What does diabase mean?
Diabase is a dark, medium-grained igneous rock composed principally of plagioclase feldspar and pyroxene, the same minerals that make up gabbro. It forms when basaltic magma cools at a moderate depth within the Earth's crust, producing crystals intermediate in size between those of fine-grained basalt and coarse-grained gabbro. Diabase commonly occurs in dikes and sills — sheet-like intrusions that cut across or lie between older rock layers — and famous examples include the Palisades Sill along the Hudson River. The rock is durable and has been widely used for crushed stone, road aggregate, and building material. In British and much European usage the identical rock is called dolerite, a terminology difference that can confuse readers of geology from different countries. Because it records the slow cooling of magma underground, diabase is valued by geologists as evidence of ancient volcanic activity.
nounA dark, medium-grained igneous rock of gabbroic composition, consisting chiefly of plagioclase feldspar and pyroxene, typically occurring as dikes and sills; known as dolerite in British usage.
- A dark, medium-grained intrusive igneous rock composed mainly of plagioclase and pyroxene, equivalent to dolerite in British terminology.
"The ancient walls were built from blocks of weathered diabase quarried from the nearby ridge."
"The Palisades Sill along the Hudson River is formed largely of diabase."
"Prospectors traced the copper vein through layers of altered diabase."
Rarely pluralized; as a mass noun denoting a rock type it usually appears without an article or plural. A plural form is occasionally used when referring to distinct occurrences or varieties of the rock.
"The region's diabases range in texture from fine to coarsely crystalline."
American geologists say 'diabase' and British geologists say 'dolerite' for the very same rock — one of geology's most persistent transatlantic disagreements.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of diabase
The word diabase comes from the Greek 'diabasis', meaning 'a crossing over', from 'dia-' ('through' or 'across') plus 'bainein' ('to go'). It was introduced into geological nomenclature by French mineralogist Alexandre Brongniart in the early nineteenth century. The name alludes to the rock's tendency to occur as dikes crossing other formations. The same Greek root gives English words such as 'diabetes', literally 'a siphoning through'.
Related word forms
How diabase is actually used
A technical geological term used primarily by earth scientists and quarrying professionals; it is essentially unknown in everyday conversation. The word carries no figurative or informal usage, and its meaning shifts regionally: American literature on the subject may use 'diabase' for rocks British sources classify as dolerite or microgabbro.
Easily confused with diabase
Basalt is a fine-grained extrusive volcanic rock, while diabase is a coarser-grained rock that cooled slowly within the crust.
Dolerite is not a different rock at all but simply the British term for the same rock Americans call diabase.