TOPIC XIII INTERPRETING GEOLOGIC HISTORY How Old Absolute
TOPIC XIII INTERPRETING GEOLOGIC HISTORY
How Old? ? • Absolute Dating – Determine the actual age • Relative Dating – Determine which thing is older
Original Horizontality • Sedimentary layers are formed horizontally • Any change from that position is evidence of crustal movement since their formation
Principle of Superposition • Older layers are on the bottom unless overturning has occurred
Intrusions & Extrusions • Intrusion – Magma squeezes into existing rock and crystallizes • Extrusion – Lava flows on the surface and solidifies
Dating Intrusions/Extrusions • Intrusions/Extrusions are always younger than any rock they cut through • Look for Contact Metamorphism
Intrusion or Extrusion? ? ?
Inclusions • An older piece of rock inside an igneous rock • Inclusions are always older than the rock they are in
Structural Features • Faulting, folding, or tilting – The rock is always older than the feature that affects it
Veins • Mineral deposit in a rock
Unconformity • Buried erosional surface • You know that some of the rock is missing, but there is no way to know how much
Correlation • Showing that rocks from different locations are the same age • Correlation by exposed outcrops • Correlation by similarity • Correlation by use of fossils
Fossils • Fossils – Remains or evidence of once living things – Bones – Shells – Imprints
Index Fossils • Must meet 2 criteria: 1. Must have lived over a wide geographical area 2. Must have existed for a relatively short time
Which of these fossils would make the best index fossil?
Other Ways to Correlate • Volcanic Ash – Spreads all over and falls in a short period of time • Meteorite Deposits – Also spread and fall quickly
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