diapir

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Sandstone dykes

Most of these features that rise more or less vertically through other rocks are volcanic in origin (see our post on Ship rock at http://on.fb.me/1GZOKfo), consisting of lava that has been squirted upwards through the crust, usually along pre existing faults or lines of weakness. The sedimentary versions are more complex, forming via a variety of processes. They are found all over the world in a wide variety of rock types. They vary from centimetres to metres in width, with the average being a few cm. The most obvious variety forms when a natural crack in the ground formed by erosion (such as frost wedging or dissolution of limestones), or an earthquake, gets filled with a different kind of sediment.

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Un diapir (du grec diapeirein, percer au travers) est une structure résultant de la remontée de roches plus légères à travers des roches plus denses. Elle est constituée de roches facilement déformables, telles que le sel, ou encore le gypse, les magmas, les boues, etc. remontant vers la surface sous l'effet de la poussée d'Archimède.

Se rencontrant en géologie, ce phénomène est en réalité la manifestation d’un mécanisme bien plus général en physique des fluides connu sous le nom d’instabilité de Rayleigh–Taylor.”

Source : Futura Sciences