A dyke is a body of rock that cuts across the layers of its surroundings. It presents a very high aspect ratio, which means that its thickness is usually much smaller than the other two dimensions. Thickness, indeed, can vary from sub-centimeter scale to many meters, whereas lateral dimensions can extend over many kilometers. What defines a dyke is that it cuts across the bedding planes of the existing rock it intrudes. Because dykes cut across pre existing rocks they yield relative geochronological data about the location where they outcrop, i.e a dyke is always younger than the rock it cuts across. Dykes are usually high angle to near vertical in orientation, but subsequent tectonic deformation may rotate the including sequence.
Dykes can vary in texture and their composition can range from diabase or basaltic to granitic or rhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vast volumes of mantle-derived magmas through fractured lithosphere throughout Earth history.
If the dyke is wide enough, it is common to have variations in the amount of time it took to cool. Along the edges, where the dyke rapidly lost heat to the surrounding rock, cooling is relatively quick and the resulting texture is correspondingly fine grained. This fine grained edge, called a chill margin, then acts as an insulator, allowing the central core of the dike to cool much slower, resulting in a coarser texture.
At the published coordinates, you will face a series of igneous dykes. In order to log this EarthCache, you will need to perform some careful observations (caution: do not go close to the dykes; all observations can be done in safe conditions from GZ).
Nas coordenadas publicadas poderá observar uma série de filões. Para logar esta EarthCache terá de efectuar algumas observações no local (atenção: não se aproxime em demasia dos filões; todas as observações podem ser feitas em segurança a partir do ponto zero).
1st: What is the thickness of the largest dyke? a) <1m; b) 2-3m; c) >5m?
1: Qual a espessura do filão mais largo? a) <1m; b) 2-3m; c) >5m?
2nd: What is the orientation of this portion of the dyke (i.e. N-S, E-W, SE-NW,…)?
2: Qual a orientação predominante nesta parte do filão (ex: N-S, E-W, SE-NW,…)?
3rd: What is the thickness of oldest rock (granitoid) between this dyke and the thinner dyke further east? a) 1m; b) 5m; c) 30m?
3: Qual a espessura da rocha encaixante (granitóide) entre este filão e o filão menos espesso, imediatamente a Este? a) 1m; b) 5m; c) 30m?
4th: Can you see any chill margins in the largest dyke? If so, what is their approximate thickness?
4: Consegue ver margens de arrefecimento no filão mais espesso? Se sim, qual a sua espessura aproximada?
Email me your answers through my GC profile and I will contact you to allow/deny the log of this cache.
Envie-me um email com as suas respostas através do perfil do GC e será contactado para logar, ou não, a cache.
Want to know more about dykes and observe other intrusion effects nearby?
Quer saber mais acerca de filões e observar estes fenómenos nas proximidades?
Visit Complex Geometry – DP /EC28 and “X” – DP / EC31