All metamorphic rock begins in hot, molten magma. All metamorphic rock IS igneous rock. Igneous intrusive rock is rock that has taken a long time to cool and harden without exposure to the atmosphere. Normal igneous intrusive rock has larger crystal growth, with crystals usually growing to visible size. Igneous intrusive rock has not been exposed to the atmosphere, where rapid cooling, chemical actions, and other atmospheric conditions create much different types of rock.
All metamorphic rock begins as igneous rock. The cooled and hardened igneous intrusive rock is then subjected to massive subsurface pressure which builds up heat. The combination of pressure, heat, some chemical reactions, and time forces igneous intrusive rock to develop the specialized and unique characteristics that separate metamorphic igneous intrusive rock from other types of igneous intrusive rock. And crystal growth is inhibited, which can give metamorphic rock a finer grain and texture.
Magma is mostly silicon and oxygen, with some microscopic crystals and gasses. When it cools without exposure to the atmosphere, crystals grow larger, gasses are less likely to vent off or solidify into liquids, and the weight of more and more cooling magma, or other disturbances in the earths crust creates the conditions for metamorphosis.
One characteristic of metamorphic rock is the flattened texture, striations, or foliation of the components of the rock. Foliation is strongly developed and obvious in some metamorphic rocks where the layers of a rock can literally be peeled or chipped off in sheets. These rocks are classified as schist. Slate is a well known schist that is metamorphosed shale. Marbles are metamorphic rock that can be metamorphosed sandstone, quartzite, or limestone.
Simply put, metamorphic rock can change back and forth into magma and igneous rock by being melted and either extruded into the atmosphere or by remaining underground to cool and harden. Magma is the only natural substance that holds enough heat to melt all of a metamorphic rock back down into silicon, oxygen, gasses and microscopic crystals in super hot and liquid form. In other words, it is all melted to liquid form, and the process starts again. Only this time, there may be different components to the cooled rock. It will now harden and form in a different manner, under different conditions, and with different results. After that, it is all igneous rock that will have to go through metamorphosis again.
Another change that can happen to metamorphic rock is that some man made or natural tumult can cause the rock to be exposed to the atmosphere, where the heat and pressure are gone, but the atmospheric and man made forces can work on the metamorphic rock. All of the natural actions that affect igneous extrusive rock can be applied to the igneous intrusive metamorphic rock, including breaking it down over time into special forms of crushed rock, gravel, soil, sand and silt.
Geology.com “Metamorphic Rocks”