William E. Ferguson

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.

Amount of Time Required for Some Geologic Processes and Events

Some geologic processes can be documented using historical records (brown area is new land from )

Uniformitarianism The present is the key to the past. — James Hutton Natural laws do not change— however, rates and intensity of processes may.

Two ways to date geologic events 1 RELATIVE DATING (relative position of fossils, structure) 2 ABSOLUTE DATING (isotopic, tree rings, varves, etc.)

RELATIVE GEOLOGIC TIME Steno Laws (1669) developed to arrange rock units in time-order Principle of Superposition Principle of Original Horizontality Law of Cross -Cutting Relationships Law of Inclusions Laws apply to both sedimentary and volcanic rocks.

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

Jim Steinberg/Photo Researchers Oldest rocks Youngest rocks Principle of Superposition

Principle of Original Horizontality Layered strata are deposited horizontal or nearly horizontal or nearly parallel to the Earth’s surface.

Principles of original horizontality and superposition

Law of Cross-Cutting Relationships A rock or feature is younger than any rock or feature it cuts across.

Law of Cross-cutting Relationships Fig. 9.9

LAW OF INCLUSIONS Included rocks are older than surrounding rocks.

PRINCIPLE OF FAUNAL SUCCESSION Principle of Faunal Succession - groups of fossil plants & animals have followed one another in a definite & discernable order so certain fossil assemblages characterize a specific time. INDEX FOSSILS - fossils used to correlate a specific time period Based on distinct preservable parts, lived a short time, in a specific environment with wide distribution - MICROFOSSILS

Ammonite FossilsPetrified Wood Chip Clark Tom Bean

Using Fossils to Correlate Rocks

Correlating beds using index fossils

Unconformity A buried surface of erosion Separates much older, eroded strata from younger ones Hiatus - the time gap or the time lost in the record

Unconformitites - 3 kinds Disconformity - undeformed beds Nonconformity - sedimentary over igneous or metamorphic rx. Angular Unconformity - overlying tilted beds

Formation of a Disconformity

South rim of the Grand Canyon 250 million years old 550 million years old 1.7 billion years old Paleozoic Strata Precambrian

South rim of the Grand Canyon 250 million years old 550 million years old Nonconformity 1.7 billion years old

Nonconformity in the Grand Canyon

Vishnu Schist (~1700 million years old) Tapeats Sandstone (~550 million years old)

Angular unconformity, Grand Canyon

The Great Unconformity of the Grand Canyon Geoscience Features Picture Libraryc

Formation of an Angular Unconformity

Reconstructing Relative Sequence of Events

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

South rim of the Grand Canyon

Generalized Stratigraphic Section of Rocks Exposed in the Grand Canyon after: Beus & Moral (1990)

Some of the Geologic Units Exposed in the Grand Canyon Michael Collier

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

The Geologic Column and Time Scale

Absolute geochronology Adds numbers to the stratigraphic column based on fossils. Based on the regular radioactive decay of some chemical elements.

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.

Isotopes Different forms of the same element containing the same number of protons, but varying numbers of neutrons. i.e. i.e.: 235 U, 238 U 87 Sr, 86 Sr 14 C, 12 C

Naturally Occurring Isotopes of Carbon

Electron Capture Beta Decay Alpha Decay

Production and Decay of Radiocarbon

Radioactive Decay of Rubidium to Strontium

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

Proportion of Parent Atoms Remaining as a Function of Time

Geologically Useful Decay Schemes ParentDaughterHalf-life (years) 235 U 207 Pb4.5 x U 206 Pb0.71 x K 40 Ar1.25 x Rb 87 Sr47 x C 14 N5730

PROBLEMS NEED A CLOSED SYSTEM!!! –MINERAL MAY LEAK PARENT OR DAUGHTER –MINERAL MAY BE CONTAMINATED WITH EITHER PARENT OR DAUGHTER

Another Clock Paleomagnetism Earth’s magnetic field reverses every half million years Reversals are recorded in rocks that are forming at that time - seafloor Time scale calibrated by both relative & absolute time methods

Earth’s Magnetic Field

Lavas record magnetic reversals

Calculating Relative Plate Motion

1871

1968

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 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 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 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 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 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 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 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 number ofnumber of number ofD/P half-livesparentsdaughters

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 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.

Frequently used decay schemes have half-lives which vary by a factor of > 100 parentdaughterhalf life (years) 235 U 207 Pb4.5 x U 206 Pb0.71 x K 40 Ar1.25 x Rb 87 Sr47 x Sm 144 Nd106 x 10 9

What if the rates have varied?  time rate of decay  What we think happened:

What if the rates have varied?  time rate of decay  What we know didn’t happen:

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 40 K decays to 40 Ar (a gas) Zircon: ZrSiO 4 ion radius (Å) Zr U Pb

Principle of Lateral Continuity Map view

Principle of Lateral Continuity Map view

Principle of Lateral Continuity Map view

Sedimentation of Beds A-D Beneath the Sea Fig. 9.6

Uplift and Exposure of D to Erosion

Fig. 9.6 Continued Erosion Removes D and Exposes C to Erosion

Fig. 9.6 Unconformity: a buried surface of erosion Subsidence and Sedimentation of E over C

Fig. 9.8 Sedimentation of Beds A-D Beneath the Sea

Fig. 9.8 Deformation and Erosion During Mountain Building

Fig. 9.8 Erosional Surface Cuts Across Deformed Rocks

Fig. 9.8 Subsidence and Subsequent Deposition Buries Erosional Surface Angular Unconformity

Fig Schlumberger Executive Communications

Sequence A forms during lower sea level Fig. 9.11a

Fig. 9.11b Sequence B forms during higher sea level

The Geologic Time Scale Fig. 9.13

Areas with Potentially Hazardous Amounts of Radon