Chaos? What chaos?
The surrounding hill of the Sanctuary of Our Lady of Guia presents great and well visible granite outcrops. These consist of large portions of the granitic massif that remain uncovered from the ground, due to weathering and subsequent erosion, and are exposed to the surface. The systematic action of water and wind in this type of rock leads to the creation of very unique landscapes that are called Blocks Chaos. This landscape is well present in the north of the country and in the great mountains.
Granite, the rock abundantly in the coordinates published on the cache page, is an intrusive / plutonic magmatic rock. Forming from the consolidation of the magma in depth, this rock presents crystals visible to the naked eye: feldspar, quartz and mica.
But how is it that, being formed in depth, it is on top of the mountain?
he appearance of these granite outcrops can be explained based on weathering and erosion over thousands of years. During this period, a rise may occur, that is, an elevation of layers of the earth's crust due to compressive forces. Through this elevation, the suprajacent layers (those that are above), are subjected to the most intense weathering. At the scale of a few thousand years they can be partially or totally removed and reveal granite outcrops. Why are not these weathered too? In fact, they are. What happens is that, as it is a magmatic rock, it has an enormous resistance to erosion and it stands out to the rest of the landscape.
Formation of Block Chaos
Due to the high pressures that the rock is subjected to during its formation, weak points and fractures are formed in its structure. When it comes out, and as the pressure is lower, the granite ends up fracturing at the weak points originated previously. The rock begins to swell and thus forms a network of fractures, usually parallel and perpendicular. This structure, in Geology, is called diaclasses.
Diaclasses can be destabilized at the mineral and structural levels by means of weathering. This can be:
Physical Weathering - when wind, water, roots and thermal aplitudes help to fracture the rock;
Chemical Weathering - when chemical reactions occur with water that lead to the release of feldspar ions and to the disintegration of structural components;
It turns out that the edges of the rocks are the parts that are more exposed to this process of weathering and erosion because they are more protruding. Thus, over millions of years, the edges of the rocks are being "filed" by these phenomena. This leads to the formation of rounded structures that are so common in granite landscapes such as this and which are called blocks. Once formed the block the meoteorization and the erosion do not stop. What happens is that erosive agents will act evenly throughout the block, keeping the shape rounded as the block gets smaller.
The rounded blocks may later slide down the scattering, scattering and forming that chaos