Fine- to medium-grained, dark gray to black intrusive igneous rock. Diabase is one of the dark rocks known commercially as “black granite.” It is extremely hard and tough and is commonly quarried for crushed stone, under the name “trap.” Chemically and mineralogically, diabase closely resembles the volcanic rock basalt, but it is generally somewhat coarser grained.
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The Palisades Sill which makes up the New Jersey Palisades on the Hudson River, near New York City, is an example of a diabase sill. The dike complexes of the Hebridean Tertiary volcanic province which includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullion region of Ireland, and extends across northern England contains many examples of diabase dike swarms. Parts of the Deccan Traps of India, formed at the end of the Cretaceous also includes dolerite. It is also abundant in large parts of Curaçao, an island off the coast of Venezuela.
In Western Australia a 200 km long dolerite dyke, the Norseman–Wiluna Belt is associated with the non-alluvial gold mining area between Norseman and Kalgoolie, which includes the largest gold mine in Australia, the Super Pit gold mine.
The vast areas of mafic volcanism/plutonism associated with the Jurassic breakup of Gondwanaland in the Southern Hemisphere include many large diabase/dolerite sills and dike swarms. These include the Karoo dolerites of South Africa, the Ferrar Dolerites of Antarctica, and the largest of these, indeed the most extensive of all dolerite formations worldwide, are found in Tasmania. Here, the volume of magma which intruded into a thin veneer of Permian and Jurassic rocks from multiple feeder sites, over a period of perhaps a million years, may have exceeded 40,000 cubic kilometres. In Tasmania alone dolerite dominates the landscape.