Shale is forms from the compaction of silt and clay-size mineral particles that have settled at the bottom of seas, lakes, and swamps. Unlike other sedimentary rocks such as sandstone that form from larger sand-size particles, shale's fine-grained composition makes it an extremely soft and fragile rock.
The clay minerals and limited amounts of silt that compose shale have plate-shaped particles that lie flat when deposited in water. As these particles accumulate on the seafloor and are buried by more and more sediment, the weight compresses them together into solid rock. However, the platy clay minerals do not tightly interlock like the more granular minerals of sandstone. This gives shale a very fissile structure, meaning it easily splits into thin layers along the original planes of the flat clay particles.
Freshly exposed surfaces of shale often appear laminated with countless subparallel layers reflecting the fine incremental accumulations of sediment that formed it. The weakness between these thin laminae causes shale to have very low toughness and high anisotropy, meaning its strength differs greatly depending on the direction of stress. Because the layers provides strength mainly when compressed vertically, shale has a much lower tendency to fracture perpendicular to the bedding planes. However, very little stress is required to split the layers apart.
The same property that allows shale to split into thin sheets also makes it very prone to crumbling when exposed at the earth's surface. The interlayer cohesion is so weak that pieces of shale can be easily scraped off with a knife or even bristles of a toothbrush. Freeze-thaw cycles, plant roots, and weathering stresses that have negligible effects on gritty sandstones and crystalline rocks are sufficient to break apart shale's fragile structure. Its clay-rich composition also makes shale vulnerable to chemical weathering processes that further degrade the mineral crystals.
Shale's soft, flaky, and delicate nature require it to stay buried to maintain its integrity. The extensive network of horizontal splits act like faults on a tiny scale, drastically reducing the rock's overall strength. While shale's fissility is useful for humans looking to cut it into blocks or slabs, the rock's fragile physical properties also makes shale prone to weakening and collapse when disturbed from its naturally stabilized condition underground. Understanding these unique properties helps explain why shale has the reputation of being the weakest and softest sedimentary rock.
