Geologic Time Chapter

Prepared by Iggy Isiorho for

Dr. Isiorho

Time and Geology

The Key to the Past

Uniformitarianism – Principle that geologic processes operating at present are the same processes that operated in the past. The principle is stated more

succinctly as “The present is the key to the past.” Actualism – The principle that the same processes and natural laws

that operated in the past are those we can actually observe or infer from observations as operating at present. Under present usage, uniformitarianism has

the same meaning as actualism for most geologists. Numerical age – Age given in years or some other unit of time. Relative time – The sequence in which events took place (not

measured in time units).

Relative Time

The geology of an area may seem, at first glance, to be hopelessly complex. A non-geologist might think it impossible to decipher the sequence of events that created such a geologic pattern. However, a geologist has learned to approach seemingly formidable problems by breaking them down to a number of simple problems. As an example, the geology of Grand Canyon can be analyzed in four parts: (1) horizontal layers of rock; (2) inclined layers; (3) rock underlying the inclined layers (plutonic and metamorphic rock); and (4) the canyon itself, care into these rocks.

Principles Used to Determine Relative Age

Original Horizontality Superposition Lateral continuity Cross-cutting relationships

Original Horizontality

Original Horizontality – The deposition of most water-laid sediment in horizontal or near-

horizontal layers that are essentially parallel to the earth’s surface.

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Superposition

Superposition – A principle or law stating that within a sequence of

undisturbed sedimentary rocks, the oldest layers are on the bottom, the youngest on the top.

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Lateral Continuity

Lateral Continuity – Principle that states that an original sedimentary

layer extends laterally until it tapers or thins at its edge.

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Cross-cutting Relationships

Cross-cutting Relationships – A principle or law stating that a

disrupted pattern is older than the cause of disruption.

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Other Time Relationships Inclusions – A fragment of rock that is

distinct from the body of igneous rock in which it is enclosed.

Correlation

In geology correlation usually means determining time equivalency of rock units. Rock units may be correlated within a region, a continent, and even between continents. Various methods of correlation are described below along with examples of how the principles we described earlier in this chapter are used to determine whether rocks in one area are older or younger than rocks in another area.

Physical Continuity

Finding physical continuity – that is, being able to trace physically the course of a rock unit – is one way to correlate rocks between two different places. In Fig. 8.13, the prominent white layer of cliff-forming rock, the Coconino Sandstone, exposed along the upper part of Grand Canyon can be seen all the way across the photograph. You can physically follow this unit for several tens of kilometers, thus verifying that, wherever it is exposed in Grand Canyon, it is the same rock unit.

Similarity of Rock Types Under some circumstances, correlation between two regions can be

made by assuming that similar rock types in two regions formed at the same time. This method must be used with extreme caution, especially if the rocks being correlated are common ones.

Correlation by similarity of rock types is more reliable if a very unusual sequence of rocks is involved. If you find in one area a layer of green shale on top of a red sandstone that, in turn, overlies basalt of a former lava flow, and then find the same sequence in another area, you probably would be correct in concluding that the two sequences formed at essentially the same time.

When the hypothesis of continental drift was first proposed (see Chapter 1) important evidence was provided by correlating a sequence of rocks (Fig. 8.16) consisting of glacially deposited sedimentary rock.

Correlation by Fossils

Faunal succession – A principle or law stating that fossil species succeed one another in a definite and recognizable order; in general, fossils in progressively older rock show

increasingly greater differences from species living at present.

Index fossil – A fossil from a very short-lived species known to have existed during a specific period of geologic time.

Fossil assemblage – Various different species of fossils in a rock.

The Standard Geologic Time Scale

Standard Geologic Time Scale – A worldwide relative scale of geologic time divisions.

The geologic time scale, representing an extensive fossil record, consists of three eras, which are subdivided into periods, which are, in turn divided into epochs. (Remember that this is a relative time scale.)

Unconformity Unconformity – A surface that represents a break in the

geologic record, with the rock unit immediately above it being considerably younger than the rock beneath.

3 types of unconformity:

Disconformities Angular unconformities Nonconformities

Disconformities Disconformities – A surface that

represents missing rock strata but beds above and below that surface are parallel to one another.

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Angular unconformities Angular unconformities – An unconformity

in which younger strata overlie an erosion surface on tilted or folded layered rock.

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Nonconformities Nonconformities – An unconformity in which an

erosion surface on plutonic or metamorphic rock has been

covered by younger sedimentary or volcanic rock.

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Numerical Age

Counting annual growth rings in a tree trunk will tell you how old a tree is. Similarly, layers of sediment deposited annually in glacial lakes can be counted to determine how long those lakes existed. But only within the few decades since the discovery of radioactivity have scientists been able to determine numerical ages of rock units. We have subsequently been able to assign numerical values to the geologic time scale and determine how many years ago the various eras, periods, and epochs began and ended.

Isotopic Dating

Radioactivity provides a “clock” that begins working when radioactive elements are sealed into newly crystallized minerals. The rates at which radioactive elements decay can be measured and duplicated in many different laboratories. Therefore, if we can determine the ratio of a particular radioactive element and its decay products in a mineral, we can calculate how long ago that mineral crystallized.

Determining the age of a rock through its radioactive elements is known as isotopic dating.

Isotopes and Radioactive Decay

Radioactive decay – The spontaneous nuclear disintegration of certain isotopes.

Daughter product – The isotope produced by radioactive decay.

Half-life – The time it takes for a given amount of a radioactive isotope to be reduced by one-

half.

Radiocarbon Dating

Because of its short half-life of 5,730 years, radiocarbon dating is useful only in dating things and events accurately back to about 40,000 years – about seven half-lives.

The technique is most useful in archaeological dating and for very young geologic events. It has even been used to date historical artifacts, such as the Shroud of Turin.

Use of Isotopic Dating

In order for an isotopic age determination to be accurate, several conditions must be met. To ensure that the isotopic system has remained closed, the rock collected must show no signs of weathering or hydrothermal alteration. Second, one should be able to infer there were no daughter isotopes in the system at the time of closure or make corrections for probably amounts of daughter isotopes present before the “clock” was set. Third, there must be sufficient parent and daughter atoms to be measurable by the mass spectrometer being used.

How Reliable is Isotopic Dating? Half-lives of radioactive isotopes, whether short-

lived, such as used in medicine, or long-lived, such as used in isotopic dating, have been found not to vary beyond statistical expectations. The half-life of each of the isotopes we use for dating rocks has not changed with physical conditions or chemical activity nor could the rates have been different in the distant past. It would violate laws of physics for decay rates (half-lives) to have been different in the past.

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