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

Another exception: Igneous Intrusions contact metamorphism on all sides Another exception: Igneous Intrusions The igneous rock is younger than the rock unit on top of it

Igneous intrusion is the youngest rock unit here contact metamorphism Igneous intrusion is the youngest rock unit here

This is not an exception to the law of superposition, why? 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

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