How Rock Is Remade

Start with the raw material. Mudstone is a fine-grained sedimentary rock made from lithified clay and mud, and it is told apart from shale chiefly by lacking fissility, the tendency to split along parallel layers. It is the kind of unchanged parent rock that the next stops transform.

Slate is the first step of change: a shale-type rock altered by low-grade regional metamorphism. Compression makes fine clay regrow as flakes in planes perpendicular to the squeeze, called slaty cleavage, which is why it splits into the flat sheets used on roofs.

Schist comes from the same kind of fine-grained parent rock as slate but at a medium grade, usually during mountain building. Its mineral grains are large enough to see with a hand lens, and platy minerals such as mica, talc and chlorite let it split into thin flakes, a texture called schistosity.

Migmatite sits at the top of the grade. Under high-grade metamorphism the rock partly melts, a process called anatexis, and the body ends up part metamorphic and part new igneous material, with a pale melt portion called the leucosome and a darker residue called the melanosome.

Now the path turns to basalt. Greenschist forms under the lowest temperatures and pressures that regional metamorphism usually produces, around 300 to 450 degrees Celsius and 2 to 10 kilobars, and gets its green color from chlorite, epidote and actinolite.

Amphibolite is the metamorphic rock of mafic parents such as basalt, built mainly from the amphiboles hornblende and actinolite with plagioclase feldspar and little or no quartz. Geologists call one made from basalt an ortho-amphibolite and one from a sedimentary parent a para-amphibolite.

Blueschist is basalt changed at relatively low temperature, about 200 to 500 degrees Celsius, but very high pressure, at depths of 15 to 30 kilometers. That combination occurs only during subduction, so the rock is direct evidence of a plate descending into the mantle, and it must be exhumed quickly to survive at the surface.

Eclogite is one of the highest-pressure metamorphic facies in the crust and mantle, garnet in a matrix of the sodium-rich pyroxene omphacite. It forms when mafic rock is subducted along thermal gradients below 10 degrees Celsius per kilometer, at pressures of 12 kilobars or more.

Hornfels shows change by heat alone. It is contact metamorphic rock baked and hardened by a nearby intrusion, without significant deformation, so its grains are fine, equal in size and randomly oriented in a mosaic called pflaster structure, and it tends to break into cubical fragments.

Skarn adds fluids to the heat. It forms where carbonate rock such as limestone or dolomite is replaced during metamorphism and metasomatism, dominated by garnet and pyroxene, and it is often enriched in metals such as copper, tungsten, iron and tin, which makes it an important source of ore.

The trail ends with strain. Mylonite is a fine-grained, compact rock made by dynamic recrystallization in ductile fault zones with large accumulated shear strain, at depths of at least 4 kilometers. It is the deep-crust counterpart of the brittle fault breccias that form nearer the surface.