Ch09 geologic time_fall2007 (1)

GEOLOGIC TIME

Chapter 9

Geologic TimeGeologic Time

• The rock layers of the Grand Canyon are like The rock layers of the Grand Canyon are like pages in a history book.pages in a history book.

• However, some pages are missing or incomplete.However, some pages are missing or incomplete.• Geologist study rocks to unravel the complexities Geologist study rocks to unravel the complexities

of Earth’s history.of Earth’s history.• Geologic events must be put into a time Geologic events must be put into a time

perspective – The perspective – The Geologic Time ScaleGeologic Time Scale..• The science that deals with determining the ages of The science that deals with determining the ages of

rocks is calledrocks is called geochronologygeochronology..

The Geologic

Time Scale

Methods of Dating Rocks

1. Relative Dating • Using fundamental principles of geology to determine

the relative ages of rocks.• Determining which rocks are older and which are

younger with respect to each other.

2. Absolute Dating • Quantifying the actual date of the rock in years via

radiometric dating.• Analysis of the breakdown of radioactive elements in

the rocks over time.

Relative Dating: Principles of Geology• Geologic events must be put into a time perspective –

the Geologic Time Scale. • Relative age dating methods were used before

numerical or absolute methods of dating were developed: Principle of Original Horizontality Principle of Lateral Continuity Law of Superposition Principle of Cross-Cutting Relationships Inclusions Unconformities (Disconformity, Non-Conformity, Angular

Unconformity) Principle of Fossil Succession Index Fossils Uniformitarianism

• Only indicates the order of events relative to each other – not how long ago they occurred.

Relative Dating: Steno’s Laws

• Principle of Original Horizontality Developed by Nicolaus Steno in 1669. Layers of sediment are generally

deposited in a horizontal position. Rock layers that are flat have not

been disturbed. Therefore, a sequence of

sedimentary rock layers that is steeply inclined from horizontal must have been tilted after deposition and lithification.

Were these deposited like this?

Law of Original Horizontality – These sedimentary rock layers have been moved from original

horizontal position by crustal disturbances after deposition


• Principle of Superposition Also developed by Nicolaus

Steno in 1669. In an undisturbed succession of

sedimentary rock layers (or layered igneous rocks), the oldest layer is at the bottom and the youngest layer is at the top.

Where would you find the youngest rocks? The oldest?

Superposition is well illustrated by the strata in the Grand Canyon


• Principle of Lateral Continuity Sediment extends laterally in all

direction until it thins and pinches out or terminates against the edges of the depositional basin.

Figure 1-6 (p. 5)Illustration of original lateral continuity. Cross-section A shows a sandstone stratum deposited within a low-lying area or sedimentary basin that received sediment eroded from surrounding uplands. Cross-section B shows the same area after erosion has exposed the sandstone on hillsides.

Relative Dating: Unconformities

• Principles of Unconformities Developed by James

Hutton

• There are two basic types of contacts between rock units: Conformable Unconformable

Relative Dating: Conformable Strata • Principles of Unconformities• Conformable Strata

A vertical sequence of rocks in which deposition was more or less continuous.

• Conformable contacts between beds of sedimentary rocks may be either:

• Abrupt or • Gradational

• Most abrupt contacts are bedding planes resulting from sudden minor changes in depositional conditions.

• Gradational contacts represent more gradual changes in depositional conditions.

Abrupt vs. Gradational

Contacts

Relative Dating: Unconformable Strata • Principles of Unconformities• Unconformable contacts (or unconformities) are

surfaces which represent a gap in the geologic record, because of either: Erosion or Nondeposition

• The time represented by this gap can vary widely, ranging from millions of years to hundreds of millions of years.

• The rock record is incomplete The interval of time not represented by strata is a

hiatus.• Unconformities represent significant geologic

events.

For 1 million years For 1 million years erosion occurrederosion occurred

removing 2 MY of removing 2 MY of rocksrocks

The Origin of an Unconformity• In the process of forming an unconformity:In the process of forming an unconformity:

Deposition began 12 million years ago (MYA), Deposition began 12 million years ago (MYA), continuing until 4 MYA.continuing until 4 MYA.

• The last column The last column is the actual is the actual

stratigraphic record stratigraphic record with an unconformity.with an unconformity.

and giving rise to and giving rise to a 3 million year a 3 million year

hiatus.hiatus.

Types of Unconformities• Angular Unconformity – Tilted rocks

are overlain by flat-lying rocks. Indicates that during the pause in

deposition, a period of deformation and erosion occurred.

• Nonconformity – Metamorphic or igneous rocks in contact with sedimentary strata. Indicates a period of uplift and erosion of

the rocks previously overlying the igneous/ metamorphic rocks prior to deposition of the younger sedimentary rocks.

• Disconformity – Strata on either side of the unconformity are parallel.

• Paraconformity – An unconformity at which strata are parallel and the contact is a simple bedding plane.

Formationof an

Angular Unconformity

Formationof an

Angular Unconformity

Insert Animation #1: Angular Unconformities and Nonconformities

Angular unconformity at Siccar Point, eastern Scotland. (A) It was here that James Hutton first realized the historical significance of an unconformity. The drawings (B) indicate the sequence of events documented in this famous exposure.

Formationof a

Nonconformity with Inclusions

Formationof an

Nonconformity with Inclusions

Insert Animation #1: Angular Unconformities and Nonconformities

Formationof an

Disconformity

Insert Animation #2: Angular Unconformities, Nonconformities, and Disconformities

Unconformities in the Grand Canyon

Relative Dating: Lyell’s Principles

• Principle of Cross-Cutting Relationships Younger features cut

across older features. • Principle of Inclusions

An inclusion is a fragment of rock that is enclosed within another rock.

The rock containing the inclusion is younger.

Principle of Cross-Cutting Relationships – Faults

Where a fault cuts across a sequence of sedimentary rock, the fault is younger than the rocks it cuts.

The sedimentary rocks are older than the fault which cuts them, because they had to be there first, before they could be faulted.

Principle of Cross-Cutting Relationships – Intrusions

Where an igneous intrusion cuts across a sequence of sedimentary rock, the sedimentary rocks are older than the igneous rock which intrudes them.

The intrusion is younger than the rocks it cuts.

What Cross-Cutting Relationships Are Illustrated in this Figure?

The Principle of Inclusions – Sedimentary Rocks

Fragments of eroded rock overlie the unconformity. These are gravel clasts or inclusions.

The pieces of gravel are older than the bed in which they are found.

The Principle of Inclusions –Igneous Rocks

A xenolith is a fragment of the surrounding rock which has broken off during an intrusion and fallen into the magma.

The xenolith is older than the igneous rock which contains it.

Comparison of inclusions in a sedimentary rock (A) with inclusions in an igneous rock (B).

Which rock unit is older in each figure? Which are gravel clasts and which are xenoliths?

Figure 2-12 (p. 21)(A) Granite inclusions in sandstone indicate that granite is the older unit.(B) Inclusions of sandstone in granite indicate that sandstone is the older unit.

Relative Geologic Age Dating

Insert Animation #70: Relative Geologic Dating #2

Interpreting a Sequence of Events

Interpreting a Sequence of Events

Determine the order in which the geologic events occurred?

• An example of how the sequence of geologic events can be determined from cross-cutting relationships and superposition.

• From first to last, the sequence indicated in the cross-section:1. First deposition of D, 2. Then faulting to produce fault B, 3. Then intrusion of igneous rock mass C, 4. Then erosion forming the unconformity,5. Followed by deposition of E.

• Strata labeled D are oldest, and strata labeled E are youngest

Interpret the Geologic History of the Grand Canyon

The Principle of Uniformitarianism• Principle of Uniformitarianism:

Developed in the late 18th century by James Hutton the “Father of Modern Geology”

States that physical laws which operate today have operated in the past.

Argued that uniform natural laws govern geologic processes (cycles) that operated over the immensity of geologic time.

““The Present is the Key to the Past”The Present is the Key to the Past”

• Principle of Fossil Succession: Developed by William Smith (late 1700's) Fossilized organisms of each age of the Earth’s

history are unique and occur in a consistent vertical order in sedimentary rocks all over the world.

Therefore, any time period can be recognized by its fossil content.

Examples: Age of Trilobites, Age of Fishes, Age of Coal Swamps, Age of Reptiles, Age of Mammals

• Geologists interpret fossil succession to be the result of evolution – the natural appearance and disappearance of species through time.

Principle of Fossil Succession

The Geologic Time Scale – Development of Plants and Animals Over Geologic Time

• Index Fossil – A geographically widespread fossil that is limited to a short span of geologic time (specific time indicator).

Principle of Fossil Succession

Dating Rocks using Overlapping Fossil Ranges

The 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 changes in plant and animal species or 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 divisions are based on changes in life forms recorded by fossils.

• These units are geochronologic units. • Geochronologic units are time units.

Geologic Time Scale. The age for the base of each division is in accordance with recommendations of the International Commission on Stratigraphy for the year 2000.

Periods: Geochronologic Nomenclature

Development of Geochronologic Nomenclature for Geolgic Periods:

• Cambrian System: Cambria (Roman name for Wales)• Silurian and Ordovician Systems: Silures and Ordovices

were ancient Celtic Tribes• Devonian System: Devonshire, England• Carboniferous System: British coal measures• Permian System: Perm Province, Russia• Triassic System: Set of three formations in Germany• Jurassic System: Jura Mountains, Franco-Swiss border• Cretaceous System: Latin for Chalk (creta)• Tertiary System: “Montes Tertiarii" of Italian Alps• Quaternary System: Soft sediments of northern France

Eons• Eons are the largest

division of geologic time. • In order from oldest to

youngest, the eons are: Hadean Eon – Origin of the

Earth – oldest rocks on Earth Archean Eon – “Ancient or

Archaic” – first single-celled organisms

Proterozoic Eon – “Beginning Life” – first multi-celled organisms (2.5 billion to 542 million years ago)

Phanerozoic Eon – “Visible Life” (542 million years ago to present)

The Precambrian

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

• The Precambrian covers ~88% of geologic history.

Eras

• The Phanerozoic Eon is divided into three Eras.

• 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 of the Paleozoic Era

• Permian Period• Carboniferous Period

(Mississippian and Pennsylvanian Periods in North America)

• Devonian Period • Silurian Period • Ordovician Period • Cambrian Period

(oldest)

Periods of the Mesozoic Era

• Cretaceous Period

• Jurassic Period

• Triassic Period (oldest)

Periods of the Cenozoic Era

• Quaternary Period (youngest – today)

• Tertiary Period (oldest)

Epochs

Periods can be subdivided into epochs.

Epochs can be subdivided into ages.

Epochs of the Cenozoic Era • Quaternary

Period Holocene Epoch

(youngest – today) Pleistocene Epoch

• Tertiary Period Pliocene Epoch Miocene Epoch Oligocene Epoch Eocene Epoch Paleocene Epoch

(oldest)

Principles of Radiometric Age 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.

Two Models of Atoms

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

What Happens When Atoms Decay?• Radioactive decay entails spontaneous

changes (decay) in the structure of unstable atomic nuclei.

• Nuclei are unstable because the forces binding the protons and neutrons together are not strong enough causing the nuclei to break apart or 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.

What Happens When Atoms Decay?

• Radioactive Decay 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 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

Subatomic Particles and Radiation Released by Radioactive Decay

• Types of Radioactive Decay Alpha Emission:

• Emission of 2 protons and 2 neutrons (an alpha particle = positively charge helium ion).

• Alpha Particles – Charge = +2 Mass = 4

Mass number is reduced by 4 and the atomic number is lowered by 2.


• Types of Radioactive Decay Beta Emission:

• An electron (beta particle) is ejected from the nucleus when a neutron splits into a proton and electron.

• Beta Particles – Charge = -1 Mass = Negligible• Mass number remains unchanged (because electrons

have practically no mass).

Atomic number increases by 1, because a neutron is a combination of a proton and an electron; therefore, the nucleus contains one or more protons than before.


• Types of Radioactive Decay Electron Capture:

• An electron is captured by the nucleus and combines with a proton to form a neutron.

• Mass number remains unchanged and the atomic number decreases by 1 (because the nucleus now contains one less proton).

As Uranium-238 (U238) to decays to Lead-206 (Pb206), 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 Rate

• Many radioactive elements can be used as geologic clocks.

• Each radioactive element decays at its own constant rate.

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

• 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.

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

Radioactive Parent Isotope

Stable Daughter Isotope

Currently Accepted Half-Life Values

Potassium-40 Argon-40 1.25 billion years

Rubidium-87 Strontium-87 48.8 billion years

Thorium-232 Lead-208 14.1 billion years

Uranium-235 Lead-207 704 million years

Uranium-238 Lead-206 4.47 billion years

Carbon-14 Nitrogen-14 5,730 years

Measuring Decay Rates• The decay rates of the

various radioactive isotopes are measured directly using a mass spectrometer.

• “Counts” the quantities of parent and daughter isotopes in a rock sample by:1. Measuring the mass of a

quantity of a radioactive element.

2. Analyzing the mass again after a particular period of time.

3. The change in the number of atoms over time gives the decay rate.

Radioactive Decay and Half-Life

Insert Animation #67: Radioactive Decay

Rocks That Can Be Dated • Igneous rocks are best for age dating:

Igneous rock crystallize minerals from magma. When the mineral crystals are formed, they “lock in”

the atoms of radioactive elements. The newly formed crystals may contain some

radioactive elements, such as Potassium-40 or Uranium that can be dated.

If the crystals remain undisturbed, fresh samples of igneous rock are not likely to have lost any daughter atoms.

The dates they give tell when the magma cooled (emplaced).

Bracketing Sedimentary Ages using Igneous Rocks

Igneous rocks that have provided absolute radiogenic ages can often be used to date sedimentary layers.

(A)The shale is bracketed by two lava flows.

(B) The shale lies above the older flow and is intruded by a younger igneous body.

Bracketing Sedimentary Ages using Igneous Rocks

Difficulties in Dating

• Radiometric dating is a complex procedure that requires precise measurement

• Sources of Error• A closed system is required: No parent or daughter

atoms gained or lost since system formation.• No daughter atoms present when the system formed.• To avoid potential problems, only fresh,

unweathered rock samples should be used.

Difficulties in Dating• Not all rocks can be dated by radiometric

methods: Grains comprising detrital sedimentary rocks are

often weathered and may have lost some of their daughter atoms.

The age of the detrital grain in a sedimentary rock gives the age of the original parent rock from which it was derived.

Metamorphic rocks have undergone trauma from intense heat and pressure that may cause the loss of some daughter atoms.

The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed – may represent age of metamorphism due to recrystallization of the minerals.

Ch09 geologic time_fall2007 (1)

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