Geological Classification of Slate

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The rock forming the earth's crust falls into three generic group's: igneous, sedimentary and metamorphic.

Heat, pressure, and chemical reactions may change either igneous or sedimentary rock into metamorphic rock, meaning "changed in form," usually into a more compact and crystalline condition, and even metamorphic rocks may be further altered to higher ranks of metamorphism.

Rocks may become plastic under great pressure and high temperature and by earth movement. They may be folded into complex forms with a banded structure. Many constitutes minerals may be dissolved, transported, and reprecipitated by thermal waters. Heat and pressure may cause recrystallization.

In this way, new rocks are formed, differing widely from the igneous or sedimentary types, and usually much harder than either. Thus shale and related rocks may be altered into slate.

The shale from which slates originate were deposited previously as clay beds. These beds of shale at first horizontal, were tilted by subsequent earth movements, and the intense metamorphism that converted these into slates folded and contracted them. Slate, then, belongs to the metamorphic group of rocks and can be defined as a fine-grained rock derived from clays and shale and possessing a cleavage that permits it to be split into two sheets.

Reference: Dimensional Stone Design Manual, Marble Institute of America

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