Determining a Sequence of Geologic Events Principle of
- Slides: 22
Determining a Sequence of Geologic Events
Principle of Uniformitarianism “The present is the key to the past. ” To understand geologic processes of the past, study geologic processes of today.
Law of Original Horizontality Sedimentary rock layers were originally deposited in horizontal layers
Law of Superposition Oldest rock layers are on the bottom
Law of Continuity of the Layers Ex. Layer C is the same rock unit separated by erosion
An exception to the Law of Superposition is a thrust fault Here older rock is on top of younger rock
Another exception is an Overturned fold OLD YOUNG OLD
contact metamorphism on all sides Another exception: Igneous Intrusions The igneous rock is younger than the rock unit on top of it
contact metamorphism Igneous intrusion is the youngest rock unit here
No contact metamorphism here This is not an exception to the law of superposition, why? There is no contact metamorphism on the top of the basalt, Therefore this is a buried lava flow and not an intrusion
Later deposition may cover this lava Lava flow seen here covering a road in Hawaii becomes part of the rock layers and follows the law of superposition
Law of Cross-Cutting Any geologic event that affects a rock layer must be younger than the rock layer it affects
Layer X is an intrusion and is younger than any other rock unit. This is an example of the Law of Cross Cutting
This fault is also younger than any other rock unit and is Another example of the Law of Cross Cutting
Volcanic ash Layers of volcanic ash also become part of the geologic record, and serve as excellent time markers
UNCONFORMITY: ROCK LAYERS ARE MISSING DUE TO A PERIOD OF EROSION Note- during deposition the region is submerged underwater During erosion periods the region is dry land unconformity
The Grand Canyon has a complex geologic history
Law of …?
Be able to explain these terms • • • Principle of Uniformitarianism Law of Original Horizontality Law of Superposition Law of Continuity of Layers Law of Cross-cutting Unconformity