Ask A Scientist Environmental Earth Science Archive

name         Bruce J.
status       other
age          40s

Question -   I understand that the earths crust is made of salt
and  rain washes this into rivers and this runs into the oceans.
wouldn't  there eventually be no more salt in the earths crust?
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Thanks for your question Bruce...  Although Earth's crust contains
chemical components that may combine to form a salt (which is a
general term used to describe the solid product of a chemical reaction
occurring between an acid and a base), the crust is not strictly salt,
per se.  The bulk mass of the crust is composed primarily of Oxygen
(45.20%), Silicon (27.20%), Aluminum (8.00%), and Iron (5.80%).  These
four elements combine in a myriad number of ways to form a class of
minerals known as the aluminosilicates, which make up the overwhelming
bulk of the crust.  The remaining 13.80% is composed of Group 1A and
Group 2A elements (Alkali Metals and Alkaline Earth Metals,
respectively), like Calcium (5.06%), Magnesium (2.77%), Sodium
(2.32%), and Potassium (1.68%), as well as other trace elements
(remaining 1.97%).  It is these last four elements from Groups 1A and
2A that, when combined with anions (negatively charged atoms) like
Chlorine and Iodine, typically form salts.  "Table salt," or sodium
chloride, is perhaps the most popular of these, but in actuality only
constitutes one of the several possibilities of what could be termed a
salt.

You are correct in stating that the salinity of large bodies of water
(fresh and marine alike) is the result of water running over (and
through) the land, dissolving soluble materials like salt, and
transporting them into lakes, seas, and oceans.  It would seem likely
that the continents would eventually become depleted of salts by this
process if it were not for the fact of plate tectonics and the
dynamics of the rock cycle.  Salts will be deposited in bodies of
water as evaporation of water increases the ratio of salt to water.
If the amount of water decreases by evaporation and the amount of
dissolved salt remains essentially constant, a portion of the salt
will become insoluble and precipitate out of solution and be deposited
on the lake bed, ocean floor, etc.

The process of plate tectonics involves, in general, movements of
individual, discrete pieces of Earth's crust, which sometimes become
consumed by the interior of Earth by the process of subduction,
whereby one plate is driven underneath another at a plate boundary
(for example: the West coast of South America) and forced to dive down
and into the hot interior of the mantle.  Oceanic plates are
particularly prone to subduction, and when they do succumb to that
fate, they carry into the hot mantle much of the sediment, including
salt, that has accumulated on their surfaces over time.  Once deep
within the hot mantle, the rock and sediment of the oceanic plate is
melted in a series of stages and then rises buoyantly upward (due to
its relatively lower density than the surrounding mantle rock) into
the over riding continental plate to produce subsurface igneous
intrusions and volcanic eruptions which add new material to the
continental land mass.  During the melting process, the salt-forming
elements recombine with other elements, predominantly Oxygen, Silicon,
and Aluminum, and form rock material that is the same as, or similar
to, the rock types from which the salt-forming elements came prior to
their erosion and transport to the sea.  This rock manufacturing
process, when coupled with the process of erosion, constitutes an
essentially consistent cycle of salt production and destruction,
whereby the salt content of the land is maintained in a limited sort
of balance.

I hope this explanation helps.
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Scott J. Badham
Department of Geology and Geophysics
University of Wyoming
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