This EarthCache takes you to Adelaide Street in London, to look at a Victorian Water Fountain. It was erected as a memorial to William Gilson Humphrey, who was the Vicar of this parish from 1855 to 1886.
The water fountain is made from granite, which is an igneous rock. Igneous rocks can be intrusive or extrusive. Intrusive rocks are those formed under the ground, such as granite, whilst extrusive rocks are those formed on the surface. For example, lava which is molten rock, is ejected into the atmosphere, and when this cools and hardens it forms rock, such as andesite.
We are looking at an intrusive igneous rock. Granite is a collection of crystals combined together, the crystals being mica, quartz and feldspar. These combine together to give the appearance of the granite. Granite forns from the cooling of molten rock under the ground, in what are called plutons. Plutons are massive chambers of molten rock, and certain parts of the UK have plutonic rocks. The Galloway Hills, and the Dalbeattie area in South West Scotland is a prime examples, their geology is formed from granite due to plutons. Now, chambers no matter how big or small have sides, a bit like a glass bottle. When the molten rock which is deep in the structure of the planet, moves upwards, it at times gathers bits of other rocks, these being from the side of the chambers. These rocks at times get mixed into the molten rock and are literally dissolved or melted bybthe immense heat, whilst others retain some of their structure. These rocks are known as xenoliths, basically an alien rock to the main rock type. These can be a variety of colours, consistency and shape, and looking at them helps to understand a bit of the geological history of a rock.
When the xenolith breaks off from it's original position it tends be be angular shaped. Imagine breaking a brick into pieces, and you get angular fragments. Now, the longer it is in the molten rock before it cools and forms granite , the more the xenolith can change. Also it depends on how fast the molten rock in the pluton is pushed towards the surface crust of the planet.
At first the xenolith could start to be affected by the molten rock around it. Thus could cause assimilation of the xenolith, this starting as rounding of edges, with eventually total rounding could take place. There can also be regional changes in the xenolith, with the outer portions showing evidence of transformation of its structure, this appearing as a glassy boundary around it, and a different outer layer compared to the rest of the xenolith. The faster the rate of ascent, and then coolling of the liquid rock, the less time there is for changes to occur in the xenolith, whilst the longer the xenolith stays in the molten rock before cooling, then the more changes occur to the xenolith.
A good indication of cooling times of the granite is the crystal size making up the granite. Faster cooling time leads to smaller crystals, whilst longer cooling time, leads to larger crystals.
The above photograph shows where you need to look, in order to answer the questions for this EarthCache.
This being an EarthCache, in order to log it, I ask that you complete the below 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 describe the xenolith in terms of shape, and colour.
2. Looking at the xenolith, is there any evidence of assimilation having taken place?
3. Would you say the xenolith was in the liquid rock before it cooled, a short time or long time? What is the rationale behind your answer?
4. Looking at the granite, would you say it cooled quickly or slowly.
5. Very nearby is a sculpture of Oscar Wilde. Please have your picture taken sat on the sculpture, or something to identify yourself (i.e name on piece of paper, you, or your device), with the sculpture in the background.