GEOLOGIC TIME HOW TO TELL TIME GEOLOGICALLY. Copyright © by Houghton Mifflin Company. All rights...

GEOLOGIC TIME

HOW TO TELL TIME GEOLOGICALLY

Copyright © by Houghton Mifflin Company. All rights reserved. 2

GEOLOGIC TIME

What is the concept of geologic time?

What are the ways we can constrain the time of Earth events?

How do geologists work out the timing of these events.

What are the basic concepts that are applied?


Geology at a Glance


GEOLOGIC TIME


GEOLOGIC TIME

The earth has a long and complex history that has involved many events.

Geologists must be able to read the geologic record in order to unravel earth history.


Magnitude of Earth History


GEOLOGIC TIME

You may not be a geologist, YET, but it is still important and interesting to be able to understand the earth record that you observe every day.


GEOLOGIC TIME

There are two basic ways to unravel a geologic record:

1. Relative sequence of events.

2. Absolute ages of events.

What is the difference? (+4)


GEOLOGIC TIME

Before about 1950, geologist relied on relative methods of dating and correlation of units to develop a geologic time scale.


GEOLOGIC TIME

Relative methods involved understanding the geologic events in a given area and recording the observed sequence of events in the rock record.


GEOLOGIC TIME

Over time, geologists attempted to correlate events and geologic units from different parts of a region to construct a history.

This was then extend over larger regions to attempt to establish a geologic time scale that could be applied over the entire Earth.


GEOLOGIC TIME

In the nineteenth century, geologists began to assemble a geologic column, which is a composite column containing, in chronological order, the succession of known strata, fitted together on the basis of their fossils or other evidence of relative age.

The corresponding column of time is the geologic time scale (refer to table 8.2 in the textbook).


Geologic Time Scale

Contrasting several dating techniques chronicling Earth’s history to produce a geologic Time Scale.

• Geologic Time Scale is divided into Eons, Eras, Periods, and Epochs.


EonsAn eon is the largest interval into which geologic time is divided.

There are four eons:

The Hadean Eon is the oldest: some of the samples brought back from the moon were formed during the Hadean Eon.

The Archean Eon follows the Hadean: Archean rocks, which contain primitive microscopic life forms are the oldest rocks we know of on the Earth.

The Proterozoic Eon follows the Archean.

The Phanerozoic Eon is the most recent of the four eons.


Eras

• Each of the eons is subdivided into shorter time units called eras.

• The Phanerozoic Eon is divided into the:

o Paleozoic era: (ancient life); mostly marine invertebrates

o Mesozoic era: (middle life) dominated by reptileso Cenozoic era: (recent life) dominated by mammals


Eras

• In the Paleozoic Era, early land plants appeared, expanded and evolved. Developing animal life included marine invertebrates, fishes, amphibians,and reptiles.

• The Mesozoic Era saw the rise of the dinosaurs, which became the dominant vertebrates on land. Mammals first appeared during the Mesozoic Era as did flowering plants.

• Mammals dominated the Cenozoic Era. Grasses evolved during the Cenozoic Era, and became an important food for grazing mammals.


Periods

• The Eras of the Phanerozoic Eon are divided into periods.

– The periods are defined on the basis of the fossils contained in the equivalent rocks.

– The two Periods are the Quaternary Period and the Tertiary Period


Epochs• Periods are further subdivided into epochs on

the basis of the fossil record.

• The Tertiary Period is divided into these epochs:– Paleocene.– Eocene.– Oligocene.

• The Quaternary Period is divided into these epochs:– Holocene.– Pleistocene.


GEOLOGIC TIME

The concept that most geologic processes happen very slowly was proposed by James Hutton (1726-1797).

Hutton also proposed a very important concept know as uniformitarianism.


GEOLOGIC TIME

What is the concept of uniformitarianism? (+3)

Why is it an important concept? (+2)

Is this concept really practical for us to use? Explain! (+3)


GEOLOGIC TIME

Modern view holds that processes that operate today have shaped the Earth through Geological Time, but rates may not have always remained constant.


Concepts of Relative Dating

Position in that sequence identifies relative age.

Basic principles:

• Relative positions of layered rocks.

• Relationships of rocks units.

• Sequence of rock units.

• Correlation of rock units and events from different areas.

• Relative-age principles.



Layered sedimentary or volcanic rocks contain important clues about past environments at and near Earth’s surface.

Their sequence and relative ages provide the basis for reconstructing much of Earth’s history. The study of strata is called stratigraphy.

The use of layered volcanic and sedimentary rocks is important for relative dating.



Most sediment is laid down in the sea, generally in relatively shallow waters, or by streams on the land.

Ash from volcanic eruptions can also be deposited on the surface.



The fact that sediment and ash is deposited in layers forms the basis for the principle of original horizontality.


Original Horizontality

What is the principle of original horizontality? (+3)



With this principle in mind, if you observe layered rocks that are inclined or disrupted in an outcrop, what can you infer? (+3)



Describe a simple teaching method by which you could illustrate this principle. (+3)


Superposition

Another important concept for relative age assignment is referred to as the principle of superposition.

What is this principle? (+3)


Superposition

Describe a simple teaching method by which you could illustrate this principle. (+3)


Superposition

Give me an example of an earth process where the principle of superposition would not apply in a layered sequence of rocks. (+3)


Lateral Continuity

What is the principle of Lateral Continuity? (+3)

Using snowfall as an example, explain why lateral continuity must happen in layered rocks. (+3)


Principle of Inclusion for Sediments

When sediments are deposited the younger units rest on top of then older units. Sometimes fragments of one unit can be entrained in another. This forms the basis for the principle of inclusion.

Explain the basic idea of the principle of inclusion (refer to page 206 in your textbook). (+3)


Principle of Inclusion for Intrusive Rocks

When bodies of magma intrude into older rocks they can also pull off pieces of the rock and include them. These are called xenoliths or foreign rocks. This allows us to determine the relative ages of dikes, sills, and other bodies of plutonic rock.


Principle of Cross-Cutting Relationship

A rock unit can be cut by another geologic unit or feature.

This forms the basis of the principle of cross cutting relationships.



Describe two geologic processes that could cause cross cutting in the geologic record. Be specific and make certain your discussion is concise and clear. (+6)



Using the concept of cross cutting which unit is cutting the other (dark unit or light unit). (+2)


Principle of Faunal Succession

Explain the principle of faunal succession. (+3)

Why is this idea so important in working out geologic history? (+ 3)


Fossils

Fossils are a valuable tool for geologists because they give us clues about past life and environments preserved in a rock unit.

Fossils can also be used for rock correlation.

Index fossils make the best time markers.


Fossils


Index Fossils

What is an index fossil?

What are three criteria that are desired in an index fossil to make it useful? (+4)


Other Key Time Markers

A time marker in a geologic record is especially useful if it is distinct, short in duration, widespread, and allows accurate correlation.

Besides index fossils, give me another example of a geologic event or process that could create this type of time marker. (+3)


Unit Correlation

Geologists can use fossils, rock sequences, and relative events to piece together the geologic history of one area.

By comparing this record to other areas a more extensive and regional record can be developed.

This is done by correlating rock records and looking for similarities in the different records to connect them.


Unit Correlation

A rock-stratigraphic unit is any distinctive stratum that differs from the strata above and below.

The basis of rock stratigraphy is the formation.

A formation is a collection of similar strata that are sufficiently different from adjacent groups of strata so that on the basis of physical properties they constitute a distinctive, recognizable unit that can be used for geologic mapping over a wide area.


Unit Correlation

Each of the boundaries of a time-stratigraphic unit, upper and lower, is uniformly the same age.

The primary time-stratigraphic unit is a system, which is chosen to represent a time interval sufficiently great so that such units can be used all over the world.


Figure 11.6


Unit Correlation

The primary unit of geologic time is a geologic period, which is the time during which a geologic system accumulated.

Correlation is the determination of equivalence in time-stratigraphic or rock-stratigraphic units of the succession of strata found in two or more different places.


Unit Correlation

Correlation involves two main tasks:

1. Determining the relative ages of units exposed within a local area being studied (identifying the same formation wherever it crops out).

2. Establishing the ages of the local rock units relative to a standard scale of geologic time.


Unit Correlation


Unit Correlation

Describe a simple class room activity to demonstrate the concept of correlation. (+5)


Gaps in the Geologic Record

No geologic record is continuous.

There are gaps created in the record by erosion or lack of geologic events in an area for a given period of time.



The many unconformities exposed in rocks of Earth’s crust are evidence that former seafloors were uplifted by tectonic forces and exposed to erosion.

Preservation of a surface of erosion occurs when later tectonic forces depress the surface.

The surface, in turn, becomes a site of deposition of sediment.



An unconformity is a substantial break or gap in a stratigraphic sequence.

Three important kinds of unconformities are found in sedimentary rocks:



1. Disconformity2. Angular Unconformity3. Non Conformity

Describe each of these unconformities and discuss what they tell you about the geologic history of a rock sequence. (+12)


Angular Unconformities and Nonconformities



What type of unconformity is shown in the figure below? (+2)


Relative Dating vs. Absolute Time

Geochronology is the study of time in relation to earth’s existence:

• Relative DatingDetermines how old a rock is in relation

to its surrounding

• Numerical DatingDetermines actual age in years



If you examine a geologic time scale you will notice that there are numbers in billions, millions, or thousands of years on the chart.

This did not come from relative dating.

These numbers come from techniques that give absolute ages of time.



• Early attempts to measure geologic time numerically were inaccurate.

– Edmund Halley suggested, in 1715, that sea salt might be used to date the ocean.

– John Joly finally made the necessary measurements and calculations in 1889. His determination of the ocean’s age, 90 million years, was not correct.

• Salts are added both by erosion and by submarine volcanism, but salts are also removed by solution.



• Lord Kelvin, a physicist, attempted to calculate the time Earth has been a solid body.

• By measuring the thermal properties of rock and estimating the present temperature of Earth’s interior, he calculated the time for the Earth to cool to its present state.

– His estimate of 100 million years is incorrect. – The Earth’s interior is cooling so slowly that it

has a nearly constant temperature over periods as long as hundreds of millions of years.


Radiometric Dating and the Geologic Column

• Through various methods of radiometric dating, geologists have determined the dates of solidification of many bodies of igneous rock.

• “Moon dust” brought back by astronauts, is 4.55 billion years old.

• The Earth was formed approximately 4.55 billion years ago.


Figure 11.15


Continent-continent collision, Mountain building, and Mountain unbuilding.


Geochronologic Methods

Radiometric age methods:

• Absolute Age Methods (e.g., U-Pb): based on radioactive decay.

• Fission Track: High speed particles emitted during radiation may pass through crystal leaving ‘tears’ within the crystal- the older the rock, the more fission tracks.

• Dendrochronology (Tree-Ring dating): Annual growth rings.

• Varve- deposited layers of lake-bottom: Paired layers of sediments.

• Lichenometry: Lichens grow at a fairly constant rate.

• Cosmogenic isotopes: Used in dating land features.

• Magnetic Polarity Time Scales: based on magnetization in rocks.


Radioactive Age Determinations

• In 1896, the discovery of radioactivity provided the needed method to measure the age of the Earth accurately.

• Different kinds of atoms of an element that contain different numbers of neutrons are called isotopes.– Most Isotopes of the chemical elements found in

Earth are generally stable and not subject to change.



• A few isotopes, such as 14Carbon, uranium, rubidium potassium and samarium, are radioactive.

o Radioactivity arises because of instability within an atomic nucleus.

o If the ratio of the number of neutrons (n) to the number of protons (p) is too high or too low, the atomic nucleus of a radioactive isotope will transform spontaneously to a nucleus of a more stable isotope of a different chemical element.



• The process is called radioactive decay.

o An atomic nucleus undergoing radioactive decay is said to be the parent.

o The product arising form radioactive decay is called a daughter.


Figure 8.19: Bracketing ages.


Radioactive Decay

• Radioactive decay can happen in five ways:

1. Beta decay: emission of an electron from the nucleus.

2. Positron emission: emission of a particle with the same mass as an electron but with a positive charge.

3. Electron capture: by capture into the nucleus of one of the orbital electrons, a process that decreases the number of protons in the nucleus by one.


Radioactive Decay

4. Alpha decay: emission from the nucleus of a heavy atomic particle consisting of two neutrons and two protons called an α (alpha) particle.

5. Gamma ray emission: emission of γ rays (gamma rays), which are very short-wavelength, high-energy electromagnetic rays.

• Gamma rays have no mass, so gamma ray

emission does not affect either the atomic number or the mass number of an isotope.


Figure 8.17: Unstable atomic nuclei decay (continued).


Rates of Decay and the Half-Lives of Isotopes

• The rate at which radioactive decay occurs varies among isotopes.

• Decay rates are unaffected by changes in the chemical and physical environment.

• The decay rate of a given isotope is the same in the mantle or in a sedimentary rock.

• In radioactive decay, the proportion—fraction or percentage—of parent atoms that decay during each unit of time is always the same.


Figure 8.17: Unstable atomic nuclei decay.


Figure 8.17: Unstable atomic nuclei decay (continued).



• The rate of radioactive decay is measured in terms of half-life, the amount of time needed for the number of parent atoms to be reduced by one half.

• At the end of each unit of time (half-life), the number of parent atoms has decreased by exactly one-half.



COMPLETE CLASS EXERCISE

1. Obtain two types of colored beads.

2. Assume the RED beds are Parent atoms and Blue beads are daughter atoms.

3. Assume that each half life of decay is 30 seconds.

4. Exchange beads every 30 seconds and tell me how many of each of the two colors of beads you have.

5. How many half lives have occurred before you can no longer exchange beads.

6. How much time has elapsed. (+6)


Radioactive Decay


Using Radioactivity to Measure Time

Radioactivity in a mineral is like a clock.



The length of time this clock has been ticking is the mineral’s radiometric age.



• Many natural radioactive isotopes can be used for radiometric dating, but six predominate in geologic studies:

o Two radioactive isotopes of uranium plus radioactive isotopes of thorium, potassium, rubidium and carbon are used.

o In practice, an isotope can be used for dating samples that are no older than about six half-lives of the isotope.


Figure 8.18: Radioactive decay.


Figure 8.20: Loss of daughter isotopes.


Figure 8.20: Loss of daughter isotopes (continued).


Radiogenic Isotope Methods

What types of rocks or earth materials is radioactive isotope (not carbon dating) age methods most useful? (+3)



What are some important assumptions we make when we use these methods? (+3)



What are the strengths of these methods? (+3)



What are the weaknesses of these methods? (+3)


Factors Affecting Isotope Dating Results

• Isotope dating is more useful for igneous rocks: Clock is set when igneous rock crystallizes locking the radioactive isotopes within its crystal lattice

• Rock/Mineral must be a closed system: Atoms of parent and daughter are still present in rock/mineral being dated

• Condition of parent Material: Fracture, weathering and migrating ground water

• Age of Substance: Enough measurable daughter isotope, use appropriate radioactive isotope


Radiocarbon Dating

• 14C is especially useful for dating geologically young samples.

• The half-life of radiocarbon is short—5730 years—by comparison with the half-lives of most isotopes used for radiometric dating.

• Radiocarbon is continuously created in the atmosphere through bombardment of 14C by neutrons created by cosmic radiation.


Radiocarbon Dating

• Though some variations have been identified, the proportion of 14C is nearly constant throughout the atmosphere and biosphere.

• Living organisms have the same proportion of 14C In their bodies as exists in their environment.

• No carbon is added after death, so by measuring the radioactivity remaining in an organic sample, we can calculate how many half-lives ago the organism died.


Radiocarbon Dating


Magnetic Polarity Time Scale

• Certain rocks become permanent magnets as a result of the way they form.

• Magnetite and certain other iron-bearing minerals can become permanently magnetized.

• Above a certain temperature (called the Curie point), the thermal agitation of atoms is such that permanent magnetism is impossible.

• Below that temperature, however, the magnetic fields of adjacent iron atoms reinforce each other.



• As solidified lava cools, the temperature will drop below 580oC, the Curie point for magnetite.

• When the temperature drops below the Curie point, all the magnetite grains in the rock become tiny permanent magnets with the same polarity as Earth’s field.

• All lava formed at the same time records the same magnetic polarity information.



• The Earth’s polarity has shifted in the past. A period in which polarity remains stable is called a magnetic chron.

• The four most recent chrons have been named for scientists who made great contributions to studies of magnetism. The four chrons below occurred during the last 4.5 million years. From the most recent to the oldest:

– Brunhes.– Matuyama.– Gauss.– Gilbert.


Primordial Gasses

• Studies of volcanic gases provide other clues to the age of the Earth.– Three gases, 40Ar (daughter of 40K), 3He, and 36Ar

(both primordial gases trapped in Earth from the solar nebula), are being released, but they are not being recycled.

– Because they accumulate in the atmosphere, their growing proportion can be used to estimate the age of the Earth.


Numerical Age

Isotope Dating relies on the rate of decay of radioactive isotopes within a rock

• Radioactive isotopes have nuclei that spontaneously decay emitting or capturing a variety of subatomic particles

• Decaying radioactive isotope- parent isotopes decay to form daughter isotopes

• Half-life- is the time it takes for half the atoms of parent isotope to decay

• Some radioactive isotopes with daughter productsU-238 => Pb-206; K-40 => Ar-40; C-14 => N-14


Figure 8.4b: Sedimentary structures.


Figure 8-h-1: An organism buried in sediment.


Figure 8.10: Three types of unconformities.


Figure 8.10: Three types of unconformities (continued).


Figure 8-h-2: Hypothetical view of early earth.


Figure 8.12: Grand canyon.


Figure 8.13: Sedimentary rock sequences.


Figure 8.16: Hypothetical landscape.


Figure 8.24: Fisson tracks.


Figure 8.25: Correlation of tree-ring sections.


Figure 8.26: Origin of lake varves.


Figure 8.26: Origin of lake varves (continued).


Figure 8.27: Age of lichen.


Figure 8.28: Cosmogenic isotopes.


Figure 8.31: Rocks underlying hypothetical landscape.


Figure 8.32: The geologic time scale.


Figure 8-h 03: Extinctions graph.


Figure 8-eoc-1: Hypothetical landscape.

GEOLOGIC TIME HOW TO TELL TIME GEOLOGICALLY. Copyright © by Houghton Mifflin Company. All rights...

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