This Earthcache is about Xenoliths
Xenolith from the ancient Greek, “xenos” (foreign) and “lithos” (stone). It is defined as an inclusion of pre-existing rock into an igneous rock. Xenoliths are fragments of the 'country rock' or fragments of an earlier solidified portion of the igneous rock with a slightly different composition. Xenoliths may also show signs of interaction with the igneous rock.
This part of the world is very stable geologically speaking but it wasn't always the case. Between 490 and 390 Ma the whole area was subjected to a mountain building event called the Caledonian Orogeny. The word Orogeny comes from the ancient Greek ὄρος oros, mountain and γένεσις genesis or creation, origin. In the later phase of this event three ancient continents LAURENTIA and BALTICA eventually came together with a third AVALONIA and bringing about the closure and disappearance of the IAPETUS Ocean and forming one landmass. The boundary called the Iapetus Suture can be traced as a geological fault going from the Shannon estuary through Louth, the Isle of man, Scotland and on up the west coast of Norway. When continental plates collide a lot of energy is released due to subduction as one plate disappears beneath another and creating hotspots. Related to this activity about 405 Million years ago a series of eight granite plutons were injected into the Precambrian Dalradian rocks which underlies much of this part of Donegal. Indeed these continental collisions were accompanied by massive granite emplacements around the country.
Caledonian orogeny, Caledonia being the Latin name for Scotland
The bedrock here beneath your feet is a type of granite, part of a granite pluton which intruded into a previous pluton which had previously intruded into the very old Precambrian Dalradian bedrock that surrounds this area. The Ardara pluton is roughly circular in shape
These granite emplacements or plutons intruded into the existing rock by forcing their way upwards as blobs of magma or more correctly a magma diapir. A simplistic way to picture this is to imagine how a lava lamp functions. Now if magma makes it all the way to the surface it can form a volcano and is described as extrusive. But if as is often the case it cools deep below the surface it hardens to form a pluton and is described as intrusive. The reason it rises is because it is less dense than the country rock above it. As it rises forcing its way it interacts with the country rock, melting it, or pushing it out of the way. Some of country rock is assimilated and melted but not all of it is absorbed and fragments remain as foreign rock or xenoliths .
A process called Stoping describes how magma makes its way upwards forcing its way up through the surrounding rock, the physical forces and thermal expansion cause fractures to develop allowing blocks and fragments of host rock to be ripped off walls or fall from the ceiling of the magma chamber' Many of these pieces probably melted before the magma cooled, however some pieces did not melt completely and were encased in the granite when it cooled.
You will need a ruler or a measuring tape.
I set the Earthcache coordinates a few meters from the Dolmen because I didn't want them hiding the position of a Regular cache hidden close by.
The capstone is supported by three orthostats (Q1) and Q2 & Q3 are based on some small xenoliths on the underside of the capstone. The indications I am giving you are to help you locate each of them and are based on you standing in a certain spot facing the Dolmen. Rear is furthest from you with front being closest, then left right etc...
- The single Orthostat at the rear or western end looks different from the others because of its colour, what colour is it?
- Measure and tell me the size of the XENOLITH under the near tip or eastern point of capstone?
- Find and examine a XENOLITH close to the rear orthostat, it is oval, almost circular in shape, Ø 5 to 6cm. It is interesting because it is not flush (±)1cm with the rest of the cap stone. Can you tell me if it is protruding or recessed & depending on your observation is it harder or softer than the rest of the capstone?
Please send the answers using the message center and not by email because I may miss them there.
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