Regional metamorphism

Mudstone, Eildon, Victoria, Australia
Step 1 - mudstone is compressed
Mudstone is a sedimentary rock made up mainly of tiny particles of clay and quartz. Water is present in the clay particles and in the spaces between them. When the rock is compressed, the clay particles are squashed and water is squeezed out.
Source: Museum Victoria

Slate, Glenmaggie, Gippsland , Victoria, Australia
Step 2 - mudstone is compressed
As the temperature and pressure increase, tiny crystals of mica and chlorite begin to form from the clay particles. The crystals are aligned perpendicular to the direction of maximum pressure. The rock changes to slate, which has a platy appearance and can easily be split into sheets.
Source: Museum Victoria

Andalusite Schist, Inkerman lead, south of Dunolly, Victoria, Australia
Step 3 - schist forms
As the temperature and pressure increase further, the mineral crystals become large enough to be visible to the naked eye. The slate becomes schist, which is still made up of mainly platy minerals like mica and chlorite but has other minerals forming larger crystals.
Source: Museum Victoria

Staurolite Mica Schist, Broken Hill District, New South Wales, Australia
Step 4 - large, course crystals form
With further metamorphism, the texture of the rock becomes much coarser and develops mineral banding. The large platy crystals of mica and large crystals of staurolite that form are stable at higher pressures and temperatures.
Source: Museum Victoria

Migmatite, Snowy Mt Highway, between Adelong and Tarcutta, New South Wales, Australia
Step 5 - the rocks partially melt
The pressure and temperature may become so high that parts of the rock start to melt. When these molten parts solidify again they form paler veins and patches, and the rock is known as migmatite.
Source: Museum Victoria