Aging and Leaching of Igneous Rocks

At equilibrium, the least soluble substance in a system that can form will precipitate. Much phosphate contained in sea water is precipitated as tricalcium orthophosphate or hydroxyl apatite, Caio(P04)6(OH)2, and fluorapatite, Caio(P04)6(F)2. Oceans floors are covered with these deposits and are referred to as marine pellets. There are many ways in which this problem may be approached, but it is obvious that if phosphates are to be leached from igneous rocks, large boulders will leach very slowly. Smaller particles of rock caused by grinding, weathering, and aging solubilize more rapidly than larger particles. As a first approximation, rates of solubilization are proportional to fresh surfaces of solubilized rocks. 

As a casual observation, it appears that the artificial mountain lakes in New Zealand, used to generate power and supply water for irrigation, are filling with glacial silts at a rapid rate. Treatment before the problem becomes acute could extend the life of these lakes indefinitely. Without proper treatment it is doubtful whether these artificial lakes will be useful for more than a few decades. The peoples of New Zealand have clearly voiced their opposition to continuing to increase the elevation of dams in order to solve electrical power problems that are certain to come. Once a lake is filled completely there is no choice but to increase the elevation of the dams, if the lake is to continue to serve a useful function. 

It can be assumed that the rate of formation of new surfaces to be solubilized is proportional to the total surface of a mass. This merely means that small lumps can be crushed to finer particles more rapidly than larger boulders can be broken up, crushed, and ground to fine particles with much increased surface area. Within limits, small particles grow in surface area more rapidly than larger particles and satisfy the relation 

As more mass of igneous rock is reduced to smaller and smaller particles, the more rapidly will it be reduced to finer particles. Rates of formation of new surface are logarithmically greater as more surface forms, 

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