The simple answer: because of soil and rock erosion.
The more detailed answer: Sodium chloride (or common salt) is a naturally occurring chemical compound, as are a variety of mineral salts. When it rains, water rinses minerals from rocks and soil (including salts) into rivers and streams. The rivers, in turn, eventually carry these minerals into the ocean. As you know, water evaporates. Salt does not. So when the water evaporates from the ocean to form clouds, the mineral deposits are left behind. Over time, accumulating salt deposits have made ocean water salty.
Salts in the sea also come from hydrothermal vents (when seeping water gets heated by the earth and dissolves minerals from the ocean floor), underwater volcanoes, and from space (which is why even "fresh" water isn’t completely salt-free).
So does the ocean get saltier every day? One would think so, but scientists say no. Ocean water is currently about 3.5% salt, which it has been for millions of years. Most scientists agree that because new minerals form on the sea floor at the same rate as salt is added, the ocean’s salinity remains about the same.
Bet you didn't know . . . |
Marine
animals are affected by changes in salinity. That's why lobsters will die
if kept in pens too close to land during the spring run-off. |
This, of course, begs yet another question: since rivers and streams also flow into lakes, why aren’t most lakes salty? The reason for this is because most lakes have inlets and outlets. Lakes are generally just temporary storage areas where water flows in one end and out the other. This rinsing action keeps the salt concentration very low. Some lakes, however---like the Great Salt Lake in Utah and the Dead Sea between Israel and Jordan---don’t have outlets. So these bodies of water act just like little isolated oceans. Water flows in, evaporates, and leaves salt deposits behind. Because these lakes are so much smaller than the world’s vast oceans, the salt concentration is even greater. Both the Great Salt Lake and the Dead Sea are about 10 times saltier than the ocean!
To see how evaporation works, collect a cup of seawater from the ocean (or make your own by dissolving salt into a cup of fresh water). Now place it in a bright window and wait a few weeks for the water to evaporate. The crust left behind in the glass will demonstrate how salt accumulates in the ocean!
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Key Study Words |
•
Salinity |
Fun with Salt:
• Make salt dough
• View
sea salt magnified 4,000 times under a microscope!
• Grow salt crystals with the easy directions at . . .
(ScienceNet)
(Cool
Science for Children)
(Great
Events #6)
(Exploratorium)
(San
Diego Natural History Museum)
(Bizarre Stuff's crystal
page)
You can also order crystal growing kits from our online Nature Learning Store!
Copyright © 2001 by John W. Schlim Jr.
Salt
Institute
(multidisciplinary curriculum for high school students)
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