Most likely our planet originally was a mass of molten material. When this material cooled, a thin film solidified on the surface just as a film forms on the surface of milk after boiling. As our planet was cooling but still fluid, heavy materials such as metals sank towards the center of the planet, so that the other materials that were left on the surface were of a fairly uniform composition. They cooled and hardened, forming what is now called igneous rocks and minerals. Here are some interesting facts about the earth itself:
- The core of our earth, which is about 3.500 km thick, is made of liquid iron and nickel;
- Around this core is a viscous layer made of nickel and magnesium, called the mantle. It is approximately 2.800 km thick;
- On top of this mantle is the planet's surface, a very thin layer called the "skin", only 10 to 70 km thick. This surface area is called the rind or bark. It is solid, and consists of three layers:
1 - The deepest layer, called the basaltic layer, is composed mostly of magnesium and silicon, and forms the bottom of our oceans;
2 - On top of the basaltic layer is a layer of granite, called the continental crust. It is composed of at least 75% silicon and aluminium. These two oxides are the main elements found in clay;
3 - Above these two layers lies the sediment surface area.
According to geologists (Millot, 1983; Galàn, 2006) there are three main geological processes at the origin of clay. These are:
* inheritance : (local or moved) Clay forms from silicate rocks in the deep and is or is not moved to the surface by tectonic movement. Silicate rocks are granite's, gneiss, mica schist, shale, lava, feldspar and mica;
* transformation : (second generation clay) Clay is transformed from the original inheritance clay by evolving and adapting to a new environment through a process called aggradation or degradation;
* neoformation : clays form a new from dispersed elements such as ions, combining at the bottom of a lake for example.
In addition to the above three mechanisms, other processes called diagenesis and metamorphism can change the clay further
Millot G (1964). Géologie des argiles, Masson, Paris.
Galàn E (2006). In Handbook of clay science, chapter 14, Elsevier ed.
Caillère S, Hénin S, Rautureau M (1982). Minéralogie des argiles, 2e édition, 2 tomes, Masson , Paris.