Indroduction :
You are facing a monument dedicated to a “Lillois” (Lille inhabitant) but what interests us here is a type of rock present in one of its side. This rock is called Xenolith, which is a piece of rock trapped in another kind of rock.
Xenolith: Word composed of xeno- (foreign) and -lithe (stone) in ancient Greek, literally "foreign stone."
Most of the time, a xenolith is integrated in the magma rock while it was cooling. Magma is the molten rock beneath the crust, it emerges as lava during a volcanic eruption. The rock that forms from cooled magma is called igneous rock. There are different types of xenoliths integrated into the igneous rock.
Formation:
During the formation of magma, its movements, its storage in a magma chamber or during an eruption, "foreign” rock can be mixed with magma without melting. For example, in the case of a volcanic eruption, the magma rises to the surface of the earth through deep cracks (schematically similar to pipes) between the crust and mantle. When the melt is directed to the surface, some rocks are torn from the walls of the "pipe" in which magma moves. Likewise, crystals torn from the sides of the "pipes" by the magma are called xenocrysts. However, some crystals in igneous rocks can be formed by chemical reactions between the magma and the xenolith.
Generally, xenoliths are visible through their different colors than the rest of the rock, they also have a density different from the surrounding igneous rock. The size of the xenoliths can be very varied, and measure the size of a grain of sand, a football balloon or several meters in length.
Xenoliths and xenocrysts are often identified by the names of the two rock types involved. A peridotite xenolith in basalt lava flow, for example, means that a piece of peridotite is present into the basalt rock. Peridotite is generally yellow and dense, while the basalt is usually grey and shiny.
Xenoliths and xenocrysts are very important because they provide valuable information about the geology of the Earth's mantle. Scientists are studying the chemical properties of xenoliths to understand how deep they were formed. Many xenocrysts were created hundreds of kilometers inside the Earth, well below deepest wells and mine.
At such depths, some information on the state of the mantle and the Earth's crust would be impossible to get without the study of xenoliths and xenocrysts. Among the characteristics studied by geologists one can cite temperature, pressure, the contents and movements of materials.
One can also qualify by “xenolith” a piece of rock trapped in a piece of sedimentary rock, but this is quite rare. Xenoliths were also discovered in meteorites. In this case, inclusions in meteorites were formed from collisions with other objects outside the Earth's atmosphere.
Reminder on "Earthcaches": There is no container or logbook on the given coordinates. Just visit the site and answer the questions by e-mail. There is no need to wait for my reply, feel free to log immediately after. If the answers are incorrect, I will let you know. Happy caching!
To validate your visit:
1-. What kinds of rocks form this geological curiosity? (Hosting rock and xenolith).
2-. Estimate the size of this inclusion (example 9x10 cm)
3-. Give an hypothesis explaining easily why it is possible to have a rock embedded in another rock type and thus form a xenolith.
4-. Give me the phrase at the bottom of the monument.
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