Geology Field Studies from Lleyn to Plynlimon Yr Arddu and Llyn Gwynant 15 Bedded Pyroclastic formation Intrusive rhyolite Lower Rhyolitic Tuff Microgabbro Figure 432: Cwm Eigiau mudstone, siltstone Felsite Field excursions. Nant Ffrancon siltstone In this chapter we continue our investigation of fracture zones (fig. 433). The Snowdon centre is the Snowdon Volcanic Group by examining the unique in lying at the intersection of a series of Snowdon volcanic centre, which was one of the deep fractures, which may explain the crustal three active centres during the second phase of instability in this location which led to the most Ordovician volcanic activity in Snowdonia. prolific eruptions of any of the late Ordovician volcanic centres in Wales. Deep crustal fractures are thought to be very important in controlling the distribution of Following the discharge of the Pitts Head Tuffs volcanoes in North Wales. Both the Llwyd Mawr from the Llwyd Mawr centre to the west, activity and Crafnant centres are associated with particular moved to the Beddgelert area as the Snowdon 250 Chapter 15 Yr Arddu and Llyn Gwynant centre began to develop. Magma accumulated in several major northeast-southwest fractures (fig. a high level chamber beneath central Snowdonia, 434). Volcanoes in the area of Yr Arddu erupted causing uplift of a horst of crustal rock bounded by ignimbrite flows and coarser pyroclastic ashes of the Yr Arddu Tuff formation. Conwy valley Menai Strait fault zone CRAFNANT Figure 433: Relationship of Llanberis Pass volcanic centres fault zone to deep crustal fractures SNOWDON Gwynant fault zone LLWYD Nantmor MAWR fault zone Rhobell fault zone Cwm Pennant Vale of Ffestiniog fault zone fault zone Trawsfynydd fault zone surface fault volcanic centre deep crustal fracture zone As volcanic activity continued, the focus of the The collapsed caldera was oval in shape with a eruptions moved north-eastwards along the longer axis of about 12 kilometres, extending central fault zones to Snowdon and the Llanberis furthest in the direction of the northeast- Pass. As magma continued to accumulate in the southwest fracture zones. Much of the material high level magma chamber, the overlying crust erupted from volcanic islands accumulated on the became structurally weakened. Collapse took sea bed in the central subsiding area of the place within a ring fracture to produce a caldera caldera, but ash flows also travelled outwards on the sea bed. Eruptions continued, creating a across the sea floor to the areas of Cwm Idwal, series of emergent volcanic islands around the Dolwyddelan and the Conwy valley. Subsidence caldera fracture. Enormous volumes of ashes were was greatest in the northern area of the caldera, erupted, first as the felsic Lower Rhyolitic Tuff with the largest thickness of ashes accumulating Formation, then the mafic Bedded Pyroclastic around Snowdon and the Llanberis Pass. formation, and finally the felsic Upper Rhyolitic Tuff formation. 251 Geology Field Studies from Lleyn to Plynlimon Nantmor Gwynant fault zone fault zone Hebog-Idwal fault zone Yr Arddu fault Llanberis Pass fault zone Cwm Pennant caldera SNOWDON fault zone ring faults Gwynant fault zone Nantmor fault zone Figure 434: Evolution of the Snowdon volcanic centre. (above) Early uplift. (below) Late caldera collapse. Many sill intrusions of both felsic and mafic the crust may have changed over time. composition reach the surface around the In the field excursions for this chapter we visit: the Snowdon volcanic centre, outcropping as arcs mountain of Yr Arddu, composed of ignimbrites outside the caldera margin. These are cone- and pyroclastic ashes of the Yr Arddu Tuff sheets, formed as different magmas were formation; the copper mines of Cwm Bychan, and forcefully intruded upwards and outwards from the Aberglaslyn pass where the Lower Rhyolitic the high level magma chamber. The magma in the Tuff is exposed; and the Gwynant valley at Llyn chamber may have become gravity stratified, with Dinas where we see breccias which accumulated silica content varying with depth, or the within the subsiding sea floor caldera. composition of the melt rising from deeper in 252 Chapter 15 Yr Arddu and Llyn Gwynant Yr Arddu 4 miles: approximately 3 hours © Crown copyright 2019 OS 100061048 Nant Ffrancon siltstone Felsite Lower Rhyolitic Tuff, felsic Intrusive rhyolite Figure 435: Cwm Eigiau sandstone Microgabbro Field excursion. Cwm Eigiau mudstone, siltstone This excursion examines the sediments underlying Start: A car park is provided in Croesor village the Snowdon volcanic centre, which have been [SH632447]. intruded by cone sheets, then continues upwards through the ignimbrites and coarse pyroclastic 1: Take the road through the village past the ashes of the Yr Arddu Tuff formation. We arrive at chapel in the direction of Yr Arddu. Pass through a the mountain summit of Yr Arddu where a volcanic gate at the end of the road to join a footpath vent is well exposed. which climbs up the hill through woodland. Yr Arddu lies on the line of a deep crustal fracture A rocky ridge to the right of the footpath is formed which has become a zone of subsidence. Volcanic by a sill of porphyritic microgranite. This is a cone ashes are preserved within a synclinal trough along sheet which dips north-westwards towards the the fracture, interspersed with sandstones and centre of the caldera. It is one of a group of cone finer sediments of the Cwm Eigiau formation. sheet intrusions which outcrop along the Croesor valley, producing the impressive mountain peak of Cnicht at the head of the valley. 253 Geology Field Studies from Lleyn to Plynlimon Figure 43 6: (left) Ridge above Croesor village formed by a sill of microgranite. (right) Detail of the porphyritic microgranite, with larger feldspar crystals in a fine quartz-feldspar matrix. 2: As the path reaches the top of the woodland, ridge in central Snowdonia, as buoyant magma take the footpath branching to the right along a filled the underlying chamber. broad valley with the peak of Yr Arddu to the Continue up the valley to the point where the north. waste tips of old slate workings in the Nant Slates, siltstones and fine-grained sandstones Ffrancon formation are seen. The slate produced outcrop alongside the path, and have been shown would have been of poor quality, perhaps used by fossil evidence to belong to the Caradoc stage only locally for roofing cottages and agricultural of the upper Ordovician. The siltstones and buildings. sandstone have current bedding and parallel 3: Leave the footpath and cross the valley laminated bedding which are characteristic of a towards the mountain of Yr Arddu, passing shallow near-shore environment. This suggests outcrops of the Lower Rhyolitic Tuff above the the uprise and emergence of the early volcanic stream. Figure 4 37: (left) Ignimbrite sequence exposed in the cliffs above the stream. Columnar jointing is visible. (right) Unwelded ash layer at the base of an ignimbrite flow. The Lower Rhyolitic Tuff formation consists mainly unwelded basal ash layer (fig.437). This grades up of ignimbrite flows, laid down on the margin of the into a more massive welded flow, which may volcanic ridge either subaerially or in shallow develop columnar jointing. water. Individual flows often begin with a thin, 254 Chapter 15 Yr Arddu and Llyn Gwynant 4: Head for the prominent grassy slope which At the head of the grassy slope, on a flatter area climbs the side of Yr Arddu, following the line of a below the summit slopes of Yr Arddu, is another fault. By the dry stone wall at the base of the outcrop of very coarse rhyolite blocks within an ash slope, examine outcrops of a very coarse matrix. This may again represent a debris flow pyroclastic deposit. This contains blocks and slabs from the edge of the volcanic cone. of rhyolite in an ash matrix, and exhibits a crude 5: Continue to the summit, crossing rocky terraces flow structure. It is likely that this material is a composed of flow banded rhyolite. We have now mixture of ash and rubble which slid off the side of reached the main rhyolite dome of the Yr Arddu the volcano and accumulated in shallow water volcano. The banding picks out the flow paths of around the edge of the volcanic island. the very viscous magma ascending through the volcanic neck, which were contorted into tight folds in places as the magma was squeezed around obstructions in the vent. Figure 438: Vent agglomerate, Yr Arddu. Ascend the grassy slope, observing the pyroclastic rocks in the cliff face above the path. Ashes with different dominant particle sizes are seen, and may be the products of a mixture of explosive air fall eruptions vertically upwards from a vent, and horizontal outflows of ash. Figure 440: (top) Outcrop of flow folded rhyolite below the summit of Yr Arddu. (bottom) Detail of flow folding. Figure 439: Pyroclastic deposits, Yr Arddu. 255 Geology Field Studies from Lleyn to Plynlimon We can now produce a model for volcanic activity extrude, forming a dome with rapidly solidifying at Yr Arddu. Earlier eruptions were explosive, margins. Blocks and finer material slumped from producing a mixture of ignimbrite ash flows and air the margins of the dome under gravity, forming the fall pyroclastics. As the activity came to a close, the coarse debris flows seen earlier on the slopes of magma became more viscose through loss of the mountain. steam and other gases. The rhyolite continued to flow foliation and flow folding gravity slumping of rubble and ash Figure 441: Rhyolite eruption. From the summit of Yr Arddu, we get a panorama 6: Descend from the summit to an area of flatter across the surrounding valleys and mountains. Yr heathland to the south-west.