Charity_Lab8.docx - RELATIVE AGE DATING GEOL 1122: Lab #8...

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RELATIVE AGE DATINGGEOL 1122: Lab #8 (Part I)When you arranged historic photos of GSU in order from oldest to youngest, you were applying principles ofrelative age dating, just as you would in determining the history of a sequence of rocks. You analyzed photos ofGSU locations as older or younger in comparison to other photos and you looked for clues in the photos such ashairstyles, clothes, and cars from previous decades just as you might study a rock layer for fossils to provide insightas to a specific moment in history. Here you’ll learn about the principles of relative age dating in more depth andhow to apply them to determine the order of events when studying a cross-section of rock layers. Just as you did inclass for historic GSU photos, you’ll write the letters of rock layers and related events from oldest to youngest.PRINCIPLES OF RELATIVE AGE DATINGRelative dating involves putting geologic events in order to determine the age of a rockrelativeto other rocks,that is, assigning whether a rock is older or younger than other rocks. The entire geologic time scale is actuallybased on theprinciples of relative datingbecause until recently, methods had not yet been developed tocalculate numerical ages for rocks. Instead, ages of rocks were dated as “Jurassic” or “Devonian”, names whichrefer to certain periods in earth history when specific groups of fossils are known to have existed. The principlesof relative datingare the rules that guide geologists when they interpret the geological history of a series of rocks.Principle of Superposition:Sedimentary rocksform when sediments such as sand, silt, or clay settleout of the water column and fall to the bottom of abasin, creating layers that become lithified. In anysequence of undeformed sedimentary rocks, layers atthe bottom are going to be older than the ones near thetop, because rocks at the bottom of a sequence musthave been deposited first (in order for other rocks tohave formed on top of them). See Figure 1a where Ais the youngest rock and E is the oldest rock.Principle of Original Horizontality:Sedimentsunder the influence of gravity, are almost alwayslaid down in horizontal layers called beds (as theyfall to the bottom of a basin when settling out of thewater column). Thus, if you see rock layers that aretilted, it can be assumed that they have been changedfrom their original position (Figure 1b).Figure 1: Principles of relative dating show that the rocks in a)are horizontal compared to b) after having been tilted due to atectonic event.Figure 2: Principles ofrelative dating focusingon cross-cuttingrelationships. (a)Faulting of rocks. RocksA-E are older than thefault as it cuts throughthe rock layers. (b)Igneous intrusions andlava flows. Theseigneous features areyounger than rocks A-E.Principle of Cross-Cutting Relationships:Rock layers that are changed due cross-cutting phenomena includefaults that break through rocks and igneous intrusions. The first scenario happens when a fault forms due to

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Term
Fall
Professor
NoProfessor
Tags
Radioactive Decay, Radiometric dating

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