Time and Geology

Sir Charles Lyell

Image source: www.mnsu.edu/emuseum

The Key to the Past

Relative Time- “this rock is older than that”Principles Used to Determine Relative Age• Unconformities• Correlation• The Standard Geologic Time Scale• Index Fossils

Absolute Time- “this rock is 28 million years old”

Principles of radioactive decay• Instruments • The age of the Earth

Important Figures in Geologic Time

• James Hutton (1726-1797): Native of Edinburgh, Scotland. Father of modern Geology. Published “Theory of the Earth” in 1785 in which he outlined that geological features and ancient rocks could be explained by present-day physical and chemical processes.

• Charles Lyell (1797-1875): Rebelled against prevailing thought, which was rooted in Biblical interpretation and Catastrophism. His main contribution

was the development of Uniformitarianism (Actualism). “The present is the key to the past…”

• Modern view holds that processes that operate today have shaped the Earth through Geological Time, but rates may not have always remained constant.

Important Relative Age Dating Principles

• Original Horizontality: all beds originally deposited in water formed close to horizontal

Superposition: within a sequence of undisturbed sedimentary or volcanic rocks,

layers become younger, upward

Lateral Continuity: original sedimentary layers extend laterally until it thins out at

edgesrocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.

Cross-cutting Relationships: disruptions in any rock sequence occurred after the youngest established event in the

undisturbed sequenceIe. A rock or fault is younger than any rock (or fault) through

which it cuts

Sedimentary Deposition

Intrusion

Tilting & Erosion

Subsidence and New Marine Deposition

Missing Formation

Dike Event

Erosion and Exposure

Subsidence & Deposition

Fluvial Deposition

Complex Subsurface Geology

Contact Relations