What is slate, after all?

Slate is extremely fine-grained mudstone—fine because sedimented in a deep-marine environment, in which current levels are extremely low and grain size proportionally small—that has been subjected to low-grade regional metamorphism (mountain building) and compression. Fossils still survive in slate, unlike in higher-grade metamorphic rocks; yet the tectonic pressures are enough to recrystallize and realign flakes of ‘platy minerals’ (mica, clay, etc.) along parallel planes, from their original randomly jumbled orientations in the mudstone, creating what is called ‘foliation.’ As Wikipedia reports, “[slate] is the finest-grained foliated metamorphic rock. . . . The foliation in slate, called ‘slaty cleavage,’ is caused by strong compression in which fine-grained clay forms flakes to regrow in planes perpendicular to the compression.” These planes, which split into flat surfaces, reflect the regularity and geometry of tectonic forces interacting with the regularity and geometry of clay mineral chemistry; they are not to be confused with layers of sedimentary bedding.

At higher metamorphic grade, slate becomes ‘phyllite,’ with the foliation characterized by larger mica grains; ‘schist’ has larger mica grains still. Such rocks are usually more crumbly and flaky than slate, in which only the reorientation and parallel flaking of the smallest clay grains determines foliation, and in which no larger mineral grains exist, being neither deposited in the original sedimentary environment, nor yet created through further metamorphosis.

The strengths and uses of slate have everything to do with the slow, steady, non-turbulent, and indeed gentle processes and pressures involved in its creation: first, the low-energy sedimentary environment of a deep ocean; second, the low-grade metamorphism of the margin of a growing mountain belt.