Presentation on theme: "Relative Dating : Which Came First?"— Presentation transcript:
1Relative Dating : Which Came First? Earth Systems Chapter 12
2Objectives: Explain how relative dating is used in geology. Explain the principle of superposition.Describe how the geologic column is used in relative dating.Identify two events and two features that disrupt rock layers.Explain how physical features are used to determine relative ages.
3The Principle of Superposition Geologists try to determine the order in which events have happened during Earth’s history.They rely on rocks and fossils to help them in their investigation.Relative datingThe process of determining whether an event or object is older or younger than other events or objects.
4Layers of sedimentary rock, such as the ones shown below, are stacked like pancakes. As you move from the top to the bottom in layers of sedimentary rock, the lower layers are older.SuperpositionPrinciple that states that younger rocks lie above older rocks, if the layers have not been disturbed.
5Some rock sequences have been disturbed by forces within the Earth. Disturbing ForcesNot all rock sequences are arranged with the oldest layers on the bottom and the youngest layers on top.Some rock sequences have been disturbed by forces within the Earth.These forces can:Push other rocks into a sequenceTilt or fold rock layersBreak sequences into moveable parts.
6The Geologic Column Geologic Geologists use the geologic column to: an ideal sequence of rock layers that contains all the known fossils and rock formations on Earth, arranged from oldest to youngest.Geologists use the geologic column to:Interpret rock sequencesIdentify the layers in puzzling rock sequences.
8Disturbed Rock Layers Geologists often find features that cut across existing layers of rock.assign relative ages to the features and the layers.The features must be younger than the rock layers because the rock layers had to be present before the features could cut across them.faultintrusion
9Events That Disturb Rock Layers Geologists assume that the way sediment is deposited to form rock layers — in horizontal layers — has not changed over time.If rock layers are not horizontalsomething must have disturbed them after they formed.Four ways that rock layers may become disturbed.A faultbreak in the Earth’s crust along which blocks of the crust slide relative to one another.An intrusionmolten rock from the Earth’s interior that squeezes into existing rock and cools.
10Foldingoccurs when rock layers bend and buckle from Earth’s internal forces.Tiltingoccurs when internal forces in the Earth slant rock layers.
11Gaps in the Record -- Unconformities Missing EvidenceSometimes, layers of rock are missing, creating a gap in the geologic record.Unconformitybreak in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.
12Types of Unconformities Most unconformities form by both erosion and nondeposition, but other factors may be involved.Geologists place them into three major categories:DisconformitiesNonconformitiesAngular unconformitiesDisconformities exist where part of a sequence of parallel rock layers is missing.
13Nonconformities Angular Unconformities exist where sedimentary rock layers lie on top of an eroded surface of nonlayered igneous or metamorphic rock.Angular Unconformitiesexist between horizontal rock layers and rock layers that are tilted or folded.
14Rock-Layer PuzzlesRock-layer sequences often have been affected by more than one geological event or feature.For example, intrusions may squeeze into rock layers that contain an unconformityDetermining the order eventsis like solving a jigsaw puzzle.piece together the history of the Earth.