Geologic timeGeologic time

Geologic Time

Geologic time scaleRelative dating principlesAbsolute dating w/ radioactivity Correlation of rock layers 

• The relative geologic time scale has a sequence of eons eras periods epochs – but no numbers

indicating how long ago each of these times occurred

Relative Geologic Time Scale

Geologic time scale

Eons

Phanerozoic - last 570 million years

Cenozoic - recent life

Mesozoic - age of middle life

Paleozoic - ancient life

Precambrian - from birth of Earth up to before complex life forms developed

Eras

0-66.4 m.y.a

66.4-245 m.y.a

245-570 m.y.a

(mammals, humans)(mammals, humans)

(dinosaurs, 1st scrawny mammals)(dinosaurs, 1st scrawny mammals)

(fish, trilobites, clams, corals, ferns )(fish, trilobites, clams, corals, ferns )

(algae, bacteria, some fossils without shells like jellyfish)(algae, bacteria, some fossils without shells like jellyfish)

“Telling time geologically”

Earth’s history concealed in rocks

Goal of geology: unraveling Earth’s history

Principle time keeping devices:

Relative dating - putting rocks/events in proper order

Absolute dating - determining event’s actual time

Relative Dating: Key principles

Relative Dating: Key principles

Law of superpositionLaw of superposition

Principle of original horizontalityPrinciple of original horizontality

Principle of cross-cutting relationshipsPrinciple of cross-cutting relationships

InclusionsInclusions

UnconformitiesUnconformities

A bed of rock is older than that above, younger than that belowA bed of rock is older than that above, younger than that below

Law of superpositionLaw of superposition

Principle of original horizontalityPrinciple of original horizontality

Principle of cross-cutting relationshipsPrinciple of cross-cutting relationships

InclusionsInclusions

UnconformitiesUnconformities

A bed of rock is older than that above, younger than that belowA bed of rock is older than that above, younger than that below

Relative Dating: Key principles

Sedimentary layers deposited flatSedimentary layers deposited flat

Law of superpositionLaw of superposition

Principle of original horizontalityPrinciple of original horizontality

Principle of cross-cutting relationshipsPrinciple of cross-cutting relationships

InclusionsInclusions

UnconformitiesUnconformities

A bed of rock is older than that above, younger than that belowA bed of rock is older than that above, younger than that below

Sedimentary layers deposited flatSedimentary layers deposited flat

Faults & dikes are younger than beds they crossFaults & dikes are younger than beds they cross

Relative Dating: Key principles

InclusionsInclusions

UnconformitiesUnconformities

Fragments of one rock enclosed in anotherFragments of one rock enclosed in another

Relative Dating: Key principles

Fragments of one rock Fragments of one rock enclosed in anotherenclosed in another

InclusionsInclusions

UnconformitiesUnconformities

Fragments of one rock enclosed in anotherFragments of one rock enclosed in another

Break in time in depositionBreak in time in deposition

(Conformable layers: deposited w/out interruption)

Relative Dating: Key principles

• An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.

Relative Dating: Key principles UnconformitiesUnconformities

3 types Angular unconformity

Disconformity

Nonconformity

• A disconformity is when two sedimentary rock layers are separated by an erosional surface.

• A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks.

• An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.

Relative Dating: Key principles UnconformitiesUnconformities

3 types

Angular unconformity

Disconformity

Nonconformity

Tilted/folded sedimentary rocks overlain by younger, more flat layers

Deposition

Folding/Uplift

Erosion

Subsidence/more erosion

Recipe for an angular unconformity

Relative Dating: Key principles UnconformitiesUnconformities

3 types

Angular unconformity

Disconformity

Nonconformity

Layers on either side of unconformity are parallel

Relative Dating: Key principles UnconformitiesUnconformities

3 types

Angular unconformity

Disconformity

NonconformityInterface between sedimentary layers and metamorphic or igneous rock

Unconformity Types

Relative Dating Principles

Correlation of rock layers

Matching up rocks from different places that are similar in age

Physical criteria

Fossils

Correlation of rock layers

Physical criteria (OK for short distances)

position of a bed in sequence of beds

distinct/uncommon minerals

Correlation of rock layers

Fossils

fossil organisms succeed each other in systematic fashion

any time period can be determined by fossil content

“principle of fossil succession”

Correlation of rock layers

“Index fossils”

geographically wide-spread

short span of time

T&L Figure 8.10

Overlapping Ranges of Fossils

corr

elat

ed s

eque

nce

Mostlynonmarinelimestone

1Kansas

2Indiana

3Ohio

4Pennsylvania

Absolute dating w/ radioactivity

Atommodel

Chemistry review

nucleusele

ctro

ns

protronsneutrons

# protrons = atomic #,defines the element

# neutrons can vary:“isotopes”

Absolute dating w/ radioactivity

Parent DaughterNow 1000 0 Next year 500 500 2 yrs from now 250 750 3 yrs from now 125 875 10 yrs from now 1 999

Example: an isotope with a half life of 1 year

# of atoms

Radioactive Decay

Absolute dating w/ radioactivity