancient Hindu scripts, ~2,200 years ago: Age of
Earth (and the universe) is ~4.3 Ga.
• Chaldeans, Neo-Babylonian empire, ~1,500
years ago: Earth is 2 Ma.
• Various (>200) biblical theologians, ~1,850 to
350 years ago: Earth is ~5,477 to 8,897 years
old.
A quantitative calculation… Based on an assumption.
1700’s – Beginning of Calculations Based on
Observations of the Natural World
• Benoit de Maillet, France, 1748: Earth is ~2.4 Ma,
based on observed sea-level decline.
• Comte de Buffon, France, 1774: Earth is ~75,000
years old, based on cooling of iron spheres.
• Mid to late 1800’s – Age of the Earth became the
most hotly debated subject in the sciences.
• Physicists – calculations based on cooling of
initially molten Earth and salt deposit
accumulations. Earth 10’s to 100’s Ma.
1800’s – Great Debate in the Sciences
• Geologists and Biologists – calculations
based on accumulation of sediments and the
fossil record. Earth is several Ga’s.
• Lord Kelvin, 1862, published the first of
several heat-flow calculations for cooling of
Earth. Earth is 20 to 400 Ma.
• Geologists and biologists closer to being
correct, but discovery of radioactivity by
physicists provided the key to determining
this!
Kelvin commanded great respect in the
scientific community.
Lord Kelvin – at center
One of Kelvins calculations, based
on Fouriers Law of heat flow, 1890.
Discovery of Radioactivity
• Latest 1800’s to early 1900’s.
• Rutherford and Soddy, 1902, published “The
cause and nature of radioactivity”.
• Radioactivity of K, Th, U supplies internal heat,
invalidates Kelvin’s cooling calculations.
• But, radioactive decay provides the basis for
modern isotopic dating (along with advent of
mass spectrometers in 1940’s to 1950’s,
Dempster, Bainbridge, Nier).
What geologic events can be dated?
• Timing of volcanic eruptions
• Formation of fossils
• Formation of ore deposits
• Timing of metamorphism
• Timing and rates of uplift of mountains
• Emplacement and crystallization history of magmas
• Formation of young geologic surfaces
• Age of groundwater
• Timing of climate changes
• Timing of geomagnetic polarity changes
• Timing of glacial periods
• The list goes on and on and on….
How Are The Absolute Ages of Rocks Determined?
• Elements consist of different isotopes – atoms with the same number of
protons, but different numbers of neutrons.
• Some isotopes are radioactive and naturally decay. These decays
produce an isotope of a different element.
• The original, radioactive isotope is called the parent, and the new
isotope is called the daughter.
• The rates of radioactive decay have been repeatedly measured for
decades and are well known.
• Absolute dating is based on a determining how the ratio between
parent and daughter isotopes change with time.
• Mass spectrometers are used for isotopic analysis.
• Absolute dating works best for igneous and metamorphic rocks.
How Are The Absolute Ages of Rocks Determined?
• We must measure the isotopic
abundances.
• A radioactive parent isotope
decays to a stable daughter
isotope.
• If we know the rate of decay, we
can use the ratio of the two to
calculate the age of the rock or
mineral they are contained in.
• Half-life – the amount of time it
takes for ½ of the parent
isotopes to decay to the
daughter isotope.
How Are The Absolute Ages of Rocks Determined?
Here is a simple way of understanding the concept of a half life - t½
t½
# parent atoms (P)
# daughter atoms (D)
D/P
0
128
0
0
1
64
64
1
2
32
96
3
3
16
112
7
4
8
120
15
5
4
124
31
6
2
126
63
7
1
127
127
How Are The Absolute Ages of Rocks Determined?
D* = No (1 - e−λt )
Growth curve of daughter
Decay curve of parent
N = No e−λt
How Are The Absolute Ages of Rocks Determined?
Summary of isotopic systems useful in geology
Secondary Ion Mass Spectrometry
Cameca Ion Microprobe at UCLA, used for U-Pb dating.
Noble Gas Mass Spectrometry
K-Ar Geochronology Laboratory (NIGL) at UNLV
How Are The Absolute Ages of Rocks Determined?
Four common isotopic dating techniques and the time spans
they can be used to measure. The longer the t½, the older the
applicable range is.
Fig 7.18
How Do We Know Isotopic Dating Works?
• Ages confirmed by historical observations.
• Ages agree with the Principle of Superposition.
• Ages on one rock determined by multiple dating
methods in different laboratories agree.
• Ages are consistent with known geologic or
solar system history.
How Do We Know Isotopic Dating Works?
• Dating rocks produced during historic events.
Eruption of Mt. Vesuvius which destroyed
the city of Pompeii, occurred 1930 years ago.
How Do We Know Isotopic Dating Works?
How Do We Know Isotopic Dating Works?
Ages agree with Principle of
Superposition, older rock layers
at bottom, progressively younger
going towards the top.
From McDougall and Brown, 2008, Geochronology of the Pre-KBS Tuff
Sequence, Omo Group, Turkana Basin, Journal of the Geological Society
Of London, v.165, p. 549-562.
How Do We Know Isotopic Dating Works?
From McDougall and Brown, 2008, Geochronology of the Pre-KBS Tuff
Sequence, Omo Group, Turkana Basin, Journal of the Geological Society
Of London, v.165, p. 549-562.
How Do We Know Isotopic Dating Works?
• Ages determined for the same rock using
multiple isotopic systems with analyses
conducted in many different laboratories
worldwide.
• Example: Acasta Gneiss, NW Territory, Canada
– Ages range from 3.94 ± 0.09 Ga to 4.03 ±
0.06 Ga.
• Samples analyzed by U-Pb, Sm-Nd, Rb-Sr
methods in 11 different laboratories over a
period of 20 years.
How have we determined the age of Earth?
• Oldest rocks from many continents worldwide – 3.6 to 4.0 Ga.
• Oldest intact rock – Acasta Gneiss - 4 Ga.
• Zircons from western Australia – up to 4.4 Ga - oldest Earth
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material dated.
Meteorites (~70) dated by numerous methods since 1950’s –
4.53 to 4.58 Ga (formed during same accretionary process
that formed Earth early in solar system history).
Oldest rocks returned from the Moon – 4.4 to 4.5 Ga (formed
very soon after Earth accreted).
All these data are consistent with formation of the Earth at
4.54 Ga – currently accepted age.
This age is consistent with astrophysicists estimate of 11-13
Ma for formation of our galaxy, and 14-15 Ga for the age of
the universe.
Combining Relative and Absolute Dating
• Sedimentary rocks are
not easily dated, but
igneous rocks are.
• What are relative dates
for these rocks?
• What if we obtained
ages of 25 Ma for the
lava and 20 Ma for the
dike?
•Age of shale and
limestone at bottom is
>25 Ma.
• Age of shale and
sandstone at top is
between 25 and 20 Ma.
Using Relative and Absolute Dating to
Define Geologic Time Scale Boundaries
What if?
Batholith = 110 Ma, Dike B = 85 Ma, Dike A = 20 Ma
1) Layers of sedimentary rock
are deposited, with the
oldest at the bottom.
2) Fault A cuts across these
rocks.
3) Erosion occurs.
4) Renewed deposition of
sedimentary rocks, starting
with conglomerate.
5) Fault B cuts across all of the
sedimentary rock layers.
6) Magma forms an igneous intrusion which cuts across sedimentary rocks and fault B.
7) Dike B intrudes, cutting across the igneous intrusion, fault B, and forming a sill.
8) Dike A intrudes, cutting across sedimentary rocks and the sill. This may have fed
volcanoes at the surface.
9) Erosion produced the current landscape.