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.

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.

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.

Geologic TimeOur understanding started with simple observations in the field.

Argon Geochronology Laboratory (NIGL) at UNLV

Now the determination of geologic time is a quantitative science called Geochronology.

Important Questions – Geologic Time

• How do we determine the order of geologic 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?

• 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

Understanding Geologic TimeRelative 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.

• 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.


• 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 isyounger.

• Principle of Lateral Continuity – rock layers are continuous until encountering an obstruction

How Do We Determine Relative Ages of Rocks?What if the rocks are in different areas???

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.

• 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.



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.

So, rocks found anywhere which contain the assemblage of fossils in A or B above must be the same age.

Up Younger

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


• 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



• 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.

• 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 ScaleRelative ages of fossils defined intervals of geologic time. We use the geologic principlesdiscussed earlier to correlate these rock layers on the Earth.

The Geologic Time ScaleOnce a time scale was constructed it is possible to determine the age of a rock anywhere simplyby 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.

angular unconformityangular unconformity

An angular unconformity is where two layers of rock meet that are inclined at different angles to one another.

A disconformity is a gap between two sedimentary layers that are parallel. Erosion, but no tilting.

disconformitydisconformity


A nonconformity is where sedimentary or volcanic rocks lie directly on igneous or metamorphic rocks.

nonconformitynonconformity


“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 EarthEarly Thoughts on the Age of the Earth

• Zoroaster, Persia, ~3,600 years ago: Earth is Zoroaster, Persia, ~3,600 years ago: Earth is

12,000 years old.12,000 years old.

• Ancient Hindu scripts, ~2,200 years ago: Age of Ancient Hindu scripts, ~2,200 years ago: Age of

Earth (and the universe) is ~4.3 Ga.Earth (and the universe) is ~4.3 Ga.

• Chaldeans, Neo-Babylonian empire, ~1,500 Chaldeans, Neo-Babylonian empire, ~1,500

years ago: Earth is 2 Ma.years ago: Earth is 2 Ma.

• Various (>200) biblical theologians, ~1,850 to Various (>200) biblical theologians, ~1,850 to

350 years ago: Earth is ~5,477 to 8,897 years 350 years ago: Earth is ~5,477 to 8,897 years

old.old.

A quantitative calculation… Based on an assumption.

1700’s – Beginning of Calculations Based on 1700’s – Beginning of Calculations Based on Observations of the Natural WorldObservations of the Natural World

• Benoit de Maillet, France, 1748: Earth is ~2.4 Ma, Benoit de Maillet, France, 1748: Earth is ~2.4 Ma, based on observed sea-level decline.based on observed sea-level decline.

• Comte de Buffon, France, 1774: Earth is ~75,000 Comte de Buffon, France, 1774: Earth is ~75,000 years old, based on cooling of iron spheres.years old, based on cooling of iron spheres.

• Mid to late 1800’s – Age of the Earth became the Mid to late 1800’s – Age of the Earth became the most hotly debated subject in the sciences.most hotly debated subject in the sciences.

• Physicists – calculations based on cooling of Physicists – calculations based on cooling of initially molten Earth and salt deposit initially molten Earth and salt deposit accumulations. Earth 10’s to 100’s Ma.accumulations. Earth 10’s to 100’s Ma.

1800’s – Great Debate in the Sciences1800’s – Great Debate in the Sciences

• Geologists and Biologists – calculations Geologists and Biologists – calculations based on accumulation of sediments and the based on accumulation of sediments and the fossil record. Earth is several Ga’s.fossil record. Earth is several Ga’s.

• Lord Kelvin, 1862, published the first of Lord Kelvin, 1862, published the first of several heat-flow calculations for cooling of several heat-flow calculations for cooling of Earth. Earth is 20 to 400 Ma.Earth. Earth is 20 to 400 Ma.

• Geologists and biologists closer to being Geologists and biologists closer to being correct, but discovery of radioactivity by correct, but discovery of radioactivity by physicists provided the key to determining physicists provided the key to determining this!this!

Kelvin commanded great respect in the Kelvin commanded great respect in the scientific community.scientific community.

Lord Kelvin – at center

One of Kelvins calculations, basedon Fouriers Law of heat flow, 1890.

Discovery of RadioactivityDiscovery of Radioactivity

• Latest 1800’s to early 1900’s.Latest 1800’s to early 1900’s.• Rutherford and Soddy, 1902, published “The Rutherford and Soddy, 1902, published “The

cause and nature of radioactivity”.cause and nature of radioactivity”.• Radioactivity of K, Th, U supplies internal heat, Radioactivity of K, Th, U supplies internal heat,

invalidates Kelvin’s cooling calculations.invalidates Kelvin’s cooling calculations.• But, radioactive decay provides the basis for But, radioactive decay provides the basis for

modern isotopic dating (along with advent of modern isotopic dating (along with advent of mass spectrometers in 1940’s to 1950’s, mass spectrometers in 1940’s to 1950’s, Dempster, Bainbridge, Nier).Dempster, Bainbridge, Nier).

What geologic events can be dated?What geologic events can be dated?

• Timing of volcanic eruptionsTiming of volcanic eruptions• Formation of fossilsFormation of fossils• Formation of ore depositsFormation of ore deposits• Timing of metamorphismTiming of metamorphism• Timing and rates of uplift of mountainsTiming and rates of uplift of mountains• Emplacement and crystallization history of magmasEmplacement and crystallization history of magmas• Formation of young geologic surfacesFormation of young geologic surfaces• Age of groundwaterAge of groundwater• Timing of climate changesTiming of climate changes• Timing of geomagnetic polarity changesTiming of geomagnetic polarity changes• Timing of glacial periodsTiming of glacial periods• The list goes on and on and on….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.

• 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.



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

D* = No (1 - e−λt ) Growth curve of daughter

Decay curve of parentN = No e−λt


Summary of isotopic systems useful in geology


Secondary Ion Mass SpectrometrySecondary Ion Mass Spectrometry

Cameca Ion Microprobe at UCLA, used for U-Pb dating.

K-Ar Geochronology Laboratory (NIGL) at UNLV

Noble Gas Mass SpectrometryNoble Gas Mass Spectrometry

Fig 7.18

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.


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.

• Dating rocks produced during historic events.


Eruption of Mt. Vesuvius which destroyedthe city of Pompeii, occurred 1930 years ago.



From McDougall and Brown, 2008, Geochronology of the Pre-KBS TuffSequence, Omo Group, Turkana Basin, Journal of the Geological SocietyOf London, v.165, p. 549-562.

Ages agree with Principle ofSuperposition, older rock layersat bottom, progressively youngergoing towards the top.


From McDougall and Brown, 2008, Geochronology of the Pre-KBS TuffSequence, Omo Group, Turkana Basin, Journal of the Geological SocietyOf London, v.165, p. 549-562.


• 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.

• 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

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.

How have we determined the age of Earth?

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

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.

What if? Batholith = 110 Ma, Dike B = 85 Ma, Dike A = 20 Ma

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.

Documents

relative ages of rocks

younger rocks

absolute ages of rocks

older rocks

oldest rocks

volcanic rocks

geologic time slide

ages of geologic events