This EarthCache takes you to the Stone Jetty in Morecambe. In good weather a lovely place, in bad weather go prepared with waterproofs. We are here to look at the two stone benches shown in the below photograph.
The benches are made from granite. Now, there are other EarthCaches in the area which deal with granite, but we are here to look at the flow dynamics of the magma which when cooled formed the granite.
🪨 What is granite?
Granite is an igneous rock. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock forms by crystallisation either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.
Granite is an interlocking matrix of quartz, feldspar, and mica crystals.
◻️ Quartz. This is a clear or greyish mineral, and is the most common mineral found on the surface of the earth.
◻️ Mica. This is a black or sivery mineral, which looklike flakes.
◻️Feldspar. This comes in a range of colours. Pink or white feldspar is known as as orthoclase, whilst grey is known as plagioclase. The pink colour, comes from K-Spar, which is also known as potassium feldspar.
Feldspar is often the component in granite that influences its colour, as feldspar can be white, pink and red. Its crystals, as shown in the below image, can be blocky, elongated or rectangular. Looking at the feldspar crystals can help understand about the activity of magma during the formation of granite.
You will notice long white linear crystals, these are feldspar, and can be described as phenocrysts. A phenocryst is a crystal in igneous rock, that is larger than the grains of the rock that it forms part of.
🤔 Flow dynamics of the magma.
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. As it cools crystals starts to form. 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, such as phenocrysts to slow down, and become caught in a layer. This creates whst is known as laminar flow, which then presents as a streak when the magma has cooled and formed an igneous rock, in this situation granite. Ay times a distinct streak might not be apparent, but if the phenocrysts are looked at they are aligned with each other in the same direction.
🤔 Flow structures.
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 solid state and thus granite formed.
This being an EarthCache, in order to log it, I ask that you complete the above tasks. Please send the answers 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 look at the top surfaces of both benches, it is helpful if you walk around each one.
(a) Please describe the distribution of the feldspar phenocrysts, is it uniform throughout?
(b) Is there any evidence of laminar flow in the granite, if so how would you describe the flow structures.
(c) What causes phenocrysts to become aligned?