I. California K-6 Standards for Geologic Time

II. Goals of this Module

III. Relative dating of events involving tilted or folded sedimentary rocks

IV. Relative dating of rocks by the Law of Superposition

V. Relative dating by the Law of Cross-Cutting Relationships

VI. Relative dating of rocks by their Fossil Content

VII. Absolute dating of rocks by decay of radioactive elements

VIII. Laboratory Exercises

IX. Possible Essay Questions

X. Practice Questions

1. Investigation and Experimentation

1. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept, students should develop their own questions and perform investigations. Students will:

1. interpret events by sequence and time from natural phenomena (e.g., relative ages of rocks and intrusions).

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• To learn the rules by which the relative ages of rocks are determined and to practice applying the rules in laboratory problems.
• To learn how the absolute ages of rocks are determined and to practice calculating the absolute ages of rocks in laboratory problems.
• To learn the definitions of and proper spelling for scientific terms that are used commonly in newspapers, magazines, and books, and on television and radio programs in discussions about geologic time.

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• The Principle of Original Horizontality states that most sedimentary rocks are originally deposited in a nearly horizontal orientation.
• Therefore, for sedimentary rocks that are tilted or folded, it follows that the tilting or folding event has come after deposition of the rocks.

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• Rocks at the bottom of a pile of sedimentary rocks are older than those at the top of the pile.
• In some parts of the world, sedimentary layers can be traced for long distances so that the relative positions of the various layers and, hence, their relative ages can be established.

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• Any rock unit or geologic feature that cuts across or disrupts another rock unit or geologic feature is younger than the rock unit or geologic feature that it cuts or disrupts.
• Plutonic igneous intrusions are always younger than the rocks that they cut across or intrude.

• Events that produce a plutonic igneous intrusion -

1. Formation of the rocks that subsequently will be intruded.
2. Intrusion of magma into the pre-existing rocks.
3. Cooling of the magma to form a dike, sill volcanic neck or batholith.

• Faults are always younger than the rocks that they cut across or displace.

• Events that produce a fault -

• Rocks above an unconformity (erosional surface in the rock record) are always younger than the rocks below the unconformity.

• Events leading to an angular unconformity -

• Events leading to a nonconformity -

• Fossils are the remains or other evidence, preserved in rocks, of animals and plants that lived in past times.

• Skeletal parts of animals or plants
• Soft parts of animals or plants
• Impressions of animals or plants
• Tracks, trails, and burrows of animals



Source for Diagram: http://www.emory.edu/GEOSCIENCE/Graphics/B-Therotrack2.jpg

• Fossil evidence of animals



Source for Diagram: http://www.emory.edu/GEOSCIENCE/Graphics/B-Coprolite.jpg

• Paleontology is the study of life that existed in the past and is preserved in the form of fossils.
• Processes responsible for the formation of fossils -

• Preservation - little or no chemical or physical change

• Recrystallization - minerals in skeletal part of organism change crystal form, but not composition
• Permineralization - cavities in skeletal part of organism are filled with mineral deposits
• Replacement - minerals in skeletal part of organism are replaced with a new mineral
• Carbonization - soft parts of organism lose their hydrogen and oxygen and remain as a thin film of carbon
• Mold formation - skeletal part of organism is dissolved, leaving a cavity that is the exact shape of the original skeleton
• Cast formation - a mold is filled with mineral deposits, creating a replica of the original skeleton

• Using the fossil record to determine relative age of rocks -

• By keeping a careful record of where various fossils are found and by applying the Law of Superposition to that fossil record, geologists and paleontologists have discovered that each different type of fossil has a limited geologic life span and occurs only within a small part, or range, of all the sedimentary rocks deposited throughout geologic time.
• The above discovery is referred to as the Principle of Faunal Succession.
• By using the Principle of Faunal Succession, widely separated or isolated sedimentary rocks, to which the Law of Superposition cannot be applied, can be correlated (recognized to be of about equal age) if they contain the same types of fossils.

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• Radioactive isotopes in minerals are created at the time that the mineral cools from a magma or a lava into an igneous rock.
• A newly created isotope (parent), being unstable because it has the wrong number of neutrons, begins to decay immediately into the stable form (daughter).
• The decay rate is constant, so careful calculations of the amounts of the parent and the daughter isotopes in a mineral will allow calculation of the age of the rocks in years.
• The decay rate of each radioactive isotope is stated in terms of its half-life, which is the time in years that it takes for one-half of the original parent to decay into the daughter.
• Decay rate curve -

• Example - A certain ancient lava flow contains in one of its minerals a radioactive element that has a half-life of 10,000 years. Analysis shows that only 20% of the unstable parent element remains and 80% has decayed into the stable daughter element. How old is the lava flow?

• Remember! When the time representing one half-life has elapsed, one-half of the original parent isotope will remain; during elapse of the time representing the second half-life, one-half of the remaining one-half of the parent will decay, leaving one quarter of the original parent isotope; etc.

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• DETERMINING GEOLOGIC HISTORYFROM GEOLOGIC CROSS SECTIONS
• DETERMINING THE ABSOLUTE AGE OF ROCKS FROM GIVEN DATA

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• List the sequence of historical events that occur before and during the production of a plutonic igneous intrusion.
• List the sequence of historical events that occur before and during the production of a fault.
• List the sequence of historical events that occur before and during the production of an angular unconformity.
• List the sequence of historical events that occur before and during the production of a nonconformity.
• In addition to the skeletal parts of animals and plants, what other features found in rocks are considered fossils?
• Describe the seven ways in which fossils are preserved.
• Describe and illustrate with a graph how rocks are dated by radioactive methods.

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X. Practice Questions

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