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 phenocrysts within granite, what makes these phenocrysts unique from other forms of granite, how these phenocrysts are developed, and what makes them unique. All observations can be made from the grassy area in the cemetery, near the large marble monument. Please do not tread on any headstones while making your observations. Parking is available in the immediate vicinity of the location and it is approcimately a 150-200 ft walk from available parking to view the phenocrysts.
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
At GZ, you will be standing near several large ground-level granite monuments. Notice the size of the crystals that make up these unique monuments. Notice how the shape of the phenocrysts (crystals that make up the granite) are almost perfectly circular and form a very striking pattern. These are the monumenets you will need to examine closely to determine your answers to the questions below.
Questions to Answer
1. Observe the various colours of the monument. What colour are the largest crystals you see, and how big is the largest one you see? What colour is the matrix you see, and are there visible crystals within the matrix, or is it too fine to see with the naked eye?
2. Have a look at 2-3 upright granite monumnets in the immediate vicinity of this monument. Compare the crystal sizes and colours. Are there any other noticable differences that set these prophyritic monument with large phenocrysts apart from the rest of the nearby granite stones?
3. Based on the information in the cache page and your observations, do you feel that this is a good example of phenocrysts? Why or why not?
4. Mandatory: Include a photograph of yourself, your GPS, a signature item, thumbs up, etc with one of the monuments. 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:
The Porphyritic Texture of Igneous Rocks
The granite slab before you displays a porphyritic texture, a term used to describe igneous rocks that contain crystals of noticeably different sizes. In a porphyritic rock, some minerals have grown into much larger crystals called phenocrysts, while the rest of the rock is composed of smaller, fine-grained crystals known as the matrix or groundmass.
In this particular example, you can see white phenocrysts rich in quartz standing out clearly from the darker, finer-grained background minerals. The contrast in crystal size tells a fascinating story about how the rock cooled and solidified from molten magma deep within the Earth.
How Porphyritic Rocks Form
Porphyritic rocks form through a process called igneous differentiation, and they develop in two stages of cooling:
Stage 1 – Slow Cooling Deep Underground:
As magma begins to cool slowly beneath the surface, the first minerals to crystallize form large, well-defined crystals — the phenocrysts you see here. These crystals typically measure 2 mm or more across. In this rock, the large white quartz crystals grew during this early cooling stage.
Stage 2 – Rapid Cooling Near the Surface:
The remaining molten material then cools much more quickly — either at a shallow depth or after erupting at the surface. This rapid cooling produces the fine-grained groundmass that surrounds the phenocrysts.
This two-stage cooling history explains why the rock contains two distinct crystal sizes — a hallmark of porphyritic texture.
Fractional Crystallization and Magma Evolution
The growth of large phenocrysts occurs during a process known as fractional crystallization. As magma cools, minerals crystallize in a specific order based on their melting points. The early-formed crystals often differ in composition or density from the remaining melt. If they are denser, they may settle out, forming layers of crystals called cumulates. However, if an eruption or other disturbance interrupts the process, or if the crystals and melt have similar densities, the phenocrysts remain suspended in the magma.
When the magma finally solidifies, the phenocrysts are “frozen” within the finer matrix, preserving this record of the rock’s complex cooling history.
What Porphyritic Texture Tells Us
The term porphyritic describes a texture rather than a rock type. A porphyritic rock can have almost any mineral composition or color — what defines it is the contrast in crystal size.
Porphyries can be either:
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Aphanitic, where the groundmass is so fine-grained it appears smooth (as in volcanic rocks like basalt), or
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Phaneritic, where both the phenocrysts and the groundmass are visible to the naked eye (as in granitic rocks like the one here).
In this granite, the white quartz phenocrysts stand out clearly within a coarser, darker background of other silicate minerals. This indicates that the magma cooled in multiple stages, with large quartz crystals forming first, followed by the remaining minerals solidifying more quickly around them.
Historical Note
The importance of porphyritic texture as evidence of multi-stage magma cooling was first recognized by Canadian geologist Norman L. Bowen in 1928. His work on magma crystallization, known as Bowen’s Reaction Series, remains a cornerstone of modern igneous petrology.
Naming and Classification
Igneous rocks are often described based on the type and abundance of phenocrysts they contain.
Because the large, light-colored crystals in this stone are quartz, the rock could be referred to as a Quartz Porphyritic Granite — a granite displaying prominent quartz phenocrysts within a finer crystalline groundmass.
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.
