Presentation on theme: "How do we know what happened first?"— Presentation transcript:
1 How do we know what happened first? Relative DatingHow do we know what happened first?
2 Historical Developments James Hutton ( ) “Father of Modern Geology”native of Edinburgh, Scotlandeducated as a medical doctor in Leiden (1749)passionate about scientific inquiry“Theory of the Earth” -- processes are slow; take a long timeCharles Lyell ( )Scotsman who attended Oxford Universityfather was an avid naturalistrebelled against prevailing thought of “catastrophism”/”Neptunism”.“Principles of Geology” -- popularized Hutton’s viewsidea of “uniformitarianism” --same processes operating today occurred in the past….the present is the key to the past….
3 The Key to the Past Relative time Absolute time order of events or objects from first (oldest) to last (youngest)she is older than he is; she was born first and he was born lastAbsolute timeage of events or objects expressed numericallyshe is twenty-one and he is nineteenstudy of timing of geologic events and processes is geochronology
4 The Key to the Past Relative Time- “this rock is older than that” Principles Used to Determine Relative AgeUnconformitiesCorrelationThe Standard Geologic Time ScaleIndex FossilsAbsolute Time- “this rock is 28 million years old”Principles of radioactive decay
5 relative time and relative order apply simple concepts to determine…• original horizontality• superposition• lateral continuity• cross-cutting relationships• inclusions• unconformities
6 relative age dating concepts original horizontalityall beds originally deposited in water formed in horizontal layerssediments will settleto bottomand blanketthe sea floor
7 Superposition: within a sequence of undisturbed sedimentary or volcanic rocks, layers become younger, upward
8 Lateral Continuity: original sedimentary layers extend laterally until it thins out at edges rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.
10 Relative Age Dating Concepts cross-cutting relationshipsa disrupted pattern is older thanthe cause of the disruptione.g. an intrusion is youngerthan the rocks it intrudes
11 Relative Age Dating Concepts inclusionsfragments of other rocks contained in a body of rockmust be older than thehost rocke.g.xenoliths in granite are olderthan granite and2) pieces of rock inconglomerate are olderthan conglomerate
12 Relative Age Dating Concepts unconformitiesA gap in the geologic record -- “gap” may be an amount of time or amount of missing sectionconformity• relatively continuous deposition• deposition of a sequence of parallel layers• contacts between formations do not represent significant gaps in time
14 Relative Age Dating Concepts different types of unconformities1. angular unconformity• contact separates overlying younger layers from tilted older layers• sequence of layersis not parallel• contacts between formationsmay represent significantamounts of timeangular unconformity
17 Relative age dating concepts different types of unconformities2. disconformity• contact separates beds (formations) that are parallel• sequence of layersis parallel• contacts betweenformationsmay represent significantamounts of time• missing time is difficult to recognize
18 Relative age dating concepts different types of unconformities3. nonconformity• strata deposited on older crystalline (metamorphic/igneous) rock• erosion surface on igneous/metamorphic rock covered bysedimentary rocks• large gap ingeologic recordnonconformity
19 Unconformity Types Using Grand Canyon as Example
21 Relative Age: Correlation How is this done?faunal succession (correlation by fossils)fossil species succeed one another through the layersin a predictable orderindex fossilshort-lived organism;points to narrow rangeof geologic timefossil assemblagegroup of fossilsassociatedtogether