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1 Relative Age Dating Absolute Age Dating Age Dating Using Magnetic Records The Age of the Earth

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.

2 Earth History

Two ways to date geologic events

1) (, structure) 2) absolute dating (isotopic, tree rings, etc.)

3 Relative vs Absolute Age • Usually geologists first establish relative ages then try to get absolute age dates • Determining relative age relies on a number of geologic principles that were developed during the 17th to early 19th centuries

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.

4 Principle of Superposition

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

Principle of Superposition Sedimentary rocks are deposited in a layer-cake fashion:

Layer 4 Layer 3 Layer 2 Layer 1 Each layer is older than the one above and younger than the one below

5 Principle of Superposition

Youngest rocks

Oldest rocks

Principle of Original Horizontality

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

6 Principles of original horizontality and superposition

7 Principle of Lateral Continuity

Layered rocks are deposited in continuous contact.

Principle of Lateral Continuity

Map view

8 Principle of Lateral Continuity

Map view

Principle of Lateral Continuity

Map view

9 Correlation • Process used to tie separated strata together

• Based on matching physical features such as – Physical continuity - trace of rock unit – Similar rock types - marker beds, coal seams, rare minerals, odd color

Correlation • Within sedimentary layers there are often the remains of small animals (fossils) • Fossils are quite useful for correlating between two sections that are not laterally continuous

10 Ammonite Fossils Petrified Wood

Correlation • Fossils represent living creatures that have evolved through time, so when we find a of the same type in two different areas, we are pretty sure that the rocks are about the same age

11 CorrelatingCorrelating bedsbeds usingusing indexindex fossilsfossils

Using Fossils to Correlate Rocks

12 Unconformity

A buried surface of

13 Layers of rock are said to be conformable when they are found to have been deposited essentially without interruption

Unconformity represents missing time in the geologic record

Sedimentation of Beds A-D Beneath the Sea

14 Uplift and Exposure of D to Erosion

Continued Erosion Removes D and Exposes C to Erosion

15 Subsidence and of E over C

Unconformity: a buried surface of erosion

Formation of a Disconformity

16 First type of unconformity

South rim of the Grand Canyon

17 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 Canyon 250 million years old

550 million years old 1.7 billion years old Nonconformity

18 Nonconformity in the Grand Canyon

Nonconformity in the Grand Canyon

Tapeats Sandstone (~550 million years old)

Vishnu Schist (~1700 million years old)

19 Second type of unconformity

Siccar Point, Scotland

20 Siccar Point, Scotland

Buried and tilted erosional surface

Sedimentation of Beds A-D Beneath the Sea

21 Deformation and Erosion During Mountain Building

Erosional Surface Cuts Across Deformed Rocks

22 Subsidence and Subsequent Buries Erosional Surface

Angular Unconformity

Formation of an Angular Unconformity

23 Third type of unconformity

Principle of Cross-Cutting Relationships • A rock unit must always be older than any feature that cuts or disrupts it – If a rock unit is cut by a fracture • The rock itself is older than the fracture that cuts across it

24 Cross-cutting Relationships

Relative Geologic Dating

25 The

• 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

TheThe GeologicGeologic TimeTime ScaleScale

26 Absolute geochronology

• Add numbers to the stratigraphic column based on fossils. • Based on the regular radioactive decay of some chemical elements.

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

27 NaturallyNaturally OccurringOccurring IsotopesIsotopes ofof CarbonCarbon


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

28 Proportion of Parent Atoms Remaining as a Function of Time

Isotopic dating • Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters). • The rate at which this decay occurs is constant and known. • 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.

29 Radioactivity and Absolute Time

30 31 Radioactive Decay of Rubidium to Strontium by β emission

Production and Decay of Radiocarbon

32 of Radiocarbon of Radiocarbon Production and Decay Production and Decay

Major Radioactive Elements Used in Isotopic Dating

33 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

Radiometric Dating

• We can calculate geologic age if – The half-life of a radioactive isotope is known – The parent/daughter ratio can be measured – There is no loss of an isotopes from the system • e.g., 222Rn is an intermediate daughter product in the 238U decay series to 208Pb

34 Oldest rocks and minerals on Earth Narryer Gneiss, Western Australia • Zircons in a metamorphosed sandstone dated at 4.35 to 4.40 Ga Acasta Gneiss, Northwestern Canada and the Nuvvuagittuq , Northern Quebec • Rocks dated at 3.80 to 4.28 Ga Several other regions dated at 3.8 Ga by various methods including Minnesota, Wyoming, Greenland, South Africa, and Antarctica.

Age of the Earth Although the oldest rocks found on Earth are 4.4 Ga, we believe that the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.4 Ga have been destroyed (the rock cycle) or are presently covered by younger rocks.

35 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. These data suggest that the present chemical composition of the crust must have evolved for more than 4.5 Ga.

The geologic timescale and absolute ages Isotopic dating of intebedded volcanic rocks allows assignment of an absolute age for fossil transitions

36 The big assumption

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

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 systems all of which have given the same answer. However, scientists continue to critically evaluate this data.

37 Bracketing ages

Radiometric dates provide absolute ages to the Geologic Column

38 Earth’s Magnetic Field

Magnetostratigraphy • Technique that works best in volcanic rocks • Time scale based on polarity reversal of Earth's magnetic field • Major problem is that Earth's magnetic field has been constant for the past 700,000 yrs (no reversals), so this doesn't work for very young rocks

39 Magnetization of Magnetite

Lavas record magnetic reversals

40 MagneticMagnetic reversalsreversals overover thethe pastpast 2020 millionmillion yearsyears

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

41 The Geologic Column and Time Scale