This is an urban EarthCache, to help show people some interesting geology without having to trek into the wilds of the Dales, Scotland or the urban metropolis of Manchester or Leeds. So what are are we here to see, well first you need to find some granite steps.
Granite is a common type of igneous rock. Igneous rock is one of the three main rock types , the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallisation either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. Granite can be white, pink, or gray in colour, depending on their mineralogy. Granitic rocks mainly consist of feldspar, quartz, mica and amphibole minerals, which form an interlocking matrix of crystals.
Though we are not here to look at the granite in general , we are here to look at one aspect of it, an item called a Xenolith. So what is a Xenolith? If you look at the side profile of the steps, you will notice a large mark, which stands out from the rest of the granite. This is a xenolith. Xenolith is greek for a foreign rock, bascially it is a piece of rock trapped in magma. As explained above, granite is an igneous rock, this is a type of rock formed through the cooling and solidification of magma. A xenolith is a rock embedded in magma while the magma was cooling. As the molten material rises, it tears off bits and pieces of the magma pipe, or roof / wall of a pluton in which it was travelling in which it is traveling. These bits and pieces, trapped in the magma but not melting into it, become xenoliths. Crystals that are torn from the sides of magma pipes are called xenocrysts. As the magma flows out on to the surface of the Earth, or remains as a body within the crust, it is cooled, and various types of igneous rocks could be formed. Xenoliths usually have a different colour and density when compared to the surrounding rock, they can be small or large.
Now there are EarthCaches which look at xenoliths, but we are here to look in a bit closer detail.
There are different types of xenolith:
Peridotite Xenoliths. Periodotite is a coarse grained igneous rock, which consists mainly of the olivine and pyroxene, which are minerals. It is a dominant rock in the upper part of the mantle of the Earth. It can be bright to dark green with some specks of black, though some peridotitic rocks may be blue, brown, and red.
Gabbro Xenoliths. Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. It is formed from magnesium and iron rich magma, deep beneath the crust of the earth. It can be a dark grey coloured rock.
Pyroxenite Xenolliths. These are often coarse grained, with individual crystals , which can be up to 17cm in length. Pyoxenite is formed as an accumulation of crystals at the base of the magma. Pyroxenites can be found as layers in peridotite masses. Pyroxenite is uncommon at the surface because it occurs predominantly deep in the crust or in the mantle. It can be black . dull coloured, but may also contain green or red crystals.
This being an EarthCache, in order to log it, I ask that you answer some questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile. You will need to use a compass to complete some of the tasks.
1. Please tell me what shape the xenolith is?
2. How wide is the xenolith at its widest point?
3. Using the above information, what type of xenolith is it?
4. There is a distinct white crystal in the xenolith, what shape is it, and what is its width in mm?