Diabase

Diabase is a dense, dark-colored mafic intrusive igneous rock with a fine- to medium-grained texture. It is compositionally similar to basalt, consisting mainly of plagioclase feldspar and pyroxene, with minor amounts of amphibole or olivine and accessory minerals such as magnetite, ilmenite, apatite, and zircon. The rock is typically free of quartz.

Diabase forms when magma intrudes into the crust and cools at relatively shallow depths, producing bodies such as dikes and sills. The rapid cooling in these settings yields a fine-to-medium grained texture, though some diabase bodies are more coarse-grained and are termed dolerite in some regions. A porphyritic variety may contain larger crystals of plagioclase amid a finer groundmass.

Geologically, diabase is widespread and commonly occurs in swarms of intrusive rocks associated with continental rifts, plume-related magmatism, and other tectonically active regions. It is often dark gray to black and can weather to a blocky surface with relatively uniform jointing, including columnar patterns in thick, uniform bodies. Distinctions between diabase and dolerite are regional: both describe similar mafic rocks, with dolerite more often used for slightly coarser-grained varieties and diabase for finer-grained ones.

Economically, diabase is quarried for crushed stone and aggregate, road ballast, and, in some cases, dimension stone for facing or paving. It is valued for durability and hardness, though it contains no metallic ore. In field terminology, diabase is the intrusive counterpart to basalt, its extrusive volcanic equivalent.