Q:

How are sills formed?

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Quick Answer

Sills form when lava travels between two layers of sedimentary rock. Sills are intrusions of magma that spread underground between the layers of another rock formation. They are found in formations such as hogbacks, mesas and cuestas. Due to their formation taking place beneath the Earth’s surface, sills consist of platonic igneous rocks.

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Full Answer

A sill is an intrusive rock formation. These formations occur when magma solidifies underground and then intrudes into the overlying rock formation. They are distinct from similarly formed domes and dikes because sills are horizontal to the surface.

A sill is tabular or sheet-like in shape because the magma intrudes in a horizontal formation before solidifying. The texture of a sill is a function of the time it takes for the magma to cool and solidify. In general, the longer it takes the magma to cool, the greater the size and extent of the formation. If a sill cools quickly, the texture of the formation is typically smooth and the mineral crystals which compose the still are not visible to the naked eye. However, if the magma takes a long time to cool, large visible crystals will appear.

Some of the most breathtaking sills can be found in Scotland and on the south shore of Nova Scotia.

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