QUIZ . According to the scientific community how old is the Earth? What type of rock are the oldest know rocks? What are Time units largely based on? Name the smallest division of geologic time Name the greatest expanse of geologic time.
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QUIZ • According to the scientific community how old is the Earth? • What type of rock are the oldest know rocks? • What are Time units largely based on? • Name the smallest division of geologic time • Name the greatest expanse of geologic time. • True or False the three earliest eons are known as Precambrian. EXTRA CREDIT 7. True or False in Greekmesomeans recent.
Interpreting Geologic Age • Uniformitarianism: principle that states geologic processes occurring today have been occurring since Earth formed • For example the geological process that formed an ancient beach (wave erosion) has not changed
How to Determine Geologic Ages Relative age dates & Numerical (absolute age) dates
Relative Age Dating Placing rocks and events in their proper sequence of formation Unable to determine exactly how many years ago an event occurred Gain understanding about geologic events Use several ways to determine age called principles of relative dating
Principles of Relative Dating • Original Horizontality • Superposition • Cross-cutting Relationships • Inclusions
Horizontality • Layers of sediment are generally deposited in a horizontal position • Rock layers that are flat have not been disturbed (deformed)
Superposition Sedimentary or layered igneous rocksOldest rocks are on the bottom
Cross-cutting relationships • The intrusion is younger than the rock it cuts across • Younger features cut across older features • Similar to faulting
Inclusions A piece of rock that is enclosed within another rock Rock containing the inclusion is younger Example a rock has eroded, the sediment might be transported & redeposited a distance away
Unconformities • Break in rock record produced by erosion and other processes; result of constant surface changes • 3 different types • Disconformity • Nonconformity • Angular conformity
Disconformity • Horizontal layer of sedimentary rock overlies another horizontal layer of sedimentary rock • Easy to identify when eroded surface is uneven
Nonconformity • Layer of sedimentary rock overlies a layer of Metamorphic or igneous rock
Angular unconformity • Horizontal sedimentary layer overlies tilted sedimentary layer
What Do you Think??The Kaibab limestone layer rims the top of the Grand Canyon in Arizona but it is also found more than 100 km away at the bottom of Zion National Park in Utah How do geologist know that these layers which are far apart from each other formed at the same time?
Correlation of Rock Layers • Matching of rocks of similar ages in different regions is known as correlation • Correlation often relies upon fossils • similar sedimentary strata in widely separated areas • identified and correlated by distinctive fossil content
Key Bed Correlation Rocks or sediment layers used as a maker is called a key bed Example: Volcanic ash, meteorite strike
Fossil Correlation • Fossils indicate similar times of deposition even though the layers are of different materials or a distant away