The erosion of the rock composing the earth's crust produces Clay. This erosion is universal, permanent and continuous. Every day more clay forms than man could extract for all its industrial uses.
Originally the Earth was a mass off matter in fusion. The heaviest materials, like metals, sunk in the deeper layers. This process formed surface layers of nearly uniform chemical composition. The Earth then cooled down and the upper layer solidified forming a crust, surrounding a very hot interior mass. The vertical thrust of the rock, compressed and lifted during the cooling process, created the first mountains. The rock formed by the cooling of this matter in fusion is called igneous rock.
The earth's crust is essentially composed of a very small number of oxides. Silica and aluminia account for 75 % and these two oxides are also the principal elements composing clay. During the cooling of the Earth various minerals were formed. Variations in the composition of the matter in fusion and various conditions of cooling created a great variety of minerals. Of the hundreds of identified minerals, the majority is relatively rare and only five of them make up more than 90 % of the earth's crust. Feldspar is by far the most common mineral.
Approximately two billion years ago began the process of geological transformation affecting igneous rocks; the gaseous atmosphere and the earth's crust reacted with each other. The moisture contained in the atmosphere, which was originally vapor, condensed into torrential rain that fell during millions of years.
This rain was the most significant factor of the geological transformations. Water gradually washed out of the rock all the soluble matter that it contained and transported it to the sea, filling the oceans. The presence of salt in the oceans demonstrates the capacity of rainwater to dissolve minerals. In addition to its chemical action like a solvent, water also has a mechanical action. The abrasive effect of precipitations, freezing and thawing, as well as the crushing of the rock by torrents, streams and rivers disintegrated the rock by splitting it in increasingly smaller particles.
This erosion process literally dissolved mountains by forming large deposits of sediment on the floor of oceans and lakes. The abrasive effect of moving glaciers and particles transported by the wind are other factors of geological transformation, of less importance.
These layers of stratified sediment were submitted to heat and pressure, transforming them slowly to metamorphic rock, which in turn was submitted to new violent pushes, becoming new mountainous masses, to be eroded again.
Clay is the product of this geological upheaval. While the mountains were disaggregated and swept along by water, their debris, increasingly small, ended up settling in calm estuaries or lakes, where they formed layers of particles of similar size. These sediments that lost almost all of their soluble elements can later emerge thanks to a reduction in water.