This fine granite obelisk stands in the centre of Euston Gardens in Fleetwood. The plaque on the front is now difficult to read but a little internet research reveals the inscription to be
"‘Erected by public subscription to the memory of James Abram and George Greenall who lost their lives in the storm of November 1890 whilst heroically endeavouring to save others’."
"Xenolith" literally means "foreign rock" and gives an insight into the formation of igneous rocks a long time ago.
Have a look at the pillar directly in front of you at GZ. Please make all observations from the path, there's no need to enter the flower bed - what you are looking for is clearly visible from the path.
1) Which side or sides contain Xenoliths - if we say the side bearing the inscription is the front, is it the front, rear, left or right?
2) Describe how it/they look different to the surrounding rock - what do you notice compared to the granite that makes up the memorial?
3) Using the information in the text below, based on how the granite was formed, do you think the Xenolith(s) are older, or the same age as the rest of the stone making up the memorial?
4) Is the granite an example of an intrusive or extrusive igneous rock?
5) Take a photo of yourself at GZ, or an identifying item, being careful not to reveal any of the answers to the questions.
Please submit your answers via message though the Geocaching website or via email - there is no need to wait for a response before logging your find.
Xenoliths
Xenoliths, are foreign rock inclusions found within igneous rock formations, such as granite. Their formation begins deep within the Earth's crust, where molten magma slowly cools and solidifies to form granite. During this process, pockets of magma may intrude into existing rock formations, incorporating fragments of the surrounding rock into the cooling magma. These fragments, known as xenoliths, are often of different composition and texture compared to the surrounding granite. As the magma cools and crystallises over time, the xenoliths become trapped within the granite, preserving a snapshot of the geological history of the area.
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.
Igneous rocks are formed by the cooling of molten rock and are classified into two categories: intrusive and extrusive.
-
Intrusive Igneous Rock: This type forms when molten rock cools beneath the Earth's surface. The slow cooling process allows for the development of large crystals. The longer the cooling period, the larger the crystals that form.
-
Extrusive Igneous Rock: This type forms when molten rock cools on the Earth's surface. Exposure to the elements, such as rain, causes the rock to cool quickly. As a result, any crystals that form are typically very small and often not visible to the naked eye.