DOLERITE IS EQUIVALENT to basalt and gabbro in composition, although it is intermediate between them in grain size. A fine- to medium-grained rock, dolerite is from one-third to two-thirds calcium-rich plagioclase feldspar, the remainder being principally pyroxene. It has a silica content of less than 55 per cent, and its quartz content is usually less than 10 per cent. Magnetite and olivine may be present: if olivine is present, the rock is called olivine-dolerite. Extremely hard and tough, it occurs in dikes and sills intruded into fissures in other rocks.
- Rock type Mafic, plutonic, igneous
- Major minerals Calcium plagioclase, pyroxene
- Minor minerals Quartz, magnetite, olivine
- Color Dark gray to black, often mottled white
- Texture Fine to medium
The dolerite “bluestones” of the inner circle of Stonehenge were transported 240 miles (385km) from Wales to Wiltshire in England. They were brought by sea, river, and over land. Exactly what significance was attached by the builders to this particular stone is unknown, but it must have been considerable to justify the enormous effort required to get the bluestones to the site. Stonehenge was constructed in 3000–2000BC.
Igneous rock formed below the Earth's surface, a form of basalt, containing relatively little silica (mafic in composition). Dolerite is a medium-gra
(Dolerite) Colour When fresh it is black, dark grey or green; may be mottled black and white. Grain size Medium. Texture Occasionally...
A dark-coloured hypabyssal igneous rock, the medium-grained equivalent of gabbros, occurring mainly as dykes, sills, and plugs. It contains...