The Critical Zone

Geologic Time


Geologists have a much different sense of time than most of our fellow citizens of Earth. It is not unusual for me to tell friends and family about my studies of the most recent episode of past greenhouse warmth, ~50,000,000 years ago, as an analog to Earth's not-too-distant globally warmed future, and in return be subject to gaping jaws or glazed eyes. Geologists' perception of time derives from our study and understanding of the history of Earth and the vastness of geologic time. Often these concepts conflict with Biblical interpretations of geologic time; nevertheless, the majority of geologists know and accept that features like the Grand Canyon did not form in 6,000 years. Personally, I find the vastness of geologic time to be much more inspiring! But like the rest of you, I still wish my weekends and vacations were longer.

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The Grand Canyon from Lipan Point, South Rim.
Credit: Tim White, 2004.

Recall from the lesson on soils that time is one of the five natural state factors of soil formation. A parent material long subjected to the forces of climate, topography, and biota is more likely to develop into soil than one subjected to only brief exposure to those factors. Following from that, the structure, texture, and horizonation of a soil can and often will become more developed and mature with time. Also, remember from the paleoclimatology lesson that paleosols exist and provide a record of the history of the Critical Zone through time. Billions-of-years-old paleosols are known to exist, providing witness to the evolution of the Critical Zone.

To learn about various aspects of geologic time, including the methods that allow us to measure geologic time and their development, visit and read:

View the figure below, being sure to note the human figure in the Pleistocene Epoch—human history represents only a small fraction of all of Earth history, yet as we will learn in future lessons, we have quickly become the dominant force in shaping Earth's surface.

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Geologic time spiral illustrating the vastness of geologic time, beginning with the formation of the universe and culminating in the present; note the relatively short duration of humanity's presence within the long history of life on Earth.
Source: USGS

Reading assignment

As important to understanding the vastness of geologic time is the perspective it brings to understanding the rates of surface processes in the Critical Zone. Visit "Chapter 6: Geologic Time, Geologic Processes Past and Present - Uniformitarianism" and focus on the description of uniformitarianism versus catastrophism.

Also, see "Geological Time" for issues relating to teaching about geologic time and the notion that it requires innovation in our sense of reality.