Interpreting Earth’s Rocks to Determine Its Past History.

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Presentation transcript:

Interpreting Earth’s Rocks to Determine Its Past History

Geologic Time Relative dating - sequence of geologic events in a region (order of past events) Absolute dating – numeric- time in years; use radioactive isotopes. How do we know how old something is?

James Hutton: The father of modern geology ( ) Uniformitarianism: processes acting on earth today have done so in the same way as in the past “The present is the key to the past.”

Examples of uniformitarianism Petoskey stones Striations Rock formation Paleoclimates Earthquake location Grand Canyon What else?

Determining Geologic History: Geologists are like CSI detectives wanting to know Where?, When?, Why?, How? Sometimes very little “evidence” remains (exposure, erosion) Evidence has been moved or tampered with! Rocks have been moved (faults, folds) Usually no witnesses!

Principle of Superposition Oldest layer on bottom (if tectonically undisturbed) Younger Older

Principle of Superposition In horizontal rock layers, the oldest and first formed is on the bottom. 5 is the youngest because it is on the top, closest to the surface.

The Grand Canyon

Principle of Original Horizontality Folding happened AFTER rocks were deposited

folded rock layers = compression by tectonic activity

Cross-Cutting Relationships An igneous intrusion (or fault) is younger than the rock layers that it cuts across. fault oldest These are younger than the surrounding rock youngest

Write the order of events that have taken place above. List (oldest - 1 to youngest - 6)

Cross-Cutting Relationships Order the events

? A C B D E Order the rock layers above from oldest to youngest. Is this an example of absolute or relative dating?

Which came first? The first layer deposited and the oldest is D, followed by C, A, B, and E. D D D C C A B B E

practice

Index fossils Fossils that lived for a short time can be used to date layers Found world wide Examples?

Order the layers 1-7

Unconformities = “Missing Time” Result of erosion

Angular Unconformities Folded layers on bottom, flat layers on top angular unconformity ( rock layers have been eroded away) rock layers are eroded new horizontal layers are deposited

FEDCBA Angular Unconformity

The Great Unconformity – Grand Canyon, AZ

Granite sedimentary rock layers Cambrian Precambrian Nonconformity – igneous rocks on bottom; overlain by sedimentary rocks

Nonconformity

Disconformity DISCONFORMITY:Missing rock layers (sedimentary) Very hard to see

Disconformities

Let’s Play…. Name the UNCONFORMITY!

Siccar Point, Scotland

granite sandstone shale limestone

granite sandstone shale limestone

Resources Bend%20National%20Park%20-%20Syncline%20-%20Javalina%20Creek%20- %2020%20June% jpghttp:// 20Bend%20National%20Park%20-%20Syncline%20-%20Javalina%20Creek%20- %2020%20June% jpg class.unl.edu/geol101i/images/igneous%20images/Black%20Canyon%20Gunnison.jpghttp://www- class.unl.edu/geol101i/images/igneous%20images/Black%20Canyon%20Gunnison.jpg