TOPIC XIII INTERPRETING GEOLOGIC HISTORY

How Old?? Absolute Dating Relative Dating Determine the actual age 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 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: Must have lived over a wide geographical area 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