Geology 111G/Lecture 13

Measuring Geologic Time

Geologic Time Scale

Relative Time vs. Absolute Time

Radiometric Dating Techniques

Potassium-Argon

Uranium-Lead

Radiocarbon

Placing Ages on the Geologic Time Scale

I. Geologic Time Scale: Sequential arrangement of the geologic time units (periods) that were originally defined from rocks with unique fossil content.

A. The major boundaries of the geologic time scale represent important changes in the history of life. It has formal subdivisions named after geographic localities where the rocks of those time intervals were first defined.

1. Eon: Largest subdivision of geologic time.

2. Era: Major span of time whose unifying feature is the continuity of life forms during that interval. Eras end at the great extinction events of Earth history.

2. Period: Spans of time that correspond to intervals of rocks named for the localities where they were first studied. A period contains distinctive assemblages of life forms or fossils. For example, Allosaurus and Apatosaurus are typical assemblages of Jurassic dinosaurs, whereas T. rex and Triceratops are Cretaceous dinosaurs.

II. Relative Time vs. Absolute Time.

A. Relative Time: The arrangement of geologic events and time periods in their proper order. This is done by using the stratigraphic techniques learned in the previous lecture. One does not need to know anything about the duration of those time periods in years.

B. Absolute Time: The age of a geologic event in years before present or the duration of a time period in years; also, the time in years since the beginning or end of that period.

III. Radiometric Dating Techniques: Means by which absolute ages may be assigned to minerals that contain radioactive isotopes; this ultimately allows numerical time values to be assigned to period boundaries.

A. Prior to the discovery of techniques that measure radioactive decay, geologists attempted to estimate the age of the Earth by assuming that sedimentary rocks accumulate at certain constant or average rates and then dividing that rate into the total known thickness of the geologic column. The thickness of the column was determined by correlating rocks from all over the world.

1. Estimates ranged from 3 million to 1.5 billion years, probably due to differences in estimated sediment accumulation rates and differences in estimates of how thick the rocks of the world are.

2. Radioactivity: Process by which the nucleus of an unstable (radioactive) isotope is spontaneously transformed to the nucleus of a stable isotope or another radioactive isotope through gain or loss of a subatomic particle. This process is termed radioactive decay.

a. Parent: isotope being transformed

b. Daughter: isotope being produced.

c. The rate at which radioactive decay proceeds, and thus parent isotopes are destroyed and daughter isotopes are formed, is unaffected by the chemical and physical environment. The percentage of parent material converted to daughter isotopes is constant per time unit.

d. Half life: The time required for half of the parent atoms in a sample to be reduced by one half. The values of half lives for various isotopes have been measured carefully. For example, the half life of Uranium 238, which decays to form lead 206, is 4.5 billion years.

e. Determining age. The ratio of parent to daughter isotope is measured, which determines how many half lives the isotopes have experienced. This in turn allows calculation of the age of the mineral in which these isotopes occur.

B. Potassium-Argon Technique. This dating technique utilizes minerals that contain potassium (K). It is best for igneous rocks, particularly silicic rocks (especially granite and rhyolite) that contain potassium feldspar, biotite and or hornblende.

1. 40K decays to 40Ar; Argon is an inert gas which does not tend to bond with other atoms in the mineral. In magma and in hot, newly formed minerals, 40 Ar diffuses out of the mineral lattice.

2. Upon cooling through the mineral’s blocking temperature, the temperature at which Argon no longer escapes, the previously Argon-free mineral begins to accumulate 40Ar by decay of 40K. The half life of this process is 1.3 billion years. By measuring the ration of 40L to 40Ar the amount of time since the mineral cooled through that blocking temperature can be determined.

3. Technique is good for hornblende (amphibole), biotite, and whole volcanic rocks, which have K in their glassy groundmass.

C. Uranium-Lead Technique. This technique utilizes the decay of 238U to 206Pb and 235U to 207Pb. Uranium occurs in very small amounts in some mineral grains, such as zircon, found in silicic to intermediate volcanic and plutonic rocks. Half life of 238U to 206Pb is about 4.5 billion years, about the age of the earth. This technique is quite precise, and because of the long half life of the reaction, has been used to find the oldest rocks known on earth

D. Radiocarbon (14C) Technique: This technique utilizes the decay of 14C to 14N, a gas that is lost to the atmosphere and so cannot be measured in the sample. The half life of this decay reaction is short, 5,730 years.

1. 14C is constantly being created from 14N in the atmosphere due to neutron (cosmic ray) bombardment of atmospheric gas. The newly formed 14C mixes with 12C and 13C, the stable isotopes of carbon, to achieve a constant ratio with those isotopes.

2. Living organisms have this same carbon balance because they ingest carbon, but upon death the 14C begins to decay back to 14N by the process of beta decay, wherein the nucleus loses an electron, or beta par.

3. To determine the age of fossil organic matter, the radioactivity of the remaining 14C is measured, rather than measuring 14C/14N, because the nitrogen escapes.

4. The short half life of this process restricts the utility of the technique to the last 70,000 years. Beyond that, there is not enough 14C in a sample to measure.

5. Technique is good for wood, bone, charcoal, tissue, textiles.

III. Placing ages on the Geologic Time Scale: Combining radiometric techniques and the stratigraphic principles of last lecture, we can start putting absolute ages on the boundaries of the time scale. To do this, igneous rocks in the stratigraphic section are needed, either deposited along with the strata, such as tuffs, or cutting across the strata, such as plutons.