Transcript Chapter 3 - HCC Learning Web

Chapter 3
Time and Geology
Finding the age of rocks:
Relative versus Actual Dating
The science that deals with determining
the ages of rocks is called geochronology.
Methods of Dating Rocks
1. Relative dating - Using fundamental principles
of geology (Steno's Laws, Fossil Succession,
etc.) to determine the relative ages of rocks
(which rocks are older and which are younger).
2. Actual (Absolute) dating - Quantifying the date
of the rock in years. This is done primarily by
radiometric dating (or detailed analysis of the
breakdown of radioactive elements within the
rocks over time).
Geologic Time Scale
• The geologic time scale has been
determined over many years of research
through relative dating, correlation,
examination of fossils, and radiometric
dating.
• Boundaries on the time scale are placed
where important changes occur in the
fossil record, such as extinction events.
Geochronologic Units
The geologic time scale is divided into a number of
types of units of differing size. From the largest
units to the smaller units, they are:
• Eons
• Eras
• Periods
• Epochs
These units are geochronologic units.
Geochronologic units are time units.
The
modern
geological
time scale
Eons
Eons are the largest division of geologic
time. In order from oldest to youngest, the
three eons are:
• Archean Eon - "ancient or archaic" - oldest
rocks on Earth
• Proterozoic Eon - "beginning life" (2.5 billion
to 542 million years ago)
• Phanerozoic Eon - "visible life" (542 million
years ago to present)
Precambrian
The Archean and Proterozoic are together
referred to as the Precambrian, meaning
"before the Cambrian Period."
The Precambrian encompasses 87% of
geologic history.
Eras
There are three eras within the eon called
Phanerozoic. Eras are divided into geologic
periods. In order from oldest to youngest, the
three eras are:
• Paleozoic Era - "ancient life" (such as trilobites)
• Mesozoic Era - "middle life" (such as dinosaurs)
• Cenozoic Era - "recent life" (such as mammals)
Periods
Eras are divided into periods.
Paleozoic Era
• Permian Period
• Carboniferous Period
(split into Mississippian and Pennsylvanian
Periods in the United States)
• Devonian Period
• Silurian Period
• Ordovician Period
• Cambrian Period (oldest)
Mesozoic Era
• Cretaceous Period
• Jurassic Period
• Triassic Period (oldest)
Cenozoic Era
• Quaternary Period
• Neogene Period
• Paleogene Period (oldest)
On maps and in publications prior to 2003, you will
see the two periods of the Cenozoic Era listed as:
• Quaternary Period
• Tertiary Period (oldest)
The former Tertiary Period is now split into two.
Epochs
Periods are subdivided into epochs.
and
Epochs are subdivided into ages.
Epochs of the Cenozoic Era
• Quaternary Period
– Holocene Epoch
– Pleistocene Epoch (oldest)
• Neogene Period
– Pliocene Epoch
– Miocene Epoch (oldest)
• Paleogene Period
– Oligocene Epoch
– Eocene Epoch
– Paleocene Epoch (oldest)
Chronostratigraphic units
Chronostratigraphic units represent the
actual rocks deposited or formed during a
specific time interval.
Chronostratigraphic units are sometimes
called "time-rock units."
Chronostratigraphic units
Chronostratigraphic units include:
•
•
•
•
•
Eonothem (all rocks corresponding to a given eon)
Erathem (all rocks corresponding to a given era)
System (all rocks corresponding to a given period)
Series (all rocks corresponding to a given epoch)
Stage (all rocks corresponding to a particular age)
Periods and Systems
Geochronologic units (time units) have the same
names as their chronostratigraphic units (timerock units).
For example, Cambrian Period is a time unit,
and Cambrian System is a time-rock unit.
The rocks of the Cambrian System were
deposited during the period called Cambrian.
Principles of Radiometric Dating
Review of Atoms
Atom = smallest particle of matter that can
exist as a chemical element.
The structure of the atom consists of:
• Nucleus composed of protons (positive
charge) and neutrons (neutral)
• Electrons (negative charge) orbit the
nucleus
• Various subatomic particles
Model of the atom
Ions
Most atoms are neutral overall, with the
number of protons equaling the number of
electrons.
If there is an unequal number of protons
and electrons, the atom has a charge
(positive or negative), and it is called an
ion.
Atomic Number
Atomic number of an atom = number of
protons in the nucleus of that atom.
Example: The atomic number of uranium
is 92. Uranium has 92 protons.
Mass number
Mass number is the sum of the number of
protons plus neutrons.
Example: Uranium-235 has 92 protons
and 143 neutrons.
The mass number may vary for an
element, because of a differing number of
neutrons.
Isotopes
• Elements with various numbers of neutrons are
called isotopes of that element.
Example: Uranium-235 and Uranium-238
• Some isotopes are unstable. They undergo
radioactive decay, releasing particles and energy.
• Some elements have both radioactive and nonradioactive isotopes.
Examples: carbon, potassium
What happens when atoms decay?
• Radioactive decay occurs by releasing
subatomic particles and energy.
• The radioactive parent element is unstable and
undergoes radioactive decay to form a stable
daughter element.
• Example: Uranium, the parent element,
undergoes radioactive decay, releases
subatomic particles and energy, and ultimately
decays to form the stable daughter element,
lead.
Radioactive Parent Isotopes and
Their Stable Daughter Products
Radioactive Parent Isotope
Stable Daughter Isotope
Potassium-40
Argon-40
Rubidium-87
Strontium-87
Thorium-232
Lead-208
Uranium-235
Lead-207
Uranium-238
Lead-206
Carbon-14
Nitrogen-14
Radioactive Decay of Uranium
As Uranium-238 decays to lead, there are
13 intermediate radioactive daughter
products formed (including radon,
polonium, and other isotopes of uranium),
along with and 8 alpha particles and 6
beta particles released.
Radioactive Decay of Uranium
Subatomic Particles and Radiation
Released by Radioactive Decay
•
Alpha particles – atomic weight = 4; atomic number =
(The same as the nucleus of a helium atom. Has + charge of 2.)
•
Beta particles – an electron that is released when a
neutron splits into a proton and an electron.
(Like all electrons, the mass is negligible and there is a positive
charge.)
•
Gamma rays – electromagnetic waves much like x-rays,
but higher frequency
(Like all electromagnetic waves, including light, there is no charge
or mass associated this photon or "particle.")
Radioactive Decay
Naturally-occurring radioactive materials
break down into other materials at known
rates. This is known as radioactive decay.
Radioactive Decay Rate
• Many radioactive elements can be used as
geologic clocks. Each radioactive element
decays at its own constant rate.
• Once this rate is measured, geologists can
estimate the length of time over which decay has
been occurring by measuring the amount of
radioactive parent element and the amount of
stable daughter elements.
Mass Spectrometer
• The quantities and masses of atoms and
isotopes are measured using an
instrument called a mass spectrometer.
• The decay rates of the various radioactive
isotopes are measured directly using a
mass spectrometer.
Decay Rates are Uniform
• Radioactive decay occurs at a constant or
uniform rate.
• The rate of decay is not affected by changes in
pressure, temperature, or other chemicals.
• As time passes, the number of parent atoms
decreases and the number of daughter atoms
increases at a known rate.
Half-Life
• Each radioactive isotope has its own
unique half-life.
• A half-life is the time it takes for one-half of
the parent radioactive element to decay to
a daughter product.
Half-Lives for Radioactive
Elements
Radioactive Parent
Stable Daughter
Half-life
Potassium-40
Argon-40
1.25 billion yrs
Rubidium-87
Strontium-87
48.8 billion yrs
Thorium-232
Lead-208
14 billion years
Uranium-235
Lead-207
Uranium-238
Lead-206
704 million
years
4.47 billion
years
Carbon-14
Nitrogen-14
5730 years
Rate of decay for Uranium-238
Rate of decay for Potassium-40
Rocks That Can Be Dated
Igneous rocks are best for age dating.
The dates from crystals in igneous rocks
tell us when the magma cooled.
When the magma cools and crystallizes,
the newly formed crystals usually contain
some radioactive elements, such as
Potassium-40 or Uranium-238 that can be
used for radiometric dating.
Minerals That Can Be Dated
Potassium-40 decays and releases Argon40 gas, which is trapped in the crystal
lattice.
Potassium-40 is found in these minerals:
– Potassium feldspar (orthoclase, microcline)
– Muscovite
– Amphibole
– Glauconite (found in some sedimentary rocks)
Minerals That Can Be Dated
Uranium may be found in:
• Zircon
• Urananite
• Monazite
• Apatite
• Sphene
Dating Sedimentary Rocks
Radioactive mineral grains in sedimentary
rocks are derived from the weathering of
igneous rocks. The radiometric dates of
these grains give us the time of cooling of
the magma, which formed the original
igneous rock.
These dates do not tell us anything about
the age of the sedimentary rock.
Dating Sedimentary Rocks
If the sedimentary rock contains a mineral
that formed at the same time as the rock
formed, then it may be possible to use that
mineral to obtain a radiometric age date.
The sedimentary mineral glauconite
contains potassium, and can be used for
radiometric dating (employing the
potassium-argon technique).
Dating Sedimentary Rocks
The ages of
sedimentary rocks
and fossils are
determined using
both relative and
absolute dating.
Dating Sedimentary Rocks
For example, the
age of the shale
layers in both
instances is
between 110 and
180 million years.
Dating Fossils
The ages of fossils
in a sequence of
sedimentary rocks
can be determined
using both relative
and absolute
dating.
Dating sedimentary rocks and
fossils
1. Locate a sequence of sedimentary rocks that
contains some igneous rocks (such as a lava
flow, volcanic ash bed, intrusion, or underlying
igneous rock).
2. Determine radiometric dates for the igneous
rocks.
3. Use relative dating to determine the relative
ages of the sedimentary rocks. Bracket the age
of the sedimentary rocks using two igneous
rocks with known ages.
Dating sedimentary rocks and
fossils – cont'd
4. Correlate the sedimentary rocks with
sedimentary rocks in another area that contain
the same fossils. They are correlated (or
"matched up") on the basis of the fossils they
contain. They must contain the same species
of fossils.
5. Using this method, the age of the rocks in
other areas is determined indirectly, from the
ages of the fossils they contain.
The geologic time scale was compiled by using
this method repeatedly at many locations
around the world.
The geologic time scale is a composite vertical
sequence representing all known rock units and
their fossils, worldwide, in sequential order.
Absolute ages of rocks have been determined
through radiometric dating where possible.
The geologic time scale provides a calibrated
scale for determining the ages of rocks
worldwide including their fossils.
Carbon-14 dating
1. Cosmic rays from the sun strike
Nitrogen-14 atoms in the atmosphere
and cause them to turn into radioactive
Carbon-14, which combines with oxygen
to form radioactive carbon dioxide.
Carbon-14 dating
2. Living things are in equilibrium with the
atmosphere, because radioactive carbon
dioxide is absorbed and used by plants.
The radioactive carbon dioxide gets into
the food chain and thus the carbon cycle.
All living things contain a constant ratio
of Carbon-14 to Carbon-12. (about 1 in a
trillion).
Carbon-14 dating
3. At death, Carbon-14 exchange ceases
and any Carbon-14 in the tissues of the
organism begins to decay to Nitrogen-14,
and is not replenished by new Carbon14.
The change in the Carbon-14 to Carbon12 ratio in fossil material is the basis for
this kind of radiometric dating.
Carbon-14 dating
• The half-life is so short (5730 years) that
this method can only be used on materials
less than 70,000 years old.
• Assumes that the rate of Carbon-14
production (and hence the amount of
cosmic rays striking the Earth) has been
constant over the past 70,000 years.
Carbon-14
formed from
Nitrogen-14
and its fate in
the natural
environment.
Rubidium-Strontium Method
• When Rubidium-87 expels a beta particle,
it becomes Strontium-87.
• This method provides a useful check on
the potassium-argon method of dating.
Rubidium-Strontium Method
• Strontium-86 is not a radioactive isotope,
and it is employed in this method as well.
• Using a mass spectrometer, the ratio of
Rubidium-87 to Strontium-86 and the ratio
of Strontium-87 to Strontium-86 is
determined for several samples.
• This is plotted on a graph and the line thus
determined is called an isochron.
Rubidium-Strontium Method
• The slope of the line permits computation
of the age of the mineral crystals being
studied.
In this instance,
the slope angle of
the isochron suggests
an age of 1.725 billion
years for a granite from
Sudbury, Ontario.
Fission Track Dating
• Charged particles from radioactive decay
pass through a mineral's crystal lattice and
leave trails of damage in the crystal called
fission tracks.
• These trails are due to the
spontaneous fission
(or radioactive decay) of
the uranium nucleus.
Fission Track Dating
Procedure:
– Enlarge tracks by etching in acid (to view with
light microscope) - or view them directly with
electron microscope
– Count the etched tracks (or measure the density
of such tracks in a given area of the crystal)
The number of tracks per unit area is a
function of age and uranium concentration.
Fission Track Dating
Useful in dating:
• Micas (up to 50,000 tracks per cm2)
• Other uranium-bearing minerals and
natural glasses
The Oldest Rocks
The oldest rocks that have been dated are
meteorites. They date from the time of the
origin of the solar system and the Earth,
about 4.6 billion years old.
The Oldest Rocks
Moon rocks have similar dates, ranging in
age from 3.3 to about 4.6 billion years.
The oldest Moon rocks are from the lunar
highlands (lighter-colored areas on the
Moon), and may represent the original
lunar crust
The Oldest Rocks
The oldest dates of Earth rocks are 4.36
billion-year-old detrital zircon grains in a
sandstone in western Australia.
These grains probably came from the
weathering and erosion of 4.36 billionyear-old granite that must have been
exposed at the time the sand grains were
deposited.
Other Old Earth Rocks
1. Southwestern Greenland (granite; 4.0 b.y.)
2. Minnesota (metamorphic rocks; 4.0 b.y.)
3. Northwest Territories, Canada (gneiss;
4.04 b.y.)
4. Hudson Bay, northern Quebec (zircons;
4.28 b.y.)
Still older rocks on Earth may remain to be found
and dated using radiometric methods.
Why are Earth Rocks Younger than
Meteorites and Moon Rocks?
The Earth is geologically active. The older rocks may have
been eroded away or destroyed by tectonic forces.
Older rocks may remain deeply buried under sedimentary
rocks, or under mountain ranges.
Older rocks may have been heated, metamorphosed, or
melted, and their isotopes "reset" to the time of the
later events of heating, metamorphism, or melting.