The geological arrow of time

The relationship between the evolution of man and the geological processes shaping the planet is complex and involves an interplay between two contrasting images of time, namely the linear (arrow-like) model and the cyclic model. Over certain time scales, the familiar patterns of cyclic time impinge most obviously on humans, such as the daily rising and setting of the Sun, the monthly lunar cycle, and the year long orbital period of the planet around the Sun. In contrast, the timescales for real geological changes, such as the formation of mountain ranges and the drift of continents, takes place over millions of years and in an irreversible way.

This contrast led to different researchers discussing these timescales with contrasting metaphors. In his influential book, Time’s Arrow, Time’s Cycle, the science writer Stephen J. Gould analysed the mythologies invoked by various researchers in their accounts of such phenomena. He wrote

Deep time is so alien that we can really only comprehend it as a metaphor. And so we do in all our pedagogy. We tout the geologic mile (with human history occupying the last few inches); or the cosmic calendar (with Homo Sapiens appearing but a few moments before “Auld Lang Syne”) . . . John McPhee has provided the most striking metaphor of all (in Basin and Range): Consider the earth’s history as the old measure of the English yard, the distance from the king’s nose to the tip of his outstretched hand. One stroke of a nail file on his middle finger erases human history.

[Gould, 1987]

Here, ‘deep time’ is a reference to the relatively enormous timescale associated with the Earth’s geology, measured in units of billion years, compared to human evolutionary scales, which are measured in units of tens of thousands of years for the development of civilization and a few million years for the emergence of our species.

 
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