Welcome to my Earthcache! An Earthcache is a special type of geocache where there is no container to find - instead you are looking for a unique geological feature of the area and need to answer questions, as well as posting a picture, in order to claim the find. The goal of this Earthcache is to educate visitors about gneiss, how it was formed, and what makes it different than other forms of granite. All observations can be made from the cemetery roadway near the specimens you will be studying. Parking is available in the immediate vicinity of the location, but please do not block traffic in the cemetery.
EARTHCACHE REQUIREMENTS
As with all of my ECs, I am not looking for PhD thesis level responses, but I am hoping that you take some time to enjoy the area and learn something new. Please include a list of all cachers with your answer, if answering for more than one caching name. There is no need to send individual answers.
To claim a 'find' for this Earthcache you must answer the following questions and send your answers in a message or email to the owner using the link at the top of the page. You can log your find with a photo at GZ. Send your answers to the tasks. I will be in contact if there is a problem, no need to wait for a response as long as the required photo is included in your log.
Observational Task
Within the Burford Pioneer Cemetery, you will be visiting three monuments - Hepp, Tota and Malcolm. After visiting each of the three stones, you will need to answer the following questions about your observations, and compare the stones as per the below:
Questions to Answer
1. Based on your observations and the reading, which of the three monuments show the greatest percentage of mafic material in it's composition? Which ones shows the highest percentage of felsic material?
2. Based on your observations, which of the three highlighted monuments underwent the greatest shearing forced during the metamorphic process? Why do you feel that this stone underwent the greatest forces?
3. If we assume that the stones are in their natural position (the top of the monument would have been the top of the rock within the earth's crust as it was developed), which direction do you think the nonhydrostatic stress was applied to the stones as they were developing?
4. Mandatory: Include a photograph of yourself, your GPS, a signature item, thumbs up, etc at the monument. You do not need to show your face in the photo, but your photo must be unique to you. If you are caching with a group you can use the same photo, but each log must upload a photo.
Geology Lesson
What is Gneiss?
Gneiss (pronounced "nice") is a type of metamorphic rock that changes a lot over time. It starts as other rocks, like rocks formed from volcanoes (igneous) or rocks made of sand and mud (sedimentary). Then, it gets squeezed and heated deep inside the Earth. This makes it change into gneiss. This rock is formed under pressures ranging from 2 to 15 kbar, sometimes even more, and temperatures over 300 °C (572 °F). Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage. In other words, it is a metamorphic rock composed of mineral grains easily seen with the naked eye, which form obvious compositional layers, but which has only a weak tendency to fracture along these layers
- How it forms:
- Gneiss forms when rocks are heated and cooled, causing changes to the chemical composition of the rock
- It needs a lot of pressure and heat to change, and may be changed by single indicents (such as volcanos) or over many millions of years
Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, or sedimentary gneiss. The gneiss you are looking at in this cemetery are all granite gneiss. Gneiss rocks may also be named after a characteristic component such as garnet gneiss, biotite gneiss, or albite gneiss, which are named based on the primary minerals that copmpose the rock.
There are two main types of gneiss - Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock.
- What it looks like:
- Gneiss has stripes or bands of different colors.
- It's made of tiny bits of minerals you can see.
- It doesn't break evenly like some other rocks.
There are two basic types of metamorphic rocks.
Foliated metamorphic rocks such as gneiss, phyllite, schist, and slate have a layered or banded appearance that is produced by exposure to heat and directed pressure.
Non-foliated metamorphic rocks such as hornfels, marble, quartzite, and novaculite do not have a layered or banded appearance.
Gneissic Banding
The minerals in gneiss are arranged into layers that appear as bands in cross section. The darker bands have relatively more mafic minerals (such as magnesium and iron). The lighter bands contain relatively more felsic minerals (such as feldspar or quartz, which contain more of the lighter elements, such as aluminium, sodium, and potassium).
The banding is developed at high temperature when the rock is more strongly compressed in one direction than in other directions (nonhydrostatic stress). The bands develop perpendicular to the direction of greatest compression, also called the shortening direction.
A common cause of nonhydrodynamic stress is the subjection of the protolith (the original rock material that undergoes metamorphism) to extreme shearing force, a sliding force similar to the pushing of the top of a deck of cards in one direction, and the bottom of the deck in the other direction. These forces stretch out the rock like a plastic, and the original material is spread out into sheets.
Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers of sandstone (lighter) and shale (darker), which is metamorphosed into bands of quartzite and mica.
- Where it's found:
- You can find gneiss in very old parts of the Earth's crust.
- Some of the oldest rocks on Earth are gneiss.