When metamorphic rocks undergo pressure, how are they changed?
Metamorphic rocks are formed when a rock (sedimentary, igneous or a previous metamorphic rock) comes under high pressure and/or temperature. Pressure and temperature forces the atoms to form new minerals and thereby a new kind of rock. It's not necessary fragmented, but the rock rather morph through recrystallization into a new state in response to the pressure and temperature as the material reorganize. 

Recrystallization doesn't remove any material, it's only a physical reorganization to compact the rock. If some change of the chemical composition occurs, it's called metasomatism. In metasomatism atoms are actually moved from one part of the rock or formation, often with water involved. 

A good petrologist can usually determine what kind of parent rock, protolith, that have been metamorphosed and to what degree, but sometimes it can be difficult do recognize the difference. At lower degrees of metamorphism, features of the original rock is preserved. E.g. you can see ripples and even fossils in slates or deformed structures from the source rock, but at higher degrees of metamorpism it's increasingly difficult to imagine how the parent material looked like. Gneiss formed from sandstone and granite can look very similar as the lihology represents the conditions of metamorphism, not the protolith. 

It can also be difficult to recognize a metamorphic rock. A weathered slate and shale or even granite and gneiss can appear very similar in the field. However, the processes to form the rocks are different and with a closer look at the minerals it can be possible to understand what kind of process that formed the rock. 


    (1, Blueschist facies
    2, Eclogite facies
    3, Prehnite-pumpellyite-facies
    4, Greenschist facies
    5, Amphibolite-facies
    6, Granulite facies
    7, Zeolite facies
    8, Albite-epidote-hornfels facies
    9, Hornblende-hornfels facies
    10, Pyroxene-hornfels facies
    11, Sanidinite facies)

The temperature and pressure determine what metamorphic rock you get, but the chemical composition is inherited from the protolith. Degrees of metamorphism are called metamorphic facies. This diagram shows at what depth and temperature a particular rock is formed. 

Igneous rocks are formed when a melt hardens to crystals and sedimentary rocks are formed from sediments. All rocks weather if exposed to water and air, and weathered material is transported by rivers to oceans where sandstone can be formed from the sand at the beach and shale from the finer sediments further away from the coast. Igneous rocks can be formed as hot magma intrudes the crust and slowly cools down and minerals are formed or form lava at volcanoes. This is known as the rock cycle and is one of the fundamental, but complicated, concepts in geology. You can read more about the rock cycle e.g. here to learn more about rock types and how they are formed.