That's a Gneiss Variation! EarthCache

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. This EarthCache helps visitors recognize how minerals, pressure, temperature, and time interact to create one of Earth’s most striking metamorphic rocks. The colours in gneiss aren't just beautiful—they're geological clues, revealing the intense processes that occur deep within Earth's crust. This EarthCache will encourage observation, interpretation, and an appreciation of how dynamic our planet really is by viewing four different gneiss headstones in Resurrection Catholic 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, but note that each cacher must upload their own photo to their log.

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.

1. Describe the Gneiss: Within 10m of GZ, there are 4 different gneiss stones - Vieira, Noonan, Gallant and Vieira. What is the texture and appearance of the four gneiss stones? Are the stones clearly banded or faintly layered?

2. Identify the Colours: List at least two distinct colours you see in the four different gneiss stones. Are the bands sharp or blurry?

3. Link Colour to Minerals: Based on your observations and the information in the description, suggest which minerals are responsible for the light and dark bands at this site.

4. Speculate on Formation: What type of gneiss do you think these are (orthogneiss, paragneiss, augen gneiss)? What clues led you to that conclusion?

5. Photo: Mandatory: Include a photograph of yourself, your GPS, a signature item, thumbs up, with one of the stones visible in the background. You do not need to show your face in the photo, but your photo must be unique to you. Each log must include their own photo. 

Earth Science Lesson:

Gneiss is a high-grade metamorphic rock that forms under extreme heat and pressure, transforming pre-existing rocks (protoliths) into a distinctly banded rock. The banding in gneiss results from the segregation of mineral phases during recrystallization, leading to alternating layers of light and dark minerals.

Types of Gneiss:

There are several classifications of gneiss, often based on origin or composition:

1. Orthogneiss – Derived from igneous parent rocks such as granite.

2. Paragneiss – Formed from sedimentary parent rocks like shale or sandstone.

3. Augen Gneiss – Contains large, eye-shaped mineral grains ("augen" means "eyes" in German).

4. Heterogeneous Gneiss – Features irregular banding and mixed mineral compositions.

Colour Variations in Gneiss – What Do They Mean?

The colours in gneiss are not random; they reflect its mineral composition, which in turn tells a story about the rock’s origin and the conditions of metamorphism.

1. Light-Coloured Bands

Typically composed of:

• Quartz – grey to translucent

• Plagioclase feldspar – white to grey

• Potassium feldspar (orthoclase/microcline) – pink to salmon-red

These minerals are generally felsic (silica-rich), which are stable at high temperatures and pressures but do not contribute dark colours.

2. Dark-Coloured Bands

Typically contain:

• Biotite mica – dark brown to black

• Amphibole (hornblende) – dark green to black

• Pyroxene – black or very dark green

These minerals are mafic (magnesium- and iron-rich), and their darker colours reflect their chemical makeup. Mafic minerals concentrate into bands due to mineral segregation during metamorphism.

3. Reddish or Pink Tones

These often result from:

• High potassium feldspar content (orthoclase, microcline)

• Iron staining or oxidation in fractures or along cleavage planes

• Granite protoliths, which are rich in pinkish feldspar

4. Greenish Tints

Indicative of:

• Chlorite – often forms during retrograde metamorphism (as rock cools)

• Epidote – common in medium-grade metamorphism

• Actinolite or amphibole – part of the greenstone facies
  These green minerals often form when mafic rocks like basalt are metamorphosed.

5. Blue or Grey Tones

Rare, but possible with:

• Glaucophane – a blue amphibole found in blueschist facies rocks

• Graphite – in carbon-rich gneiss, may give grey to steel-blue sheen

Why the Colours Vary:

The specific combination and abundance of these minerals depends on:

• The composition of the original rock

• The temperature and pressure of metamorphism

• The presence of fluids during metamorphism

• Chemical reactions that occur as minerals become stable under new conditions

Thus, every band and colour in gneiss is a record of geological history—a physical fingerprint of deep Earth processes.

This cache was placed by a PROUD Platinum Earthcache Master.