What is slate, after all?

Slate is extremely fine-grained mudstone that has been subjected to low-grade regional metamorphism (mountain building) and compression. In commercial slate deposits, the fine grain size is due to sedimentation in a deep-marine environment, in which current levels are extremely low and grain size proportionally small. VT Weathering Green slate is material originally deposited in major crustal rift basins that formed on the margin of an earlier, smaller version of the North American continent during the breakup of the supercontinent Rodinia, in late precambrian and Cambrian time. Later, as this material was weakly metamorphosed—folded, faulted, sheared, stressed, heated, and re-crystallized, but not intensely enough to form large mica grains—it became the durable and workable slate that exists today. Metamorphosis was part of the processes of continental accretion and collision that gave rise to the Appalachian mountain belt over the course of the Mesozoic. Much later, in the Cenozoic, uplift, erosion, and glaciation exhumed the rocks of VT’s ‘Slate Valley,’ bringing them to the bedrock surface.

In all slates, the tectonic pressures are enough to recrystallize and realign the flakes of ‘platy minerals’ (mica, clay, etc.) along parallel planes, from their original randomly jumbled sedimentary orientations, creating metamorphic ‘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 (cleavage) determined not by tiny clay grains, but by larger mica grains created through metamorphosis. ‘Schist’ has larger mica grains still. Large grains of mica make such rocks more crumbly 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 metamorphism.

Slate’s combination of metamorphic recrystallization, on one hand, and very small grain size, on the other, make it a relatively uniform and workable material with extraordinary longevity. At the same time, since all geology is local as well as global, the qualities and properties of slate will vary from region to region, quarry to quarry, bed to bed, block to block, and—albeit slightly—from individual slate to individual slate.