Why is the Ocean Salty?
You may know that the oceans cover about 70 percent of the Earth's surface, and that
about 97 percent of all water on and in the Earth is saline—there's a lot of salty water on
our planet. By some estimates, if the salt in the ocean could be removed and spread
evenly over the Earth's land surface it would form a layer more than 500 feet (166 meters)
thick, about the height of a 40-story office building (Source: NOAA). But, where did all this
salt come from? If you get into folk stories and mythology you will see that almost every
culture has a story explaining how the oceans became salty. The answer is really very
simple. Salt in the ocean comes from rocks on land. Here's how it works...
From precipitation to the land to the rivers to the sea
The rain that falls on the land contains some dissolved carbon dioxide from the
surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid. The
rain physically erodes the rock and the acids chemically break down the rocks and carries
salts and minerals along in a dissolved state as ions. The ions in the runoff are carried to
the streams and rivers and then to the ocean. Many of the dissolved ions are used by
organisms in the ocean and are removed from the water. Others are not used up and are
left for long periods of time where their concentrations increase over time.
The two ions that are present most often in seawater are chloride and sodium. These two
make up over 90% of all dissolved ions in seawater. By the way, the concentration of salt
in seawater (salinity) is about 35 parts per thousand. In other words, about 35 of 1,000
(3.5%) of the weight of seawater comes from the dissolved salts; in a cubic mile of
seawater the weight of the salt, as sodium chloride, would be about 120 million tons. And,
just so you don't think seawater is worthless, a cubic mile of it also can contain up to 25
pounds of gold (at a concentration of 0.000005 parts per million). Before you go out and try
alchemy on seawater, though, just think about how big a cubic mile is (1 cubic mile
contains 1,101,117,147,000 gallons!).
Rivers discharge mineral-rich water to the oceans. Satellite view of La Plata River
discharge to the Atlantic Ocean. One way minerals and salts are deposited into the oceans
is from outflow from rivers, which drain the landscape, thus causing the oceans to be salty.
Credit: NASA
Salt comes up from below, too
Rivers and surface runoff are not the only source of dissolved salts. Hydrothermal vents
are recently-discovered features on the crest of oceanic ridges that contribute dissolved
minerals to the oceans. These vents are the exit point on the ocean floor from which sea
water that has seeped into the rocks of the oceanic crust has become hotter, has dissolved
some of the minerals from the crust, and then flows back into the ocean. With the hot
water comes large amounts 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 seawater.
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.
The Mariana Arc is part of the Ring of Fire that lies to the north of Guam in the western
Pacific. In 2004, scientists exploring the NW Eifuku volcano near the Mariana Islands
reported seeing small white chimneys emitting a cloudy white fluid near the volcano's
summit, as well as masses of bubbles rising from the sediment around the chimneys. In
this picture you can see masses of minerals and carbon dioxide escaping from the earth's
crust into the ocean. These vents contribute dissolved minerals to the oceans, which is
one reason the oceans are salty. Credit: NOAA
Source: Argonne National Laboratory and The University of Texas at Dallas.
https://www.usgs.gov/special-topic/water-science-school/science/why-ocean-salty?qtscience_center_objects=0#qt-science_center_objects