17th-18th centuries dominated by "catastrophism". Earth's age was calculated to be about 4000 years old. Geological landforms were believed to be shaped by great catastrophes.
James Hutton - Theory of the Earth put forth a principle that came to be known as the doctrine of uniformitarianism. It states:
the physical, chemical and biological laws that operate today have also operated in the geological past.
Hutton cited verifiable observations to support his ideas (scientific method).
Sir Charles Lyell (1830-1872) - Principles of Geology interpreted and publicized the doctrine of uniformitarism.
The Earth is considered to be 4.6 billion years old based on evidence from radiometric dating of zircons: oldest know terrestrial minerals.
There are two basic techniques for dating geological events:
Relative dating means placing rocks in their proper sequence of formation.
Law of Superposition: in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it.
Principle of Original Horizontality: Most layers of sediment are deposited in a horizontal position.
Principle of Cross-Cutting Relationships: Intrusions are younger than the rocks affected.
Inclusions: pieces of one rock unit that are contained in another.
Conformable: rock layers deposited without interruption.
Unconformity: long period during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed.
Three types:
A fossil is a trace of prehistoric life.
Principle of fossil succession: fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content.
index fossils - widespread geographically and limited to a short span of geological time.
Divisions of Time: Eons > Eras > Periods > Epochs