One from the archives (from 2013, with small additions). Karen Z on Snowdonia
The British Isles have an extreme diversity of geology. Travel 20 miles from just about anywhere, and you’ll find a region with a very different history. Almost every geological division of time left a mark somewhere here. I live on ice age sediment, thick, back-breaking yellow clay to be specific. But 10 miles from here are remnants of ancient coral reefs. The oldest rocks are well over 2 billion years. There is desert sand, deep ocean sediment, remnants of ancient Himalayas, flood basalts. The ancient tin mines of Cornwall are a geological inheritance. There is an ancient volcano, in the Lake District, a name which draws attention to its tranquility and beauty, rather than its volcanicity. There are reasons for this geological wealth. The British Isles were often on the edges of plates, where the action was. Not always – when you are on the edgeof an ocean, that ocean may one day vanish and you’ll find yourself in the dry centre of a newborn supercontinent. It has happened here. But one day that continent will break again, and often that break will be where the parts last came together. The ocean will return, and the British Isles once more find themselves on the edge. The isles are most at home when the ocean is close.
Snowdon is the highest mountain in Wales and an extinct volcano. The summit is easily accessible on foot, by train or you can practice your mountaineering skills there – as apparently did Sir Edmund Hillary to train for the ascent of Mt Everest. I walked it; mountaineering is not one of my skills.
Snowdon is well worth a visit as it is in a national nature reserve for rare flora and fauna.
Wales has had a marked influence on geology. Early geologists defined periods based on the names of Welsh villages. “Ordovician” is named after a Celtic tribe, the Ordovices.
Snowdon formed during the Ordovician period. It is comprised of tuff with sedimentary rocks and igneous intrusions, folded into a syncline. Around 450 million years ago, a caldera formed, producing ash flows of rhyolitic tuff deposits up to 500 metres (1,600 ft) thick. The current summit at 1,085 metres (3,560 ft) is near the northern edge of the caldera.
Much of Wales was under water in the Ordivician period. Wales was in a back arc basin between a subduction zone and in front of the Midland Platform. The basin had both submarine and sub aerial volcanoes. While I have focussed on the volcanic activity, there was a lot of sedimentary activity occurring as well so igneous and sedimentary rocks often form consecutive layers.
England and Wales were part of the Avalonia micro plate; Scotland was then sited on the Laurentia plate and did not join England and Wales until much later during the Caledonian Orogeny. In the early Ordovician period, Avalonia was a volcanic arc on the northern edge of Gondwana where the Iapetus ocean crust subducted under the Gondwana plate. As the Iapetus Ocean closed, Avalonia broke off from Gondwana and moved northward to eventually meet the Laurentia and Baltica plates.
The collision of the plates resulted in the Caledonian Orogeny around 490 ma to 390 ma, the building of a chain of mountains which stretched from the Appalachians, through Snowdonia and the Lake District to Norway.
For Avalonia, volcanism of the Tremadoc era, c. 510 ma, was island arc, whereas the subsequent volcanism of the Llanvirn and Caradoc eras was characteristic of a back arc environment. Wales, itself, was the site of a back arc basin with voluminous calc-alkaline basaltic and rhyolitic volcanic activity which ended with the meeting of the three terranes in the late Ordovician. Acidic lavas were produced by subduction and basaltic lavas were produced by thinning of the crust of the back arc basin.
Early volcanic activity in the Tremadoc was subaerial, followed by a period of submarine activity in the Caradoc and subaerial again in the Ashgill.
The Snowdon Volcanic Corridor
The Snowdon volcanic corridor was built in two phases: the Llewelyn volcanic group and the Snowdon volcanic group. These are separated by sedimentary rocks.
The Llewelyn group had five main formations: Conway – rhyolite and ash flow lavas; Foel Fras – andesitic lava and tuffs; Foel Grach – basaltic – andesitic lava; Braich Tu Du – acidic ash flow and rhyolitic tuff; and, the Capel Curig – formation of both subaerial and submarine acidic ash flow and tuffs.
The Snowdon Volcanic Group had three centres: Llwyd Mawr – an emerging volcanic island that produced acid ash flow tuffs that were partially contained in a subsiding caldera; Snowdon, itself; and, Crafnant –deep water acidic submarine tuffs. Snowdon evolved as initial ash flow tuffs from a series of fissures south east of the volcano. The caldera subsided as more ash was erupted. This was followed by pumice and rhyolite. Ash flow tuffs were partially contained by the caldera.
Basaltic rocks also occur alongside acidic: both intrusive and extrusive basalts are found and also hyaloclastites. The sequence is acidic followed by basaltic and then a final rhyolitic phase. It is bimodal: there are basaltic and rhyolitics, but not much in between.
At the end of the Caradoc, most volcanic activity ceased, although there were some minor eruptions later in the region. Successive orogeny episodes led to mineralisation of the faults in the region and further uplift. There are no rocks in the area that are younger than the Triassic period. Any that might have been deposited have since been eroded. Glaciation during the Cenozaic Ice Age and subsequent erosion from wind and rain formed the current landscape, revealing the underlying geology of Snowdonia.
The original caldera was 10 km across. It erupted three main sequences: the lower Lower Rhyolitic Tuff Formation, the Bedded Pyroclastic (basaltic) Formation, and the Upper Rhyolitic Tuff Formation. The basaltic melts came from a compositionally heterogeneous (volcanic arc) mantle which had as much as 60% crystallization during ascent. This fraction of crystallization still leaves SiO2 contents close to that of the primitive magma, so that the magmas remain basalt or basaltic andesite. Snowdon is heterogeneous, showing there were multiple magma chambers where this happened to a different degree. There may have been 500 km3 of magma in the chambers. The basalt erupted in two phases, the second one when the caldera underwent resurgence. The second phase had multiple basaltic flows. There was a third basaltic intrusion but that magma reached the surface only after it had evolved into rhyolite.
The rhyolitic magmas formed by crustal melt merging with one or more of these basaltic chambers, followed by further crystallization. This happened after all three basaltic intrusions. Three main rhyolitic layers formed in the million years after the first basaltic phase. The first one was 2km3, but the second ash layer was much larger and may have exceeded 603. This eruption caused the collapse of the first caldera. The resurgence after the caldera formation gave rise to the Bedded Pyroclastic Formation, comprising basaltic eruptions. This was followed by (and perhaps interspersed with) several rhyolitic eruptive phases, which together ejected 20 km3. The rhyolitic eruptions came from shallow, short-lived magma chambers associated with dome formation. One could envisage a mush left behind underneath the collapse caldera, with a series of smaller chambers within the mush or above it, closer to the surface.
And then it ended. Almost half a billion years later, we can still walk around the remnants of this ancient volcanics.
“British Regional Geology Wales”, M F Howells, British Geological Survey, 2007
“Geology of Snowdonia”, Matthew Bennett, The Crowood Press, 2007.