The learning point of this EarthCache is to get the geocacher to become familiar with igneous rocks and identify which type the memorial is made from.
Everything you need to answer the questions is available by attending the co-ordinates and 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.
Welcome to the African and Caribbean War Memorial!
On 22 June 2017, a memorial to the African and Caribbean soldiers of World War One and World War Two was unveiled in Windrush Square, Brixton. It is believed to be the only national memorial to African and Caribbean service personnel in the United Kingdom.
An estimated two million African-Caribbean soldiers fought in both world wars. Countries across the Caribbean and Africa were profoundly affected during the world wars, sending manpower, materials, and funds to aid the war effort. More than 165,000 troops from the African continent alone died during the conflicts. The memorial had previously been on display at the Black Cultural Archives in Brixton in 2014. The unveiling and dedication ceremony, which marked Windrush Day, was attended by war veterans, Commonwealth High Commissioners, the UK Defence Secretary, the Mayor of London and other dignitaries.
The Voice newspaper reported, "Proceedings opened with instrumental offerings from the Ministry of Defence military band, London All Stars Steel Orchestra and African and Caribbean drummers. These were followed by neutral prayer, delivered by two children, as well as a Christian prayer and a Muslim one. A libation was given by Professor Gus John, a priest of African traditional religious persuasion – an ode to the diverse beliefs nurtured across this country. The ceremony began with a spectacular, traditional military salute and a display of flag and ensigns for each regiment by Commonwealth defence representatives of land, air and sea. This was followed by African commemorative war music and dance. Medals were given out to the ex-servicemen and women who made extraordinary contributions to the war effort and public life."
The sculpture, in Windrush Square, Brixton, was designed by Jak Beula of the Nubian Jak Community Trust. It is formed of two 6ft (1.8m) long obelisks, with a combined weight of just under five tonnes and is inscribed with the name of every African and Caribbean regiment that served in the two wars.
Each of the two obelisks is made of Scottish whinstone and weigh approximately 0.75 tons. The sculpture also includes a pyramid-shaped plinth made of Ancaster stone which weighs approximately 1.5 tons.
Whinstone is not an example of a rock per se, but is more a term used in the quarrying industry to describe any rock that is hard and dark-coloured. It is called whinstone because ‘whin’ is the sound made when the rock is hit by hammers. The rock that these obelisks are made of was mined in the Pentland Hills and is igneous in origin.
Igneous rocks, as opposed to sedimentary and metamorphic rocks, is 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.
Underneath the earth’s surface and closer to the heat source deep underground, the magma cools slowly. Because of this, when you have rocks that cooled from magma, you can easily see the different mineral crystals in the rock. The crystals are relatively large. These rocks are intrusive igneous rocks because they cooled slowly 'inside' the earth. These type of rocks are called (defined as) phaneritic because of their crystal texture. In other words, phaneritic rocks are comprised of large crystals that are clearly visible to the eye with or without a hand lens or binocular microscope.
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. Rocks that cooled quickly 'outside' the Earth are called (defined as) aphanitic because of their crystal texture.
Rocks that are have large and small crystals mixed together (mixed phaneritic and aphantic) are called porphyritic.
The colour of an igneous rock is related to the colours of the minerals present. Rocks that are rich in silica usually contain considerable quartz and light-coloured feldspar. In general, the darker the rock, the more coloured minerals they contain. Rocks rich in magnesium and iron tend to contain olivine, pyroxene, amphibole, and biotite. These minerals tend to be much more darkly coloured. A rock that contains less than 30% coloured minerals is considered light and may be called felsic (from feldspar and silica). With 30-60% colored minerals, it is called mafic (from magnesium and ferro, meaning iron) and with greater than 60%, ultramafic. If there are holes evident in the rock, the rock is referred to as vesicular.
Key words from this lesson:
- phanertic - rocks containing relatively large crystals
- aphantic - rocks containing relatively small crystals
- porphyritic - rocks containing a mixture of large and small crystals
- vesicular - rocks with visible naturally-occurring holes present
- felsic - a light coloured rock that contains less than 30% coloured minerals
- mafic - a dark rock containing 30-60% coloured minerals
- ultramafic - a dark/green rock containing more than 60% coloured minerals
To log this cache, please visit the memorial at 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:
- Look at the crystals in the obelisk. To identify the speed the rock cooled, please measure the average length and width of the crystals.
- Describe the colour and type of mineral in the whinstone that has been used.
- Are the crystals in the obelisks phanertic, aphantic or porphyritic?
- Are the obelisks felsic, mafic or ultramafic?
- With reference to the diagram, identify which type of igneous rock the obelisks are made of.
- Optional, take a photo of yourself and/or your GPS in the general area of this earth cache.
Good luck, and thanks for visiting this EarthCache!
