Metamorphic rocks

Slate, phyllite, schist and gneiss can all start as the same mudrock; what differs is how hot it got, from about 150 °C to above 550 °C.

Metamorphism means change in form. A metamorphic rock started as some other rock, igneous, sedimentary or an earlier metamorphic one, and was changed by high heat, high pressure, hot mineral-rich fluids or, most often, a combination of these. Such conditions are found deep in the Earth and where tectonic plates meet. The BGS adds that rocks may stay in these settings for millions of years, long enough for their minerals to change physically, chemically or both.

Changed, but never melted

The key condition is that the rock does not melt. If it did, it would become magma and then igneous rock. Instead, minerals recrystallize in the solid state: atoms rearrange into new minerals, or react with fluids that seep in. The result is usually denser and more compact than the original. Rocks that come close to melting may partly melt; Physical Geology calls such a mixed rock of metamorphic and igneous material a migmatite.

Foliation: stripes from squeezing

When rock is squeezed harder in one direction than another, flat or long minerals such as mica grow lined up at right angles to the push. The rock gets a striped or sheet-like look called foliation, which records the direction of the pressure. Physical Geology lists the foliated rocks in order of grade, or intensity of metamorphism, for a mudrock such as shale. Slate forms first; its mica flakes are too small to see, but it tends to break into flat sheets. Phyllite has been heated more and its micas give a sheen. In schist the mica crystals are big enough to see. In gneiss the minerals have separated into light and dark bands.

Some rocks do not develop foliation however hard they are squeezed, because their minerals are not flat or long. Limestone is mostly calcite, and its grains will not line up; heated, it recrystallizes into marble, and any fossils it held are destroyed. Pure calcite gives white marble; impurities give the swirls that the word marbled describes. Sandstone becomes quartzite, its quartz grains welded together by extra silica. The tables above, from Physical Geology, also show that granite barely changes until high grade, because its minerals stay stable up to several hundred degrees.

A single outcrop can show the whole story. Physical Geology describes a slate boulder in the Canadian Rockies where the original sedimentary bedding is still visible as light and dark bands, while the slaty foliation cuts across those bands at a different angle. The bedding records how the mud was laid down; the foliation records the direction in which the rock was later squeezed.

Baked at the edges

Not all metamorphism needs deep burial. When hot magma pushes into cooler rock, the rock next to it is baked without being squeezed and without melting. This is contact metamorphism. Its rocks, such as hornfels, are usually not foliated, because there was no strong push from one direction. Over a large region, where plates collide and rocks are both buried and squeezed, regional metamorphism produces the foliated slates, schists and gneisses.