This EarthCache takes you to urban Ilkley, just off The Grove. Ilkley can be a busy bustling place, though down here there is some respite from the hassle and bustle.
The below photograph shows the area that we are concerned with.
🤔 So, what are we here to look at?
As you wander around Ikley, it is very apparent that this Victorian Spa town does not have a lot of igneous rocks. Many of the towns and cities which heralded either industry or relaxation, have ornamental architecture in the form of granite. Strangely, Ilkley does not have this, and when compared with the likes of Keighley and Bradford, it is literally devoid of igneous rocks.
Ilkley is dominated by sedimentary rocks, these being rocks which have formed by the accumulation of sediment, in the case of Ilkley, it is mostly sand which accumulated and thus sandstone was formed.
Here, we have the chance to examine some igneous rock, namely 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 formed of crystals of different minerals which together form the rock. These minerals are quartz, mica and feldspar.
🔴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 potassium feldspar.
Examining granite can give an indication as to how quickly or how slow the rock formed. This is done by looking at the crystals which form the rock.
Crystals which have well defined appearance are known as Euhedral. Crystals which are not well defined or who don't have any apparent structure are known as Anhedral.
Granite can be coarse grained, and also fine grained. To be defined as coarse grained means you should be able to see individual crystals, whilst fine grained means the opposite, whilst they are there, they are difficult to discern by the naked eye.
The rate of cooling of the magma, and thus formation of the granite affects whether there is a coarse or fine texture.
🪨Slow cooling.
Slow cooling of the magma allows for larger crystals to form, thus giving a coarse texture.
🪨Fast cooling.
Fast cooling leads to smaller crystals and thus a fine texture.
🪨Mixed rate of cooling.
The magma may cool at different rates, and this will lead to a variation in the texture of the granite, with both coarse and fine textures present.
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 look at the granite cobbles on the indicated area. The colour of granite can be affected by the type of feldspar, would you say that there is orthoclase or plagioclase feldspar. What is the rationale for your answer?
2. Would you describe the granite cobbles as Euhedral or Anhedral?
3. Looking at the granite cobbles, would you say the granite cooled quickly or slowly? What is the rationale for your answer?