This geological feature caught my eye when setting an AdLab stage here and I researched the formation and was interested in what I read.
What is a Xenolith?
These are dark blocks of finer grained rock enclosed in the tonalite (a type of granite), and merging into it to varying extents – they were plucked from the walls of the expanding magma chamber, baked, and melted to varying degrees. When melted completely, of course, they became part of the magma.
So, a xenolith is a piece of rock trapped in another type of rock.
Most of the time, a xenolith is a rock embedded in magma while the magma was cooling. Magma is the molten rock beneath the Earths crust that emerges as lava during a volcanic eruption. The rock that forms from cooled magma is called igneous rock. Xenoliths are different types of rock embedded in igneous rock.
Xenoliths are torn from deep cracks, or pipes, in the Earths surface. Magma rises to the Earths surface through these pipes between the Earths crust and mantle. As the molten material rises, it tears off bits and pieces of the magma pipe in which it is traveling. These bits and pieces, trapped in the magma but not melting into it, become xenoliths.
Wait, what is a xenocryst then?
Crystals that are torn from the sides of magma pipes, and swept along and embedded in the magma are called xenocrysts. They are a type of xenolith.
As magma erupts or flows from the Earths surface, it is cooled by exposure to air or water. Lava cools fairly quickly, and various types of igneous rocks are formed. Xenoliths are usually visible. They have a different color and density than the surrounding igneous rock. Xenoliths can be as small as a grain of sand or as large as a football, and as long as several meters.
Xenoliths and xenocrysts are affected by temperature. A xenolith may lose its unique qualities if it melts into the surrounding magma. As it cools, the material may cease being a xenolith at all and become a metamorphic rock. Metamorphic rock is rock that has changed from one form (sedimentary or igneous) to another.
Xenoliths and xenocrysts are often identified by the names of the two rock types involved. A peridotite xenolith in a basaltic lava flow, for instance, means a chunk of the rock peridotite is embedded in basalt rock. The peridotite is usually yellow and dense, while the basalt is usually grey and light.
Xenoliths and xenocrysts provide valuable information about the geology of the Earths mantle. Scientists study the chemical properties of xenoliths to understandthe depth at which they were formed. Many xenocrysts were created hundreds of kilometers within the Earth, far below the deepest mines and wells. The information about the condition of the mantle at these depths would be impossible to understand without xenoliths and xenocrysts. Some of the features studied by geologists are temperature, pressure, construction, and movement within the Earth's surface.
The Earthcache tasks.
Having read the above information, and observed the visible face of the cenotaph above the sentence; 'To commemorate the re-location and re-dedication of this memorial', you should notice an impurity embedded in the granite rock.
Question 1. Do you think you are looking at a xenolith or a xenocryst? Please give an reason for your choice.
Question 2. There are other examples visible around the sides of this cenotaph, please compare to the observed sample. Are they the same colour and size as the observed sample? If not, please describe how they differ.
Question 3. How does the basic granite face compare to the observed embedded xeno-item(s)? Please describe colour, shape etc.
Please message me your three answers to the questions above, if you need any help please let me know.
Question 4. Please provide a photograph (with the base of the plinth of the cenotaph showing) of yourself or a personal item to prove you visited the site. A personal item such as a handwritten name, or trackable is an option for those who do not want to photograph themselves. This picture should be uploaded with your found it log.
You can log your find as soon as you like after your visit, and your answers have been submitted, don't forget - if you need any help with the questions please let me know.
Information sourced from National Geographic Resource Library