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Granite at Waverley Bridge EarthCache

Hidden : 02/11/2024
1 out of 5
1 out of 5

Size: Size:   other (other)

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Geocache Description:

This earthcache will have you looking at some granite slabs to work out how they were formed a long time ago.

Look for the street sign saying "Waverley Bridge" and have a closer look at the slab its attached to,

1) Can you describe the stone that makes the slab - pay attention to the colours, what it feels like to touch and the size of the crystals that make up the stone.

2) Using your answer to question 1 and the information below, would you say the rock has cooled slowly or quickly?

3) Given your answer to question 1 and the information below, would you say the slabs contain more plagioclase or orthoclase feldspar?

4) Take a photo of yourself at GZ, or an identifying item, being careful not to reveal any of the answers to the questions. 

Please submit your answers via message though the Geocaching website - there is no need to wait for a response before logging your find.

Granite is a coarse-grained, intrusive igneous rock that is predominantly composed of quartz, feldspar, and mica minerals. One of the key components of granite is feldspar, a group of minerals that includes both orthoclase and plagioclase. The presence of orthoclase or plagioclase in granite depends on the specific geological conditions during its formation. These feldspar minerals contribute to the overall composition and appearance of granite, with orthoclase imparting a pinkish hue and plagioclase ranging from white to gray.

In igneous rocks such as granite, the size of crystals is intricately tied to the rate at which the molten magma solidifies. Consider it akin to the process of cooling a liquid to form ice. Swift cooling leads to the development of minute crystals, resembling the rapid freezing of small water droplets in an ice tray. On the other hand, gradual cooling affords more time for crystal growth, resulting in larger, well-defined crystals akin to the formation of sizable ice crystals in a slow-freezing process.

If magma cools quickly, for example when lava erupts from a volcano, then many crystals form very quickly, and the resulting rock is fine-grained, with crystals usually less than 1mm in size.

If magma is trapped underground , it cools slowly because it is insulated by the surrounding rock. Crystals have more time to grow to larger size and crystals 2mm and larger can form.

For example, the picture below is Shap Granite, formed in the Lake District - the coin for scale here shows a slow cooling crystal as it's large in size.


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