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Geologic Time. Marble demo Some Index Fossils Coin Toss Sheet Color Copies of Expected Values for X 2 in homework Time Scale.doc. Determining geological ages. Relative age dates – placing rocks and geologic events in their proper sequence

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geologic time

Geologic Time

Marble demo

Some Index Fossils

Coin Toss Sheet

Color Copies of Expected Values for X2 in homework

Time Scale.doc

determining geological ages
Determining geological ages
  • Relative age dates – placing rocks and geologic events in their proper sequence
  • Numerical dates – define the actual age of a particular geologic event (termed absolute age dating)
principles of relative dating developed by nicolaus steno in 1669
Principles of relative datingDeveloped by Nicolaus Steno in 1669
  • 1. Law of superposition
      • In an undeformed sequence of sedimentary or volcanic rocks, oldest rocks at base; youngest at top

Niels Steensen (Nicolas Steno)

Steno recognized the organic origin of fossils and sketched a theory of geological strata, which he used in an attempt to reconstruct Tuscany's geological development

steno s 2nd principle of relative dating
Steno’s 2nd principle of relative dating
  • Principle of original horizontality
      • Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting)
steno s 3rd principle of relative dating
Steno’s 3rd principle of relative dating
  • Principle of cross-cutting relationships

Chunks of this “country rock” have broken off and are visible in the intrusion

3rd principle of relative dating
3rd principle of relative dating
  • Principle of cross-cutting relationships

This fault shows the offset

of the two sides. See the key beds? Notice this side is lower

unconformities loss of rock record
Unconformities (loss of rock record)
  • An unconformity is a break in the rock record produced by erosion and/or non-deposition
  • Types of unconformities
    • Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement) with time lost
    • Angular unconformity – tilted rocks overlain by flat-lying rocks
    • Disconformity – strata on either side of the unconformity are parallel (but time is lost)
slide9

Layered

sedimentary

rocks

(a)

8_9

Nonconformity

Igneous

intrusive rock

Metamorphic

rock

(b)

Younger

sedimentary

rocks

Angular

unconformity

Older, folded

sedimentary

rocks

(c)

Disconformity

Trilobite

(490 million years old)

Brachiopod

(290 million years old)

slide10

Development of a Nonconformity

Pennsylvanian sandstone over Precambrian granite is a nonconformity

slide13

Horizontal younger sediments over tilted older sediments

Cambrian Tapeats sandstone over Precambrian Unkar Group

What type of unconformity is this?

Grand Canyon in Arizona

slide14

Cross Cutting Relationships in strata

Zoroaster Granite across Vishnu Schist

correlation of rock layers
Correlation of rock layers
  • Matching strata of similar ages in different regions is called correlation

http://www.uwsp.edu/geo/faculty/ozsvath/images/stratigraphy.jpg

slide16

Correlation of strata in

southwestern United States

Sections are incomplete

Match with fossils

Matching lithology is risky, discussion

slide17

How impression fossils form (the most common type)

8_10

Shells

settle on

ocean

floor

Cast forms when mold

is filled in with mineral

water

Rock broken

to reveal

fossil cast

Rock broken

to reveal external mold

of shell

Shells

buried in

sediment

Mold, or cavity,

forms when original

shell material

is dissolved

correlation of rock layers with fossils
Correlation of rock layers with fossils
  • Correlation often relies upon fossils
      • Principle of fossil succession (Wm. Smith)
        • fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it
      • Index Fossils - useful for correlation
        • Existed for a relatively brief time
        • Were widespread and common

http://www.csun.edu/~psk17793/ES9CP/ES9%20fossils.htm

  • Most fossils are just impressions. A few may have small amounts of some original tissue
determining the ages of rocks using overlap of fossils
Determining the ages of rocks using overlap of fossils

Overlap time span is shorter

than that of any one fossil.

geologic time scale
Geologic time scale
  • The geologic time scale – a “calendar” of Earth history
      • Subdivides geologic history into units
      • Originally created using relative dates
  • Structure of the geologic time scale
      • Eon, Era, Period, Epoch
slide22

Geologic Timescale

Homework

Learn Timescale.doc less Epochs

Divisions based on fossils

Eon, Era, Period, Epoch

geologic time scale24
Geologic time scale
  • Structure of the geologic time scale
      • Names of the eons
        • Phanerozoic (“visible life”) – the most recent eon, began about 545 million years ago
        • PreCambrian (Cryptozoic)
          • PreCambrian subdivisions:
          • Proterozoic – begins 2.5 billion years ago
          • Archean – begins 3.8 bya
          • Hadean – the oldest eon begins 4.6 bya

Read from bottom to top – Oldest to Youngest

geologic time scale25
Geologic time scale
  • Precambrian time
      • Nearly 4 billion years prior to the Cambrian period
      • Long time units because the events of Precambrian history are not know in detail – few fossils, most rock modified
      • Immense space of time (Earth is ~ 4.6 Ga)
      • PreCambrian spans about 88% of Earth’s history
geologic time scale26
Geologic time scale
  • Structure of the geologic time scale
      • Era – subdivision of an eon
      • Eras of the Phanerozoic (visible life”) eon
        • Cenozoic (“recent life”) begins ~ 65 mya
        • Mesozoic (“middle life”) begins ~ 248 mya
        • Paleozoic (“ancient life”) begins ~ 540 mya
      • Eras are subdivided into periods
      • Periods are subdivided into epochs
using radioactivity in dating
Using radioactivity in dating
  • Importance of radiometric dating
      • Allows us to calibrate geologic timescale
      • Determines geologic history
      • Confirms idea that geologic time is immense
slide28

Radiometric Age Determinations

show Earth not as old as Moon, Meteorites

  • Included in some sediment from NW Australia, detrital grains of the mineral Zircon that are 3.96 billion years old. The dates are based on datable Uranium in the Zircons.
  • Similar dates are known from Yellow Knife Lake, NWT, Canada
  • Claims of older zircons 4.4 by.
radiometric age determinations of the earth
Radiometric Age Determinations of the Earth
  • However, the age of the Earth is thought to be about 4.5 - 4.6 billion years
  • Based on the dates obtained from meteorites and samples collected on the moon, assumed to have formed at the same time.
recall isotopes
Recall Isotopes
  • The number of protons in an atom's nucleus is called its atomic number –defines “element”
  • Protons + neutrons called atomic weight
  • The number of neutrons can vary
  • Atoms of the same element with different numbers of neutrons are called isotopes.Some are radioactive
slide31

Radioactive

parent nucleus

Decay process

Daughter

nucleus

p

p

p

p

p

p

p

Atomic mass decreases

by 4; atomic number

decreases by 2

p

p

p

p

p

p

p

Proton

Neutron

p

Alpha particle

Emission of 2 protons and 2 neutrons (alpha particle)

(a)

Alpha decay

p

p

p

p

p

p

Atomic mass not changed

much; atomic number

increases by 1 because

Neutron becomes proton

p

p

p

p

p

p

p

p

p

p

p

p

p

Beta particle

An electron (beta particle) is ejected from the nucleus

(b)

Beta decay

p

p

p

Atomic mass not changed much;

atomic number

decreases by 1

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

electron combines with a proton to form a neutron

Beta particle

(c)

Electron capture

using radioactivity in dating32
Using radioactivity in dating
  • Parent – an unstable radioactive isotope
  • Daughter product – stable isotopes resulting from decay of parent
  • Half-life – time required for one-half of the parent isotope in a sample to decay into stable daughter product
a radioactive decay curve
A radioactive decay curve

1/2 = 50% parent: 1 half-life has passed

1/2x1/2 = 1/4 = 25% parent: 2 half-lives have passed

1/2x1/2x1/2 = 1/8 = 12.5% parent: 3-half-lives have passed

MARBLE DEMO

slide35
How do we actually “date” a rock?
  • Collect sample
  • Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids
  • Measure parent/daughter ratio of target isotopes - mass spectrometer
  • Substance heated – Ions – move in Electrical Field, curved in Magnetic
slide36

Mineral

Dating a crystal

crystal

1

Mineral crystal

formed in igneous

rock

Parent

atoms

Igneous rock

2

buried beneath

younger rocks;

daughter atoms

formed by

normal decay

(3) We calculate age

based on half-life

Daughter

atoms

Usual Case

8 22b
8_22b

But IF:

Deep burial and

3

metamorphism

during mountain

building causes

daughter atoms

to escape from

crystal

Heat

Resets the clock

Rock looks as if it just formed: it looks young

Age found dates from metamorphic event

After mountain

4

building ends,

accumulation of

daughter atoms

in crystal

resumes

Easily recognized,

useful in studying

metamorphism

Metamorphism Case

dating sediments without fossils superposition cross cutting
Dating sediments without fossils: Superposition, Cross-cutting

Wasatch Fm. younger than 66 my

Mancos Shale and Mesa Verde Fm.

older than 66 my

Rule of Cross-cutting

Morrison Fm older than 160 my

(superposition)

Radiometric Dating with Igneous Rocks

Or Bracket between fossiliferous layers

slide39

Basalt Lava flow 2

200 mya

We can bracket this

limestone’s age

between 209 and 200

mya

Lava flow 1

209 mya

Even better: we get lucky. A layer we need to date is between two datable beds

So we have and upper and lower bound on the age of this limestone:

slide40
Dating with carbon-14 (Carbon Dating)
  • Half-life only 5730 years
  • Used to date very young rocks
  • Carbon-14 is produced in the upper atmosphere
  • Useful tool for geologists who study very recent Earth history
slide41

Cosmic rays

bombard

atmospheric atoms

Atoms split into

smaller particles,

among them neutrons

Carbon-14

Neutrons strike

nitrogen atoms

Nitrogen atom gains a neutron and loses a

Proton; becomes carbon-14

C-14 mixes with atmospheric oxygen

to produce CO2

C-14 absorbed

by living

organisms

CO2 taken up

by plants, water

CO2 dissolved

in water

C-14 intake ceases when organism

dies; C-14 concentration decreases

slide42

Tree Rings both modern and past 2000 years

Years of age

500

Annual-ring similarities

show correlation

400

Current year

200

150

100

50

Tree

growth

rings

A

D

C

B

A

B

Sediment layers

with tree logs to

be collected for

dendrochronology

C

D

Buried tree

logs

slide43

Southern lakes track glaciation

8_28

Dating with Lake Varves

Very

little

or no

runoff

Heavy

runoff

into

lake

Ice

Winter

Summer

Clear water

Turbid water

Winter layer

(fine, thin, and

dark-colored)

Summer layer

(coarse, thick, and

light-colored)

Lake deposits, fossil plants C14. Fossil tree pollen track climate.