Following the Great War and the tragedy of the Iolaire disaster, a public meeting was held in Stornoway in January 1920 with the aim of preserving the memory of the 1,151 Lewis men known to have made the supreme sacrifice for their country. The memorial takes the form of a Scottish Baronial Tower rising to a height of over 85 feet. Internally, the tower is divided into an arched entrance chamber 20 feet high, and four upper chambers accessed by square and circular steel stairs and by granite turnpike stairs to the turret. A separate chamber was allotted to each of the four parishes of Lewis and in each, bronze plaques were mounted, bearing the inscriptions of every one of the fallen. The stairs allowed visitors to access the view of the Parishes from whence the dead had come. The dressed work is of fine axed Aberdeenshire granite, the walls are of native Gneiss and the floors of reinforced concrete. Sadly due to water damage the memorial was closed to the public in 1975. Despite renovations it was deemed still not fit to allow the public to enter the tower so the refurbished plaques were mounted outside the tower, on granite stones. A viewing path and seating area was also provided. This last project was completed in time for Armistice Day 2002.
Now to this EarthCache, take a walk into the memorial rock circle and have a look at the rock containing the Barvas parish panel. It is the sixth on the left as you enter by the path. Here we are looking at metasomatism in rocks.
Metasomatism in Rocks: Definition, Process, Identification, and Colour Indicators
What is metasomatism?
Metasomatism is a chemical alteration process where the mineral composition of a rock is changed due to the influx of hydrothermal or metamorphic fluids. This process involves the removal or addition of elements via fluid-rock interaction, resulting in the growth of new minerals that were not originally present. Unlike simple metamorphism, which typically involves changes due to pressure and temperature alone, metasomatism is driven primarily by fluid chemistry and element mobility.
How does matasomatism occur?
Metasomatism usually takes place in the Earth's crust under conditions where fluid pathways are available, such as: along faults and fractures, around igneous intrusions. In subduction zones and deep crustal environments.
Types of Metasomatism
1. Hydrothermal metasomatism
-Description-: Hydrothermal metasomatism occurs when hot, mineral-laden fluids interact with existing rocks and cause chemical changes. These fluids are often expelled from magma chambers or circulate through cracks and fractures in the rocks.
- -Formation-: The high temperatures and pressures of these hydrothermal fluids can dissolve and transport elements from one location to another. As the fluids interact with the surrounding rocks, they deposit new minerals and alter the existing mineral assemblages. This can lead to the formation of ore deposits such as gold, copper, and silver.
2. Metasomatism in subduction zones
- -Description-: Metasomatism in subduction zones is a process that occurs at convergent plate boundaries where one tectonic plate is forced beneath another. This process involves the release of fluids and melting of minerals in the subducted plate, which then interact with the overriding plate.
- -Formation-: As the subducted oceanic plate descends into the mantle, the release of fluids and melting minerals causes metasomatic changes in the overlying mantle wedge. This can lead to the formation of new minerals and alteration of the rock composition in the mantle wedge, influencing volcanic activity and magma generation in subduction zones.
3. Contact metasomatism
- -Description-: Contact metasomatism occurs when rocks are altered by the heat and chemical fluids from the intrusion of a hot igneous body. The surrounding rocks are "baked" and chemically altered by the high temperatures of the intruding magma.
- -Formation-: The intruding magma introduces heat and volatile components that can react with the surrounding rocks. This can result in the formation of new minerals through recrystallization and alteration of pre-existing minerals. Contact metasomatism can lead to the formation of distinct mineral assemblages known as metamorphic aureoles around the igneous intrusions.
4 Diffusional metasomatism
- -Description-: This involves the movement of atoms and ions through a solid rock matrix due to chemical potential gradients. This process occurs in the absence of significant fluid flow or infiltration. In essence, elements migrate from areas of higher chemical potential to areas of lower chemical potential within the rock, leading to changes in its composition
-Formation-: This occurs by the diffusion of a solute through a stagnant solution of fluid, the metasomatism in this situation happens as chemicals change concentration in the fluid which is contained within a rock.
5 Infiltrational metasomatism
- -Description-: Metasomatism where materials are transported and deposited within a rock mass by a fluid phase infiltrating from a remote source. This process contrasts with diffusion metasomatism, which involves chemical changes occurring within a rock due to internal chemical potential gradients. Infiltration is driven by external fluid pressure gradients, allowing for more extensive transport of matter compared to diffusion.
-Formation-: occurs when a material in solution, infiltrates through rocks. is a type of metasomatism that takes place by the transfer of material in solution, infiltrating through the host rocks.
During the metasomatic process:
Introduction of fluids rich in components like Si, K, Na, Ca, Mg, Fe, etc.
Element migration, where certain elements are leached out and others introduced.
Replacement of original minerals by new stable minerals suited to the new chemistry.
Distinct colour zones in rock exposures Veining by minerals like quartz, calcite, or epidote Halos or alteration zones around intrusions or shear zones Mineralogical changes—e.g., feldspars replaced by sericite, or olivine replaced by serpentine Textural features like pseudomorphs or recrystallized areas Look for progressive zoning or abrupt mineralogical boundaries, often coinciding with fractures or contacts.
Colours of Metasomatic Alteration and Their Mineral Causes Different types of metasomatism are often visually recognizable based on the colour of alteration halos, which reflect specific minerals and fluid chemistries.
Below is a summary: Colour Typical Minerals Elements Added Associated Alteration Type: Green Chlorite, epidote, actinolite, serpentine Fe, Mg, Ca Propylitic alteration (common in volcanic and porphyry systems)
Red/Brown Hematite, limonite Oxidized Fe Oxidation zones, supergene enrichment.
White/Grey Quartz, sericite, kaolinite, albite Si, Al, Na Phyllic or argillic alteration.
Yellow/ Orange Goethite, jarosite, limonite Partially oxidized Fe Acid-sulfate systems or weathered zones.
Black/Dark Grey Biotite, magnetite, pyroxene Fe, Mg Potassic or mafic-rich environments
Pink K-feldspar (orthoclase, microcline) K= Potassic metasomatism near intrusions.
Examples of Metasomatic Alteration Environments Skarns: Form at the contact between igneous intrusions and carbonate rocks; rich in garnet, pyroxene, epidote.
Greisens: Form in granitic rocks; contain quartz, muscovite, tourmaline, and topaz. Serpentinization: Occurs when ultramafic rocks are altered to serpentinite via water and magnesium-rich fluids.
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 us via e-mail or the Geocaching Message Centre. You are free to log your find once you have contacted us. You don't have to wait for a reply. If there are any questions about your answers, we will get back to you.
Logs without answers or photo will be deleted. Please don’t include close up pictures in your logs that may answer the questions.
Q1 Describe the type of metasomatism you can see in the rock.
Q2 Explain how it has been formed.
Q3 Which mineral(s) are causing the colour in the rock.
Q4 Post a picture of you/ your mascot or pet with the memorial tower.
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