Sydney Xenolith
Tasks:
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Describe the shape, size, and color of the xenoliths you see here.
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Examine whether the xenoliths have a distinct boundary from the granite or gradually blend into it.
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Explain how the xenoliths likely became incorporated into the rock, based on the formation explanation.
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Please take a picture of yourself or an object on site to prove your visit and post it with your log.
Log this cache as “Found it” and send me the answers to the questions to “silka03.earthcache@gmail.com”, my profile e-mail or via Geocaching Message Center. Not in your log!
You may then log immediately. I will contact you if something is wrong.
If there are no answers after 14 days I have to delete your log.
Have fun!
Welcome to this EarthCache, which explores a fascinating geological phenomenon: xenoliths in granite. Here at this location, under a covered structure supported by granite columns, numerous dark inclusions can be seen in the gray rock. These inclusions are xenoliths—fragments of older rock that were incorporated into a rising magma body and preserved within it. In this EarthCache, you will learn more about the formation of granite, the creation of xenoliths, and their geological significance.
Geological Formation of Granite: Granite is an intrusive igneous rock (plutonite) that forms through the slow cooling and crystallization of magma within the Earth's crust. This magma originates from the upper mantle or lower crust and is often generated by the partial melting of continental crust. Due to the slow cooling process, large crystals of minerals such as quartz, feldspar, and mica develop. The granite you see here is part of an ancient pluton that was exposed at the surface through geological processes.
How Do Xenoliths Form? Xenoliths (from the Greek: "foreign rock") are rock fragments that were captured by rising magma. They originate from older surrounding rocks and can be either igneous or metamorphic in origin. As magma ascends, it tears off existing rock fragments. These fragments become engulfed in the hot molten material and may begin to react or undergo changes. Some xenoliths are altered through chemical processes, while others remain largely unchanged. Since the magma cools and solidifies slowly, these foreign rocks become permanently embedded within the granite. The xenoliths here are often composed of darker, mafic rocks, which differ significantly in chemical composition from the granite.
Geological Significance: Xenoliths provide valuable insights into the composition and evolution of the Earth's crust, as they often represent deeper rock layers. They help geologists understand mantle processes and interactions between magma and crustal rocks. In Sydney, xenoliths can be observed particularly well since many structures and monuments are built from locally sourced granite.
picture: https://www.mineralienatlas.de/lexikon/index.php/BilderGeordnet?lang=en&rockid=141
https://www.mineralienatlas.de/lexikon/index.php/Xenolith?lang=de
https://de.wikipedia.org/wiki/Xenolith
https://www.mineralienatlas.de/lexikon/index.php/RockData?lang=de&rock=Granit