While completing this EarthCache please do not move any rocks, disrupt soil or pose risks to wildlife. Please read the entire description.
A migmatite is a formation of rocks composed of both metamorphic and igneous components. Layers of granite occur within metamorphic rocks, most commonly gneiss, schist or amphibolite.
The layers in the rock occur due to different geological processes.
Igneous rocks such as granite form by crystallisation of minerals from molten magma. However, the metamorphic rocks form by recrystallization of minerals without melting. This leads to differences in colour, texture and crystal structure.
Geologists aren't exactly sure on how the two coexist in a single rock. What is known is that migmatites form at the threshold between high-grade metamorphic recrystallization and complete igneous melting. Migmatites are partially melted during formation, a geological process known as "Anatexis". Evidence of anatexis is usually left behind, either on a macroscopic or microscopic scale (visible or not visible).
Some migmatites are formed by intrusion of liquid granitic melt into a pre-existing metamorphic rock. When this occurs, the granite inclusions have sharp contact points with the metamorphic layers.
In other cases, boundaries between metamorphic and igneous rocks can be gradual or indistinct. This suggests that not all migmatites form during a single phase of melting and recrystallization. Some take several phases to appear the way they do.
Migmatites are most commonly found in plate tectonic settings, where continental crust has been subject to high temperatures and pressures.
Logging Requirements:
Observe the migmatites at GZ and answer the following questions:
1. Are the layers in the stone obvious or indistinct? What does your observation tell you about the formation of the stone?
2. Describe the colour, texture and crystal structure of both the metamorphic and igneous rocks. Does either of them vary?
3. Can you see any signs of anatexis?
4. Take a photo of you, your stamp or personal item with GZ in the background.
Please send answers via the Geocaching message centre or to my email, answers posted in the logs will be deleted.
You can log the find immediately, if answers are supplied to CO within 14 days.
Be wary of kids locations at all times and observe signs nearby. You don't need to go near any edges or anywhere unsafe to complete this EarthCache.
Happy caching!
Extra information regarding caching on Kangaroo Island:
The EarthCache site is not accessed by the beach by any point. However, if you choose to visit the beach during your visit to Vivonne Bay, please be aware of beach-nesting birds.
Follow the simple steps to help protect the wildlife:
• Walk below the high tide line during breeding season (between September and April)
• Be respectful of signs and fencing
• Keep dogs on leashes and away from known feeding and breeding areas
• Avoid driving 4WDs on beaches
More information can be found at:
https://www.australiangeographic.com.au/topics/wildlife/2021/03/beach-nesting-birds-are-under-attack-and-they-need-our-help/
We need to respect nature while geocaching so please be responsible at all times and respect all areas, not just sensitive ones.
As per the Geocaching Creed, "Leave no trace" and "Observe all laws".
I have reached the GSA's highest level:
