This is an urban EarthCache in which geocachers are invited to examine a memorial plaque and become more familiar with gneiss and ductile flow.
Everything you need to answer the questions is available by visiting the location and by reading this lesson. I don’t think you will have to research anything extra online, although you’re welcome to do so if you wish to.
Keywords for this lesson
orthogneiss - gneiss formed by the metamorphism of igneous rocks
paragneiss - gneiss formed from the metamorphism of sedimentary rocks
foliated - layered
At GZ you will find a memorial plaque to Richard St. Barbe Baker OBE, who was born here in the village of West End in 1899. The memorial was installed here in 2013. There is more information about the memorial plaque after the Earth science lesson below.
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. Metamorphic rocks are formed inside the Earth by temperature and pressure changes that affect existing rocks.
The type of rock around the bronze portrait is called gneiss. Gneiss is a high-grade metamorphic rock, meaning it was formed under extreme heat and pressure. Gneiss is foliated and is usually quite distinctive due to its banding. This banded appearance and texture, rather than its mineral composition, is what defines a gneiss. The principal minerals in gneiss are quartz and feldspar. They occur together in layers which are separated from each other by thin drawn-out bands of mica. A gneiss often has a light colour. The bands are of varying composition. The bands are formed under the extreme (high-grade) pressure when the mineral grains recrystallise and separate them into bands, which makes them more stable in that environment. Gneiss forms in temperatures above 320°C.
Gneiss is often formed by regional metamorphism at convergent plate boundaries. This means it is formed by the metamorphic process occurring on rocks over a wide area along tectonic plates that are moving towards each other.
Gneiss is a metamorphic rock, but the protolith (the original rock that was metamorphised) could be igneous or sedimentary. Orthogneiss is formed by the metamorphism of igneous rocks; paragneiss results from the metamorphism of sedimentary rocks. As a very general rule, orthogneiss often has a coarser-grained background matrix whilst parageniss often might have thinner flow bands and a finer-grained background matrix.
Gneiss usually doesn't split along planes of weakness, like many other metamorphic rocks do. This makes it useful in road construction, dimension stones or if brightly polished, architectural stone.
The dark minerals in gneiss sometimes show an orientation determined by the pressures of metamorphism. Multiple orientations might show evidence of multiple deformation episodes.
Ductile flow
Rocks along active plate boundaries are subjected to physical stress. They can experience squeezing (compression), stretching (tension), or pushing in different directions (shear stress). How rocks respond to physical stress depends on the type of stress, the rate at which it is applied, and the environmental conditions of the rocks, such as their temperature and depth.
Generally, rocks respond to stress in one of two ways: they break, or they bend.
Deformation is the change in shape or position of a rock body in response to stress.
When a rock breaks, it is called brittle deformation. When rocks bend or flow, it is called ductile deformation, or ductile flow.
When rocks deform in a ductile manner, they may bend or fold, and the resulting structures are called folds. Folds result from compressional stresses or shear stresses acting over considerable time. Because the strain rate is low and the temperature is high, rocks that might be considered brittle can behave in a ductile manner and folds can form.
The banding in gneiss is evidence of the folding and deformation associated with ductile flow. In some gneisses, the banding can extend for long distances in straight, regular lines. In others it can be curved, folded, contorted, or faulted, often in a complex and beautiful manner. Examples of such intricately folded and compressed gneisses is evidence of repeated dynamic movements.
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. Please look at the stone that the memorial plaque is set onto. Please describe the pattern of the bands in the gneiss (think about the thickness, shapes, colourations, crystal size, or anything else you would like to comment on.)
2. Do you think this is gneiss or schist? How did you come to this conclusion? Is it a good example? Please let me know what evidence you have used to reach that conclusion.
3. Thinking about your answer to task 1 (the banding,) is there evidence of more than deformation episode, and if so, what is the evidence?
4. From your observations, do you think this is an orthogneiss or a paragneiss? Please explain your answer.
5. Please take a photo of your or your GPS at GZ.
Good luck, and thanks for visiting this EarthCache!
The memorial project cost £180,000, with the bronze bas-relief made by sculptor Jill Tweed. She also produced "The Railwayman" statue in Eastleigh town centre and the bronze "Angel of Mons" on the town's War Memorial.
Mr Barbe Baker was a forestry adviser, silviculturist and Founder of the Men of the Trees in 1922. Today, this organisation is known as the International Tree Foundation.
In the 1920s, he became the Assistant Conservator of Forests in Kenya and Nigeria and was concerned about the rapidly decreasing fertility of the land. He believed it was necessary to plant more trees to combat the shifting methods of agriculture and he initiated the idea of voluntary tree planting by local communities.
In his later years, Mr Barbe Baker travelled extensively around the world, lecturing and writing to convey his message about the importance of trees. Through his determination and energy he influenced the protection of the giant redwoods in California and the replanting of large areas in northern Africa. He also worked to stimulate a new world-wide direction for forest conservation.
In 1978 Mr Barbe Baker received an OBE for his work. He planted his last tree in Saskatoon, Saskatchewan, Canada on 5th June 1982 in a ceremony celebrating World Environment Day, and died four days later.
