Hull Geological Society


Local Geology
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Common urban rocks -

(a work in progress)

Basalt, Dolerite and Gabbro

These three igneous rocks pretty much have the same chemistry and mineralogy. The are all black or very dark grey. The main difference between them is the grain size. Basalt is fine grained and Gabbro is course grained. This is due to the rate of cooling of the molten magma. Basalt is a volcanic rock that cooled rapidly when erupted from a volcano, so the crystals did not have much time to grow. . Dolerite was intruded into existing rocks as sills and dykes so it was more insulated than basalt and developed larger crystals. Gabbro cooled very slowly in the magma chamber so the crystals had plenty of  time to grow.

If you look at basalt closely you will not normally be able to see the crystals with the naked eye, but you might be able to with a hand lens. This tough rock can be seen being used as cobble stones in to streets of Hull's Old Town and sometimes in the Avenues and Dukeries. It is also used in cemeteries for memorials and takes a high glossy polish; this makes it hard to photograph because you can see the reflection of the camera and photographer! On modern memorials the lettering is often picked out in white or gold; pictures can be etched in using a laser.

cobble stones in the Old Town


Copyright - Mike Horne and Hull Geological Society 2020

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