Laboratory Manual for Introductory Geology 4e

To help visualize the billions of years of geologic time, it may help to compare its
span to a more familiar one: if all of geologic time was condensed into a single year,
each second would represent 145 years.
1.5 Rates of Geologic Processes
Some geologic processes happen quickly. A meteorite impact takes a fraction of a
second; most earthquakes are over in a few seconds; and a landslide or explosive
volcanic eruption can happen in minutes. But others occur so slowly that it’s difficult
to recognize that they are happening at all. It often takes decades before
anyone notices the slow downhill creep of soil; layers of mud only an inch thick
may require thousands of years to accumulate in the deep ocean; and satellite
measurements show that continents are moving 1 to 15 cm/yr. The slow rates of
important geologic processes were the first clue that the Earth must be very old,
and geologists understood this long before they were able to measure the ages of
rocks (see Chapter 12).
Even small changes can have big results when they are repeated over enormous
expanses of time, as you will see in Exercise 1.9. And at the very slow rates at which
some of these processes take place, familiar materials may behave in unfamiliar
ways. For example, if you drop an ice cube on your kitchen floor, it shatters into
pieces. But given time and the weight resulting from centuries of ice accumulation,
the ice in a glacier can flow slowly, at tens of meters per year (FIG. 1.10a). Under
geologic conditions and over long enough periods, even layers of solid rock can be
bent into folds like those shown in FIGURE 1.10b.
FIGURE 1.10 Solid Earth materials behave in unexpected ways over long periods and
under appropriate conditions.
(a) Flowing ice in Athabasca Glacier, Alberta, Canada.
(b) Folded sedimentary rocks in eastern Ireland.
You will learn later that mountains and oceans are not permanent landscape features.
Mountains form by uplift or intense folding, but as soon as land rises above
the sea, streams, ice, and wind begin to erode it away. When the forces that cause the
uplift cease, the mountains are gradually leveled by the forces of erosion. Oceans
are also temporary features. They form when continents split and the pieces move
apart from one another, and they disappear when the continents on their margins
collide. Exercise 1.9 examines the rates at which these processes operate and gives
insight into the life spans of mountains and oceans.
1.5 RATES OF GEOLOGIC PROCESSES
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