Water And Oceanography

Why are Oceans Salty but Rivers and Lakes are not



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"Why are Oceans Salty but Rivers and Lakes are not"
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Salts, that are many kinds of chemical materials are present in dissolved states in lakes, rivers, seas and oceans. This means that the abundance of these substances will create specific tastes, such as saltiness. However, the amount of salt in each watery body varies significantly from place to place. Generally it is believed that salt makes up around eighty five percent of the ocean's total chemical make up. Sodium chloride Is also present in rivers and lakes, but its concentration is much lower so its extremely difficult to taste any of it. For example, it is considered that there is about ten billion tonnes of gold dissolved in the ocean among other rare materials.

HOW WATER BECAME SALTY

A few billion years ago, as the Earth was still undergoing complete formation a lot of outgassing occurred due to large volcanic activity, which resulted from geological instability. This outgassing carried a lot of water vapour into the primitive atmosphere, which cooled on its way up and then precipitated out when the planet and its atmosphere cooled. As it rained for many cventuries and even milleniums. The water accumulatd into geological depressions which are now seas and oceans.

On its way through the rivers and into the oceans, water did the following to accumulate and concentrate all the salts and minerals that are currently present in liquid water:

a) Over millions of years the cooled rock of the Earth's crust broke up and was carried away by rains and rivers into seas and oceans where it gradually dissolve.

b) In hydrothermal vents and hot springs along mid-ocean ridges ocean water circulates in hydrothermal vents and undergoes powerful reactions with the hot basalt, then it returns greatly changed in chemical composition.

c) As volcanoes erupted on the ocean floor, sea water mixed and dissolved with the molten rocks and thus injecting fresh new minerals into the water's chemical composition. Such as salt.

d) Over time rivers also helped to increase the concentration of minerals in the oceans. It is estimated that throughout the world, rivers dump around four billion tonnes of dissolved salts into the ocean every year.

e) Salinity maintains a low concentration at the poles of the planet, because of large amounts of fresh water that constantly melts and dissolves the salts in the oceans.

THE HYDROLOGICAL CYCLE CONTRIBUTES TO A STABLE MINERAL CONCENTRATION

a) Water in the seas and oceans evaporates and is carried up into the atmosphere. This vapour is made up of what used to be liquid water, snow and ice. b) The water vapour condenses into clouds. These clouds move around the world and cone they become heavy and rich with water they precipitate out as rain, hail or snow.

c) As this lands all over the world it carries of and dissolved rocks, dead material and other minerals from the surface of the planet, and then runs of in rivers back into the oceans.

d) The cycle then repeats itself.

SALINITY OF THE OCEANS IS CONSTANT

Water being one of the best disolvents should eventually dissolve the entire surface of the planet and mix it thoroughly into the oceans, thus increasing the water's salinity indefinitely. However, this does not happen because:

a) During plate tectonics, the less dense continental plates float over the other plates an thus forcing large amounts of minerals, salts and other chemicals in those lower plates to be trapped deep within the crust of the Earth and some of the mantle. This helps the oceans greatly in maintaing a constant salt concentration.

b) Life in the oceans such as Benthics, Foraminifera and other living organisms such as mollusks, oysters, etc extract calsium and other salts to build exoskeletons, shells and homes. As they die this risidual structures accumulate on the bottom of the oceans and remain there for many centuries or even many thousands of years, until they begin to dissolve. This ensures that the concentration of chemicals in the water is fairly constant.

RIVERS, LAKES AND SOME SEAS HAVE A MUCH LOWER SALINITY THAN THE OCEANS

a) Precipitaion supplies rivers with water and impurities that are present in the atmosphere.

b) As the river water flows, it moves through many different soils, rocks and other materials that act as filters, thus greatly reducing the amount of salts and other chemicals in the water. However, some does remain and is carried of into the oceans to maintain the salt levels.

c) As some rivers enter lakes they slow down thus more of the chemicals accumulate in the lakes. However, since the lakes are not deep enough or large enough, there isn't a significant change in the chemical constitution of the lakes.

d) Some lakes like the Dead sea and the Great salt lakes have enormous salt and chemical concentration, because they do not have any vents in the form of rivers coming of them. This means that the only way water can escape is through evaporation. During this process the heavier salts are left behind, thus increasing the lakes salt concentrations.

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