This EarthCache is located within a cemetery. Please be respectful and sensitive to the location whilst completing the Tasks, and, if challenged, please freely explain about geocaching. It may be worth pointing out that it is not a physical box you are looking for, but are simply studying the rock visible to everyone. Please, no night caching.
Everything you need to answer the questions is available by attending the co-ordinates and reading this lesson. There's no need to research anything extra online, although you’re welcome to do so if you want to.
At GZ you will spot a tall memorial that is a very different colour to all the other stones around it. This is the final resting place of Mary Isabella Nunn. It is made of igneous rock.
Igneous rocks
Igneous rocks are formed through the cooling and solidification of magma or lava. . Volcanoes contain molten rock - above the surface (exterior) this is called lava and below the surface (interior) this is called magma. Pockets of magma come up from the mantle/main vent and push their way through weak spots and vents in the earth's crust, often under volcanoes. When magma is pushed up through a volcano that's erupting, the magma becomes lava as it exits out of the crater.
Rocks that come from magma become intrusive igneous rocks, and the rocks that come from lava become extrusive igneous rocks. The differences are due to many factors, including the geochemistry and cooling rate, among many other factors.
Since magma is located inside the Earth, the overlying rock and sediment act as a blanket that keeps the magma warm and allows it to cool slowly. There are many other factors that keep the magma warm including radioactive decay, the great pressure upon it, and time. Lava, on the other hand, exits the volcano and cools much faster on the surface.
When looking at igneous rocks with the naked eye, you can often see crystals within their matrix. Sometimes, these are only visible with a microscope. The rate of cooling affects the speed these mineral crystals grow. Generally speaking, the longer the rock is allowed to cool, the bigger the mineral crystals that are formed.
Lying on top of the earth’s surface, lava cools more quickly, so there is less time for mineral crystals to form and grow. This means extrusive igneous rocks have much smaller crystals, which sometimes can only be seen with a microscope.
Examples of igneous rocks:
Granite is a commonly found igneous rock. It includes the minerals quartz, (which has a clear/glassy colour,) potassium feldspar, (which has a pink/red colour) and biotite mica, (which has a black colour.) Granite is an intrusive igneous rock, formed under the Earth’s surface as magma cools. When looking at granite, we can look at the size of the crystals in the rock and conclude how quickly the magma cooled: broadly speaking, the slower the cooling, the larger the crystallised minerals are within the granite. Granite can take millions, if not hundreds of millions of years to cool and form. Some of the crystals in granite can be significantly larger than the surrounding crystals. These larger crystals are called phenocrysts. The phenocrysts in the granite are formed as the molten rock cooled very slowly, deep underground. At some point the molten rock migrated upwards towards the surface and started to cool much more quickly, and/or the geochemistry changed too. This two stage cooling process results in the formation of the much smaller crystals which surround the larger phenocrysts. The smaller crystals tend to be more cramped together, resulting in less regular or defined crystal shapes.
Gabbro is also an intrusive igneous rock that is formed in a similar way to granite, though the chemistry of the magma that forms it is quite different. It is also formed as the magma cools slowly beneath the surface over millions of years. Gabbro typically has a very dark colour, from black to dark green. It often is coarse-grained (meaning it has larger crystals, over 1cm,) which is caused by it's slow cooling. The most important minerals in gabbro are plagioclase (which has a grey colour) and pyroxene (which is black.) Unlike many other igneous rocks, gabbro usually contains very little quartz.
Basalt is an extrusive igneous rock, meaning it was formed by lava that came out from beneath the surface of the Earth from a volcano. (Once magma has been erupted onto the surface of the earth, it is termed lava.) Basalt is the extrusive equivalent of gabbro. Once the lava has reached the surface of the Earth, it often cools much more quickly than igneous rocks formed beneath the surface of the Earth. Because the cooling happened much more quickly, the crystals within it are a lot smaller, (around 1mm.) Again, basalt typically has a very dark colour, because it lacks quartz.
To log this cache, please visit the published co-ordinates and answer the questions below. Once you have obtained the answers, please send them to me via email or through the Message Centre. You are free to log your find once you have contacted me. You don't have to wait for a reply. If there are any questions about your answers, I’ll contact you.
Logs without answers will be deleted. Please don’t include close up pictures in your logs that may answer the questions.
- Look at the crystals in the stone. What colour mineral has the biggest crystals? What is their size and shape?
- Describe the makeup of the stone. (For example, you could include the colour of the stone, the percentage of the stone that each of the main minerals makes up, and the feel or texture of the stone.)
- Has this stone cooled in one stage, or more than one? Explain your answer.
- Having observed the memorial, and with reference to the lesson, what type of stone do you conclude it is made of?
- Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache, but please do not give any answers away.
