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Name: Bruce J.
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: August 28, 2004


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?



Replies:
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.

Scott J. Badham
Department of Geology and Geophysics
University of Wyoming



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