ch11_geologic_time_students
TRANSCRIPT
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?
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
Methods of Relative Dating
5) Unconformities - “missing
time”
- gap of time in rock record at
that location
- usually buried erosional surface
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
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)
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
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
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.