dyke

A dyke is a sheet-like body of igneous rock that has forced its way into a fissure in older host rock and solidified in place. The dyke cuts across the grain of the surrounding strata, often at a steep angle or even vertically, which distinguishes it from a sill (which runs parallel to bedding). Dykes range from hairline fractures to structures metres wide and can extend for hundreds of metres or more. They form when molten material under pressure finds a path of weakness and cools before it can retreat.

In mining, dykes matter because they change rock properties, affect ore distribution, and alter the stability of excavations. A dyke of basalt or dolerite running through softer sedimentary rock creates a hard, brittle zone that breaks differently under stress. Some dykes carry ore; others are barren country rock that must be worked through. The contact between the dyke and host rock is often a zone of weakness where water seeps or fracturing initiates.

Recognition and measurement

A dyke is identified by its sharp contact with surrounding rock, its uniform composition along its length, and the baked or altered zone in the host rock adjacent to it. Miners map dykes by their strike and dip, noting width and composition. Thick dykes (over 10 metres) may be worked as separate units; thin ones are noted as hazards. Sampling across a dyke shows a clear compositional boundary, unlike a fault zone which typically shows cataclastic material and fraying contacts.

The term comes from the Dutch and Scandinavian word for a wall or embankment, transferred to geology because the dyke stands proud like a wall once surrounding softer rock is eroded away. Regional usage varies: dike is the standard American spelling, dyke the British and Australian, though both refer to the same geological feature. Dykes are studied extensively in structural geology and are valued by mineralogists as windows into the composition of magma at depth.