Lecture 7
GEOLOGIC TIME
Historical Notes
Catastrophism.
Catastrophism influenced the study of geology
during 17th and18th centuries.
Catastrophics believed that the Earth's landscape
was developed by great catastrophes.
Modern geology.
The late 1700's is known as the beginning of
modern geology.
During this time, James Hutton published Theory
of the Earth, which set forth the doctrine of uniformitarianism.
Uniformitrianism
states that the physical, chemical, and biological laws that operate today have
also operated in the geologic past.
Relative
Dating: relative dating means placing rocks in their proper sequence of
formation, which formed first, second, and last. Relative dating does not tell
the actual age of a geological event or material, only that it occurred prior
and after another event
The law of
superposition states that in an undeformed sequence of sedimentary rocks,
each bed is older than the one above it and younger than the one below it.
The principle
of horizontality states that most layers are deposited in a horizontal
position.
The principle
of cross cutting relationships states that when faults or magma cut through
rock material, the material the fault or magma cuts through is older than the
fault or magma.
Inclusions are pieces of one rock unit that are
contained within another; the rock mass adjacent to the one containing the
inclusions must have been there first in order for the rock fragments to be
included.
Unconformities represent a long period during
when deposition ceased, erosion removed previously formed rocks and then
deposition resumed (represents a gap in geologic time). Some types of
unconformities are: angular unconformity, disconformity, and nonconformity.
Correlation
of rock layers is the determination of the equivalence in geologic age and
stratigraphic position of two rock units in different locations. Correlation
may be based on rock properties, such as color, porosity, etc., or fossils.
Fossils: are the remains or traces
of prehistoric life and are extremely important inclusions in sediment and
sedimentary rocks. Some topics to keep in mind when studying fossils:
The conditions favoring preservation.
Fossil types and correlation of fossils from one
rock unit to an other.
The principle of fossil succession.
Geologic time scale:
refer to the introduction section for general information about geologic time.
Absolute dating
Radiometric
methods: Unlike relative dating,
this method provides an absolute range of time for age determination.