Modern Menhirs EarthCache
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This is an urban EarthCache in which geocachers are invited to examine some modern standing stones in a housing development in northern Cambridge. The learning point of this EarthCache is to get the geocacher to become familiar with metamorphic rocks and identify which type the standing stones are made from.
Everything you need to answer the questions is available by visiting the location and by reading this lesson. I don’t anticipate you will have to research anything extra online, although you’re welcome to do so if you wish to.
Here are some keywords for this lesson:
magma - molten rock
lithosphere - the Earth’s crust
protolith - the original, unmetamorphosed rock from which a given metamorphic rock is formed
slaty cleavage - extremely closely spaced, parallel planes of weakness that give slate its ability to split into very thin, platy layers.
schistosity - layered foliation with medium to large grained flakes in a sheetlike orientation
Introduction to rocks
Minerals make up rocks. Rocks are formed in many different types of environment. These can be on, or within the Earth's crust. There are three types of rock, and each is formed in a different way.
Igneous rock is formed within the Earth’s crust, or on it’s surface. It is formed by the cooling of magma (molten rock.)
Sedimentary rocks are formed on the Earth’s surface from the products of weathering which then becomes cemented or deposited.
Metamorphic rocks are formed inside the Earth by temperature and pressure changes that affect existing rocks.
All three types of rock make up the Earth’s lithosphere, the outermost layer. The lithosphere averages about 100 kilometres in thickness.
More on metamorphic rocks
Metamorphic rocks are igneous, sedimentary, or preexisting metamorphic rocks that are now inside the Earth. Sometimes they are within the crust and sometimes the upper mantle. At that depth, rocks are subject to great pressure and temperature. Although very great, the temperature is still not high enough to melt the rocks, because otherwise igneous rock would form. The pressure is much greater than that required solely to break the rocks up. In fact, the pressure is so high that it changes the chemical make up of the rocks by forcing the elements in the minerals to change position. This may take thousands or millions of years.
Metamorphic rock needs either pressure or temperature, or both of these occurring together, to form.
Metamorphism is an isochemical process, which means the chemical composition is mostly unchanged from that of the protolith (original rock.) The main difference is the recrystallisation of the minerals into a new form. New structural features are often found in the metamorphosed rocks, such as slaty cleavage or schistosity.
Different grades of temperature and pressure will cause the same original rock to form very different metamorphic rocks.
Slate vs schist... and gneiss
Slate and schist are two common metamorphic rocks that are often confused with each other.
Slate
Slate, which forms from a sedimentary rock like shale or mudstone, appears as a very dense and smooth rock. It is a hard rock with a well-developed slaty cleavage. This means it has extremely closely spaced, parallel planes of weakness that give slate its ability to split into very thin, platy layers. Slate does not contain minerals visible to the naked eye. They are present, but slate is so fine grained you would need a microscope to see the minerals in it. Slate is a product of low grade metamorphism, which means that it can form in temperatures and pressures less than other types of metamorphic rocks. Low grade metamorphic rocks typically are very fine grained. Slate can be many different colours, often black, blue, green, red and brown. It is smooth to the touch.
Schist
If more pressure and temperature are applied to a slate, it could turn into schist, which does have visible layers of minerals. Schist is characterised by coarse grained foliation (layers or bands.) The minerals (often mica crystals) are large enough to be seen with the naked eye. Schist is therefore a medium to high grade metamorphic rock, so the grain size is larger (coarser.) The minerals and banding in schist are a lot more visible to the naked eye than in shale. The colour of schist varies, is often shiny, and often alternates in lighter and darker bands. It is usually fairly smooth.
Gneiss
If even higher temperature and pressure are applied, the schist could turn into gneiss, another type of metamorphic rock, which shows visible bands (foliation) of minerals. These bands are light and dark coloured, and are made from alternating felsic and mafic layers. The cleavage is poorly developed here. Like schist, gneiss is a medium to high grade metamorphic rock.
This table should summarise everything!
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.
Questions:
1 Look at the stones. How many can you see?
2 Please identify the tallest one and describe it (dimensions, colour, texture, foliation, grain, any visible minerals, shininess, distinguishing features, etc.)
3 Is there any evidence of slaty cleavage or schistosity?
4 With reference to the lesson and the diagram, please distinguish the type of metamorphic rock you find here. Is it slate, schist or gneiss? What leads you to this conclusion?
5 Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache.
Good luck, and thanks for visiting this EarthCache!
Additional Hints
(No hints available.)