Geologic time scale

Relative dating Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures. Methods for relative dating were developed when geology first emerged as a natural science. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events. The principle of uniformitarianism states that the geologic processes observed in operation that modify the Earth’s crust at present have worked in much the same way over geologic time. In geology, when an igneous intrusion cuts across a formation of sedimentary rock , it can be determined that the igneous intrusion is younger than the sedimentary rock. Different types of intrusions include stocks, laccoliths , batholiths , sills and dikes. The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.

Geologic time scale

Early history[ edit ] In Ancient Greece , Aristotle BCE observed that fossils of seashells in rocks resembled those found on beaches — he inferred that the fossils in rocks were formed by living animals, and he reasoned that the positions of land and sea had changed over long periods of time. Leonardo da Vinci — concurred with Aristotle’s interpretation that fossils represented the remains of ancient life.

Steno argued that rock layers or strata were laid down in succession, and that each represents a “slice” of time.

If the tops of the rock units within the folds remain pointing upwards, they are called anticlines and synclines , respectively.

Geologic time scale

For many geologic applications, isotope ratios of radioactive elements are measured in minerals that give the amount of time that has passed since a rock passed through its particular closure temperature , the point at which different radiometric isotopes stop diffusing into and out of the crystal lattice.

Geologic time scale

A typical USGS field mapping camp in the s Today, handheld computers with GPS and geographic information systems software are often used in geological field work digital geologic mapping.

Geologic time scale

The most influential of those early attempts championed by Werner , among others divided the rocks of Earth’s crust into four types:

Geologic time scale

This table is arranged with the most recent geologic periods at the top, and the most ancient at the bottom.

Logically a younger layer cannot slip beneath a layer previously deposited.

These structural regimes broadly relate to convergent boundaries , divergent boundaries , and transform boundaries, respectively, between tectonic plates.

This can remove signs of the original textures of the rocks, such as bedding in sedimentary rocks, flow features of lavas , and crystal patterns in crystalline rocks.


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