Introducció

Exactly as presented to our sight (figure 1), the Earth is the result of the extraordinary diversity of the internal and external geodynamic processes that the planet has undergone since its inception, which go back to some 4 500 million years (Ma). The Earth was formed from a dust and gas interstellar cloud together with the Sun and the other planets of the Solar System. On the Earth there are no traces of rocks from an earlier history; the Solar System, mainly the other planets of the group known as inner or terrestrial planets (Mercury, Venus, and Mars), the Moon and the meteorites, give information about this stage. The oldest mineral dated up to now is a zircon crystal found in Australia, which has a radiometric age of 4 300 Ma. The oldest rocks, from the region of the Great Slave Lake in Canada, have been dated at 3 960 Ma. In Catalonia, we have more than 550 Ma of geological history recorded in the outcroping rocks.

Figure 1: Present day image of the Earth

Observing a world map in certain detail, it could be said that the shapes of the coasts of some of the emerged land are like the “negative” of the coastal shapes of the land situated in front; it seems that they could fit together, as if they were pieces of a puzzle. The continents form part of the tectonic plates and, in fact, some of the plates fit perfectly. It is know that some plates move with respect to others. Sometimes they separate, and sometimes they move closer and sometimes collide or slip and rub against each other. The result is the distribution of the seas and the emerged land changes with time, but the volume of the Earth remains constant. The space that remains between the plates that separate is occupied by volcanic materials coming from the interior of the Earth, which constitute the oceanic crust; these areas are occupied by the oceans in which marine sediments are deposited. The collision between plates produces deformations of their edges, like gigantic wrinkles, creators of reliefs. These are the great mountain ranges. Their formation process lasts some tens of millions of years, and involves a great amount of energy.