Download Geology of the Hawaiian Islands

Dynamic Earth
Class 3
17 January 2006
Any Questions?
Radiometric Dating
Key principle: Half Life = time required
for 1/2 of the nuclei in a sample to
decay
 Earth formed about 4.6 billion years
ago

Youngest rocks
Oldest rocks
Principle of Superposition
Principle of Original Horizontality
Principle of Cross-cutting Relationships
CORRELATION
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Process used to tie separated strata
together
Based on matching physical features such
as
 Physical continuity - trace of rock unit
 Similar rock types - marker beds, coal
seams, rare minerals, odd color
CORRELATION
Within sedimentary layers there are
often the remains of small animals
(fossils)
 Fossils are quite useful for correlating
between two sections that are not
laterally continuous

Paleontology
The study of life in the past based on
fossilized plants and animals.
Fossil: Evidence of past life
Fossils preserved in sedimentary rocks are
used to determine:
1) Relative age
2) Environment of deposition
Ammonite Fossils
Chip Clark
Petrified Wood
Tom Bean
Using Fossils to Correlate Rocks
CORRELATION

Fossils have evolved through time, so
when we find a fossil of the same type in
two different areas, we are pretty sure
that the rocks are about the same age
CORRELATION

This technique is not very useful in
Hawaii -- Why?
 Fossils helpful in sedimentary rocks,
but usually no fossils in volcanic rocks
Radiometric
dates provide
absolute ages
to the local
stratigraphy
Radiometric dating of lava flows provides
absolute dating of glacial events in Hawaii
Bracketing ages
Magnetostratigraphy

An alternate method for absolute age
dating that works well with volcanic
rocks
Magnetostratigraphy
Technique that works best in volcanic
rocks
 Time scale based on polarity reversal of
Earth's magnetic field
 Major problem is that Earth's magnetic
field has been constant for the past
700,000 yrs (no reversals), so this
doesn't work for very young rocks

Earth’s
Magnetic
Field
Magnetization of
Magnetite
Lavas
record
magnetic
reversals
Magnetic
reversals
over the past
20 million
years
Magnetic time scale
0-700,000 -- Normal
 700,000 - 2.5 my -- Reversed
 > 2.5 my -- Normal
 Ko`olau lavas mostly reversed in
polarity, so they must be older than
700,000 yrs, but younger than 2.5 my
 Lavas on Kaua`i and in Wai`anae
Range show normal polarity, so they
must be older than 2.5 my

The Geologic time scale
Divisions in the worldwide
stratigraphic column based on
variations in preserved fossils
 Built using a combination of
stratigraphic relationships, crosscutting relationships, and absolute
(isotopic) ages

The Geologic
Column
and
Time Scale
Hominoid fossils can
be dated using both
radiometric and
paleomagnetitic
techniques.
Generalized
Stratigraphic
Section of
Rocks
Exposed in
the Grand
Canyon
Some of the Geologic Units Exposed in
the Grand Canyon
Michael Collier
The first ~4 billion years of
Earth’s history was nearly
devoid of life – this time is
known as the Precambrian.
After the Precambrian, life began to
develop very rapidly.
The Paleozoic lasted for 300 my.
Dinosaurs dominated the Mesozoic,
which lasted for almost 200 my.
After the dinosaurs died out,
mammals began to dominate in the
Cenozoic.
These are the main subdivisions
that you should know.
The geologic
timescale
and absolute ages
Isotopic dating of intebedded
volcanic rocks allows
assignment of an absolute age
for fossil transitions
Bracketing ages
The geologic
timescale
and absolute ages
Isotopic dating of intebedded
volcanic rocks allows
assignment of an absolute age
for fossil transitions
What are these subdivisions
based on?
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Boundaries are marked by mass extinctions
Cretaceous-Tertiary boundary (death of the
dinosaurs): 65 million years
Permian-Triassic boundary (the biggest
mass extinction of them all): 250 million
years
Start of the Cambrian (first hard animal
parts--nice fossils): 550 million years
The Geologic
Column
and
Time Scale
How do we know
when it started?
What is the age of
the Earth?
Many methods have been used to
determine the age of the Earth
1) Bible: In 1664, Archbishop Usher of
Dublin used chronology of the Book of
Genesis to calculate that the world began
on Oct. 26, 4004 B.C.
2) Salt in the Ocean: (ca. 1899) Assuming
the oceans began as fresh water, the rate
at which rivers are transporting salts to the
oceans would lead to present salinity in
~100 m.y.
Many methods have been used to
determine the age of the Earth
3) Sediment Thickness: Assuming the rate of
deposition is the same today as in the past,
the thickest sedimentary sequences (e.g.,
Grand Canyon) would have been deposited in
~ 100 m.y.
4) Kelvin’s Calculation: (1870): Lord Kelvin
calculated that the present geothermal
gradient of ~30°C/km would result in an
initially molten earth cooled for 30 – 100 m.y.
Flawed assumptions
• Bible is not a science text or history book
• Salt is precipitated in sedimentary
formations
• Both erosion and non-deposition are
major parts of the sedimentary record
• Radioactivity provides another heat
source
The heat inside the Earth
The discovery of radioactivity at the turn of
the century by Bequerel, Curie, and
Rutherford not only provided the source of
the heat to override Kelvin’s calculations
but provided the basis for all later
quantitative estimates of the ages of
rocks.
Oldest rocks on Earth
Slave Province, Northern Canada

Zircons in a metamorphosed granite dated
at 3.96 Ga by the U-Pb method
Yilgarn block, Western Australia

Detrital zircons in a sandstone dated at 4.10
Ga by U-Pb method.
Several other regions dated at 3.8 Ga by
various methods including Minnesota,
Wyoming, Greenland, South Africa, and
Antarctica.
Age of the Earth
Although the oldest rocks found on Earth
are 3.96 Ga (or even 4.1), we believe that
the age of the Earth is approximately 4.6
Ga. All rocks of the age 4.6 to 4.0 Ga
have been destroyed (the rock cycle) or
are presently covered by younger rocks.
Three types of rocks
Igneous
Sedimentary
Metamorphic
Igneous rocks make up most of the earth
sedimentary rocks make up most of the
surface.
Major Rock Groups
IGNEOUS
SEDIMENTARY
METAMORPHIC
High Relief of Andes
Low Relief of Western Australia
The
Rock
Cycle
Age of the Earth
Although the oldest rocks found on Earth
are 3.96 Ga (or even 4.1), we believe that
the age of the Earth is approximately 4.6
Ga. All rocks of the age 4.6 to 4.0 Ga
have been destroyed (the rock cycle) or
are presently covered by younger rocks.
Age of the Earth
This is based on the age of rocks brought
back from the Moon (4.4 Ga), and
meteorites (4.6 Ga), that are thought to
be good representatives of the early solar
system as well as more complicated
geochemical modeling. This data
suggests that the present chemical
composition of the crust must have
evolved for more than 4.5 Ga.
The big assumption
The half-lives of radioactive
isotopes are the same as they
were billions of years ago.
Test of the assumption
Meteorites and Moon rocks (that are
thought to have had a very simple
history since they formed), have been
dated by up to 10 independent isotopic
systems all of which have given the
same answer. However, scientists
continue to critically evaluate this data.
Age of the Earth
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We have not found a way to determine the
exact age of the Earth directly from Earth rocks
Earth's oldest rocks have been recycled and
destroyed (rock cycle)
We have been able to determine the probable
age of the Solar System and to calculate an
age for the Earth
Assume the Earth and the rest of the solid
bodies in the Solar System formed at the same
time and are, therefore, of the same age
Age of the Earth
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Oldest rocks have been dated by a number of
radiometric dating methods and the consistency
of the results give scientists confidence that the
ages are correct to within a few percent
Interestingly, these rocks are not from any sort
of "primordial crust" but are lava flows and
sediments deposited in shallow water

Indicates that Earth history began well before these
rocks were deposited
Age of the Earth
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Moon rocks returned to Earth on the Apollo
missions are generally 4.4-4.5 billion years old
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Minimum age of our nearest planetary neighbor
More than 70 meteorites of many different types
date 4.53 to 4.59 billion years old
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No rock cycle on the Moon
Primitive objects provide best ages the formation of
the Solar System
So, the best age for the Earth comes not from
dating individual rocks but by considering the
Earth and meteorites as part of the same
evolving system
Any Questions?
Thursday
Homework #1 is due – check web
page
 First video: Dating the Earth

Big Island Field Trip

Need deposit of $50 on February 17th