Earth History • Rocks record geological events and changing life forms of the past.
Feb 21, 2016
Earth History• Rocks record geological events and
changing life forms of the past.
Uniformitarianism• Principle of Uniformitarianism
“The present is the key to the past.”
• Geologic forces at work today also occurred in the past. Only the rate, intensity and scale have changed.
Relative dating
• Sequence in which events occurred, not how long ago they occurred.
• The law of superposition: rocks on the bottom are older than the ones on top
Ordering the Grand Canyon’s History
Principle of Original Horizontality
• The principle of original horizontality means that layers of sediment are deposited in a horizontal position.
Principle of Cross-Cutting Relationships
• The principle of cross-cutting relationships states that the fault or intrusion is younger than the rocks affected.
The Principle of Cross-Cutting A magma intrusion or a fault is younger than the rock it cuts across.
Inclusions• Inclusions are rocks contained within other
rocks.• Rocks containing inclusions are younger than
the inclusions they contain.
Unconformities
• An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.
1. An angular unconformity indicates a period of deformation (folding or tilting) and erosion occurred.
How did this happen?!!
Formation of an Angular Conformity
Unconformities
2. A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks
3. A disconformity is when two sedimentary rock layers are separated by an erosional surface.
A Record of Uplift, Erosion, and Deposition
Correlation of Rock Layers• Correlation is establishing the
equivalence of rocks of similar age in different areas.
Correlation of Strata at Three Locations