Weathering and relief

There are three types of weathering: physical, chemical and biological. During physical weathering, sediments in rivers, beaches and deserts, for instance, break up and become progressively rounded due to collision between them. The development of fractures in rocks is also important in physical weathering. Fractures allow the circulation of fluids, in particular water, in the rock. The closer the network of fractures, the greater the amount of water that circulates, leading to more intense weathering. Sometimes, minerals form from the circulating water and put pressure on the rock as they grow, contributing to the enlargement of fractures. On the other hand, when water freezes in the fractures, its volume increases about 9%. This also increases pressure on the fracture, which enlarges. When water becomes liquid again, it penetrates in the new open spaces and progressively alters the rock as a result from the successive cycles of freezing and melting. Heat also favours physical weathering, particularly in deserts, once extreme temperature variations between day and night take place and might contribute to the origin of fractures in rocks. When plants grow, the volume of their roots in fractures increases, and this also contributes to their enlargement.

Chemical weathering is the result of chemical reactions between the minerals that form the rocks and the water and gases that are present in the atmosphere. Not all minerals react similarly because they chemical composition and crystalline structure are different. Iron minerals, for instance, tend to oxidize quickly in the presence of water and oxygen leading to changes in their physical and chemical properties. Climate also greatly influences chemical weathering because chemical reactions are faster in higher temperatures and the existence of abundant water. Thus, chemical weathering is more significant in humid tropical climates than in cold regions.

Living organisms in rock fractures and at their surface can be responsible for the existence of certain chemical compounds that contribute to chemical weathering. The outcome of physical and chemical alteration caused by living organisms is called biological weathering.

Rocks that are subjected to weathering become fragile and are dismantled in a process called erosion. Weathering and erosion are distinct but complementary processes. Weathering favours erosion, and in its turn, erosion contributes to expose new rock surfaces to weathering.

The relief in this LNM

There are multiple factors that contribute to weathering. In rocks constituted by harder minerals, weathering is usually slower and therefore they are more prominent in the landscape (esq. A). Such is the case of quartzites; these metamorphic rocks are almost constituted by quartz which is highly resistant to weathering. In this LNM, metamorphic rocks give rise to aligned landforms with a NW-SE direction that stand out in the landscape (esq. B).

References:

Pereira, P; Henriques, R.; Brilha, J. & Pereira, D.I. (2019). Conteúdos científicos para a caracterização dos 8 monumentos naturais locais” enquadrado no projeto Geoparque Litoral de Viana do Castelo – 2ª fase. Município de Viana do Castelo, Relatório Final GEOSITE, 273 p.