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Chapter 8

Time and Geology

GEOL 101

Introductory Geology

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Determining Geological Ages

Relative age dates – placing rocks and events in their proper sequence of formation

Numerical (absolute age) dates – specifying the actual number of years that have passed since an event occurred

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Principles of Relative DatingLaw of superposition

• Undeformed section of sedimentary or layered igneous rocks

• Oldest rocks are on the bottom

Principle of original horizontality• Layers of sediment are generally deposited

in a horizontal position• Rock layers that are flat have not been

disturbed (deformed)

Principle of cross-cutting relationships• Younger features cut across older features

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Superposition Strata in the Grand Canyon

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Horizontality

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Cross-cutting Relationship

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Cross-cutting Relationship

Which crater is youngest?

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Cross-cutting Relationships

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Principles of Relative Dating

Inclusions

A piece of rock that is enclosed within another rock

Rock containing the inclusion is younger

Unconformity

Break in rock record produced by erosion and/or nondeposition of rock

Represents period of geologic time

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Principles of Relative Dating

Types of unconformities• Angular unconformity – tilted rocks

(disturbed) are overlain by flat-lying rocks

• Disconformity – strata on either side of the unconformity are parallel

• Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata

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Angular Unconformity

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Angular Unconformity

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Angular Unconformity

Animation

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Disconformity

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Disconformity

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Nonconformity

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Nonconformity

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Nonconformity

Inclusions

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Grand Canyon Unconformities

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Interpret Geologic HistoryAnimation

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Correlation of Rock Layers

Matching of rocks of similar ages in different regions is known as correlation

Correlation often relies upon fossils• similar sedimentary strata in widely

separated areas• identified and correlated by distinctive

fossil content• Principle of fossil succession

– fossil organisms succeed one another in a definite and determinable order

– time period recognized by its fossil content

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Correlation of Rock Layers

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Fossils Ages of Rocks

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Radioactivity: spontaneous changes (decay) in the structure of atomic nucleiBasic atomic structureTypes of radioactive decayParent, daughter product, half-lifePrinciple of radioactive datingSources of errorCarbon-14 (radiocarbon) datingImportance of radiometric dating

Radiometric Dating

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Basic Atomic Structure• Nucleus

– Protons: positively charged particles with mass– Neutrons: neutral particles with mass– Electrons: negatively charged particles, orbit nucleus

• Atomic number– An element’s identifying number– Number of protons in the atom’s nucleus

• Mass number– Sum of protons & neutrons in atom’s nucleus

• Isotope– Variant of the same parent atom– Differs in the number of neutrons– Different mass number than the parent atom

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Types of radioactive decay• Alpha emission

– Emission of 2 protons & 2 neutrons (alpha particle) – Mass number is reduced by 4 and the atomic

number is lowered by 2• Beta emission

– Electron (beta particle) is ejected from the nucleus– Mass number remains unchanged and the atomic

number increases by 1• Electron capture

– Electron is captured by the nucleus– Electron combines with a proton to form a neutron– Mass number remains unchanged and the atomic

number decreases by 1

Radiometric Dating

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Radiometric Decay

Electron Capture

AlphaEmission

BetaEmission

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Parent – an unstable radioactive isotope

Daughter product – the isotopes resulting from the decay of a parent

Half-life – the time required for one-half of the radioactive nuclei in a sample to decay

Radiometric Decay

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Uranium Isotopes

ParentU238

DaughterPB206

Atomic # - 92Mass # - 238

Atomic # - 82Mass # - 206

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Principle of radioactive dating• The percentage of radioactive toms that

decay during one half-life is always the same (50 percent)

• However, the actual number of atoms that decay continually decreases

• Comparing the ratio of parent to daughter yields the age of the sample

Radiometric Dating

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Radioactive Decay Curve

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Radioactive Decay Curve

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Radiometric Dating

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Sources of error• A closed system is required• To avoid potential problems, only fresh,

unweathered rock samples should be used

Carbon-14 (radiocarbon) dating• Half-life of only 5730 years• Used to date very recent events• C14 is produced in the upper atmosphere

Radiometric Dating

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C14 Production and Decay

Neutron Capture

BetaEmission

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Importance of radiometric dating• Radiometric dating is a complex

procedure that requires precise measurement

• Rocks from several localities have been dated at more than 3 billion years

• Confirms the idea that geologic time is immense

Radiometric Dating

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Other Dating Methods

Shell GrowthRings

TreeRings

Rhythmic Layering in Glaciers

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Cross-Dating Principle in Dendrochonology

Youngest Older

Oldest

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Geologic Time Scale

Geologic time scale – Earth history calendar • Subdivides geologic history into units• Originally created using relative dates

Structure of the geologic time scale• Eon – the greatest expanse of time

– Phanerozoic – “visible life”, most recent eon

– Proterozoic – before life

– Archean – ancient

– Hadean – mythical subterraneon world of departed spirits, oldest eon

– Precambrian: Proterozoic, Archean, Hadean

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Geologic Time ScaleEra – subdivision of an eon

Precambrian – no era subdivisions

Eras of the Phanerozoic eon• Cenozoic (“recent life”)• Mesozoic (“middle life”)• Paleozoic (“ancient life”)

Eras are subdivided into periods

Periods are subdivided into epochs

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Geologic Time Divisions

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Geologic Time Scale

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Geologic Time ScalePrecambrian time

• Nearly 4 billion years prior to the Cambrian period

• Not divided into smaller time units 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

Visualizing vast period of geologic time

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Evolution vs Geologic Time

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Visualizing Geologic Time

Twice around the equator

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Geologic Time Scale Difficulties• Not all rocks can be dated by radiometric

methods– Grains comprising detrital sedimentary rocks are

not the same age as the rock in which they formed– The age of a particular mineral in a metamorphic

rock may not necessarily represent the time when the rock formed

• Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages

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Sedimentary Ages using Radiometric Dating

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Stratigraphic Dating