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Two main processes act to change the nature of sedimentary rocks during metamorphism. The first, realignment, affects mostly shales. The crystals of clay minerals that compose shales are tiny, flat sheets. When subjected to pressure, these crystals reorient at right angles to the direction of pressure. Metamorphic rocks which exhibit crystal grains with strong orientation are termed foliated. Many sedimentary rocks, such as sandstones or limestones, do not have grains which have preferred orientation. During compression, they will not undergo realignment, and remain unfoliated.

As pressure and temperature become more intense, all sedimentary minerals will begin to recrystallize into new mineral combinations as a result of metasomatism, the migration of ions and elements in the solid condition. Sedimentary rocks whose composition is limited to a single mineral will not have the chemical freedom to form different minerals, and will simply form larger crystals of the same mineral. Such is the case of sandstones composed entirely of the mineral quartz, and of limestones composed of calcite. The metamorphic rocks formed from these, quartzite and marble, respectively, differ from the parent sedimentary rock, not in composition, but in crystal size.

Clay minerals, being much more complex in composition, also form new crystals, differing from the original clays not only in size, but also in composition. These new silicates are, in fact, the same minerals (or closely related ones) which are formed during the crystallization of a magma. Thus, clays (depending of course on available elements) metamorphose into micas and feldspars. If rich in iron, the clays will change into pyroxene- and amphibole-rich rocks. The table below shows what the most common rocks become, as a result of metamorphism:

Original Rock	After realignment	After recrystallization
Conglomerate	Little change	Metaconglomerate then Quartzite
Breccia	Little change	Metabreccia then Quartzite
Sandstone	Little change	Quartzite
Siltstone	Little change	Quartzite
Limestone	Little change	Marble
Coal	Anthracite	Graphite
Shale	Slate	Phillite, Schist and Gneiss**
Basalts	Little change	Greenschists and Blueschists

**The main difference between a schist and a gneiss is that in gneisses the various minerals have segregated into distinct bands.

In a way, one can consider metamorphism to be the opposite of weathering. One is a surface process, the other operates at depth. While one may carry this contrast into great detail, it is well to remember that on a more fundamental level, both processes do the same thing. In both, earth materials readjust into more stable configurations, better suited and more stable in their new environments.

Ultimately, if sufficient heat is added, metamorphic rocks remelt to form magmas. Conceptually, this brings us back to igneous rocks, the starting point of our discussion.