This is an urban EarthCache in which geocachers are invited to examine the wall cladding of a building. The learning point of this EarthCache is to get the geocacher to become familiar with gneiss, and to learn more about ductile flow.
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.
Keywords for this lesson
orthogneiss - gneiss formed by the metamorphism of igneous rocks
paragneiss - gneiss formed from the metamorphism of sedimentary rocks
foliated - layered
regional metamorphism - metamorphism affecting rocks over an extensive area as a result of the large-scale action of heat and pressure
convergent plate boundaries - a tectonic boundary where two plates are moving towards each other
high-grade rock - metamorphic rocks formed under extreme temperature and pressure
At GZ you will find a building clad with a fairly spectacular rock. This rock is sold commercially as Kashmir Granite, but in fact is not granite at all. Neither does it come from Kashmir. It is in fact a different kind of rock, called gneiss, and is quarried in Tamil Nadu, southern India. It is an unusual and striking example of a gneissic rock, which we’ll now learn a bit more about.
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.
Gneiss is a high-grade, foliated metamorphic rock, which is usually distinctive due to its black and white banding. This banded appearance and texture, rather than its composition, is what defines a gneiss. The principal minerals in gneiss, quartz and feldspar, occur together in layers which are separated from each other by thin drawn-out bands of mica. A gneiss usually has a light colour, although this is not necessarily the case. 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; that is to say, the metamorphic process occurring on rocks over a wide area along tectonic plates moving towards each other.
Gneiss is a metamorphic rock, but the protolith (original rock that was metamorphised) might be igneous or sedimentary. Orthogneiss is formed by the metamorphism of igneous rocks; paragneiss results from the metamorphism of sedimentary rocks. Unfortunately, in the absence of anything to compare it to at GZ, and in the absence of specialised geological equipment, it is difficult to say which is found here.
Since gneiss is a rock type that is defined by its oriented texture, rather than by its mineralogy or chemical composition, qualifying terms are often added to the rock name. Various varieties of gneiss have received distinctive names, most of which are self-explanatory. Geologists describe banded-gneiss, lenticular gneiss, biotite-gneiss, hornblende-gneiss, granite-gneiss, diorite-gneiss, and syenite-gneiss, among others. With these specialised terms, the composition is indicated by the first word, and the texture by the second.
Gneiss usually doesn't split along planes of weakness like many other metamorphic rocks. 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 cladding stone of this building. (This might be best done on the wall closest to Penrhyn Avenue.) Please describe the pattern of the bands in the gneiss.
2. What are the colours and widths of the largest adjacent bands you see?
3. Describe the rock. You can refer to colour, grains, the regularity of the banding, the typical thickness of light vs dark bands, or anything else you wish to comment on.
4. How do you know this is a gneiss? Is it a good example?
5. Is there evidence of more than deformation episode, and if so, what is the evidence?
6. In the gneiss, there are numerous large crystals. These are garnets. Please describe their size and colour.
7. 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!
