Principle of Uniformitarianism The same laws and processes operating on Earth throughout geologic time are the same laws and processes operating today Weather/Climate Mountain building Volcanism Earthquakes Erosion Deposition

Law of Superposition Younger layers of rock are deposited on top of older layers of rock. (Relative Dating)

What is the relative age of the lava and the road? Why can’t an absolute age be determined?

Stratigraphic Sequence Zion Canyon, UT Younger Rock Layers Stratigraphic Sequence Older Rock Layers

Intrusion An exception to the Law of Superposition!

Formation of Sedimentary Rocks

A B C D Which layer is a) Oldest? b) Youngest?

Law of Original Horizontality Sedimentary rocks are originally deposited in horizontal layers

Later, they can be bent or tilted due to collisions between tectonic plates.

Law of Lateral Continuity erosion may remove some areas of rock layers, but they can be assumed to belong to a larger original sequence.

Original Horizontality Grand Canyon, AZ Original Horizontality Lateral Continuity

How did these Southern Alberta Hoodoos form?

Unconformity: layers may be missing due to erosion, the boundary left behind where such a gap is missing is called an unconformity.

Rank the rock layers from oldest to youngest. Identify the unconformity.

Uncomformities – Intrusive Igneous Rock Volcanic rocks that melt and cut their way through rocks, must be younger than the layers they cut through.

The Devils Tower in Wyoming is an example of a volcanic neck The Devils Tower in Wyoming is an example of a volcanic neck. Magma was intruded into the neck of a volcano and cooled to form an intrusive igneous rock. As the magma cooled it contracted and cracked into the vertical columns shown in the picture. This feature, called columnar jointing, is described further on in this module. After intrusion of the volcanic neck, the volcano became extinct and eventually, after a long period of time, eroded completely away, leaving only the hard, resistant rock in the volcanic neck to stand up above the surrounding countryside.

Rank the rock layers from oldest to youngest. Identify the unconformity.

versus Law of Superposition RELATIVE DATING: Determining which rock layers and fossils are older and which are younger (i.e. determining which era/period/epoch a fossil or layer belongs to). Law of Superposition Low of Original Horizontality Law of Lateral Continuity Law of Cross-Cutting Relationships versus Radiometric Dating ABSOLUTE DATING: Determining age (in years) of a rock layer or fossil

Determining which fossil species appeared in what order in time. Relative Dating Techniques - Fossil Succession Determining which fossil species appeared in what order in time.

Index fossils should be: common easily recognizable Relative Dating Techniques - Correlation Index Fossils: fossils that can help determine the relative ages of rocks and correlate rock sequences from different locations. Index fossils should be: common easily recognizable occurred over a large area relatively short time

William Smith (1769-1839)

The Grand Record

Forces of Catastrophe