Chapter 8

 

GEOLOGIC TIME

 

Introduction

 

Relative dating.  Involves placing geologic events in a sequential order as determined from their position in the geologic records.

Absolute dating.  Results in specific dates for rock units or events expressed in years before the present.  Radiometric dating is the most common method of obtaining absolute ages.

 

Age of Earth

 

Age of Earth was debated for a long time:

 

-mid 17th century, James Ussher decided Earth created in 4004 B.C. based on literal interpretation of the bible; therefore very little time for all features on Earth to form.

 

-Early scientific attempts: melted iron spheres; sediment accumulation rate; salinity of oceans.

 

James Hutton

 

"father of modern geology"; first to study rocks in detail & recognize immensity of time needed to produce features of the Earth; 1785 published book "Theory of the Earth".

 

-Uniformitarianism- geological processes operating now are the same processes that have operated in the geologic past ("the present is the key to the past").

-Hutton recognized that processes were very slow and required a very old age for the Earth.

 

-Lord Kelvin- (respected English physicist) said in 1866 that he proved uniformitarianism wrong; assumed Earth originally molten and gradually lost heat over time;

calculated that Earth must be between 20 to 400 MY old (therefore too little time for all geologic features on Earth to form).

 

-geologists either had to accept Kelvin's age and squeeze all geologic events into 400 MY or abandon the concept of uniformitarianism.

 

-Radioactivity- discovered ~40 years later; found to be a heat source within the Earth, therefore Kelvin’s calculations were invalid because Earth has not gradually

cooled over time as Kelvin assumed

- shortly after discovery, radioactive dating of rocks confirmed a very old age for the Earth (fig. 8.1).

 

 

 

 

 

Relative Dating

Age of something relative to something else; sequence of events.  Prior to radiometric dating, this was the only tool geologists had to interpret Earth history;

relied on several fundamental principles.

 

Fundamental Principles of Relative Dating (fig. 8.3)

1.   Superposition- in a sequence of sedimentary rock, layers get younger from bottom to top

 

2.   Original horizontality- layers of sediment deposited in water are approximately horizontal (fig. 8.2); if not horizontal, tilted after deposition.

 

3.   Lateral continuity- sediment extends laterally in all directions until it thins and pinches out or terminates against the edge of basin.

 

4.   Cross-cutting relations- disrupted rocks are older than the cause of disruption (figs. 8.3, 8.4).

 

5.   Inclusions- fragments (inclusions) of rock within a layer of rock are older than the rock layer itself (fig.8.5)

 

6.   Fossil succession- fossil assemblages (groups) succeed one another through time in a regular and predictable order (any period of geol. time can be

recognized by fossils present) (fig. 8.6).

 

Unconformity- buried surface of nondeposition or erosion (fig. 8.7)

 

-Disconformity- unconformity between parallel beds (fig. 8.8).

 

-Angular unconformity- unconf. between older tilted/folded rocks and younger rocks (fig. 8.9).

 

-Nonconformity- unconformity between igneous or metamorphic rocks and overlying sedimentary rocks

(may look like intrusive igneous contact, but no "baking") (fig. 8.10).

 

Apply principles of relative dating to geologic history of an area (figs. 8.11, 8.12).

 

Correlation

To show equivalency of rocks in different areas (figs. 8.13, 8.14)

 

Different ways to correlate:

-Physical continuity- can physically trace a rock unit from one area to another.

-Similarity of rock type- 2 rocks similar from different places, may be the same.

-Position in a sequence of rock- more than 1 layer is similar in rocks from different places.

-Key bed- distinctive or unusual bed present in different areas (coal bed, volcanic ash layer).

-Correlation by fossils- same fossil or fossil assemblage in widely separated rocks are same age.

 

The shorter the range of the fossil, the more accurate the correlation.

-Guide fossil (index fossil)- fossil with a short range (lived for a short period of time).

-Assemblage range zones- use more than 1 fossil in rock layer; area of age overlap is age (figs. 8.15, 8.16).

 

Relative geologic time scale- early geologists constructed based on correlation of rocks from all over world (mostly by fossils);

only relative geologic time periods, no actual dates (fig. 8.1 w/o dates).

 

Absolute Dating

Specific age given in number of years before present.

-geologists had a relative geologic time scale and knew Earth was very old, but didn't know how old until radioactivity provided a way to directly date rocks.

 

Radiometric dating:

Radioactive decay.  Spontaneous transformation of unstable atomic nucleus to an atomic nucleus of a different element (fig. 8.18)

-rate of radioactive decay is constant and accurately known for each isotope (table 8.1).

 

Half life- time it takes for half of the atoms of the original unstable parent element to decay to atoms of a daughter element (figs. 8.19 & 8.20).

 

-to date a rock:

1. measure parent/daughter ratio (% parent remaining) with a mass spectrometer.

2. determine number of half lives that have gone by.

3. look up half life of isotope pair (table 8.1).

4. age of rock = number of half lives gone by X half life of isotope pair used

 

-What rocks can be dated?

-igneous rocks- date time of magma crystallization (fig. 8.21)

-metamorphic rocks- usually date time of recrystallization (metamorphism) (fig. 8.22)

-sedimentary rocks- generally can't date directly (made up of minerals from other rocks) (can  be dated indirectly (fig 8.27).

 

*Combining radiometric dates with known relative age relations, can put numbers on the relative geologic time scale making it an absolute geologic time scale (fig. 8.1).

 

How old is the Earth?

-oldest rocks dated are from Greenland, Minnesota, etc. and are ~3.9+ BY old

-meteorites and some moon rocks, however, dated at ~4.6 BY; believed to have formed at same time as Earth; age of Earth, therefore, believed to be ~4.6 BY.