Clays, granites and continentsA “Big History” of potassium

Résumé

Continents are a characteristic feature of the Planet Earth, not only because the existence of oceans over which they emerge, but because the composition of the rocks that form the continental masses. In spite of the particular and intriguing compo- sition, continents contain the oldest rocks of the Earth, and written in rocks is most of the history of the Planet. To decipher the messages of rocks is the essential job of geologists. To know the origin of continents, the mechanisms of formation of new continental masses and their evolution along the geological time, are keystones to better understanding the Earth, the resour- ces and natural hazards. In this work I will focus the attention in an element that has been key in the evolution of continents: Potassium. The composition of the continents has changed drastically at the end of the Archaean period, approximately 2.500 million years ago; a change marked by the composition of the rocks and by the appearance of large masses of igneous rocks rich in K: the granites. The observed changes necessarily lead to establish a connection between external weathering processes, the composition of the gas mixture in the atmosphere, at the same time related with biological activity and the genesis of magmas. A complex system that can be traced by the particular behavior of K in the processes involved throughout geological history.