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, what makes them unique, how they form, and how to recognize them in porphyritic igneous rocks. All observations can be made from the HENRY K. DZIEKANSKI monument found in Marymount Cemetery, just north of Guelph. Parking is available in the immediate vicinity of the location.

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.

At GZ, you will be standing in front of a large, distinctive gravestone monument built from a highly porphyritic granite. Notice the unusually large crystals that make up this monument. These oversized crystals are phenocrysts—the star feature of porphyritic rocks.

Questions to Answer

1. Observe the various colours of the monument. What colour are the largest crystals (phenocrysts), and how big is the largest one you can identify?

2. Have a look at the unpolished top of the monument, and the polished back of the monument. Do the phenocrysts look different in their unpolished and polished states? What do you notice about the unpolished phenocrysts that help you to identify what they are?

3. Based on the information in the cache page and your observations, do you feel this monument is a good example of a rock containing phenocrysts? Why or why not?

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, but the photo must be unique to you. If caching with a group, you may use the same photo, but each log must upload a copy.

Geology Lesson:

The word porphyritic describes igneous rocks that contain a noticeable difference in the size of the crystals that make up the rock. The large crystals are called phenocrysts, and the much smaller background crystals form the groundmass or matrix. Most porphyritic rocks have a bimodal texture—two distinct crystal sizes—with one mineral growing significantly larger than the others. Some examples even contain multiple types of phenocrysts.

How Phenocrysts Form (Expanded & Updated)

Porphyritic rocks form through a two-stage cooling process known as igneous differentiation. This happens when rising magma cools under two very different rates:

Stage 1 – Slow Cooling Deep Underground (Phenocryst Formation)

Deep within Earth’s crust, magma cools extremely slowly over thousands to millions of years. This slow cooling allows certain minerals—usually those with higher melting points, like feldspar, pyroxene, or olivine—to crystallize first.
Because they have so much time to grow, these early-formed crystals can reach sizes of 2 mm to several centimetres, producing the large, well-shaped phenocrysts seen in porphyritic rocks.

This process is driven by fractional crystallization. As each mineral crystallizes out of the melt, the chemistry of the remaining magma changes. The early-formed crystals may begin to settle if they are denser than the melt, forming deposits called cumulates. However, if magma movement, eruption, or changes in density occur, these large crystals become suspended in the melt instead of settling.

Stage 2 – Rapid Cooling Near the Surface

If the magma rises quickly or erupts as lava, the remaining melt cools much faster. Rapid cooling prevents large crystal growth, producing the fine-grained groundmass surrounding the phenocrysts.

This dramatic difference between slow and fast cooling produces the classic porphyritic texture:
big crystals frozen inside a much finer matrix.

Porphyritic Rocks and Their Significance

Porphyritic texture can occur in both volcanic rocks (like basalt or andesite) and plutonic rocks (like granite). Even magma that never erupts can become porphyritic if its cooling rate changes underground.

The presence, size, and abundance of phenocrysts help geologists classify rocks:

• Aphyric: No phenocrysts (or <1% by volume)

• Phyric / Porphyritic: Contains phenocrysts

• Named by dominant phenocryst mineral: such as K-feldspar-phyric granite

• 

The stone before you is an example of K-feldspar-rich porphyritic granite, recognizable by its oversized salmon-pink feldspar crystals.

The importance of this cooling history was first recognized by Canadian geologist Norman L. Bowen in 1928.

About this area:
Marymount Cemetery is an active cemetery in Wellington County. If there is an active funeral or celebration of life in progress, PLEASE come back at another time. Don't be the reason that caches are banned from cemeteries in the area. This cache can be accessed from 8:00am until dusk, sever days per week. 

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.