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