Geologic Time

Geologic time: A metaphor

Geologists estimate Earth is at least 4.6 billion years old. What if all those years were compressed into one calendar year?

Geologic periods in Kansas

Geologists divide the record of Earth’s past into segments—eons, eras, periods, and epochs—to make studying and understanding billions of years of geologic history more manageable.

Eons and eonothems? Periods and systems? Understanding how geologists talk about time

Terms used to describe units of geologic time and the rock layers formed during those time intervals provide a common language for talking about the past.

Mass extinctions

Mass extinctions, extraordinary events in evolutionary history, have taken place at least five times in the last 500 million years.

Clockwise, from top left: Quartzite boulder, Wabaunsee County, left by receding glaciers during Pleistocene Epoch; Castle Rock, Gove County, Niobrara Chalk, Cretaceous System; Bevier coal outcrop, Cherokee County, Pennsylvanian Subsystem, Carboniferous System; Brachiopod fossil collected in Shawnee County from Coal Creek Limestone Member, Topeka Limestone, Carboniferous System; Salt mine drill and transport, Reno County, Hutchinson Salt Member, Permian System; Fence-post limestone bed of the Greenhorn Limestone, Hodgeman County, Cretaceous System.