Xenolith is a geological term from greek "xenos" which means "foreign" and from "lithos" which means "stone". So a xenolith is an inclusion in a magma rock or in other words a piece of rock trapped in another kind of rock.
Most of the time magma rocks contain inclusions of another different rock, these inclusions are pieces of rocks met by the magma during its formation (plutonic rocks) or during its raising in the volcanic chimney (volcanic rocks). But as the minerals and crystals at the origin of both rocks are not the same, they didn't crystallise on the same way and that's why there is a rock mixed in another...
Plutonic rocks as granite or diorite are formed on depth in magmatic chambers where the magma cool and crystallise slowly. This is at the origin of the phaneritic textureof the plutonic rocks whose mineral grains are large enough to be seen with the naked eye or a hand lens.
Photo of granite and observation with a polarizing microscope (we can see the mineral grains which are joined)
Schéma of the uprooting of xenocrysts from the walls of the magmatic chamber
| This is during its cooling and its stock in the magmatic chambers that xenocrysts (=crystals which are differents from those of the magma) of the walls of the magmatic chamber can be mixed with the magma to form xenolith.
Then these rocks are climbed up to the surface thanks to tectonic movements.
Volcanic rocks as andesite or rhyolithe are not formed on the depth in fact their texture is aphanitic,which means that the grains and the crystalline structure is too fine to be seen by the unaided eye.
Such rock is formed when the material solidifies at or near the surface so that the cooling is rather rapid. Under these conditions there is not enough time for the growth of large crystals
Photo of andesite and observation with a polarizing microscope
(we can see the mineral grains and theglassy matrix)
||This is during the eruption and especially during the raising of the magma to the surface of the Earth that magma uproots xenocrysts from the walls of the magmatic chimney which forms xenoliths.
|Schéma of the uprooting of xenocrysts from the walls of the magmatic chimney during the volcanic eruption.
How do you recognize xenoliths?
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 variable: from some millimeters to several meters for the most outstanding xenoliths... We identify the xenoliths and the xenocrysts by the names of the two rocks involved:
Left : Gabbroic xenolith in a granite.
Center : Peridotite (green) mantle xenolith within a volcanic bomb.
Right : Large and rather unusual xenolith of sandstone.
Importance of the xenoliths:
Xenoliths and xenocrysts are very important for geologists indeed they give precious informations: thanks to the study of the composition of the rocks, scientists are able to determin on which depth the rock was formed and so to know the composition of the Earth's mantle.
As it is impossible for us to go to such depths (several hundreds of kilometers inside of the Earth), some informations on the Earth's crust would be impossible to obtain without xenoliths.
Reminder on "Earthcaches": There is no container or logbook on the given coordinates. Just visit the site and answer the questions by e-mail or by the Message Center.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 EarthCaching!
To validate your visit:
1-. Measure the size of one of the biggest inclusions located at the top of the rock. (see picture).
2-. Give me the name of the two kinds of rocks which form this xenolith
3-. With the help of the listing and of your observations, say me what are the conditions of the formation of the rock in which the inclusion is. Justify your answer.
4-. So deduce how the rock was included in the other to form this xenolith.
En option -. a picture of you and/or of your GPS (no spoiler of the inclusion, thanks!)
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