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. At this location, you will be observing the bricks of the entrance gate to the Burford Agricultual Society Fairgrounds. Take a moment to feel them - some are smooth and hard, while others are bumpy and look like they are full of small holes! For the purpose of this earthcache you're going to want to focus on the stones within the gate that look like they are full of small holes - these vesicles are the topic of this earthcache. This earthcache will review why they exist, how they were formed, and what they tell us about the rocks that display them.
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.
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.
Questions to Answer:
1. Describe the mineral grains you see in the rocks that contain vesicles. Compare them to the mineral grains of the other rocks that don't have as many or any vesicles presents. You can compare the colour, size, texture, or "shininess". Based on your comparison do you think these are the same type of rock, formed using the same process?
2. Focus on one specific rock within the wall with vesicles present. How many (rough count) are visible on the surface of the stone, and what is their average size (estimate in mm or cm)?
3. Based on your observations, would you expect this granite to have formed deep underground or relatively shallow (closer to surface)? Explain using evidence from texture and vesicle presence or absence.
4. Photo Requirement: Take a photo of yourself (or just your GPS device, or your geocaching name on a piece of paper, or your signature item, a thumbs up) with the stone entrance in the background. You are not required to show your face in the photo, but each geocacher should upload an image with their log indicating that they were there. You are also welcome to share other photos of your visit to this beautiful area, but at least once image should show you with the entrance way visible in the background.
Earthcache lesson:
While most granite forms deep underground, the presence of vesicles in these rocks indicates a different cooling history. These features form when gas bubbles are trapped during solidification, a process typically associated with rapid cooling at or near the Earth's surface.
What is Granite?
Granite is a coarse-grained igneous rock that forms from the slow cooling of silica-rich magma deep underground. This slow cooling process allows large, visible mineral crystals to grow, which is why most granite you encounter has a distinct "speckled" appearance. The main minerals you'll see are:
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Feldspar: Often opaque and blocky, ranging in color from pink to white or gray.
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Quartz: Translucent or glassy, typically gray, filling the spaces between other crystals.
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Biotite or other dark minerals: Small, black, and sometimes flaky crystals.
Because typical granite cools under high pressure deep in the Earth's crust, it is dense and largely free of gas bubbles.
What's a Vesicle?
Vesicles are small, bubble-like voids in igneous rock. They form when magma or lava contains dissolved gases (volatiles like H₂O, CO₂, etc.). As magma ascends and the pressure drops, these gases come out of solution and form bubbles. If the rock cools quickly, these bubbles get frozen in place before they can escape, resulting in vesicles when the rock solidifies. In contrast, granite that cools slowly and deeply underground typically lacks vesicles because high pressure keeps gases dissolved and slow cooling allows any bubbles that do form to escape.
Why This Granite Is Different
The granite at this location is different from typical, deep-seated granite because it has vesicles. This means it must have formed under specific conditions that allowed these gas bubbles to be trapped.
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Shallow Intrusion: This granite likely formed in a shallow, hypabyssal environment, such as a dike or sill. The lower pressure near the surface allowed gases to come out of solution and form bubbles.
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Faster Cooling: While not as fast as volcanic lava, the cooling rate was quicker than that of a deep pluton. This faster cooling preserved the bubbles, locking them into the rock's structure.
The presence of vesicles suggests this granite is not a part of a typical, deep-seated pluton but rather a shallower, more dynamic intrusion that had a higher volatile content. This is a special characteristic that makes this particular granite unique.
As the magma intruded closer to the surface, the pressure dropped, causing dissolved gases to come out of solution and form bubbles. The relatively faster cooling rate in this shallower setting "froze" these bubbles in place before they could escape, leaving behind the vesicles you can see today. This particular rock isn't a typical, deep-plutonic granite, but rather an example of a shallow intrusion with a high volatile content. It's a special occurrence that highlights the fact that not all granites are created equal, even within the same geological region.
Details About this Location
This area may not be accessible at all times due to private events and functions that may be happening on the fairgrounds. Please obey all requirements for paid entry, or come back another time to complete this earthcache. There is no need to enter the fairgrounds to complete this earthcache, but you do need to gather information by looking closely at the stones.
This cache was placed by a PROUD Platinum Earthcache Master.