26-Oct-2015 11:29

Relative dating determining the sequence of geologic events

Grand Canyon contains many important geologic resources, including a vast fossil record ranging from Precambrian stromatolites to Ice Age mammal bones and dung found in caves; a potentially active volcanic field in the western Grand Canyon; a geologic history ranging more than 1.7 billion years; and the canyon landscape itself as the greatest example of arid land erosion.There are two major categories of geologic dating techniques: relative dating and absolute age determinations .

organisms, geologists employ a variety of techniques.These include some that establish a relative chronology in which occurrences can be placed in the correct sequence relative to one another or to some known succession of events.Grand Canyon National Park [see Photo 1] is one of the best places in the world to gain a sense of geologic, or “deep,” time because the canyon exposes a great swath of geologic history.Rocks exposed in Grand Canyon are truly ancient, ranging from 1840 million years old (m.y.), or 1.84 billion years old (b.y.), to 270 m.y.Nonetheless, how geologists determine the age of rocks is a mystery to many members of the public, and even to many park rangers [see Photo 2], guides, and others who share the canyon’s geologic story with others. ” when geologists say a Grand Canyon rock formed 270 million years ago.Further confusion arises when one publication or geologist says, for example, that the Kaibab Formation is 270 m.y. The same questions arise for the other rock units at Grand Canyon. This article will answer these questions by providing a short primer on geologic dating methods and how they were applied to Grand Canyon rocks.

We then describe Grand Canyon rocks as belonging to three “sets,” or packages of rocks, each with unique geologic histories, and present a compilation of “best” numeric ages for Grand Canyon rocks to use when explaining Grand Canyon geology to the public.

Photo 1: Many people consider Grand Canyon National Park the world’s premiere geologic landscape and a “geologic wonder”.

The Grand Canyon landscape is geologically young, being carved within just the last 6 m.y.

There are younger geologic deposits in Grand Canyon too, such as the Ice Age fossils found in caves, a 1000-year-old lava flow in the western canyon, and even the debris flow deposits that continue form each year.

Yet, it is the canyon’s rock walls that allow people to develop their greatest perspective on geologic time, because of these rocks’ immense age, their fossil record, and because these rocks formed in environments far different than those found in northern Arizona today.

With a rock record that spans more than 1500 m.y., Grand Canyon is truly a panoramic view into the geologic past.

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