MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY
Metamorphic gneiss rocks either side of a river. Gneiss rocks form by metamorphic processes (heat and pressure at depth) that alter pre-existing igneous or sedimentary rocks. If the gneiss formed from igneous rock, it is known as orthogneiss. If it formed from sedimentary rock it is known as paragneiss. These two types can be distinguished by their colours. Orthogneiss is a light grey colour, while paragneiss is a darker, redder colour and sedimentary layers can still be seen. The rock here is mostly orthogneiss. This is the Verzasca River in Ticino, Switzerland. The rocks were formed and raised to the surface as the Alps mountain range formed, and have here been eroded and shaped by the river and the weather.
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