Ian Francis and Bruce Yardley (UK)
The violent eruption that occurred near the Pacific island of Tonga in January 2022 reminded the world of the ferocious power of volcanoes. The most destructive eruptions can bury huge areas in layers of ash and lava, generate tsunamis, and even alter the Earth’s climate by injecting vast quantities of ash and aerosol droplets high into the atmosphere.
Modern Britain is luckily far from any active volcanoes (Vesuvius in Italy, and the volcanoes of Iceland are the nearest to us), but this was not always the case: geological evidence shows that around 455 million years ago, during the Ordovician Period,an intense, but short-lived, periodof volcanic activity took place inwhat is now the Lake District.
The geological record of that activity is mainly found in Lakeland’s high and rugged central fells, stretching from Ennerdale and Wasdale in the west, across to Haweswater in the east. Most of the Lake District’s highest fells are found in this central belt, including the Coniston fells, Pillar, Great Gable, Kirk Fell, the Sca Fells, Esk Pike, Crinkle Crags, the Langdales, Helvellyn and High Street (Fig. 1).
As far back as the early nineteenth century, those familiar with the rocks and landscape of the Lake District (such as Jonathan Otley of Keswick) recognised that the rocks of the central fells differed from the dark grey slates of the northern fells around Skiddaw, and from the slates and sandstones which form the low-lying hills of the southern Lakes around Windermere. Otley called these hard rocks of the central fells ‘green slates and porphyries’.
However, it wasn’t until the early twentieth century that the volcanic origin of these rocks was finally established, and they are now known as the Borrowdale Volcanic Group. It has taken geologists 100 years to unravel their complexities, and there is still much to be discovered.
The oldest Lakeland rocks are not volcanic – they are in fact mudstones and sandstones (the Skiddaw Group), which were deposited in an ancient ocean, the Iapetus Ocean. But around 455 million years ago, the ocean retreated from what would become Cumbria, and a volcanic arc built up in its place. Increasingly violent volcanic eruptions continued for some five million years, eventually burying all of what is now the Lake District under layers of ash and lava, which together measure at least six kilometres in thickness (although the thickness of individual layers within this sequence may vary greatly).
Volcanic rocks: a very brief, pain-free, introduction
Volcanic rocks are formed from molten rock (magma) erupted at the surface. There are two main types:lavas, formed when magma flows from a volcano; and pyroclasticrocks, which consist of material thrown out explosively during eruptions.
Different volcanoes erupt different sorts of lava. Some solidify to very dark rocks, such as the basalts of Hawaii or Iceland; others to pale-coloured rock such as dacite. Basalt lavas are runny and can flow for many kilometres, while dacite builds up as slow-moving mounds of ‘sticky’ (or viscous) lava. In the Lake District, basalts are rare; initially, most of the lavas were andesite, which flowed moderate distances.
However, as time went on, there was a trend in the Borrowdale volcanics towards fewer andesite lavas, and an increasing proportion of paler, viscous dacite. At the same time, the eruptions became increasingly violent and much of the upper part of the Borrowdale Volcanic Group is composed of pyroclastic deposits or tuffs. Some, known as welded tuffs, were so hot when they formed that the fragments of rock and magma droplets fused together. Often, volcanic ash has been reworked by streams and rivers to form volcaniclastic sandstones.
Both the colour of volcanic rocks, and the ease with which their lavas flow, reflect their composition. The most abundant constituent of lavas is silica (silicon dioxide, SiO2). The mineral quartz is pure silica, but silica is also present in many other minerals and almost all lavas. At an atomic level, silica forms tetrahedral building blocks. In molten lava, these silica building blocks are mixed with various sorts of metal atoms, which latch on to the silica if they get a chance.
Nevertheless, if there is enough silica present, the silica tetrahedra link together to form long strings:the lower the proportion of silica in the lava, the shorter the strings. When the lava moves, longer silica strings tend to get tangled up, rather than moving freely, and so these magmas are more viscous. Imagine trying to pick up a mouthful of spaghetti from a plate, compared to a mouthful of macaroni – with spaghetti, you are likely to get all or nothing! Basalt lavas have the least silica (and therefore the shortest strings), so they are the runniest. Dacite, in contrast, is silica-rich and so is sticky and builds domes; while rarer rhyolite is the most viscous of all. Andesite is intermediate between basalt and dacite, in terms of stickiness.
Another important factor is the presence of gas. As magmas rise and solidify, they give off dissolved gas. Generally, silica-rich magmas contain more gas than runnier, silica-poor ones. The stickier the magma, the harder it is for gas to escape. As a result, gas pressure builds up in silica-rich volcanoes, leading to explosive eruptions. The 1980 eruption of Mount St Helens in the USA was triggered by the loss of pressure in the magma body when part of the bulging mountainside above was carried away in a landslide. But, whatever the cause, once the pressure on the molten rock is released, gas bubbles will expand and the pent-up gas will cause an explosion.
A key feature of volcanic rocks is the size of the crystals from which they are made. Some lavas cool so rapidly that crystals do not have time to grow at all, resulting in volcanic glass. In most Lake District lavas however, cooling was slow enough for tiny crystals to form, and occasionally – when magma began to cool deep inside a volcano, before being erupted – crystals had time to grow big enough to see with the naked eye. Many of the lavas erupted in the lower part of the Borrowdale volcanic sequence contain such crystals (usually a few millimetres across, and white or grey in colour). These are crystals of plagioclasefeldsparand they are typical of andesite lava (Fig. 2).
Many modern volcanoes erupt andesite lavas and, on a world map, they occur in long, gently curving lines. Some are at the margins of continents, such as the Andes of South America (from which the word ‘andesite’ is derived), while others are island chains, such as the Aleutian Islands in the North Pacific, or the Lesser Antilles in the West Indies. Because of their shape, these chains of volcanoes are called volcanic arcs; and they develop where two tectonic plates, converge. As one plate is carried down into the Earth’s interior, it is heated and gives off water, which triggers melting in the plate above.
Andesite is typical of arc volcanism; it is believed that the andesites of the Borrowdale Volcanic Group were erupted from arc volcanoes near the southern margin of the Iapetus Ocean. The sequence of Borrowdale volcanic rocks, from andesites early on, to increasingly violent eruptions of rhyolites and dacites through time, can also be seen in modern-day volcanic arcs.
The Borrowdale Volcanic Succession
The lower part of the Borrowdale volcanic sequence is dominated by lavas erupted from low-lying volcanoes.The lavas, known as the Birker Fell Formation (named after a hill in southwest Lakeland), spread out across the land surface as extensive flows, between 10m and 200m thick. In places, these lava flows give rise to a distinctive step-like topography, seen very clearly in the profile of High Rigg (Fig. 3) between St John’s in the Vale and the A591 south of Keswick, or further north at Eycott Hill near Berrier (a Cumbrian Wildlife Trust reserve, with parking and public access).
At times, magma spread sideways before it reached the surface, into weaknesses between older layers, forming sheets of volcanic rock known as sills. It can be hard to distinguish sills and lava flows in the field, but it is thought that many of the andesite sheets that make up the Birker Fell succession, well displayed on Honister Crag for example, are sills.
A very characteristic feature of many of these andesite sheets is the presence of peculiar rocks known peperites. These are composed of a jumbled mixture of lava and sediment, whichformed when hot andesite magma came into contact with wet sediment, causing explosive fragmentation to occur. The presence of peperite along the upper margin of an andesite sheet (as in Fig. 4) is a sure sign that the igneous body in question is a sill rather than a lava flow.
In places, ash from eruptions settled in (or was washed into) lakesas volcaniclastic sediments, and the action of water separated out fine grained material, often preserving sedimentary structures, which give a detailed record of the sedimentary environment. One such lakebed eventually turned into the green slate of Honister, prized as an ornamental building stone since the eighteenth century, and used on the roof of Buckingham Palace.
Over time, the nature of the volcanic activity changed, and there was a trend towards increasingly explosive eruptions. Clouds of intensely hot ash surged down the flanks of volcanoes and spread across the landscape, leaving thick ash deposits, the largest of which can be traced over huge areas of the central fells. These explosions were much larger than earlier ones, driven by the rise in silica concentration in the lavas over time, and the increase in dissolved gas.
It now seems clear that the biggest explosive eruptions in the Lake District produced calderas. These large craters are formed when an eruption empties the magma chamber beneath a volcano, and the remaining surface rocks collapse into the empty chamber. Oregon’s Crater Lake is a famous recent example of a caldera (Fig. 7).
The wide extent of the ash beds, and the sheer volume of material erupted, indicates that the Lake District calderas were on a par with the largest volcanic eruptions of the past 100,000 years. Nevertheless, these ancient calderas leave no surface expression in the fells; subsequent metamorphism means that lavas and pyroclastic rocks can be of similar hardness and geological boundaries can be hard to unravel. The calderas were only identified late in the twentieth century, when geologists from Sheffield and Liverpool universities, with knowledge of active modern volcanoes, reinvestigated the Borrowdale Volcanic Group.
It seems the Lake District calderas underwent ‘piecemeal’ collapse, in which the roofs of the magma chambers fractured into large sections as they sank inwards, creating a geologically complex jumble of separate blocks.Through painstaking investigation of the ash beds in the central Lake District, many mapped in detail by BGS geologists, the volcanologists were able to determine the approximate boundaries of the calderas and reconstruct their complex eruptive histories (Fig. 8). Ancient calderas have now been identified at Langdale and Haweswater, but the largest is the Scafell Caldera, which occupies some 150 km2 between Scafell and Ambleside.
Modern calderas often contain lakes. The crater lake of Lake Taupo in New Zealand, for example, lies close to still-active volcanoes (Fig. 9); and ash from their eruptions has been carried by rivers into the lake.
In the same way, ash from the Lake District volcanoes was carried by rivers into caldera lakes to form volcaniclastic sandstone beds. The most economically important of these, the Seathwaite Fell Sandstone, has been extensively quarried for slate in the Coniston Fells, Langdale and Troutbeck.
Where the Cumbrian volcanic and volcaniclastic rocks differ from their modern equivalents is in their ubiquitous green colour. The Borrowdale Volcanic rocks were metamorphosed during the Acadian Orogeny, around 400 million years ago. This resulted in the replacement of volcanic glass, and much of the igneous mineralogy, with hydrous minerals stable at lower temperatures such as the ubiquitous chlorite, and it is this which imparts the green colour we see in the slates today.
By around 450 million years ago, the volcanic activity died out, probably because the arc accreted to the adjacent continent and subduction ceased. The volcanoes were gradually worn down by erosion, and sea levels rose, eventually submerging the volcanic landscape.
Beneath the surface
The lava and ash that spewed from the Ordovician volcanoes of the Lake District was fed from magma chambers that lay beneath the surface. Much of this magma never erupted, but slowly cooled and solidified perhaps 3 to 7km below the surface, forming granite. Although granite has a similar chemical composition to the dacite and rhyolite lavas found at the surface, the slow rate of cooling allowed time for larger crystals to grow, giving granite a coarser texture.
Uplift and erosion of the Lake District in the geological past has revealed Ordovician granites at the surface around Eskdale and Ennerdale, with a smaller outcrop at Threlkeld near Keswick. North of Skiddaw, at Carrock Fell, a distinct type of coarse-grained igneous rock, known as gabbro (similar in composition to basalt), also marks the site of a former magma chamber. Geophysical measurements show that these outcrops are part of much more extensive bodies of granite lying deep below much of the Lake District (and indeed much of the north of England).
While these Ordovician granites are contemporary with the Borrowdale Volcanic Group, there are smaller granite outcrops in Cumbria, in particular, the Skiddaw and Shap granites, which are of Devonian age, formed around 400 million years ago, towards the end of the Acadian Orogeny, when the Iapetus Ocean finally closed.
You can read more about the volcanic rocks of the Lake District, as well as many other aspects of the region’s fascinating geology in The Lake District: Landscape and Geology by Ian Francis, Bruce Yardley and Stuart Holmes (Crowood Press, 2022). This book includes excursions to localities where volcanic rocks are exposed.