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Xenoliths EarthCache

Hidden : 03/31/2019
2 out of 5
2 out of 5

Size: Size:   other (other)

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Geocache Description:

This EarthCache is located within an area frequently patrolled by Police. Please avoid acting suspiciously 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 rocks visible to everyone.


This is an urban EarthCache in which geocachers are invited to examine the plinth of an equestrian statue in central London. The learning point of this EarthCache is to get the geocacher to become familiar with xenoliths and xenocrysts.
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:

igneous intrusion - igneous rocks that form when magma cools and solidifies before it reaches the surface.
ophiolite - an igneous rock formed from the submarine eruption of oceanic crustal and upper mantle material.

Please note: this EarthCache invites you to examine the plinth of the equestrian (sat on a horse) statue of the Duke of Wellington.

Please don't confuse it with the Wellington Monument, which is only 150 metres away!

This statue portrays the hero, the Duke of Wellington, on a campaign mounted on his horse Copenhagen. The statue itself was cast in bronze by the sculptor Joseph Boehm and was constructed between 1884 - 1888. The figures at the corners of the pedestal representative different types of soldiers from around the U.K.: a Grenadier, a Scottish Highlander, an Irish Dragoon and a Welsh Fusilier.

The statue faces toward Apsley House, which was the Iron Duke's London home. The bronze it was cast from was made from melted-down French cannons.

The plinth, which is the subject of this EarthCache, is made from pink Peterhead Granite. Peterhead Granite comes from Stirlinghill Quarry, near Boddam, Aberdeenshire, Scotland. Significant quarrying began in the late 18th century and continued until 16 June 1956. Peterhead Granite was a popular stone used internationally throughout the 19th Century for monuments, architectural elements and facing stone.

The ‘pink’ variety of Peterhead Granite is coloured by orthoclase feldspar, together with smoky grey quartz and black hornblende. It is one of the Caledonian ‘Newer Granites’ and was formed around 400 million years ago. The plinth is also made of grey Rubislaw granite from Aberdeen, which is around 470 million years old. Aberdeen granite. water, the base of the hole lies below sea-level. The quarry lies within the City of Aberdeen and in the past supplied much of that city’s building stone. From looking at the small exposures around the rim the quarry it is clear that the stone is quite variably foliated but it is typically dark grey in colour and also includes xenoliths of the local schistose country rock.

In both the pink and grey sections of the plinth, there are some darker patches of a rock that does not match the granites. Geologists refer to these as 'xenoliths.'

Xenolith means “foreign rock” coming from xenos (foreign) and lithos (stone) in Ancient Greek. A xenolith is a fragment of foreign rock within a host rock. Xenoliths are different from the rock they are found within. If they are similar in nature, then these rocks are called autoliths or cognate inclusions.  True xenoliths are always older than their host rocks because they had to already exist as a solid rock fragment when the magma or lava around them solidified. This is not always the case with cognate inclusions.

A xenolith is often a rock that was embedded in magma as it was cooling. Magma is the molten rock beneath the Earth’s crust. If the magma rises above the Earth’s surface, for example during a volcanic eruption, it is referred to as lava. Lava is the extrusive equivalent of magma.

Igneous rocks form from magma and lava that has cooled. Xenoliths are different types of rock that are often embedded in igneous rock. Depending on the foreign rock material, the composition of the magma or lava, the temperatures, available time, and many other factors, the xenolith chemically equilibrates with the host rock to varying degrees.

Xenoliths are usually easy to recognise because they are often visibly different in composition, density and colour from the encompassing rock. Xenoliths are generally small in size, relative to the overall body of rock. However, xenoliths can range in size from single crystals (xenocrysts) to rock fragments of several metres. The large-scale inclusion of foreign rock strata at the margins of an igneous intrusion is called a roof pendant.

There are a number of possible sources for xenoliths and xenocrysts found at the Earth’s surface. Magma can intrude into pre-existing rocks (sometimes called country rocks) and as it does so, might pick up fragments of this country rock. Xenoliths can also be formed from fragments of the walls of a magma chamber or conduit. Xenoliths can also be picked up by lava during explosive volcanic eruptions, or can  be picked up by lava as it flows along Earth’s surface. These fragments, trapped in the magma or lava but not melting into it, become xenoliths.

A visual representation, if that makes it any clearer!

As magma erupts or flows from the Earth’s surface, it is cooled by exposure to the atmosphere or water. Lava cools fairly quickly, and different types of igneous rocks are formed. Xenoliths and xenocrysts are affected by temperature. A xenolith may lose its unique qualities if it melts into the surrounding magma. As it cools, the material may cease being a xenolith and instead become a metamorphic rock. Metamorphic rock are rocks are formed inside the Earth by temperature and pressure changes that affect existing rocks, (often igneous or sedimentary.)

Xenoliths and xenocrysts are often identified by the names of the two rock types involved. For example, a periodite xenolith in a basaltic lava flow, is a periodite fragment embedded in basalt rock. 

Xenoliths in granite

Xenoliths and xenocrysts provide valuable information about the geology of the Earth’s crust and mantle. Xenoliths can give clues about the origin and evolution of the host rock. For example, when an igneous rock contains a xenolith, geologists know that at some point the magma or lava that cooled to form that igneous rock was in contact with the foreign rock. Xenoliths are also important because they can  allow geologists to sample and study rocks which are difficult to access. For example, the material in the Earth’s mantle is not generally exposed at Earth’s surface, (except at ophiolites,) so xenoliths can give evidence about the composition of Earth’s mantle. Some xenoliths come from very deep within the Earth. For example, diamonds are formed at high pressures and temperatures, very deep within the Earth, and are then brought to the Earth’s surface as xenocrysts in kimberlite rock. Through xenoliths and xenocrysts, geologists can learn about temperature, pressure, constriction and movement within the Earth's surface.

Xenocrysts can also be formed by the mixing of dissimilar magmas, and foreign material can be incorporated late into the magma with minimal contamination. The fate of assimilated xenocrysts depend on their composition, as well as that of the host rock. 

Xenoliths most often appear as fragments of foreign rock in igneous rocks. However, xenoliths can also appear in sedimentary rocks, or even meteorites, that have been involved in collision events outside of the Earth’s atmosphere.


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.  
1. Identify the xenoliths in the plinth. On the side marked WELLINGTON, at the base of the plinth, how many xenoliths can you see?
2. What type of rock hosts the xenolith, (for example, igneous, sedimentary, metamorphic, meteorite?) Please note, this answer is not ‘Granite.’
3. Please choose one xenolith, and describe it's colour, shape and size of the xenolith?
4. Which do you think is older, the rock that makes up the pillar or the xenolith within it?
5. What is Wellington holding in his right hand?
6. 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!

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