CH. 11 – GEOLOGIC TIME

Geologic Time

Earth = 4.6 billion years old

Oldest rocks = 3.2 billion years

old

Question: How can we be sure

Earth is older than its oldest rx?

Base Units of Time

Humans = 1 year

Earth = 1 million years

Types of Geologic Dating

1) Relative Dating

- chronological order (1st, 2nd, 3rd, etc.)

Ex: Rock layers are ______than Grand Canyon itself

Types of Geologic Dating

2) Numerical Dating

- assigning an age in years to a

rock or geologic event

Ex: Grand Canyon is 10 million

yrs old

Methods of Relative Dating

1) Superposition – horizontal

layers (sed. rx & lava flows)

2) Original Horizontality

- layers tilted after deposition

Methods of Relative Dating

3) Cross-Cutting Relations

- a feature which cuts across rx

is younger than the rx it cuts

Ex: dike, fault, canyons

4) Inclusions

- fragment contained w/in a rock

is ________ than the rock it’s in

Methods of Relative Dating

Inclusions

Large-scale: Rock fragment of

sandstone in a batholith

- (batholith/sandstone) is older

Ex: Xenolith (“xeno” = stranger)

Piece of country rock found w/in

igneous rock

Inclusions

Small-scale:

Gravel in a conglomerate is

_______ than the

conglomerate

Methods of Relative Dating

5) Unconformities - “missing

time”

- gap of time in rock record at

that location

- usually buried erosional surface

Unconformities

Major John Wesley Powell

- “missing time” is like pages torn from history book

Unconformities

Three types:

a) Angular unconformity

- layers on bottom tilted at an

angle to layers on

top

Types of Unconformities

b) Nonconformity

- crystalline rx on bottom overlain

by sedimentary rx

- crystalline rx = intrusive

igneous or metamorphic

The great unconformity

Types of Unconformities

c) Disconformity

- layers above & below unconformity are parallel

- identified based on fossil content or channel cuts

Methods of Relative Dating

6) Fossil Succession

- plants & animals have changed

through time

- same sequence of major life

groups around world

Uses of Fossil Succession

a) Geologic Time Scale

- any time period can be

identified based on its fossil

content

- know eras and major life forms

- know boundary criteria

Geologic Time Scale

Paleozoic Era (“ancient life”)

“Age of Invertebrates”

- ended with largest mass

extinction in Earth’s history

(over 90% organisms were

extinct by 245 mya)

Geologic Time Scale

Mesozoic Era (“middle life”)

“Age of the Dinosaurs”

- ended with extinction of

dinosaurs (66 mya)

Geologic Time Scale

Cenozoic Era (“recent life”)

“Age of Mammals”

Will the Cenozoic Era end?

Uses of Fossil Succession

b) Correlation – matching rx of

similar ages in different regions

Methods of Correlation

i) Index fossil

Requirements:

a) widespread geographically

b) existed for a short period of

geologic time

Methods of Correlation

ii) Fossil Assemblages

*Overlapping geologic ranges

- narrows down time frame

Fossil Assemblages

Fossil A = Devonian-Tertiary

Fossil B = Cambrian-Permian

Fossil C = Pennsylvanian -

Jurassic

Age of rock:

Numerical Dating

Assigns an age in years to rx or

geologic events

One method is radiometric dating

Radioactivity = spontaneous

change w/in nucleus of atom

Radioactivity

Isotope – variable forms of same element due to different number of neutrons in atoms

Ex: Carbon

Atomic Number = 6

Atomic Mass = 12.011

Radioactivity

Unstable isotopes are

radioactive

- nucleus emits or captures

atomic particles

Radioactivity Vocabulary

1) Parent element = unstable atom

2) Daughter product = atom it turns into

Daughter Product

Belongs to a different element than parent element

- some daughter products are also unstable (radioactive)

- can take several steps to reach a stable atom

Radioactivity Vocabulary

3) Half-life = amount of time for one-half parent element to change into daughter product

*Rate of decay

NOTE: decay doesn’t imply we have less matter (atoms)

Half-life

Each parent element has a

unique half-life

Each half-life is a constant

- unaffected by changes in

temperature or pressure

Review of Radiometric Dating

1) Half-life is constant for each radioactive isotope

Ability to use radiometric dating depends on:

i) parent element available

ii) age of rock

Examples:

Parent Daughter Half-life238U 206Pb 4,500 my235U 207Pb 713 my40K 40Ar 1,300 my14C 14N 5,730 yrs

Half-life diagram

14C

Produced in upper atmosphere

Used to measure ages of organic

remains

Effective back to 75,000 yrs.

Review of Radiometric Dating

2)*IMPORTANT to have a closed system for an accurate date!

Radiometric dating is most accurate for igneous rx

- clock starts “ticking” as soon as magma crystallizes

Review of Radiometric Dating

Sedimentary rx – age obtained is

for source area of sediment

Sedimentary rock has to be

(younger, older) than this age

Review of Radiometric Dating

Metamorphic rx

- chance of losing atoms out of

system is greater

- partial melting can allow atoms

to escape (esp. Ar40)

Metamorphic Rx

If daughter product escapes

rock, its radiometric age will be

(older, younger) than it really is.

Geologic Time

Earth = 4.6 billion years old

Oldest rocks = 3.2 billion years

old

How can we be sure Earth is

older than its oldest rx?