USGS: Science for
a Changing World


Diagram of basalt flow
The basalt flow shown here obeys the Law of Superposition. It is younger than the beds below it and older than the beds above it. Note that the molten rock of the volcanic flow has baked the rock underneath it. The bed above was deposited long after the flow had cooled and hardened and has not been baked. Numeric ages from the flow and dike and relative ages from the fossils in the surrounding rocks contribute to the geologic time scale.

The granite dike (a mass of rock that cuts across the structure of the rocks around it) shown here illustrates the Law of Cross-Cutting Relations. The dike is younger than all the rocks that it cuts across and older than the rocks above it that it does not cut. Note that the contact between the dike and the rocks around it has been baked by the heat of the molten granite.

Return to The Numeric Time Scale

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Last updated 26 June 1997 (krw)
Maintained by John Watson