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 mafic enclaves, how they differ from xenoliths, why these inclusions often appear to “rust,” and why they tend to form in granites with large phenocrysts rather than the more uniform commercial varieties such as Stanstead or Barre granite. All observations can be made from the grassy area at the posted coordinates with Crown Cemetery in Puslinch, and parking is available nearby without disturbing any gravestones. This cemetery is open daily from 8:00am until dusk, 7 days per week.
EARTHCACHE REQUIREMENTS
As with all of my ECs, I am not looking for PhD thesis level responses, but I am hoping you take some time to enjoy the area and learn something new. Please include a list of all cachers if you are logging as a group. You may log your find immediately as long as the required photo is included; I will contact you if there is a problem with your answers.
Observational Task
At GZ, you will notice that the granite monument for John Calle contain dark, rounded or irregular patches that contrast sharply with the lighter pinkish granite around them. These dark zones are mafic enclaves. Take a moment to look closely at them, noting its shape, texture, and colour, and how it interacts with the surrounding granite. You should be able to see clear differences between the fine-grained dark material and the coarser, light-coloured granitic crystals that host it.
Questions to Answer
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Describe one enclave you see at GZ. What colour is it, what shape does it have, and do you notice any rusty or reddish staining within it or on the surrounding stone?
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Compare the enclave to the granite around it. How do the crystal sizes differ?
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Based on the geology lesson, explain why the inclusion is a mafic enclave rather than a xenolith.
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Why might this enclave appear rusty or weathered compared to the surrounding granite?
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Mandatory Photo: Include a picture of yourself, your GPS, a signature item, or another personal marker at the monument. Faces are not required, but each cacher must include a photo unique to them.
Geology Lesson
Mafic enclaves are dark, fine-grained blobs or patches of mafic magma that became trapped within cooler, lighter-coloured felsic magma such as granite. Unlike xenoliths – which are solid fragments of pre-existing rock torn off and incorporated into a melt – mafic enclaves usually begin as droplets of still-molten or partially molten mafic magma. When these hotter, iron- and magnesium-rich magmas intrude into cooler granitic magma, they chill rapidly and solidify into distinct dark patches. Because both magmas are still fluid during this process, the boundaries of enclaves often appear rounded or subtly mingled rather than sharply angular.
Xenoliths form in a very different way. They are older pieces of solid rock that are physically broken off from the surrounding country rock and incorporated into a magma chamber. Because they were never molten during mingling, they usually preserve sharper, more angular outlines and exhibit little to no blending with the host magma. In contrast, the enclaves seen here are smoother and more globular, showing that they formed from magma mixing rather than physical fragmentation.
The chemical composition of mafic enclaves explains their dark appearance and their tendency to weather differently from the surrounding granite. Mafic magmas are typically rich in iron, magnesium, and minerals such as amphibole, pyroxene, and biotite. These iron-bearing minerals oxidize when exposed to air and moisture at the Earth’s surface, creating reddish-brown stains that resemble rust. This oxidation is not evenly distributed in the granite because the granitic host rock contains far less iron and therefore weathers more slowly and more uniformly. This contrast makes the enclaves much more visually prominent over time.
The granites that host enclaves also tell a story of a complex cooling history. This particular stone contains large, well-formed feldspar phenocrysts, indicating that the granite cooled in at least two stages: a slow, deep stage that allowed large crystals to grow, followed by a faster, shallower stage. Such interrupted or multi-stage cooling histories create ideal conditions for the injection and mingling of mafic magma. In contrast, more uniform granites (typically found in commercial applications such as statues, building slabs, and countertops) like Stanstead and Barre cooled more uniformly without late-stage magma mixing, therefore would be more prone to xenoliths, where the rock inclusions do not become magma and remain in their solid form. As a result, they are more homogeneous, lack large phenocrysts, and rarely contain enclaves.
AI Content Disclosure
Some of the descriptive text and/or images on this page were created with the assistance of artificial intelligence tools. All information has been reviewed, verified, and edited by the cache owner for accuracy and clarity.
This cache was placed by a PROUD Platinum Earthcache Master.
