Chapter 3 Time and Geology. Geochronology The science that deals with determining the ages of rocks is called geochronology

Chapter 3Chapter 3

Time and Geology Time and Geology

Geochronology Geochronology

The science that deals with determining the ages of rocks is called geochronology.

Methods of Dating RocksMethods 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. Absolute dating - Quantifying the date of the rock in years. This is done primarily by radiometric dating (or analysis of the breakdown of radioactive elements in the rocks over time).

Geologic Time Scale Geologic Time Scale

• The geologic time scale has been determined bit-by-bit over the years through relative dating, correlation, examination of fossils, and radiometric dating.

• Boundaries on the time scale are drawn where important changes occur in the fossil record, such as extinction events.

Geochronologic UnitsGeochronologic UnitsThe 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.

EonsEons

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)

The PrecambrianThe Precambrian

The Archean and Proterozoic are together referred to as the Precambrian, meaning “before the Cambrian Period”.

The Precambrian covers 87% of geologic history.

ErasEras

There are three eras in the Phanerozoic Eon. 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 Periods

Eras are divided into periods.

Paleozoic EraPaleozoic Era

• Permian Period

• Carboniferous Period (Mississippian and Pennsylvanian Periods in North America)

• Devonian Period

• Silurian Period

• Ordovician Period

• Cambrian Period (oldest)

Mesozoic EraMesozoic Era

• Cretaceous Period

• Jurassic Period

• Triassic Period (oldest)

Cenozoic EraCenozoic Era

• Neogene Period (youngest – today)

• 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)

These are not the same as the current two periods.

EpochsEpochs

Periods can be subdivided into epochs

Epochs can be subdivided into ages

Epochs of the Cenozoic Era Epochs of the Cenozoic Era

• Neogene Period – Holocene Epoch (youngest - today) – Pleistocene Epoch – Pliocene Epoch – Miocene Epoch

• Paleogene Period – Oligocene Epoch – Eocene Epoch – Paleocene Epoch (oldest)

Chronostratigraphic units Chronostratigraphic units

Chronostratigraphic units are the actual rocks formed or deposited during a specific time interval.

(They are sometimes called time-rock units.)

Chronostratigraphic unitsChronostratigraphic 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 an epoch) • Stage (all rocks corresponding to a particular age)

Periods and SystemsPeriods and Systems

Geochronologic units (time units) have the same names as the chronostratigraphic units (time rock units) that they represent.For example, the Cambrian System is a rock unit, and the Cambrian Period is a time unit. The rocks of the Cambrian System were deposited during the Cambrian Period.

Principles of Radiometric DatingPrinciples of Radiometric Dating

Review of Atoms 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

Two models of atoms Two models of atoms

IonsIons

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 NumberAtomic Number

Atomic number of an atom = number of protons in the nucleus of that atom.

Example: The atomic number of uranium is 92. It has 92 protons.

Mass numberMass 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.

IsotopesIsotopes

• 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 non-radioactive isotopes. Examples: carbon, potassium.

What happens when atoms decay?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 Radioactive Parent Isotopes and Their Stable Daughter Products 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 UraniumRadioactive 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 UraniumRadioactive Decay of Uranium

Subatomic Particles and Radiation Subatomic Particles and Radiation Released by Radioactive Decay Released by Radioactive Decay

• Alpha particles - large, easily stopped by papercharge = +2 mass = 4

• Beta particles - penetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays.

charge = -1 mass = negligible• Neutrons - highly penetrating; no charge mass = 1• Gamma rays (high energy x-rays) - Highly

penetrating electromagnetic radiation; can penetrate concrete. Lead shield can be used. Photons (light). No charge or mass.

Radioactive DecayRadioactive Decay

Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.

Radioactive Decay RateRadioactive 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 SpectrometerMass Spectrometer

• The quantities and masses of atoms and isotopes are measured using an instrument called a mass spectrometer.

• The mass spectrometer came into use after WWI (1918). This led to the discovery of more than 200 isotopes.

Measuring Decay RatesMeasuring Decay Rates

The decay rates of the various radioactive isotopes are measured directly using a mass spectrometer.

Basically, the mass of a quantity of a radioactive element is measured. Then after a particular period of time, it is analyzed again. The change in the number of atoms over time gives the decay rate.

Decay Rates are UniformDecay Rates are Uniform

• Radioactive decay occurs at a constant exponential or geometric rate.

• The rate of decay is not affected by changes in pressure, temperature, or other chemicals.

• The rate of decay is proportional to the number of parent atoms present.

Half-Life Half-Life

• Each radioactive isotope has its own unique half-life.

• A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.

Half Lives for Radioactive Elements 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-207 704 million years

Uranium-238 Lead-206 4.47 billion years

Carbon-14 Nitrogen-14 5730 years

Decay Curve for Uranium-238Decay Curve for Uranium-238

Decay Curve for Potassium-40Decay Curve for Potassium-40

Rocks That Can Be Dated Rocks That Can Be Dated

Igneous rocks are best for age dating.

The dates they give tell when the magma cooled. When the magma cools and crystallized, the newly formed crystals may contain some radioactive elements, such as potassium-40 or uranium that can be dated.

Minerals That Can Be DatedMinerals That Can Be Dated

Potassium-40 decays and releases argon gas, which is trapped in the crystal lattice.

Potassium-40 is found in these minerals: – Potassium feldspar (orthoclase, microcline) – Muscovite – Amphibole – Glauconite (greensand; found in some

sedimentary rocks)

Minerals That Can Be DatedMinerals That Can Be Dated

Uranium may be found in: – Zircon – Urananite – Monazite – Apatite – Sphene

Dating Sedimentary RocksDating Sedimentary Rocks


Radioactive mineral grains in sedimentary rocks are derived from the weathering of igneous rocks. Their dates are the time of cooling of the magma that formed the igneous rock.

The date does not tell anything about when the sedimentary rock was deposited.


If the sedimentary rock has a mineral that formed on the seafloor as the rock was cemented, then it may be possible to age date it.

The greensand mineral, glauconite, contains potassium, and can be dated using the potassium-argon technique.


The ages of sedimentary rocks and fossils are determined using both relative and absolute dating.

Dating FossilsDating Fossils

The ages of fossils in a sequence of sedimentary rocks can be determined using both relative and absolute dating.

Dating sedimentary rocks and Dating sedimentary rocks and fossilsfossils

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. Get a radiometric date for the igneous rocks.

3. Use relative dating to determine the relative ages of the sedimentary rocks. Bracket the sedimentary rocks between two igneous rocks of known age.

Dating sedimentary rocks and Dating sedimentary rocks and fossils – cont’dfossils – cont’d

4. Correlate the sedimentary rocks with sedimentary rocks in another area which 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 established by doing this repeatedly for 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 by examining their fossils.

Carbon-14 datingCarbon-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.


2. Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle.

All living things contain a constant ratio of carbon-14 to carbon-12. (1 in a trillion).


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 C-14.

The change in the carbon-14 to carbon-12 ratio is the basis for 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.

Fission Track Dating Fission Track Dating

• Charged particles from radioactive decay pass through mineral's crystal lattice and leave trails of damage called fission tracks.

• These trails are due to the spontaneous fission (or radioactive decay and breakdown) of uranium.

Fission Track DatingFission Track DatingProcedure to study: – Enlarge tracks by etching in acid (to view with

light microscope) – Or view with electron microscope – Count the etched tracks (or measure track

density in an area)

The number of tracks per unit area is a function of age and uranium concentration.

Fission Track DatingFission Track Dating

Useful in dating:

• Micas (up to 50,000 tracks per cm2)

• Tektites (glassy rocks produced when meteorite impact melts bedrock forming molten droplets which cool quickly as they are thrown into the atmosphere.)

• Natural and synthetic (manmade) glass

The Oldest Rocks 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 RocksThe 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 RocksThe Oldest Rocks

The oldest dates of Earth rocks are 4.2 billion-year-old detrital zircon grains in a sandstone in western Australia.

These grains probably came from the weathering and erosion of 4.2 billion-year-old granite that must have been exposed at the time the sand grains were deposited.

Other Old Earth Rocks: Other Old Earth Rocks: 1. Southwestern Greenland (4.0 b.y. granites) 2. Minnesota (4.0 b.y. metamorphic rocks) 3. Northwest Territories, Canada (3.96 b.y.

Acasta gneiss) 4. Beartooth Mountains, Montana (3.96 b.y.

zircons in quartzite) 5. China (3.8 b.y.) 6. South Africa (3.7 b.y.) 7. West Africa (3.6 b.y.)

Still older rocks may remain to be found and dated

Why are Earth Rocks Younger than Why are Earth Rocks Younger than Meteorites and Moon Rocks?Meteorites and Moon Rocks?

1. The Earth is geologically active. The older rocks may have been eroded away.

2. Older rocks may have been deeply buried under sedimentary rocks, or beneath thrust sheets.

3. Older rocks may have been heated, metamorphosed, or melted, and their dates "reset" to the time of heating, metamorphism, or melting.

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Chapter 3 Time and Geology. Geochronology The science that deals with determining the ages of rocks is called geochronology