Events in the Earth’s history, such as the deposition of sediments, eruptions of volcanoes, or the evolution and extinction of animals and plants, are interpreted by geologists from the records they have left in sequences of rocks.
Geologists may refer to the age of rocks either by their absolute or numerical age (ranging from thousands to billions of years), or by a relative age (named episodes in Earth history, similar to the way in which historians refer to periods such as the Ming dynasty).
Nonconformity – older metamorphic or intrusive igneous rocks in contact with younger sedimentary strata 1. Given enough time, remains may be petrified (literally “turned into stone”) 4. The percentage of radioactive atoms that decay during one half-life is always the same: 50 percent b.
Angular unconformity – tilted rocks are overlain by flat-lying rocks b. William Smith (late1700s–early 1800s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content 2.
Disconformity – strata on either side are parallel c. Principle of fossil succession – fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content a.
The age of the carbon in the rock is different from that of the carbon in the air and makes carbon dating data for those organisms inaccurate under the assumptions normally used for carbon dating.
This restriction extends to animals that consume seafood in their diet.
Early in the 17th century, Irish Anglican Archbishop James Ussher made a calculation, based on a literal interpretation of the Bible, that the Earth was created in the year 4004 BC.