Keywords for this lesson:
fracture - any separation in a geologic formation
fissure - a fracture or crack in rock along which there is a distinct separation
groundwater - water that exists underground in saturated zones beneath the land surface
This EarthCache invites geocachers to look more closely at the paving slabs at the entrance to a Cambridge hotel. The learning point of this EarthCache is to familiarise the geocacher with the processes that form manganese oxide dendrites.
Everything you need to answer the questions is available by attending the co-ordinates and reading this lesson. There's no need to research anything extra online, although you’re welcome to do so if you want to.
At GZ, (the entrance to a hotel,) you’ll find some iron stained paving slabs made of granite. Please ignore the large red lines visible in the rock - you are looking for the smaller, dark structures visible in many of the slabs. These are manganese oxide dendrites.
Dendrites
manganese dendrites
The term "dendrite" comes from the Greek word "dendron", meaning "tree". Dendritic growth is used to describe branched projections and is a very common phenomenon in nature. Trees grow branches and roots out from a main trunk, window frost reaches out to cover entire window panes, and rivers form intricate drainage patterns. These are all forms of dendritic growth, and yet another type of growth can be found on rock surfaces. This is the type referred to in this EarthCache.
Geological dendrites form as naturally occurring fissures in the rock are filled by percolating mineral solutions in the groundwater. They form when groundwater that is rich in manganese, iron, and other minerals, flows along fractures and bedding planes within various rock types, depositing dendritic crystals as the solution flow passes.
Dendrites vary in colour depending upon the type of mineral deposited.
Many dendrites have been mistaken for fossils (leaves or branches) and are therefore often used as an example of a pseudofossil, (fake fossil).
To log this cache, please visit the published co-ordinates and answer the questions below. Once you have obtained the answers, please send them to me via email or through the Message Centre. You are free to log your find once you have contacted me. You don't have to wait for a reply. If there are any questions about your answers, I’ll contact you.
Logs without answers will be deleted. Please don’t include close up pictures in your logs that may answer the questions.
Please remember: ignore the large red lines visible in the rock - you are looking for the smaller, dark structures.
1. Look at the manganese dendrites in the granite slabs. Describe their distribution in the rock - are they all linear, are they formed in a pattern, or does it just look random?
2. Observe and describe the manganese dendrites. What shape, size and colour are they? Do they remind you of any biological structures?
3. Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache, but please do not give any answers away.
