Transcript powerpoint

Geologic Time
How long has this landscape looked like this? How can you
tell? Will your grandchildren see this if they hike here in 80
years?
The Good Earth/Chapter 8: Geologic Time
Geologic Time Importance – Fossil Fuels
Petroleum and coal are only found
in rocks young enough to have had
abundant life to produce
petroleum and coal.
The Good Earth/Chapter 8: Geologic Time
Geologic Time Importance - Diamonds
Diamonds are only found in very
old parts of earth.
The History of (Relative) Time
Paradigm shift: 17th century – science was a baby and geology as a discipline did
not exist. Today, hypothesis testing method supports a geologic (scientific) age
for the earth as opposed to a biblical age.
Structures such as the oldest Egyptian pyramids (2650-2150 B.C.) and the Great
Wall of China (688 B.C.) fall within a historical timeline that humans can relate
to, while geological events may seem to have happened before time existed!
Two Methods of Determining Geologic
Time
Relative Geologic Time – Places sedimentary
rocks in a sequence of old to young
rocks.
Radiometric (absolute or numeric) Time –
Uses radioactive isotopes to determine
the time since new minerals formed in a
rock. Usually used in igneous rocks.
Principle of Superposition
A
B
The Grand Canyon – rock layers record thousands of millions of
years of geologic history.
• In undeformed
sedimentary rocks,
the oldest rock is on
the bottom.
− Grand Canyon –
excellent model
− Which do you think
happened first – the
deposition of rocks at
location “A” or “B”.
Why?
At which location on the
picture left, A or B, are
the rocks younger?
The Good Earth/Chapter 8: Geologic Time
Principle of Original Horizontality
Red rock units
Tan rock units
Sedimentary rock deposited underwater is horizontal.
Sedimentary rock deposited on land CAN be non-horizontal.
The Good Earth/Chapter 8: Geologic Time
Principle of Cross-Cutting Relationships
Red rock units
Tan rock units
Important principles:
Superposition – rocks at the bottom are
the oldest.
Cross-cutting relationships – older rocks
may be cut by younger rocks or
features.
Inclusion – Younger rocks may
incorporate pieces of older rocks.
-More complicated histories are
represented by multiple events.
-Above: Explain the history of
the rocks using the events
deposition, erosion, and tilting.
Refer to the diagram at left for
help. (Hint: there are 4 major
events)
The Good Earth/Chapter 8: Geologic Time
Uniformitarianism
18th century - James Hutton watched the landscape of his
farmland and invented our modern concept of geologic time.
Observation:
The landscape remained unchanged with
the passage of time.
Deductions:
1) The same slow-acting geological processes that operate
today have operated in the past, meaning it takes a long time
to influence the Earth’s surface significantly
(Uniformitarianism).
2) All land should be worn flat (erosion) unless some process
renews the landscape by forming new mountains (cyclical
change).
- he called these eroded surfaces, representing gaps in time,
unconformities.
Controversial resulting message – Earth must be much older
than the commonly accepted age of 6,000 years.
The Good Earth/Chapter 8: Geologic Time
Quiz 1
Examine the following image of rock layers and answer
Questions 1 and 2 about relative time.
1. Which statement is
most accurate?
E
A
C
F
B
D
a. D is older than B
b. E is older than A
c. F is older than C
The Good Earth/Chapter 8: Geologic Time
Quiz 2
Examine the following image of rock layers and answer
Questions 1 and 2 about relative time.
2.
E
A
C
F
B
D
When did the tilting of
the layers occur?
a. After A was
deposited
b. Between deposition
of layers E and A
c. Before B was
deposited
d. Between deposition
of layers C and E
The Good Earth/Chapter 8: Geologic Time
Fossils
Geologists can correlate sedimentary rocks by comparing the
fossils found within the rocks
Fossils found in many rock
layers (long lived species) are
difficult to match to layers in
other regions.
Index fossils: species that
existed for a relatively short
period of geologic time and
found over large geographic
areas are the best for precise
correlations.
Which of the fossils in the
diagram at left (1,2, or 3) would
make the best index fossil?
Why?
The Good Earth/Chapter 8: Geologic Time
Grand Canyon Fossils
Fossils of the Grand Canyon support the
geologic interpretations
Although they do not preserve the body of an organism, tracks
are important trace fossils that tell us something about the
organisms that left them behind.
The Good Earth/Chapter 8: Geologic Time
Unconformities – Erasure of Earth History
Sedimentary rock layers are used to
determine earth history.
Rock erosion erases that record.
Places where ancient rocks were eroded
are called unconformities.
Three types of unconformities.
The Good Earth/Chapter 8: Geologic Time
Angular Unconformities
There is an angle between the
sedimentary rocks below the erosion
surface and the rocks deposited on top of
the erosion surface.
The Good Earth/Chapter 8: Geologic Time
Disconformity
The sedimentary rocks are parallel on
either side of the erosion surface.
The Good Earth/Chapter 8: Geologic Time
Nonconformity
Sedimentary rocks have been deposited
on igneous or metamorphic rocks.
The Good Earth/Chapter 8: Geologic Time
The History of (Relative) Time
Grand Canyon Rock
Sequence:
-Rocks at base are older
than rocks at top
(superposition).
-Examine lowest units –
which is older, the schist
or the granite? Why?
-Schist – metamorphic –
thought to have been the
root of an ancient
mountain belt or volcanic
arc. How did the
schist/granite get
exposed at the surface?
The Good Earth/Chapter 8: Geologic Time
Grand Canyon
Sandstone, shale, limestone progression
indicative of passive margin (rising sea level).
The Good Earth/Chapter 8: Geologic Time
Relative Time
How can we tell that the volcanism is younger
than formation of the sedimentary rocks?
The Good Earth/Chapter 8: Geologic Time
Relative Dating Review - Superposition
Relative Dating Review – Original
Horizontality
Relative Dating Review – Cross-Cutting
Relative Dating Review – Cross-Cutting
Canon St. Students
Grand Canyon and Geologic Time
Actual Geologic Time: Clocks in the Rocks
Biblical Calculation
Archbishop Ussher of Northern Ireland calculated
the age of earth as being 4004 B.C. based on
careful reading of the Bible. In 1997 Earth was
6000 years old.
Many complications.
Egyptian cat mummies vs. fossilized cats
required much more time for cat evolution.
Actual Geologic Time: Clocks in the Rocks
Evolution and Fossils
Charles Lyell used the rate of fossil change from first
fossils to present with known changes during the
Ice Age. 80 million years since the beginning of the
Cenozoic. Not too far off at 65 million years.
Actual Geologic Time: Clocks in the Rocks
Sediment Deposition Rate
Estimates of how fast sediment accumulates at the
mouths of rivers with a comparison of known
sedimentary rock thicknesses gave an estimate of a
million to over a billion years.
Actual Geologic Time: Clocks in the Rocks
Ocean Salinity
• Sir Edmund Halley (of the comet) estimated the age
of the earth based on the amount of salts in the
oceans and how much salt is transported into the
oceans by streams and rivers. Assumed the original
ocean was pure water, no loss of ocean salinity by
precipitation, or adhesion of salts to ocean clay
minerals.
• John Joly estimated 90 million years for the
accumulation of ocean salts. Important in that this
demonstrated an earth age much older than a few
thousand years.
Ocean Salinity
Actual Geologic Time: Clocks in the Rocks
Earth Cooling Rate
Lord William Kelvin (of Kelvin temperature fame; 273.14°C = 0°K) calculated how long it would
take earth to cool from an original molten state. He
calculated an age of between 24-40 million years
using black-body radiation. Just a little off.
Didn’t know about the production of heat from
radioactive decay.
Radioactivity Provides a Way to Date Rocks
(A Hot Date!)
The discovery of natural radioactivity by Henri
Becquerel allowed geologists to determine the time
it has been since new minerals formed in a rock.
Atoms are composed of a nucleus in the center with
an electron cloud surrounding the nucleus.
Isotopes are elements with different mass numbers caused
by a different number of neutrons.
What Occurs When Atoms Decay?
• When there are too many neutrons in the nucleus
it becomes unstable and radioactively decays.
– Alpha decay – an alpha particle is ejected and the mass reduced by
four (4). Two protons and two neutrons.
– Beta decay – a high velocity electron is ejected from the nucleus
when a neutron decays to a proton/electron pair.
– Gamma radiation – high energy electromagnetic wave given off.
Why Radioactivity Lets Us Date Ancient Rocks with
Confidence
• The rate of radioactive decay is
independent of temperature, pressure
and chemical environment.
• Statistical process given by the half life.
The time it takes one half of the parent
isotope to decay to its stable daughter
isotope
– Parent isotope - .
– Daughter isotope - .
Why Radioactivity Lets Us Date Ancient Rocks with
Confidence
Some minerals can contain only certain
elements based on their size and bonding
coordination. (Testable hypothesis)
– At time zero there are only parent isotopes in the
mineral. After a long period of time some of these
decay.
– Measure the quantity of parent and daughter isotopes
using a mass spectrometer.
– Graph results. A straight line implies a good date.
Mass Spectrometer
Why Igneous Rocks Give the Most Trustworthy Dates
• These rocks date the time since new minerals
formed in a rock.
– Best rocks where new minerals form are the Igneous Rocks.
–
Sedimentary rocks have rarely been used if we know that the
minerals grew during the formation of the rock.
•
Can date the age of source rocks. Stone Mountain. Bedford Canyon Formation.
– Metamorphic rocks can be used sometimes if they are high
temperature rocks where many new minerals are formed.
• Some minerals can contain only certain elements based on
their size and bonding coordination. (Testable)
– At time zero there are only parent isotopes in the mineral. After a
long period of time some of these decay.
– Measure the quantity of parent and daughter isotopes using a mass
spectrometer.
– Graph results. A straight line implies a good date.
Half Life (not the game)
Important Radioactive Isotopes
• The time it takes one half of the original parent
isotope to decay to its stable daughter isotope.
The Potassium-Argon Method
• K40 decays to Ar40. Half-life = 1.251 billion years.
• A major problem is that Ar is a gas and can be lost
from the system which would give a too young age.
The Rubidium-Strontium Method
• Rb87 decays to Sr87. Half-life = 48.8 billion years.
How Carbon-14 Enters the Environment
• Cosmic ray strikes N14 converting a proton to a
neutron and making C14.
• Carbon reacts with O2 to make CO2.
How Carbon-14 Enters the Environment
• C14O2 then is incorporated into sugar
through photosynthesis.
14C
6H12O6
+ O2
How Carbon-14 Dating Works
• The living plant or animal always has a constant
concentration of C14 while alive. After death no new
C14 enters system and C14 only decays allowing
geologists to determine the time since death.
Fission Track Dating
• Natural radioactive decay sometimes produces
high-energy particles from uranium and thorium
decay. These particles cause considerable visible
destruction near the site of the original radioactive
parent. We can count these and estimate the age of
a rock.
Using Dating Methods
• We can bracket rock layers using both relative
and radiometric dating methods.
How Old Is Earth?
Geologists have evidence that the Earth’s age is 4.5 –
4.6 billion years old. There are no rocks of this age
in Earth’s crust.
The oldest evidence of minerals is 4.36 billion years, a
mantled zircon.
Lunar samples and some meteorites are 4.5 billion
years old.
The Goosenecks of the San Juan River
• “Middle Life”.
• Triassic: named for the
tri-fold division of rocks.
• Jurassic: named for the
Jura Mountains between
Switzerland and France.
Geologic Time Scale
• Cretaceous: Latin “chalk”
found in England, France,
Holland and Belgium.