Spur Bay

The field trip was led by Dave Dorey.

A group of about 30 of us met up at the Hotel as arranged and drove (and cycled) in convoy to meet as instructed at Spur Bay. This was to see the layering effect in the St. Peters Port Gabbro and ponder its formation, this was layering in igneous rock, OK yes, it was strange, and we did ponder. Although the rock, known as a gabbro is coarsely crystalline indicating an igneous origin, many of the crystals are aligned into bands, (rhythmic black and white layering). The dark bands were predominantly of black hornblende the lighter of grey feldspar.

The darker layers showed spotted bands of large hornblende crystals, interspersed with lighter bands. The rock was generally known as Bird's Eye Gabbro. (click to enlarge)

The darker layers showed spotted bands of large hornblende crystals, interspersed with lighter bands. The rock was generally known as Bird's Eye Gabbro.

I did ask why it is called a gabbro when my limited geology ( S260 ) told me that gabbro did not have any hornblende in it, and also igneous rocks are not supposed to be layered. Despite all the confusion (on my part), it was interesting. The explanation followed a typical geological theory/conjecture sort of theme I am getting quite used to now. The magma was supposed to have settled out as it solidified.

Another interesting feature was where the banding took on strange orbicular features, which probably formed around cooler lumps of rock -"xenoliths"- although the usual dissimilar rock of the xenoliths was not apparent.