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Classroom presentations to accompany Understanding Earth , 3rd edition. prepared by Peter Copeland and William Dupré University of Houston. Chapter 9 The Rock Record and the Geologic Timescale. Geologic Time. William E. Ferguson. Geologic Time.

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classroom presentations to accompany understanding earth 3rd edition

Classroom presentations to accompany Understanding Earth, 3rd edition

prepared by

Peter Copeland and William Dupré

University of Houston

Chapter 9

The Rock Record and the Geologic Timescale

slide2

Geologic Time

William E. Ferguson

geologic time
Geologic Time

A major difference between geologists and most other scientists is their attitude about time.

A "long" time may not be important unless it is > 1 million years.

two ways to date geologic events
Two ways to date geologic events

1) relative dating (fossils,

structure)

2) absolute dating (isotopic, tree

rings, etc.)

some geologic processes can be documented using historical records brown is new land from 1887 1988
Some geologic processes can be documented using historical records(brown is new land from 1887-1988)

Fig. 9.2

slide7

Ammonite Fossils

Petrified Wood

Chip Clark

Fig. 9.4

Tom Bean

steno s laws
Steno's Laws

Nicolaus Steno (1669)

  • Principle of Superposition
  • Principle of Original Horizontality
  • Principle of Lateral Continuity

Laws apply to both sedimentary and volcanic rocks.

principle of superposition
Principle of Superposition

In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom.

principle of superposition10
Principle of Superposition

Youngest rocks

Oldest rocks

Fig. 9.3b

Jim Steinberg/Photo Researchers

principle of original horizontality
Principle of Original Horizontality

Layered strata are deposited horizontal or nearly horizontal or nearly parallel to the Earth’s surface.

principle of lateral continuity
Principle of Lateral Continuity

Layered rocks are deposited in continuous contact.

unconformity
Unconformity

A buried surface of erosion

subsidence and sedimentation of e over c
Subsidence and Sedimentation of E over C

Unconformity:

a buried surface of erosion

Fig. 9.6

slide24

The Great Unconformity of the Grand Canyon

Fig. 9.7

Geoscience Features Picture Libraryc

south rim of the grand canyon27
South rim of the Grand Canyon

250 million years old

Paleozoic Strata

550 million years old

1.7 billion years old

Precambrian

south rim of the grand canyon28
South rim of the Grand Canyon

250 million years old

550 million years old

1.7 billion years old

Nonconformity

nonconformity in the grand canyon30
Nonconformity in the Grand Canyon

Tapeats Sandstone

(~550 million years old)

Vishnu Schist

(~1700 million years old)

slide37

Fig. 9.10

Schlumberger Executive Communications

generalized stratigraphic section of rocks exposed in the grand canyon
Generalized Stratigraphic Section of Rocks Exposed in the Grand Canyon

after: Beus & Moral (1990)

the geologic time scale
The Geologic time scale
  • Divisions in the worldwide stratigraphic column based on variations in preserved fossils
  • Built using a combination of stratigraphic relationships, cross-cutting relationships, and absolute (isotopic) ages
absolute geochronology
Absolute geochronology
  • Add numbers to the stratigraphic column based on fossils.
  • Based on the regular radioactive decay of some chemical elements.
isotopes
Isotopes

Different forms of the same

element containing the same

number of protons, but varying

numbers of neutrons.

i.e.:

235U, 238U 87Sr, 86Sr 14C, 12C

half life
Half-life

The half-life of a radioactive isotope is defined as the time required for half of it to decay.

isotopic dating
Isotopic dating
  • Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters).
  • The rate at which this decay occurs is constant and knowable.
  • Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding.
geologically useful decay schemes
Geologically Useful Decay Schemes

Parent Daughter Half-life (years)

235U 207Pb 4.5 x 109

238U 206Pb 0.71 x 109

40K 40Ar 1.25 x 109

87Rb 87Sr 47 x 109

14C 14N 5730

uniformitarianism
Uniformitarianism

The present is the key to the past.

Natural laws do not change—

however, rates and intensity of

processes may.

— James Hutton

slide55

1871

Fig. 9.17

slide56

1968

Fig. 9.17

paleontology
Paleontology

The study of life in the past based on fossilized plants and animals.

Fossil: Evidence of past life

Fossils preserved in sedimentary rocks are used to determine: 1) Relative age 2) Environment of deposition

many methods have been used to determine the age of the earth
Many methods have been used to determine the age of the Earth

1) Bible: In 1664, Archbishop Usher of Dublin used chronology of the Book of Genesis to calculate that the world began on Oct. 26, 4004 B.C.

2) Salt in the Ocean: (ca. 1899) Assuming the oceans began as fresh water, the rate at which rivers are transporting salts to the oceans would lead to present salinity in ~100 m.y.

many methods have been used to determine the age of the earth61
Many methods have been used to determine the age of the Earth

3) Sediment Thickness: Assuming the rate of deposition is the same today as in the past, the thickest sedimentary sequences (e.g., Grand Canyon) would have been deposited in ~ 100 m.y.

4) Kelvin’s Calculation: (1870): Lord Kelvin calculated that the present geothermal gradient of ~30°C/km would result in an initially molten earth cooled for 30 – 100 m.y.

flawed assumptions
Flawed assumptions

• Bible is not a science text or history book

• Salt is precipitated in sedimentary formations

• Both erosion and non-deposition are major parts of the sedimentary record

• Radioactivity provides another heat source

the heat inside the earth
The heat inside the Earth

The discovery of radioactivity at the turn of the century by Bequerel, Curie, and Rutherford not only provided the source of the heat to override Kelvin’s calculations but provided the basis for all later quantitative estimates of the ages of rocks.

oldest rocks on earth
Oldest rocks on Earth

Slave Province, Northern Canada

  • Zircons in a metamorphosed granite dated at 3.96 Ga by the U-Pb method

Yilgarn block, Western Australia

  • Detrital zircons in a sandstone dated at 4.10 Ga by U-Pb method.

Several other regions dated at 3.8 Ga by various methods including Minnesota, Wyoming, Greenland, South Africa, and Antarctica.

age of the earth
Age of the Earth

Although the oldest rocks found on Earth are 3.96 Ga (or even 4.1), we believe that the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.0 Ga have been destroyed (the rock cycle) or are presently covered by younger rocks.

age of the earth66
Age of the Earth

This is based on the age of rocks brought back from the Moon (4.4 Ga), and meteorites (4.6 Ga), that are thought to be good representatives of the early solar system as well as more complicated geochemical modeling. This data suggests that the present chemical composition of the crust must have evolved for more than 4.5 Ga.

double it and add 1
Double it and add 1

number of number of number of D/P

half-lives parents daughters

0 64 0 0

1 32 32 1

2 16 48 3

3 8 56 7

4 4 60 15

5 2 62 31

the geologic timescale and absolute ages
The geologic timescale and absolute ages

Isotopic dating of intebedded volcanic rocks allows assignment of an absolute age for fossil transitions

the big assumption
The big assumption

The half-lives of radioactive isotopes are the same as they were billions of years ago.

test of the assumption
Test of the assumption

Meteorites and Moon rocks (that are thought to have had a very simple history since they formed), have been dated by up to 10 independent isotopic systemsall of which havegiven the same answer. However, scientists continue to critically evaluate this data.

frequently used decay schemes have half lives which vary by a factor of 100
Frequently used decay schemeshave half-lives which vary by a factor of > 100

parent daughter half life (years)

235U 207Pb 4.5 x 109

238U 206Pb 0.71 x 109

40K 40Ar 1.25 x 109

87Rb 87Sr 47 x 109

147Sm 144Nd 106 x 109

what if the rates have varied
What if the rates have varied?

What we think happened:

rate

of

decay

 time

what if the rates have varied73
What if the rates have varied?

What we know didn’t happen:

rate

of

decay

 time

best initial d 0
Best initial D = 0

Two ways around this problem:

1) Choose minerals with no initial daughter.

2) Use a method that tells you the initial concentration of D and P.

minerals with no initial daughter
Minerals with no initial daughter
  • 40K decays to 40Ar (a gas)
  • Zircon: ZrSiO4

ion radius (Å)

Zr4+ 0.92

U4+ 1.08

Pb2+ 1.37