Chapter 10
Geological Dating
How are the ages of rock determined? How can rocks be dated?
Absolute – vs – Relative dating
I
Absolute Dating
Scientists want to pinpoint the age of rocks to a specific date in the earth’s history. This can now be done
through a method called radiometric dating. This method depends on the ability to predict the rate at which
isotopes of particular elements decay.
II
Relative Dating
Relative dating means that rocks are placed in their proper sequence or order. Relative dating will not tell us
how long ago something took place, only that it followed one event and preceeded another. Absolute dating
did not replace relative dating; they simply supplemented them.
The most basic principle of relative dating is: The “Law of Superposition.” The law simply states that in an
undeformed sequence of sedimentary rocks, each bed is older that the one above it and younger than the one
below it.
Principle of original horizontality – It means that layers of sediment are generally deposited in a horizontal
position. Thus if we observe rock layers that are flat, they have not been disturbed and still have their
original horizontality. This law compliments the law of superposition.
III
Principle of cross cutting relationships – when rocks are cut and offset by a fault or an intrusion, the rocks
being cut must be older than the fault or intrusion..
IV
Inclusions – inclusions are pieces of one
rock unit that are contained in another.
The rock mass adjacent to the one
containing the inclusions must have
been there first in order to provide the
rock fragments.
Conformable Rock Layers
Layers of rock are said to be conformable when they are found to have been deposited essentially without
interruption.
Unconformities – When deposition of sediments has been interrupted, a break in the rock record occurs.
Unconformities represent a long period of time during which deposition has ceased, erosion removed previously
formed rocks, and then deposition resumed. In each case uplift and erosion are followed by subsidence and renewed
sedimentation.
Types of Unconformities
I
Angular unconformities – consists of tilted or folded sedimentary rocks that are overlain by younger, more
flat lying strata. Angular unconformities indicate that during the pause in deposition, a period of
deformation (folding or tilting) as well as erosion occurred.
II
Disconformity – more common but harder to identify because the rocks above and below are similar and
there is little evidence of erosion. The strata on either side of a disconformity are essentially parallel.
Cenozoic strata
No Mesozoic strata!!!!!
Paleozoic strata
III
Non-Conformities – with this type of uniformity, older igneous and metamorphic rocks are in contact with
much younger sedimentary strata. Intrusive igneous masses and metamorphic rocks originate far below the
surface. Thus, for a non-conformity to develop, there must be a period of uplift and erosion of overlying
rocks. Once exposed on the surface, the igneous and metamorphic rocks erode and form sediments and
eventually sedimentary rocks.
Correlation of Strata
In order to develop a geological time scale that is applicable to the whole earth, rocks of similar age in different
regions must be matched up. This is called correlation.
Rocks may be correlated by their distinctive features over short or wide geographical locations. Fossils, minerals,
bedding features stratigraphic order are all clues to the identification and correlation of rocks.
Fossils
Paleontology – the branch of geology devoted to the study of ancient life, is based on the study of fossils.
A fossil refers to the remains or traces of organisms preserved from the geological past.
Value of Fossils
1. As stratigraphic indicators, for correlation of deposits containing them and for determination of the relative
geological age.
2. As records of past forms of life, showing the course of modern evolutionary modification of plants and
animals during geological time.
3. As evidence of changing environments and geographic patterns during geological history.