A WALK IN THE PARK #5 ( Taibach )
THIS IS AN ACCESSIBLE EARTHCACHE WHICH EXAMINES A CRYSTALLINE ROCK WHICH IS NOT NATIVE TO THE SOUTH WALES COALFIELD.
There is street parking available but also there is a large free car park in the nearby supermarket.
In 1918 Emily Charlotte Talbot of Margam Park donated a field in the town as a memorial for the dead of the First World War (1914-18). A pillar can be found in the memorial park, in the centre of the main avenue. The park is apparently open 24/7.
The Memorial is a square-section unpolished pillar supporting a bronze statue.
The pillar is an unusual GRANITE
Granite is a coarse-grained igneous rock that forms deep within the Earth's continental crust through the slow cooling and solidification of molten magma. Because this process happens underground, the magma remains hot for a very long time, allowing large, interlocking mineral crystals to grow, giving granite its characteristic "speckled" appearance.
It is a rock with prominent crystals mainly of quartz, feldspar, and mica, which form a coarse-grained structure. The slow cooling of magma deep within the Earth's crust allows crystals to form - the slower the cooling the bigger the crystals
Quartz, often translucent and milky in appearance, imparts hardness and durability. It’s the reason why granite is such a robust and long-lasting stone. Feldspar, on the other hand, is responsible for the variety of colours we see in granite. Depending on its type and concentration, feldspar can infuse hues of pink, creamy white, or even stark green or red into the stone. Lastly, mica lends granite its shimmer, tiny flakes or sheets that glisten when light hits the surface.
Most granite is formed in areas of intense tectonic activity:
- Mountain Building (Orogeny): Large masses called batholiths often form the cores of major mountain ranges, such as the Caledonian Mountains or the Southwest of England
- Subduction Zones: Fluids from subducting oceanic plates lower the melting point of the overlying crust, generating magmas that crystallise into granite.
- Hot Spots and Rifting: In some cases, granite forms away from plate boundaries where "hot spots" melt the lower continental crust.
The Formation Process
The journey from molten magma to solid rock involves several geological stages:
- Magma Generation: Granite starts as magma rich in silica and alkali metals, typically generated in the Earth's mantle or lower crust. This often occurs at subduction zones (where one tectonic plate slides under another) or during continental collisions.
- Intrusion: Because magma is less dense than the surrounding solid rock, it rises slowly toward the surface. Instead of erupting as a volcano, it often gets trapped in pockets or "magma chambers" miles beneath the surface.
- Slow Cooling: Deep underground, the surrounding rock acts as an insulator, slowing the heat loss. It can take millions of years for the magma to fully solidify.
- Crystallisation: As the temperature drops, different minerals crystallise at different times (fractional crystallisation).
- Feldspar and Quartz are the primary components, forming the bulk of the rock.
- Mica (biotite or muscovite) and Amphibole provide the dark flecks and silvery sparkles.
- Uplift and Erosion: Granite is only seen on the surface after tectonic forces push the crust upward and millions of years of erosion wear away the miles of overlying sediment and rock.
The cooling time of granite is directly proportional to its crystal size, with slower cooling rates in large, deep-seated intrusions (plutons) allowing for the growth of large, coarse-grained crystals (>5mm), while faster cooling in smaller intrusions or surface flows produces finer grains.
Cooling Times and Crystal Size Relationships
- Coarse-grained Granite (>5mm): Form in large intrusive bodies (batholiths) where insulation from surrounding rock allows for very slow cooling over long geological timescales (often millions of years).
- Medium-grained Granite (2-5mm): Form in smaller intrusions such as sills and dykes, which cool faster than large batholiths.
- Fine-grained Granite/Rhyolite (<1mm): Form from rapid cooling, often near the surface or in smaller intrusions (dikes) where the magma is quenched rapidly, resulting in fine-grained igneous rock. It could solidify in thousands of years rather than millions.
| Place where formed (NB - the slower the cool - the larger the crystals) |
Estimated Cooling Time |
| Deep underground - large batholith |
± millions of years |
| Smaller / less deep underground intrusion |
± 500,000 years |
| Small / rapidly cooled sills and dikes nearer the surface |
± 20,000 years
|
Examples of polished granite:-
LOGGING REQUIREMENTS
Please complete the tasks and questions below, and send me your answers as you log your find.
ANSWER FOR YOURSELF _ GROUP ANSWERS WILL NOT BE ACCEPTED .
1. Which of the blocks of granite (pictured above) looks most like the pillar (a) , (b) , (c) - OR none of them? (=d)
2. What, if any, noticeable differences do you see between the pillar granite and the block(s)? Mention colour, grain and any exceptional crystals you see.
3. Estimate the percentage of each mineral in the rock. (Pick a small area to work with, say 2" or 50mm square.)
4. Based on the information above, and the rock in front of you, estimate how long the rock might have taken to cool.
5.Take a photo resembling, the first sketch above, of yourself (or personal item) with the BANDSTAND and the SCULPTURE . Not too close to avoid a spoiler! You should attach it to your log.
LOGS WITHOUT A RESPONSE AND THE REQUIRED PHOTO WILL BE DELETED.
ACKNOWLEDGEMENTS
Cadw Listing database
Geological Society of London
SandAtlas
Stone Surfaces
Wikipedia
FTF honours go to --- sniffadogz ---
PLACED FOR 'BLUE SWITCH DAY' 2026