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Geologic Time & Fossils
Geologic Time Scale – the timeline that organizes the events in
Earth’s history
Cambrian Boundary – the point at which variety of life on Earth
exploded about 550 million years ago
Eon – the largest group on the geologic time scale; billions of years
long
Era – mass extinctions mark the boundaries between eras, include:
Precambrian, Paleozoic, Cenozoic, & Mesozoic; hundreds of millions of
years long
Period – smaller blocks of time within eras showing major events;
tens of millions of years long
Epoch – divisions of the most recent periods; several million years
long
Fossils – imprints or remains of organisms that were once alive
Absolute Age – the actual age of a rock or fossil or how long ago an
event occurred
Radioactive Dating – the means of measuring the age of a material
by comparing the amount of a radioactive form of an element in a
rock or fossil with the amount of its decay product
Relative Age – the age of an object or event in comparison to
another object or event, does not give a definite date
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Law of Superposition – a scientific law that states that in
undisturbed sedimentary rock layers, older layers of rock lie beneath
younger rock layers
Intrusion – an igneous rock that cuts through older sedimentary
rocks
Unconformity – a gap in the rock record
Extinct – no longer found living on Earth, example: woolly mammoth
Mass Extinction – the disappearance of a large number of species
(50%) in a fairly short period of time, example: all dinosaurs
disappeared
Index Fossil – the fossil of an organism that existed for a relatively
short period of time, example: trilobite