Miners looking for gemstones and precious metals were the first "scientists" who needed to understand the relationships among different kinds of rocks. In 1669, Nicolaus Steno, a Danish Geologist and anatomist, described two basic geologic principles: sedimentary rocks are laid down in a horizontal manner, with younger rocks deposited on top of older rocks like layers on a cake. The oldest layer is laid down first, with the newest layer at the top. Scientists call the study of rock layering "stratigraphy."

The fossilized remains of once-living animals and plants then provided the opportunity for scientists to correlate layers of rocks with time in different geographic areas. In 1815, English canal builder and amateur geologist, William Smith, produced a geologic map of England that demonstrated what is called faunal succession: fossils are preserved in rocks in a definite order and thereby relate both rocks and time.

Earth time is divided into eons, eras, periods, epochs and ages. For example, the Phanerozoic Eon, which extends back in time some 545 million years and coincides with the appearance of animals that evolved external skeletons, like shells, and the even more recent animals that formed internal bony skeletons, like the vertebrates. These shells and bones are buried in certain sequences of rocks around the world, telling us when the creatures lived, the age of the rocks they're buried in, and what kinds of environments existed where and when. We know dinosaurs lived in the Mesozoic Era of the Phanerozoic Eon, thanks to the study of stratigraphy.

Scientists conducting research through the NSF-funded CHRONOS (Greek for "time") project are trying to better understand chronostratigraphy—Earth processes over time. They hope to answer such questions as what caused the largest mass extinction of the last 600 million years? And how did life evolve from the first simple bacteria that dominated Earth for billions of years?

Marine geologists working in two sea-going programs funded by NSF—the Integrated Ocean Drilling Program and its predecessor, the Ocean Drilling Program—are also seeking answers to these questions by drilling deep under the oceans to extract cores of sediment and using stratigraphy to shed light on Earth’s history.

To learn more:
www.chronos.org
www.iodp.org

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