This EarthCache takes you to Briggate in Leeds, to look at the cladding of a rather famous shop. In order to be in the right place you need to look for a yellow sign with 150 H 2 on it. Once located you are now in the right place to look at the stone cladding of the building. Now the cladding is formed from granite, which is an igneous rock. There are plenty of EarthCaches which describe this type of rock, which bascially is formed of interlocking crystals, so rather than repeat the record so to say, we are here to look closely at the granite, namely evidence of when it was formed.
We do not want to look at the dark panels on the base, we wants to look at the first panels up from there. You will notice that the two granites are different in terms of their colouring, one is a lighter grey than the other. Ignore the lighter one.Stand directly below the yellow sign then look to the side.
Granite is an interlocking matrix of quartz, feldspar, and mica crystals, and when first looked at may appear to be uniform, but take a step back and look for about 1 minute at the panel, and you will notice that you can actually see some linear structures in the granite.
So, what has caused these? Well, it being a city, it could have been grafitti, but not here. The linear structures are known as flow banding. Lets go back to basics here, and as they say in Cornwall : I’lltellywot its all about.
The granite that clads the building is known as cornish granite, it comes from the county of Cornwall. Now the granite was formed in what is known as a pluton, and is a type of an intrusive rock.
Intrusive rock forms within the crust from the crystallization of magma. As the magma pushes up from deep, it eventually slowly cools into a solid, and forms rock. An intrusive rock is any form of igneous rock that forms within the crust of the planet. Large areas of magma that solidify underground before they reach the surface are known as plutons.When the magma is ejected from the earths crust, such as during a volcanic eruption, the igneous rock is then known as extrusive rock.
As the magma flows and starts to cool within the pluton, parts of it become viscous, in laymans language, thicker and sticker than other parts. When an area of non viscous magma meets an area which is more viscous, it then creates friction, and a viscous layer is formed causing crystals, phenocrysts and xenoltiths to slow down, and become caught in a layer. This creates laminar flow of this layer, which then presents as a streak when the magma has cooled and formed an igneous rock, in this situation granite. A process of fractional crystallisation also can cause this, though this EarthCache is not here to delve deeper into this subject.
There are some structures in igneous rocks, that show evidence of being formed in the fluid stage of magma, these include:
(i) Primary foliation:
Sometimes many plutonic rocks are characterised by foliation. Foliation, is where the metamorphic processes squeezes and elongates the crystals forming the rock, and clear defined aligment can be seen, in laymans language it means you can see defined lines.
(ii) Banding in rocks: this may occur due to It may result from lamellar flow, from settling of minerals from a crystallized magma or from successive injections.Laminar flow is characterized by fluid particles following smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing.
(iii) Schlieren: These are wavy, streaky, irregular sheets, usually lacking sharp contact with the surrounding igneous rocks. They may be altered inclusions, segregation or may represent concentration of residual fluids into layers in a rock that had otherwise crystallised.
(iv) Aligment of crystals : as the magma flow touches the more vicous layers, then what is known as strain can occur. This can go into very much geological indepth talk, but bascially evidence of this may be seen in the alignment of crystals. However, magmatic flow micro structure s may be destroyed by fracturing, crystal plasticity (crystals also go through a solidification process, and befroe this their shape can be changed as they are plastic - able to be manipulated) and recrystallization before the magma reaches its solidus.
If you want to learn further about phenocrsyts and xenoliths there are EarthCaches in West Yorkshire which cover this subject, such as :
Igneous Bradford Microstructure
Igneous Leeds (Phenocrysts)
Igneous Bradford - Xenoliths...
Igneous Leeds....
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.
1. Please tell me what evidence there is of flow banding on the panel, and how does it compare to the panel adjacent to it, which is directly below the H sign. How many flow bands can you find on he panel?
2. Looking at the flow bands, please measure the length of the longest one, how long is it?
3. The longest flow band has at its base a distinct area of crystals, how wide is this area? Are the crystals aligned, or a mismatch of crystals?
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4. Can you see any evidence of laminellar flow? What is the rationale for your answer?