Slate
Metamorphosed mud that splits into flat sheets — the roofing material that made dense cities weatherproof.
Slate is shale that has been squeezed. Under directed pressure the clay minerals in mudstone recrystallise into micas aligned perpendicular to the direction of squeezing, and that alignment lets the rock split into sheets that can be millimetres thick and remain strong.
The critical point is that this cleavage has nothing to do with the original bedding. Slaty cleavage is imposed by the deformation and frequently cuts across the sedimentary layering at an angle, which is why a slate quarry is worked according to the cleavage rather than the bedding.
How it forms
Slate forms at the lowest grade of regional metamorphism, in the fold belts produced where continents collide. Mudstone buried and compressed at a few hundred degrees develops the mica alignment that defines it; heated further it coarsens into phyllite and then schist, losing the fine even split that makes slate useful.
It is therefore a narrow product of a narrow range of conditions, which is why good roofing slate comes from a small number of regions — north Wales, Cornwall, Brittany, parts of Spain and the Appalachians — rather than from wherever mudstone happens to have been buried.
Uses
Roofing is the use that mattered historically and still matters. A slate roof is thin, light for its coverage, effectively waterproof and lasts a century or more, and the industrial-scale quarrying of Welsh slate is what let nineteenth-century cities roof terraced housing cheaply and durably.
Slate is also used for flooring, cladding, worktops and billiard table beds, where its flatness and stability are the point. Its older uses as writing slates and school blackboards are gone, but they are the reason the word means a clean surface to write on.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Muscovite — mineral · the aligned micas that give slate its cleavage
is a source for
- Dimension stone — material · for roofing and flooring rather than walling
is used in
- Construction — industry · roofing, flooring and cladding
is an input to
- Quarrying — process · split along its cleavage by hand into roofing sheets a few millimetres thick
is sourced from
- Shale — rock · slate is shale taken further: the clay rebuilt as mica and aligned by pressure, cleaving across the original bedding rather than along it
Sources
- Material WorldOur own writing