The Earthcache
Croagh Patrick, nicknamed the Reek, is a 764 meters (2,507 ft) mountain and one of the most singular mountains in County Mayo.
Although most of the geological layers that compose it are sedimentary rocks of the Mississippian (323.2 - 358.9 Ma) and the Silurian marine molasses (419.2 - 443.8 Ma), its basal part consists of an older rock rarely observed in surface, serpentinite, a transformed rock fruit of the turbulent geological history of this Irish region.
► At the origin of serpentinite, peridotite
Peridotite is a rock composed mainly of olivine crystals and pyroxenes.
It constitutes the major part of the terrestrial mantle; she is the rock mother of it.
Ultramafica and with granular structure, peridotites are mainly composed of olivine associated with other ferro-magnesian silicates, mainly pyroxenes.
This gives it a color with different shades of green depending on the geological composition.
It is therefore a magmatic primary rock, a protolith, directly resulting from the solidification of the magma of the upper mantle.
► Hydrothermal metamorphism
Peridotite can be transformed into a secondary rock under the effect of heat and hydration, it is hydrothermal metamorphism.
Hydrothermal metamorphism is the mineralogical transformation of a rock under the effect of a change in temperature with a constant pressure before subduction and with a pressure which increases after subduction but without modification of the chemical composition of the rock, and requiring a fluid (usually water).
This hydrothermal metamorphism occurs at low pressure, at low temperature (a few hundred degrees) and in the presence of large amounts of water.
The process is in the solid state.
In the case of Croagh Patrick, this peridotite (Mantle material) was trapped here when the Iapetus Ocean closed, thus preserving part of this original material.
Iapetus Ocean was an ancient ocean that formed about 600 million years ago, at that time northwest Ireland and southeast Ireland were unseen and separated by this ocean, which was several thousand kilometers wide. About 450 million years ago the ocean effectively died.
The peridotite then turned into serpentine.
► Serpentinite
Serpentinite is therefore a secondary metamorphic rock, the result of the transformation of a protolith of peridotite under conditions of pressure and particular temperature, generally at the level of the ocean ridges.
This is a relatively rare kind of metamorphosed igneous rock.
It is a rock combining blue and greenish colors. It is mainly constituted (more than 75%) antigorite (magnesian phyllosilicate).
Its appearance is much less grainy than its protolith, and presents a massive and melted character peculiar to a metamorphic rock It also has centimetric pheno-crystals of greenish inclusions (porphyritic form).
These crystals are serpentine, a soft, green, magnesium-rich mineral (Hydrated Magnesium silicate), which is derived from the transformation of olivine and piroxene with water when exposed to the surface of the earth.
Serpentinite is a hard rock; to assess its hardness, it is possible to apply the conditions of the Mohs scale.
► Bibliographical sources
The Questions
On the side of the road when climbing to the top of the mountain, 20 meters before reaching the cattle gate, head to the right at the edge of the path to observe a block of bedrock outcrop (see blue area of the photo WP1).
- Question 1 : describe the color and texture (granular or massive) of the block of rock under the blue zone. Based on your observations, are we in the presence of the primary (protolite) or secondary (metamorphic rock) form of the rock formation. What is her name ?
- Question 2 : What is the color of the largest crystals and their shape? What is their nature? According to their size, how can you qualify them?
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Question 3 : try to scratch the block of rock with:
- a piece of copper (1, 2 or 5 cents). Can you do it ?
- a metal knife. Can you do it ?
Deduce the hardness of the rock on the Mohs scale.
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