The Origin of Slate

Abstract

Slate is a metamorphic stone formed as a result of mountain building processes. The properties or character of a slate depends on the geological setting during its origin. The depositional environments comprise mainly the passive continental margins but also back-arc basins or sedimentation in delta facies. These environments govern the sedimentary pattern and colours as well as the carbon-carbonate content in a slate. The tectonic setting and metamorphic conditions during the orogeny lead to different types of slate in respect to stone type such as slate s.s., schist or gneiss. These stones are all more or less fissile and used for roofing and for other architectonical purposes. The metamorphic processes, different tectonic settings and positions in an orogenic belt as well as the related fold styles are explained in this chapter. The main result of the deformation during folding in respect to slate is the cleavage plane, governing its fissility. The cleavage plane is strongly related to folds which occur in various forms that can be classed depending on the tectonic position in the orogenic belt. Mountain building processes comprise several phases of deformation. The cleavage plane is the result of the first phase whereas the following phases can overprint this plane more or less intensely, changing the fissility and strength of a slate. Besides folds, also faults and joints occur during mountain building, influencing the mining process as well as the properties of a slate.