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Dolerite

A dark, fine-grained igneous rock; diabase. Dolerite is a volcanic rock. While similar to basalt, it contains crystals which can be seen with a hand lens. This indicates that it cooled a little more slowly than basalt. Typically it is found in volcanic plugs which channeled the basalt to the surface. Erosion since Tertiary times, 60 million years ago, has exposed these plugs.

A fine-textured, dark-gray to black igneous rock composed mostly of plagioclase feldspar (labradorite) and pyroxene and exhibiting ophitic texture. It is commonly used for crushed stone. Its resistance to weathering and its general appearance make it a first-class material for monuments.

The most diagnostic feature is the ophitic texture, in which small rectangular plagioclase crystals are enclosed or partially wrapped by large crystals of pyroxene. As the quantity of pyroxene decreases, the mineral becomes more interstitial to feldspar. The rock is closely allied chemically and mineralogically with basalt and gabbro. As grain size increases, the rock passes into gabbro; as it decreases, diabase passes into basalt.

Dolerite is an intrusive igneous rock; that is, one which has "invaded older rock and may displace or dilate the pre-existing rock", contrasting with "extrusive", where magma reaches the Earth's surface and erupts from a volcano as lava flows which then cool. It took many years to agree on this position.

Dolerite is formed below the Earth's surface, a form of basalt, containing relatively little silica (mafic in composition). Dolerite is a medium-grained (hypabyssal) basalt and forms in shallow intrusions, such as dykes, which cut across the rock strata, and sills, which push between beds of sedimentary rock. When exposed at the surface, dolerite weathers into spherical lumps.

Dolerite is a coarse black or dark greenish crystalline rock. Crystals in dolerite can be up to 5mm in length. The minerals in dolerite are rich in iron so it weathers a dark rusty brown. Dolerite is a very hard, heavy rock that makes an excellent road stone. Dolerite is an intrusive igneous rock, cooled slowly beneath the ground. It can also be injected violently into the crust in sheet-like structures called dykes. These are associated with pulling apart of the crust, creating gaps which are filled in by molten rock which then cools to form dolerite.

Material Notes:
Igneous rock, with a grain size < 1 mm (extrusive). It is a basic rock, with a total silica content of 45-55%.

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