Xenolith in Auckland
Tasks:
1. How can you distinguish a xenolith from the surrounding granite? Describe its shape, color, and texture.
2. What does the fine-grained structure of a xenolith indicate about its origin compared to the coarse-grained granite?
3. Why do xenoliths remain preserved in the granite instead of melting completely?
4. Please take a picture of yourself or an object on site to prove your visit and post it with your log.
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In a war memorial here in Auckland, xenoliths can be seen embedded in the granite blocks. These dark inclusions stand out against the lighter granite, providing a fascinating glimpse into the geological history of the rock used in the monument.
Granite – Formation and Characteristics
Granite is an intrusive igneous rock (plutonic rock) that forms deep beneath the Earth’s surface. It develops through the slow cooling of magma within the Earth’s crust, allowing large crystals to grow. Granite is primarily composed of the following minerals:
- Quartz (transparent to gray, very hard)
- Feldspar (white, reddish, or pink, depending on composition)
- Mica (dark shiny or silvery, forming plate-like structures)
As granite is a plutonic rock, it only reaches the surface through erosion and tectonic processes. In many mountainous regions, large granite formations are exposed due to weathering and erosion.
Xenoliths – Foreign Rock Inclusions in Granite
A xenolith (Greek for "foreign rock") is a rock inclusion that differs from the surrounding igneous rock. Xenoliths form when magma picks up fragments of pre-existing rocks on its way to the surface. These fragments often originate from the surrounding country rock or deeper layers of the crust.
There are two main types of xenoliths:
- Metamorphic xenoliths – These fragments come from already transformed rocks (e.g., gneiss, schist) and may have undergone some changes due to the heat and pressure of the magma.
- Igneous xenoliths – These consist of older igneous rocks that were incorporated into the rising magma.
Since xenoliths do not fully melt into the magma, they remain visible as darker, often rounded or irregularly shaped inclusions within the granite. They are usually finer-grained than the surrounding granite because they originate from pre-solidified material.
Importance of Xenoliths for Geology
Xenoliths are valuable to geologists because they provide insights into deeper layers of the Earth's crust and their composition. They help reconstruct the history of magma ascent and rock formation in a given region.
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