About 520 million years ago, during the Cambrian period, the region where the town of Armação dos Búzios is located was part of a giant mountain chain, as tall as the Himalayas. This comparison is based on the similarities between the rocks that are found today in the Himalayan Mountains and the oldest rocks of southeastern Brazil. Nowadays the Himalayas are still rising due to the collision between the continental land plates of India and Southern Asia.

The ancient Brazilian Himalayans were also generated by the collision between continental blocks during the geological past. The South American and African land masses joined, consequently closing the ancient Adamastor Ocean that existed between these blocks. The joining of Brazil and Africa originated an even larger continent called Gondwana. This paleocontinent (palaios = ancient) was discovered by the German scientist Alfred Wegener in the beginning of the twentieth century. Land masses such as Australia, India and Antarctica were also part of this continent.

The Búzios rocks register the final event of Gondwana's formation. They consist of minerals which were re-crystallized under high pressure and temperature, a typical condition in continental mass collisions. Such extreme conditions transformed volcanic and sedimentary rocks, which used to lie under the ancient Adamastor Ocean, into metamorphic rocks. In Búzios, the most common rock is gneiss (a typical metamorphic rock). However a dark green rock called amphibolite (composed mainly of amphibole - a mineral mixture of iron and magnesium) can also be found.

Some minerals obtained in Búzios also provide clues about when this collision must have occurred. Using high precision chemical analysis, the isotopic ratios of uranium (U) and lead (Pb) were measured in minerals such as zircons and monazites. These ratios allow scientists to calculate the ages of these mineral formations by means of the isotopic dating method. The Buzio minerals were formed within the range of 520 and 495 million years ago, in the middle of the Cambrian period. By this time the earth was undergoing great biological changes with the appearance and evolution of the earliest macroscopic organisms.

The Cambrian is the first period of the Phanerozoic Eon Period (faneros = visible; zoic = life) and it is followed by the Ordovician Period, when the first fish appeared and vegetation started to cover the continental land masses. The sea level started rising considerably, which would subsequently cause marine fish to dominate fresh water environments and evolve into amphibians.

The geographical transformations were also very significant on the continental land masses, the Laurentia paleocontinet (present North America) was starting to become isolated, as occurs with Australia today. A The Brazilian southeastern region was rising due the creation of mountain ranges on the degree of Himalayans mountains. Orogeny (oros = mountains) is how the mountain formation process are called. Considering the age of minerals obtained in Búzios, this episode between 520 and 495 million years ago was named Búzios Orogenia.
		Dra. Renata S. Schmitt