Age dating of fossils

You should already understand that the lower rock strata are generally older than the strata found higher up in the rock.

When a scientist finds a section of rock that has lots of different strata, he assumes that the bottom-most layer is the oldest, and the top-most layer is the youngest.

The three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.

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We talked about relative dating of rocks and how scientists use stratigraphic succession to compare the ages of different rock layers. This is an informational tour in which students gain a basic understanding of geologic time, the evidence for events in Earth’s history, relative and absolute dating techniques, and the significance of the Geologic Time Scale.This clock representation shows some of the major units of geological time and definitive events of Earth history.For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs and many other groups of life.You may already know how to date a fossil with a rock.The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago).Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon.Scientists find out the age of a dinosaur fossil by dating not only the rocks in which it lies, but those below and above it.Sometimes, scientists already know the age of the fossil because fossils of the same species have been found elsewhere and it has been possible to establish accurately from those when the dinosaur lived.Such a preserved specimen is called a "fossil" if it is older than some minimum age, most often the arbitrary date of 10,000 years.The observation that certain fossils were associated with certain rock strata led early geologists to recognize a geological timescale in the 19th century.

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