Everyone who has been to the beach knows that seawater is
salty. Everyone also knows that fresh water in rain, rivers, and even ice is
not salty. Why are some of Earth’s waters salty and others not? There are two
clues that give us the answer. First, “fresh” water is not entirely free of
dissolved salt. Even rainwater has traces of substances dissolved in it that
were picked up during passage through the atmosphere. Much of this material
that “washes out” of the atmosphere today is pollution, but there are also
natural substances present.
As rainwater passes through soil and percolates through
rocks, it dissolves some of the minerals, a process called weathering.
This is the water we drink, and of course, we cannot taste the salt because its
concentration is too low. Eventually, this water with its small load of
dissolved minerals or salts reaches a stream and flows into lakes and the
ocean. The annual addition of dissolved salts by rivers is only a tiny fraction
of the total salt in the ocean. The dissolved salts carried by all the world’s
rivers would equal the salt in the ocean in about 200 to 300 million years.
A second clue to how the sea became salty is the presence
of salt lakes such as the Great Salt Lake and the Dead Sea.
Both are about 10 times saltier than seawater. Why are these lakes salty while
most of the world’s lakes are not? Lakes are temporary storage areas for water.
Rivers and streams bring water to the lakes, and other rivers carry water out
of lakes. Thus, lakes are really only wide depressions in a river channel that
have filled with water. Water flows in one end and out the other.
The Great Salt Lake, Dead Sea,
and other salt lakes have no outlets. All the water that flows into these lakes
escapes only by evaporation. When water evaporates, the dissolved salts are
left behind. So a few lakes are salty because rivers carried salts to the
lakes, the water in the lakes evaporated and the salts were left behind. After
years and years of river inflow and evaporation, the salt content of the lake
water built up to the present levels. The same process made the seas salty.
Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to
fall again as rain and to feed the rivers, but the salts remain in the ocean.
Because of the huge volume of the oceans, hundreds of millions of years of
river input were required for the salt content to build to its present level.
Rivers are not the only source of dissolved salts. About
twenty years ago, features on the crest of oceanic ridges were discovered that
modified our view on how the sea became salty. These features, known as hydrothermal
vents, represent places on the ocean floor where sea water that has seeped
into the rocks of the oceanic crust, has become hotter, and has dissolved some
of the minerals from the crust, now flows back into the ocean. With the hot
water comes a large complement of dissolved minerals. Estimates of the amount
of hydrothermal fluids now flowing from these vents indicate that the entire volume
of the oceans could seep through the oceanic crust in about 10 million years.
Thus, this process has a very important effect on salinity. The reactions
between seawater and oceanic basalt, the rock of ocean crust, are not one-way,
however; some of the dissolved salts react with the rock and are removed from
the water.
A final process that provides salts to the oceans is
submarine volcanism, the eruption of volcanoes under water. This is similar to
the previous process in that seawater is reacting with hot rock and dissolving
some of the mineral constituents.
Will the oceans continue to become saltier? Not likely. In
fact the sea has had about the same salt content for many hundred of millions
if not billions of years. The salt content has reached a steady state.
Dissolved salts are being removed from seawater to form new minerals at the
bottom of the ocean as fast as rivers and hydrothermal processes are providing
new salts.
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