Foliation in Metamorphic Rocks | Definition, Causes & Types

A foliation in a rock is a fabric defined by a set of minerals that are either flat or needle-like, which forms while a rock is being stressed and metamorphosed. One way to produce a foliation is by flattening – a force like that found in mountain building that simply squashes everything. Alternatively, if rocks are being sheared, as happens deep beneath fault zones where one plate of rocks is sliding beneath another, there is a rotation component that stretches minerals out in such a way that i