Presentation on theme: "Principles of Relative Dating"— Presentation transcript:
1Principles of Relative Dating What are the rocks telling us about the history of the earth?
2Two ways to tell time with rocks: RELATIVE DATING: order or sequence known, but not the actual date of occurrence. “Time Line”(ex) – older or youngerABSOLUTE DATING: actual date determined by radioactive decay“Clocks in Rocks”(ex) – 50 million years ago
11The Principle of...CROSS-CUTTING: Anything that cuts a rock must be younger than the rocka. Igneous intrusion - is younger than rock it has intruded (cut across). Pre-existing rock will undergo CONTACT METAMORPHISMb. Faults – are younger than rock displacedc. Erosion – layers that get cut are older than the erosion
17Unconformities – Buried Eroded Surfaces Sometimes layers of rock are missingThere is a break or gap of geologic time not represented by the layers in an area. The gap represents an unknown length of timeNo way of knowing exactly what happened but we do know UPLIFT exposed rocks to weathering and erosion.Rocks above unconformity are younger – rocks below older
18(1) A fault exists between outcrops A and B. Bedrock outcrops A and B are located at two different locations along the Provo River. Rock layers 1, 2, and 4 are the same in both outcrops.Which statement best explains why rock layer 3 is missing from outcrop B?(1) A fault exists between outcrops A and B.(2) Erosion created an unconformity between rock layers 2 and 4 in outcrop B.(3) A volcanic eruption destroyed rock layer 3 in outcrop B.(4) Metamorphism of outcrop A created rock layer 3.
194 steps produce an unconformity Uplift – area of crust uplifted above sea level (deposition – under water)Erosion – some time afterSubmergence (subsidence) below sea levelDeposition – new sediments deposited on top of the buried eroded surface
20Layers formed according to superposition Something happens to uplift areaErosion wears away the uppermost layersArea submerges and deposition begins againAT THE INTERFACE BETWEEN STEPS 3 AND 4 THERE IS A BURIED EROSIONAL SURFACE AKA AN UNCONFORMITY
22Sediments deposited, sea-level lowered, layers intruded, layers tilted, erosion and deposition under sea, sea-level lowered again. Click arrow to continue.
23Practice: what happened here? Click to see arrows shoot in one at a time. #1= included fragments #2=intrusion, cross-cutting #3= angular unconformity #4=erosion. This is the outcome of the previous slide.Unconformities are usually shown in block diagrams with a wavy line to represent the buried eroded surface interface between layers
28Correlation of rock layers often relies upon fossils William Smith (late l700’s) noted that rock layers in widely separated areas could be identified and correlated by their distinctive fossil contentThis led to the "principle of faunal succession“Fossils succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content
301. Which layers are the same? 2. Which layer is older E or F?3. What is correct sequence of rock layers from oldest to youngest?4. An unconformity is represented by the interface between which 2 layers?
32Index FossilsIndex fossils - any animal or plant that is characteristic of a particular span of geologic time or environment.2 criteria of good index fossils:Life form lived over a wide geographic area – horizontal distributionLife form existed for a short period of time – short vertical distribution
33Which letter would make a good index fossil? Location ALocation BLocation CRock layer 1WW ZRock layer 2W ZYZRock layer 3W XXX ZWhich letter would make a good index fossil?
34OTHER METHODS OF CORRELATION Layers of bedrock exposed (outcrops) on either sides of river valleys/excavations“walking the outcrop”Volcanic ash – large eruption – widely distributed – represents a small time intervalLIKE AN INDEX FOSSIL
35VOLCANIC ASH LAYER AT THE SAME TIME AS THE DINOSAURS BECAME EXTINCT LEAD SCIENTISTS TO LOOK FOR METEORITE CRATER Chicxulub Crater 65 mya km. Wide