How did grains in a metamorphic rock be flattend?

Answer 1

Schist, is formed originally as a sedimentary rock on the ocean bed, generally not very close to the shore. (For it often lacks coarse grains.) These are laid down by periodic floods, often with an annual cycle, unless they come from a lake. This forms banded sediments, sometimes known as varves.

These periodic floods tend to have more coarse sediment embedded when the river is in a fresh, and clayey particles when in lower flow. The clay particles are flattened by simple pressure, for these particles contain water which is then squeezed out.

Upon entering metamorphism; due to deep burial and underground heat; some of the minerals may be heated enough to form new (appropriate) minerals.

The micas are among the minerals with the lowest temperature of crystallization (=freezing) and therefore form last. The natural state of the micas is as conspicuous flat crystals. Consequently, the age of the micas is often taken as the age of that particular metamorphic event. [But could be re-set again by a subsequent thermal event.]

If you look at a cross-section of a schist, under a microscope, you may be able to determine its original state of bedding of an individual varve - coarser grains at the bottom, finer ones at the top.