Time and Geology

Key Points

• The meaning of relative age and absolute age?

• Different principles for relative time and their meaning?

• Given a cross section, student is required to reconstruct the sequence of events

• Unconformity and different types of Unconformities

• About the geological time scale: rember the names and symbols of different periods and the order of these period

• Given a cross section, student specify type of unconformity in the cross section, write the sequence of events. See example below

Tecnical Terms

Students are required to understand the technical terms, be able to explain these terms, and find the meaning in mother language (be able to explain in your own way)

angular unconformity

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

Archean Eon

The oldest eon of Earth’s history.

Cenozoic Era

The most recent of the eras; followed the Mesozoic Era

contacts

Boundary surface between two different rock types or ages of rocks.

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.

cross-cutting relationship

A principle or law stating that a disrupted pattern is older than the cause of disruption.

disconformity

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

eon

The largest unit of geological time.

epoch

Each period of the standard geologic time scale is divided into epochs (e.g., Pleistocene Epoch of the Quaternary Period).

era

Major subdivision of the standard geologic time scale (e.g., Mesozoic Era).

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.

formation

A body of rock of considerable thickness that has a recognizable unity or similarity making it distinguishable from adjacent rock units. Usually composed of one bed or several beds of sedimentary rock, although the term is also applied to units of metamorphic and igneous rock. A convenient unit for mapping, describing, or interpreting the geology of a region.

fossil assemblage

Various different species of fossils in a rock.

half-life

The time it takes for a given amount of a radioactive isotope to be reduced by one-half.

inclusion

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

index fossil

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

isotope

Atoms (of the same element) that have different numbers of neutrons but the same number of protons.

isotopic dating

Determining the age of a rock or mineral through its radioactive elements and decay products (previously and somewhat inaccurately called radiometric or radioactive dating).

lateral continuity

Principle that states that an original sedimentary layer extends laterally until it tapers or thins at its edges.

Mesozoic Era

The era that followed the Paleozoic Era and preceded the Cenozoic Era.

nonconformity

An unconformity in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.

numerical (or absolute) age /time

Age given in years or some other unit of time.

original horizontality

The deposition of most water-laid sediment in horizontal or near-horizontal layers that are essentially parallel to Earth’s surface.

Paleozoic Era

The era that followed the Precambrian and began with the appearance of complex life, as indicated by fossils.

period

Each era of the standard geologic time scale is subdivided into periods (e.g., the Cretaceous Period).

Phanerozoic Eon

Eon of geologic time. Includes all time following the Precambrian.

physical continuity

Being able to physically follow a rock unit between two places.

Pleistocene Epoch

An epoch of the Quaternary Period characterized by several glacial ages.

Prearchean (Hadean) Eon

The earliest Eon of Earth's history.

Precambrian

The vast amount of time that preceded the Paleozoic Era.

Proterozoic Eon

Eon of Precambrian time.

Quaternary Period

The youngest geologic period; includes the present time.

radioactive decay

The spontaneous nuclear disintegration of certain isotopes.

Recent (Holocene) Epoch

The present epoch of the Quaternary Period.

relative time/age

The sequence in which events took place (not measured in time units).

standard geologic time scale

A worldwide relative scale of geologic time divisions.

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.

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.

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.” Also, see actualism.

Excercise to practices

Write the name of rock layers from oldest to youngest and interpret the sequence of events that occured in the cross sections

Examples of cross sections.

Extra Reading

Watch out! this reading part is from the main objectives of the lesson

1. Uniformitarianism (actualism) implies that geologic processes operating today also operated in the past;"the present is the key to the past." Since rates of deposition and other activities are slow, the expanse of geologic time was necessarily broadened by application of uniformitarianism, but the term doesn't imply rates were uniform.

2. Absolute time provides a date in years or some other time unit to a rock, while relative time merely arranges events in a sequence.

3. Geologists think of the geology of an area in terms of the sequence of events that form its history. Four basic principles are applied to recognize the various steps in the geologic history of an area. Original horizontality implies that the rocks were horizontal when first formed, and any change from horizontal took place after deposition. Superposition implies that rock sequences get younger toward their tops. Lateral continuity states that sedimentary layers extend laterally until their edges pass into another environment reflected by different sediment types. Cross-cutting relationships imply that a disrupted rock unit is older than the cause of its disruption. Figure 8.1-8.11 and Table 8.1 apply these principles to understanding the sequence of events that developed Minor Canyon, a fictitious location similar to Grand Canyon.

4. Correlation establishes age relationships between rock units or events in separate areas. Physical continuity implies that a rock unit can be traced from one area to another. Similarity of rock types allows correlation, particularly if the character and sequence are distinctive. Correlation by fossils utilizes the observation of faunal succession: fossil species occur in a definite and recognizable succession through time. Index fossils are short-lived and widespread, but most correlations are accomplished by fossil assemblages.

5. The standard geologic time scale reflects relative time and is based on fossil assemblages. Eras are the largest divisions, followed by periods, and then epochs. Fossils become common with the beginning of the Paleozoic Era, and rocks that precede that era are called Precambrian. The Mesozoic Era succeeds the Paleozoic Era, followed by the Cenozoic Era. Those era boundaries are times of mass extinction. The Cenozoic Era includes the Holocene Epoch in which we are now living. Most geological investigations involve the use of relative time.

6. Unconformities are gaps in the geologic record developed as buried surfaces of erosion. A disconformity separates beds that parallel one another. Fossils may indicate a break in the record. Angular unconformities separate tilted older strata from horizontal younger strata. Fossils and cross-cutting relationships may be used to determine the relative time of the folding and tilting. Nonconformities separate older plutonic or metamorphic rocks from younger sedimentary rocks.

7. Absolute time provides ages, usually in years, to geological features and events. The earth is estimated to be 4.5-4.6 billion years old. The oldest rocks on earth, 4.03 billion years old, are found in northwestern Canada. The oldest dated mineral is a zircon from Australia, which is 4.4 billion years old.

8. Absolute dating is based on the decay of radioactive isotopes, particularly uranium. Decay is expressed in half-lifes, the time it takes for one-half of a given amount of radioactive isotope to be reduced to its stable daughter product. Radioactivity involves emission of alpha and beta particles and electron capture that may change the atomic number or atomic mass number of the atom. In contrast, radiocarbon dating involves the formation and breakdown of C-14. Dates are determined by comparing the amount of C-14 present to what would be expected in a living organism. The half-life is only about 5,730 years and the system only provides dates on organic materials for the last 40,000 years with accuracy.

9. Dating is based on a comparison of the amount of isotope originally present compared to the amount present at the time of the analysis and the half-life of the isotope involved. Usually the date provided by the analysis is the time that the rock or mineral became a closed system. In igneous rocks, that would date mineral crystallization, while in metamorphic rocks, it would be time of metamorphism. Absolute dates have been assigned to the geologic time scale by bracketing events whose relative time is known. It has been used to subdivide the Precambrian, which comprises the bulk of geologic time.

10. The age of the earth has been a controversial subject and several attempts were made to determine its antiquity prior to the discovery of radioactivity (biblical, rate of cooling). The age of the earth has been established as 4.5 - 4.6 billion years old based on isotopic dating of meteorites.

11. Geologic time is vast, mostly represented by the Precambrian, and human history represents an exceedingly small portion of that time.