Yr Arddu, North Wales.When people discuss volcanism they usually refer to the large spectacular erup

Yr Arddu, North Wales.When people discuss volcanism they usually refer to the large spectacular eruptions caused by more volatile magma, though small-scale volcanism can also be as interesting. The volatiles within magma affect how explosive it will be. A highly volatile magma will produce large Plinian style eruptions, whereas low volatiles create Hawaiian style volcanism. A famous explosive, Plinian style eruption can be seen from the Mt St Helens eruption in 1980. During the Ordovician (485-443Ma), small-scale volcanism atop of Yr Arddu, North Wales, deposited a series of volcanic tuffs. Tuffs are an igneous extrusive (deposited on the Earth’s surface) consisting of ash and the debris of volcanic rock (tephra). This was the result of short-lived continuous volcanism across an area of about 2Km.An unstable volcanic dome began to build on top of Yr Arddu which collapsed due to gravitational forces. These domes form from a combination of factors including high viscosity (runniness), volatile content and silica content. Collapse of this dome deposited a series of high energy, pyroclastic flows. These flows consisted mainly of ash, pumice (light coloured, porous, volcanic fragments) and fiammé (flattened pumice) exhibiting eutaxitic textures which are pictured below. These eutaxitic textures form when pumices undergo flattening, producing planar characteristics. Much of these pumices and fiammé have been weathered out, making these textures more distinctive.Voids within this unit have been filled with drusy quartz, forming nodules. These nodules range in size from 1cm to ‘baseball’ sized, also pictured below. Much of North Wales was submerged at this time, but the extrusives in the area were sufficiently voluminous to rise above sea level. It is believed that Yr Arddu was rimmed by a shallow marine environment which is indicated by the tuffs lying disconcordantly (at an angle to the underlying rocks) on top of sandstones. These flows were reworked by marine processes, which left traces in the form of ripples marks and volcanic horizons on the surface of sandstone outcrops.After these tuffs were erupted, the conduit they originated from was plugged by a siliceous rhyolite dome. The high silica content of this intrusion produced a white to grey-white surface. These volcanics formed continuously, without influence from other processes, forming a distinct characteristic of this intrusive, flow banding.The process in which flow banding formed is still debated. It may have been the convective forces within the magma causing friction between itself, producing flow banding. Flow banding at Yr Arddu is extremely distinctive and can be confused with folding. Perhaps the most distinct feature atop of Yr Arddu can be seen in the form of siliceous nodules. Silica nodules like these represent the base and tops of acidic lava flows. Compared to the Arddu Tuffs, these nodules are larger, reaching >30cm across (pictured below).So there we have it, small scale volcanism can produce some exciting horizons, particularly at outcrops in North Wales. I visited all these outcrops on my independent mapping project this summer, which I found fascinating, particularly deducing their origins through thin section analysis (looking at the rocks under a microscope). Hopefully you’ll all like them too!-Elliott Pictures are all my own.Further Reading: Michael Watkins, 2006. Yr Arddu: An investigation into the behaviour of acid magma at the surface and shallow crust. MSc Thesis, Cardiff University.For more on styles of volcanism: http://geology.com/volcanoes/types-of-volcanic-eruptions/ -- source link
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