Diabas

Diabas, or diabase, is a dark, mafic igneous rock that forms as shallow intrusive bodies and, in some cases, as fine-grained extrusive rocks. It is the intrusive counterpart to basalt and is often grouped with the dolerite family. In thin section it typically consists mainly of plagioclase feldspar and pyroxene, with possible accessory minerals such as olivine, amphibole, and magnetite. Textures range from aphanitic to fine- to medium-grained, and may be ophitic or subophitic, reflecting rapid or moderate cooling near crustal margins.

Formation and occurrence: Diabase forms when basaltic magma intrudes existing crust and crystallizes at shallow depths, producing dikes, sills, and sometimes small plutons. It is widespread in continental crust and is commonly found in dike swarms associated with tectonic rifting and volcanic-tectonic provinces. Geographically, diabase occurs on many continents in varied geological settings.

Nomenclature and use: In different regions the rock may be called diabase, diabas, or dolerite, with diabase often referring to finer-grained varieties and dolerite to coarser-grained ones; practices vary by country. Diabase tends to be hard and durable, making it a common source of crushed stone and aggregate for construction and road building. It is also used as dimension stone in some architectural applications, valued for its uniform dark color and texture.

Relation to basalt: Diabase shares a basaltic chemical composition with basalt, but forms at shallow crustal levels as an intrusion rather than at the surface as lava flows.