Geoscience Reference
In-Depth Information
lakes by rivers and streams, as well as by winds and the melting of gla-
ciers. Other material is washed or blown off the continents to be
deposited in layers at the bottom of the oceans.
In all of these modern settings, different types of debris, called sed-
iments, are laid down under different environmental conditions by
erosion and deposition. We can observe how rain and ice erode
mountains and carry the residue down from the heights in rivers
and streams to be deposited across floodplains and within ocean
basins. We can also document the kinds of sediments that are being
deposited on the floodplains and in the oceans. By comparing sedi-
mentary layers forming today to the kinds of rocks we find in ancient
rock sequences, we can identify the kind of environment in which the
ancient sediments were deposited. This is possible because sedi-
mentary rocks are simply ancient sediments that, with time and the
action of several geological processes, have become petrified. The sed-
iments that we see being transported and deposited today will even-
tually become the sedimentary rocks of tomorrow.
Such observations led some early students of geology to a rather
startling, yet intuitive, conclusion about how the earth has evolved
since it formed. In essence, they reasoned that the present is the key
to the past, because the processes of erosion and deposition that we
can observe operating today have acted more or less similarly through-
out the vast expanse of geologic history. This concept is a fundamental
dictum of geology called the principle of uniformitarianism.
Illustrative examples are provided by the many volcanoes that
have erupted in the past across the globe, generating both lava and
ash. Once cooled, some lava forms a kind of volcanic rock called
basalt that has a characteristic texture and composition of minerals.
When ancient sequences of rocks contain this same type of hardened
lava, we know that it was erupted out of an ancient volcano, whether
or not the volcano still exists.
In 1980, for example, a tremendous eruption occurred in southern
Washington State at Mount St. Helens. Millions of tons of volcanic
ash were blasted several miles up into the atmosphere, where winds
carried the ash to be deposited across the states of Washington,
Idaho, Montana, and even into the Dakotas. The ash formed a layer
of powdery dust as it settled out of the air on distant cities and land-
scapes. Again, its texture and mineral composition is quite distinc-
