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Geologic Time
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)
First principle of relative dating
Law of superposition
• Developed by Nicolaus Steno in 1669
• In an undeformed sequence of
sedimentary or volcanic rocks, oldest
rocks at base; youngest at top
Superposition illustrated by strata
in the Grand Canyon
2nd, 3rd principles of relative dating
Principle of original horizontality
• Layers of sediment are originally
deposited horizontally (flat strata have
not been disturbed by folding, faulting)
Principle of cross-cutting relationships
• Younger features cut across older ones
Cross Cutting Relationships in strata
Question: How many geologic “events” must
have occurred in this region to produce
the contacts shown here? “Events” can
Include: 1) periods of deposition; 2) uplift and
erosion; 3) burial and metamorphism or
4) igneous intrusion?
A)
B)
C)
D)
E)
4
6
8
10
12
Grand Canyon younger strata cutting across older ones
Cambrian Tapeats sandstone over Precambrian Unkar Group
Grand Canyon younger strata cutting across older ones
Cambrian Tapeats sandstone over Precambrian Unkar Group
Unconformities (loss of rock record)
• An unconformity is a break in the rock record produced
by erosion and/or nondeposition
• Types of unconformities
– Angular unconformity – tilted sedimentary rocks overlain by
flat-lying sedimentary or volcanic rocks
– Disconformity – strata on either side of the unconformity are
parallel (but time is lost)
– Nonconformity – sedimentary rocks deposited above
metamorphic or igneous rocks (basement)
Formation of an angular unconformity
An angular unconformity at Siccar Point, England
Development of a Nonconformity
The basement-cover contact near
Boulder (Pennsylvanian sandstone
over Precambrian granite) is a
nonconformity (visible on Flagstaff
Road near bouldering area)
Nonconformity in the Grand Canyon - Sediment deposited over Schist
Nonconformity in the Grand Canyon - sediment deposited over schist
What evidence tells you this is a nonconformity?
A) Non-sedimentary rocks located beneath the contact
B) Angular discordance in sediments across the contact
C) Weathering profile at contact
D) Loss of time across the contact
Nonconformity on Flagstaff Mountain Fountain Formation over granite
Weathered Boulder Creek
Granodiorite (1700 Ma)
Silver Plume
Dikes (1400 Ma)
Fountain Formation
(300 Ma)
What is the geologic history of the Boulder region as
revealed by this roadcut?
A)
Intrusion at 1700 & 1400, Deposition at 300
B)
Uplift at 2000, Intrusion at 1700 & 1400, Erosion at 1400-300, Deposition at 300
C)
Intrusion at 1700 & 1400, Uplift & Erosion between 1400-300, Deposition at 300
D)
Intrusion at 1700 & 1400, Deposition from sometime before 300 to-300, Uplift and
Erosion just before 300,
then Deposition after 300
Weathered Boulder Creek
Granodiorite (1700 Ma)
Silver Plume
Dikes (1400 Ma)
What is the geologic history of the Boulder region as
revealed by this roadcut?
A)
Intrusion at 1700 & 1400, Deposition at 300
B)
Uplift at 2000, Intrusion at 1700 & 1400, Erosion at 1400-300, Deposition at 300
C)
Intrusion at 1700 & 1400, Uplift & Erosion between 1400-300, Deposition at 300
D)
Intrusion at 1700 & 1400, Deposition from sometime before 300 to-300, Uplift and
Erosion just before 300,
then Deposition after 300
Weathered Boulder Creek
Granodiorite (1700 Ma)
Silver Plume
Dikes (1400 Ma)
Correlation of rock layers
Matching strata of similar ages in
different regions is called correlation
Correlation of strata in
southwestern United States
Sections are
incomplete
Correlation of rock layers with fossils
Correlation relies upon fossils
• Principle of fossil succession – fossil
organisms succeed one another in a
recognizable order - thus any time period
is defined by the type of fossils in it
Determining the ages of
rocks using fossils
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 – the greatest expanse of time
Geologic Timescale
Numbers
based on
absolute
age dates
Names based on fossils
Numbers based on radio
metric age dates
For exam
Know Eras
Periods and
Epochs and
Their order
Geologic Times Scale Mnemonic’s…
A.
Parents Can Only Sit Down Carefully; Perhaps Their Joints Can't Tolerate
Quickness winner by a longshot
B. Please Come Over, Sally Dean, Might Prepare Pasta Then Juice
Cantaloupes Too Quickly
C. Quoting Tim Curry Just Takes Patience Most Dancers
Seem Outstanding Counting Presley
D. Quadriplegics Tantalize Congressmen Just To Persuade Paralyzed Men
Disregarding Strippers Or Coloradans 2nd
E. Poor clueless onlookers slowly discussed many pretty perturbed thirsty joggers
cursing their quest 3rd
"Cold Oysters Seldom Develop Many Precious Pearls, Their
Juices Congeal Too Quickly." Another device for remembering
the order of the Epochs of the Cenozoic era is, "Pigeon Egg
Omelets Make People Puke Heartily."
Geologic time scale
Structure of the geologic time scale
• Names of the eons
– Phanerozoic (“visible life”) – the most recent
eon, began about 540 million years ago
– Proterozoic
– Archean
– Hadean – the oldest eon
This, believe it or not, is the rock with the oldest known minerals ever found. From
NW Australia, the rock (a conglomerate) is about 3.0 Billion years old. The rock
contains detrital grains of zircon (a mineral formed in granite in the crust) that is
4.4 Billion years old. Age of the Earth is 4.567 Billion ( ancient zircons first
discovered by Simon Wilde, Curtin Institute of Technology, Australia). Rock and
additional age dating courtesy of Steve Mojzsis) Amazing because Earth must
have differentiated quickly (<100Ma)
Geologic time scale
Structure of the geologic time scale
• Era – subdivision of an eon
• Eras of the Phanerozoic eon
– Cenozoic (“recent life”)
– Mesozoic (“middle life”)
– Paleozoic (“ancient life”)
• Eras are subdivided into periods
• Periods are subdivided into epochs
Geologic time scale
Precambrian time
• Nearly 4 billion years prior to the
Cambrian period
• Not divided into small time units because
the events of Precambrian history are not
know in detail
• Immense space of time (Earth is ~ 4.5 Ga)
Radioactivity (Used to age date rocks)
• Spontaneous changes (decay) in structure
of atomic nuclei
Types of radioactive decay
• Alpha emission
– Emission of 2 protons and 2 neutrons (an
alpha particle)
• Beta emission
– An electron (beta particle) is ejected from the
nucleus
• Electron capture
– An electron is captured by the nucleus
– The electron combines with a proton to form
a neutron
Neutron capture (A)
and Beta emission (B)
Using radioactivity in dating
Parent – an unstable radioactive isotope
Daughter product – isotopes resulting
from decay of parent
Half-life – time required for one-half of
the parent isotope in a sample to decay
A radioactive decay curve
Dating with carbon-14 (radiocarbon 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 (for me this is the
history of earthquakes).
The daughter-to-parent ratio (D/N) of two isotopes in a rock
is 7:1 and the half-life for conversion between the parent
and daughter is 100 million years. How old is the rock?
A) 200 million years
B) 300 million years
C) 500 million years
D) 700 million years
E) none of the above
The daughter-to-parent ratio (D/N) of two isotopes in a rock
is 7:1 and the half-life for conversion between the parent
and daughter is 100 million years. How old is the rock?
A) 200 million years
B) 300 million years
C) 500 million years
D) 700 million years
E) none of the above
Question worth 3 points if correct, 1 point if not…
Please work by yourself on this one…
A 400 million year old rock is dated using an isotopic
technique where the parent isotope has a half life of 100
million years. What are the percentages of parent and
daughter isotope left in the rock
Parent
A) 50%
B) 25%
C) 12.5%
D) 6.25%
E) 3.12%
Daughter
50%
75%
87.5%
93.75%
96.88%
Question worth 3 points if correct, 1 point if not…
Please work by yourself on this one…
A 400 million year old rock is dated using an isotopic
technique where the parent isotope has a half life of 100
million years. What are the percentages of parent and
daughter isotope left in the rock
Parent
A) 50%
B) 25%
C) 12.5%
D) 6.25%
E) 3.12%
Daughter
50%
75%
87.5%
93.75%
96.88%
Using radioactivity in dating
Importance of radiometric dating
• Allows us to calibrate geologic timescale
• Determines geologic history
• Confirms idea that geologic time is immense
How do we actually “date” a rock?
1. Collect sample (geologist = pack animal)
2. Process for minerals by crushing, sieve, separate
magnetically and/or with heavy liquids
3. Measure parent/daughter ratio of mineral
separates with a mass spectrometer
Dating sediments without fossils
Use volcanic rocks instead…
Question: What is the age of the Dakota Sandstone?
A) Older than 160 Million
B) Younger than 160 Million
C) Younger than 160 Million and older than 66 Million
D) Younger than 66 Million
E) None of the above
End of Chapter 8