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:
orogeny - the mechanism by which mountains are built on continents
matrix - the finer-grained mass of material in which larger grains, crystals or clasts are embedded. Also known as groundmass.
porphyry - an igneous rock consisting of large-grained phenocrysts in a fine-grained matrix.
phenocryst - a large and distinctly larger crystal larger than the matrix of the rock it is found in
dike - a fracture in the Earth’s surface, that cuts across pre-existing rocks
sill - a flat, sheet-like intrusion formed by magma pushing between pre-existing layers of rock
viscous - a thick, sticky consistency between liquid and solid
mantle - part of the Earth, located underneath the crust
differentiation - the various processes by which magma is chemically altered in processes like melting, cooling or erupting.
The RAF Bomber Command Memorial commemorates the sacrifice of 55,573 aircrew from Britian, Canada, Czechoslovakia, Poland and Commonwealth countries who died during the Second World War. It was designed by architect Liam O’Connor and was unveiled by the Queen on 28 June 2012, the year of her Diamond Jubilee. The plinth of the memorial is inscribed with the following text by Pericles: ”Freedom is the sure possession of those alone who have the courage to defend it.”
The memorial is of interest to geologists because it is quite new, and is surrounded by fresh Portland Stone, that has not been significantly weathered. However, for the purposes of this EarthCache, we are focusing on the plinth of the memorial, upon which the bronze sculpture stands.
The bronze sculpture inside the memorial is by the artist, Phillip Jackson, and depicts a flight crew. The plinth is made of a purple porphyry that was mined in Egypt. Several different types of purple porphyry have been mined in Egypt for centuries. Egyptian porphyry was highly valued by the Romans for its colour, and the Roman-era quarries there are now considered archaeological ruins. The rock here, formed from lava that erupted during the Pan-African Orogeny between 680-550 million years ago, is sometimes called ‘Imperial Porphyry.’ It is still mined today, from mountains west to the town of Hurghada, and this is where the rock in this plinth came from.
OK, let’s break it down a little...
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.
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.
Igneous rock is formed within the Earth’s crust, or on it’s surface. It is formed by the cooling of magma (molten rock.)
All three types of rock make up the Earth’s lithosphere, the outermost layer. The lithosphere averages about 100 kilometres in thickness.
More on igneous rocks
Igneous means ‘resulting from fire, produced by fire or resembling fire.’ Inside the Earth’s core there are a lot of molten rocks and molten minerals. These melt due to very hot temperatures inside the Earth. The melted material is called magma. It puts a lot of pressure on the Earth’s surface. Sometimes, the pressure breaks the surface, and this magma comes out. When it comes out, it is called lava. Some of the magma can’t come out, and it settles inside the Earth. Whatever magma cools inside the Earth changes into intrusive igneous rocks, such as granite. Lava that comes out and is cooled by the atmosphere forms extrusive igneous rocks, such as basalt.
Therefore, there are two types of igneous rocks: intrusive (from magma cooling inside the Earth’s surface) and extrusive (from lava cooling on the Earth’s surface.)
Dacite
The plinth of the memorial is made of an igneous rock called dacite. Dacite usually forms as an intrusive igneous rock, often in a dike (a fracture in the Earth’s surface) or a sill (when the magma is pushed between pre-existing layers of rock.) Dacite formations can be found all over the world. However, it can be formed in other ways. The magma that forms dacite only has a moderate silica content, which makes it quite viscous and therefore prone to explosive eruption in a volcano. Mount St Helens is a famous example of a volcanic eruption in which dacite domes formed from previous eruptions. Lava (pyroclastic flows) can also have a dacitic composition, for example the Fish Canyon Tuff or La Garita Caldera.
Dacitic magma forms when a geologically young oceanic crust is pushed (subducted) under an older continental plate, which is thicker and felsic (rich in elements that form feldspar and quartz.) The heat of this collision partially melts the younger oceanic plate and hydrothermally alters it, adding quartz and sodium. The subducted crust also interacts with the upper mantle through convection and dehydration reactions. Minerals such as talc, serpentine, mica and amphiboles are broken down. As the magma rises, chemical reactions (differentiation) continue, making it even more rich in sodium and silica. Once it has risen and begins to cool, this sodium-rich magma causes crystals to grow. The minerals that crystallise include plagioclase, quartz and hornblende. Along with other minerals found in this forming dacite gives geologists information about how the magma was formed. Geologists can also learn about the connection between oceanic and continental crusts.
Dacite consists mostly of plagioclase feldspar with biotite, hornblende, and pyroxene. Quartz can be found, either within the matrix or as rounded, corroded phenocrysts.
Dacite is often coloured grey, pale brown or yellow, with white feldspars, glassy quartz and black biotite and hornblende crystals. Some dacite, especially that which contains more pyroxene, is darker. The colour of dacite may also be affected by the presence of other minerals, such as appearing red/pink in the presence of the iron oxide mineral hematite.
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1. The plinth of the memorial is made of dacite. How was the dacite formed millions of years ago?
2. What per cent of the rock is made up of the grey, glassy quartz, and what per cent is made up of the white feldspar?
3. What mineral makes up the darker phenocrysts? What per cent of the rock does this darker mineral make up?
4. What colour is the matrix? Is this usual for dacite? What is causing this?
5. How many members of the flight team are depicted in the sculpture?
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!