Reading the Rock Record PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Rock Layers Sediments and lava flows form distinct layers as they are deposited A sedimentary layer that can be distinguished from the layer above and below is called a stratum (pl. strata) http://www.joshushund.com/terragen_01-06/wall_of_strata.jpg PSC 121 Prince George’s Community College

What Information do Strata Provide to a Geologist? To answer this question we need to look at some geologic principles. #1 Processes that occurred many millions of years ago probably happened the same way as they do today (because the science principles are the same). Uniformitarianism PSC 121 Prince George’s Community College

What Information do Strata Provide to a Geologist? #2 Sedimentary deposits or lava flows generally form horizontal layers. Principle of Original Horizontality PSC 121 Prince George’s Community College

What Information do Strata Provide to a Geologist? #3 Unless disturbed by an unusual process, younger layers will be on top of older layers. Principle of Superposition youngest oldest PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Here are the layers in the Grand Canyon shown graphically. The Bright Angel shale is younger than the Tapeats Sandstone but older than the Muav Limestone. http://www.gc.maricopa.edu/appliedscience/gjc-nsf/images/grndcynxsect.cv.jpg The Vishnu Schist is the oldest formation (made of metamorphic rocks). PSC 121 Prince George’s Community College

What Information do Strata Provide to a Geologist? #4 When an igneous intrusion or fault cuts across other rocks of any type the intrusion is younger than the layers that are cut across. Principle of Cross-Cutting Relationships PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Layers 1-4 are positioned oldest to youngest (superposition) 4 5 Formation 5 is younger than 1-4 since it cuts through them 3 2 6 1 Fault 6 is youngest because it cuts through 5 http://www.winona.edu/geology/MRW/mrwimages/Earth%20History/cross_section.jpg PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College The fault and dike are called non-conformities. Notice that the dike has incorporated pieces of other layers. #6 When a feature contains pieces of other layers it has to be younger than the layers it contains. Principle of Inclusion PSC 121 Prince George’s Community College

What Information do Strata Provide to a Geologist? #5 Fossils can be used to determine the approximate age of a stratum since specific living organisms lived over identified time periods. If a particular fossil is present in a layer, the layer must be the same age as other layers containing the same fossil, no matter where the layers occur. Principle of Faunal Succession PSC 121 Prince George’s Community College

How Can We Determine Age More Accurately? Radioactive elements are unstable and their nuclei will release particles or energy to stabilize. Each radioactive element has a half-life – the time it takes for one half of the atoms in a sample to disintegrate Some half-lives are fractions of a second while some are as much as 4.5 million years. PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Radiometric Dating After a 2nd half-life has passed half of the remaining atoms have also changed After 1 half-life has passed, half the atoms have changed or emitted energy. PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Determining Rock Ages We can use radioactive elements in the rocks to help determine age. If the rock contains a radioactive element like potassium-40, we can measure how much of this element is present. Knowing how much should be there we can determine the rock age. PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Determining Rock Ages Suppose a rock sample has 1/64th of the expected amount of radioactive material. 1  1/2  1/4  1/8  1/16  1/32  1/64 This means that the sample has been through 6 half-lives. If each half-life is 100,000 years then the rock layer is 600,000 years old. PSC 121 Prince George’s Community College

Why Aren’t All Strata Straight? Geologic processes generate pressures that cause layers to become deformed. The first type of deformation is folding. Click on image to begin animation http://www2.wwnorton.com/college/geo/egeo/flash/9_1.swf PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Folds http://www.glossary.oilfield.slb.com/DisplayImage.cfm?ID=10 PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College anticline syncline PSC 121 Prince George’s Community College

Why Aren’t All Strata Straight? Geologic processes generate pressures that cause layers to become deformed. The second type of deformation is faulting. PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Faults Faults occur when sections of Earth’s surface shift position with respect to another One side of the fault is designated as a footwall block and the other a hanging wall block PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Normal Faults PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Hanging wall moves down Hanging wall moves down NORMAL FAULT PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Reverse Faults PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Hanging wall moves upward PSC 121 Prince George’s Community College REVERSE FAULT

Strike-Slip or Transform Faults PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College TRANSFORM FAULTS – aerial view PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College Fault Animations PSC 121 Prince George’s Community College

PSC 121 Prince George’s Community College http://www.winona.edu/geology/MRW/mrwimages/Earth%20History/cross_section.jpg Can you date the features from oldest to youngest? PSC 121 Prince George’s Community College