Lesson:
This EarthCache aims to teach you about xenoliths, what they are and how they are formed.
Questions:
- Describe the stone that the fountain is made of in terms of its colour, texture and crystalline structure (size and colour)
- Identify the xenolith, imagine the fountain is a clock and the face is the center. If the xenolith was the hand, what number is it pointing towards?
- Describe the xenolith in terms of its size, colour, texture and crystalline structure (size and colour)
- Considering what you have discovered in relation to the crystal structure how do we know these rocks were formed under different conditions?
Send your answers by email or via the Message Centre.
fig 1. Example of a xenolith (not the one at GZ)
History of the location:
Henrietta park opened in 1897 to celebrate the Diamond Jubilee of Queen Victoria, the park was donated by Captain Forester under the stipulation that it always remain a green space and never be built on. The fountain itself was donated by James Brymer during a period of concern over the safety of public water supply, when many wealthy individuals donated water fountains across Bath. However, the fountain wasn't always situated in Henrietta Park, it was originally in place at the entrance to the vegetable market behind the Guildhall but was moved in 1899 when the Guildhall was redeveloped.
So, like the subject of this EarthCache, it is also a stranger in the park.
Background information:
Igneous rocks form through the cooling and solidification of magma or lava. When the magma or lava cools and solidifies into rock it can form a crystalline structure, the size of these crystals depends on the speed at which the rock cooled. When the rock cools quickly the crystal structure has less time to form and the particle size is therefore smaller, on the other hand when it cools slowly the crystals have more time to develop and larger crystals form.
fig 2. Xenolith formation diagram
Xenoliths are rock fragments that become trapped in larger rocks during their formation, they are primarily found in igneous rocks and become trapped in the rock during magma eruptions. The rock fragments are from the native rock of the area which are torn loose by the magma erupting through volcanic chambers, they are then carried along with the magma and as the magma cools they become entrapped within the rock.
