Have you ever walked down a street and just wondered at the geology of the buildings? Have you ever gone shopping and thought, just what am I looking at? Here there is an opportunity to look at something that there is not a lot of in Calderdale, an igneous rock.
You will notice that the builidng has panels of a highly polished decorative stone. So as you look around, you will probably say, yes stone, but there is something special about this stone. Look closley and you you see that it shines back at you you.
What you can see in this stone are crystals, these are known as phenocrysts, this is what we are here to see.
A phenocryst is a crystal in igneous rock, that is larger than the crystals of the rock that it forms part of. Here the panels are made out of basalt which is an igneous rock. It tends to have small crystals, due to the rapid cooling of lava near the surface, however basalt can have large crystals in it, which formed prior to the extrusion of the magma.
Phenocrysts can have different forms :
They can be euhedral, these crystals are well defined, sharp and easily recognisable edges.
They can be anhedral, which do not have well defined edges
This is meant to be an easy enjoyable EarthCache, that children can enjoy and learn from, so there is not a lot in the description , to blind you with science, it is mean to be about doing, learning and exploring, so lets do that, by completing some tasks :
(1) Please look at the phenocrysts , what colour are they?
(2) Are these phenocrysts euhedral or anhedral?
(3) Please meanure the biggest phenocyrst you can find, and tell me its dimensions.