What is shale?
Shale (Dutch: schalie) is a fine-grained, layered sedimentary rock made of silt and clay minerals. It forms when clay layers are compressed over a very long time by younger sediment deposited on top of them. The pore space shrinks, the density increases and the flat clay minerals line up parallel to each other. This hardening process is called diagenesis; the squeezing itself is called compaction.
Because of that parallel alignment, shale consists of many thin layers (lamination), so it splits fairly easily along those layers. Shale can contain fossils, and when mixed with lime it is known as calcareous shale.
Shale versus slate
The two are often confused, but they are different rocks:
- Shale is hardened clay, formed by compaction and cementation. It is a sedimentary rock.
- Slate formed from shale or mudstone through metamorphism (high pressure and temperature). It is denser and harder; shale flakes and crumbles far more readily than slate.
Application
In construction you mainly encounter shale as a raw material and in the ground:
- Raw material: shale and claystone are ground up and used to make ceramic products such as bricks and roof tiles.
- Roofing: not every split stone is frost resistant. For a roof, only use slates that are specifically tested and approved for roofing.
- Groundworks: shale layers can weather and break down once excavated and exposed to air and water. A site investigation will flag this before you design foundations.
On older roofs, slates that are flaking badly are usually a sign that the covering is nearing the end of its life.
Related Terms
- Slate
- Roof slate
- Sedimentary rock
- Clay
- Natural stone
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