Steve Schiedel, age 10, of Albany, New York, for his question:
Why don't lakes gather minerals as the ocean does?
Actually they do, though on a smaller scale. Several minerals dissolved in a fresh water lake have a salty taste. But the mixture is so diluted we cannot taste them, though a few lakes are much saltier than the salty sea. Most lakes are drained by streams that join rivers on their way to the ocean. All this running fresh water gathers minerals from the land and steals it away. Finally the loot is dumped into the sea. There it stays while more diluted minerals are added every day.
We are told that running water can wear away almost anything. It does this as it sweeps against rocks and pebbles, porous soil and soggy mud. The surface of a fresh water lake may be so calm and still that it reflects the changing sky like a smooth, polished mirror. But the water coming into it gathers minerals from the land. As a rule, a lake is part of a network of fresh water, winding its way to the sea.
The usual lake is a sort of bathtub set in a system of streams. Water flows into its basin and out again. It may be filled by a great river, by several streams or by springs that well from its floor. Its outlet may be a sizeable river or several streams too small to notice. Its moving water gathers minerals from the earth around it. The streams that keep it filled dissolve traces of chemicals along the way and bring them through the lake. Springs come from ground water that gathers still more minerals as it seeps through porous rocks below.
The continents are drained by networks of busy lakes and rivers. All this moving fresh water gathers dissolved minerals and dumps the diluted mixture into the ocean that covers three quarters of our watery world. The smiling sun evaporates moisture from its surface, but most of the chemical loot is left behind. Every day the fresh water drainage adds small helpings of salty chemicals to the big hoard and the salty sea grows saltier.
The evaporated moisture becomes gaseous vapor. It mingles with the air and creates clouds with droplets of pure, fresh water. The winds waft the clouds along and somewhere they shed their rainy moisture. When this fresher than fresh water comes tumbling down, it completes a trip around the great global water cycle. Then it sinks or drains over the ground, gathering more minerals to dump into the sea.
The vast ocean has been mineral collecting for at least four billion years. It now has enough loot to spread a 500 foot layer of salty chemicals over the continents. But some lake water is even saltier. Our famous Great Salt Lake is fed by the usual diluted streams. But it has no outlet to the sea. As the bright skies of Utah evaporate moisture from its surface, its daily quotas of minerals concentrate in the water. The amazing lake is now four or five times saltier than sea water, though it has been mineral collecting for only a few thousand years.