War Memorial #2433 ~ Sgt Peter W Thorpe EarthCache

We were on Walney, on a bit of a tour on a day with unpredicable weather. We'd not been here for more than 10 years, and as we had a wander we noticed this bench in remembrance of a fallen solder. It was different to all the other benches nearby, and I thought about why the materials here were chosen.

1) Why do you think the bench here is made out of igneous rock? Think about the location.
2) Describe any crystals you can see on the bench. Are they similar or different sizes?
3) Which of the three rock types (granite, basalt, or gabbro) is the bench most likely to be?
4) Has the rock in the bench cooled slowly, quickly, or a mix of the two? Give a reason for your answer.
5) Take a photo of yourself at GZ, or an identifying item nearby - please do not include the bench in this photograph.

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

Understanding Types of Igneous Stones

Igneous rock is a popular building material in public areas - it's extremely hard wearing and often attractive. Various different types of igneous stones are often used:

• Granite: A common intrusive igneous rock. It is typically light-coloured with visible interlocking crystals of quartz, feldspar, and mica. It forms from slow cooling of magma deep beneath the Earth's surface.
  
  

• Basalt: A common extrusive igneous rock. It is typically dark-coloured and fine-grained (crystals are too small to see with the naked eye) due to rapid cooling of lava on the Earth's surface.
  
  

• Gabbro: A common intrusive igneous rock. Like basalt, it is dark-coloured, but unlike basalt, it is coarse-grained (crystals are visible) because it cools slowly deep beneath the Earth's surface. Gabbro is the intrusive equivalent of basalt.
  
  

Understanding Crystal Size and Cooling

The crystal size of igneous rocks also gives us an idea of how quickly it cooled.

• Slow Cooling: When magma cools slowly deep underground, it allows plenty of time for large crystals to form. This results in igneous rocks with a coarse-grained texture - like granite.

• Quick Cooling: If lava erupts and cools quickly on the surface, there is very little time for crystals to grow. This leads to igneous rocks with a fine-grained texture or no crystals at all, basalt.

• Slow then Quick Cooling: Sometimes magma begins to cool slowly below the surface, forming some large crystals, before the remaining molten rock is erupted. The quick cooling on the surface then solidifies the fine-grained matrix around the pre-existing large crystals, creating a porphyritic texture.

An example of porphyritic granite.

The Moh Scale and the Hardness of Igneous Rocks

The Mohs scale is a way of measuring the relative hardness of a mineral or rock by its resistance to scratching. Granite is a very hard igneous rock, typically rating between 6 and 7 on this scale. This makes it much harder than weaker sedimentary stones like limestone and sandstone, which often rank between 3 and 4. Because of its excellent hardness and durability, granite is widely used for exposed locations such as in paving, building exteriors, and monuments, as it can withstand wear and tear from weather and human activity.