| Relative Geologic Time | Is a historical record of the rocks determined by applying the principles of uniformity, original horizontality, superposition, faunal succession and others to determine ages of rocks in comparison to each other. Led to the development of eons, eras, periods and epochs with specific names to define the given unit of time. The time scale applied to a specific eon, era or period is revised as absolute dating techniques improve. |
| Absolute Geologic Time | The use of radioactive dating techniques to determine a precise measurement of time based on the amount of radioactive material in the rock. As the rock ages the amount of radioactive material decreases. The amount of time for that decrease can be calculated provided the half-life of the element being measured is known. The half-life of each radioisotope is constant and is defined as the amount of time required for one half of that isotope to decay. |
| ORDOVICIAN |
440 million years ago |
eradicated small organisms that lived on the bottom
of the ocean. |
|
| DEVONIAN |
365 million years ago |
caused the loss of coral reefs and small marine
life-forms . |
|
| Paleozoic
Era ends |
PERMIAN |
250 million years ago |
wiped out more than 90% of marine species and
drastically affected lineages of four-footed animals on land, probably
caused by a meteorite. |
| TRIASSIC |
210 million years ago |
killed off more than a fifth of animal lineages,
both on land and in the ocean. |
|
| Mesozoic
Era ends |
CRETACEOUS |
65 million years ago |
decimated nonavian dinosaurs as well as marine
reptiles and numerous species of marine organisms, probably caused by a
meteorite. |
"A curious aspect of the theory of evolution
is that everybody thinks he understands it"
-- Jacques Monod
Any Questions:
Dr. Baker
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Last updated: January 5, 2006