As the Borrowdale Volcano reached maturity, the magma which fuelled its eruptions became thicker and stickier and the passageways and cracks along which the magma crept to the surface became clogged and blocked. Yet still molten rock and hot gases worked their way up from the depths of the Earth into the magma chamber beneath the volcano, where slowly, relentlessly, the pressure began to build. |
For many thousands of years the Borrowdale Volcano lay quiet, locked in a silent struggle to contain the fire-storm that was gathering beneath its roots. As the pressure mounted, the volcano began to shudder and groan. The lavas and tuffs of which it was made began to crack and shift uneasily along scars left by previous
eruptions. Fissures opened up sporadically, releasing brief flows of sticky lava or fountains of incandescent ash. As the churning mass of hot magma finally surged upwards, the great volcano heaved and arched its back until, with a devastating explosion, one of its flanks was torn open and thick clots of magma gushed out of the gaping wound, foaming and frothing as the pent-up gases trapped within burst their way to freedom. |
Driven by the rush of escaping gases, a seething cloud of fine ash and incandescent rubble plunged down the flanks of the volcano amid volleys of explosions. Hugging the ground, this burning cloud, this nuée ardente, swept unhindered up over ridges and down into valleys, snatching up rocks and loose debris that lay in its path to add to its ferocious power. Such was its speed that within minutes the entire landscape was smothered under a scorching, smoking layer of ash and rubble. Yet still the eruption continued unabated, billowing clouds of debris surging out of the volcano until the ash and rubble lay hundreds of metres deep, and the heat and pressure became so intense that the fragments welded together and began to flow, buckling and folding like lava. Little balls of hot pumice, frothy clots of gas-filled magma which sprayed out of the mouth of the volcano, were flattened and stretched as they became caught up in the flowing debris; today they form fiammes within the rock: dark, flame-like streaks of glass within the fine, pale ash.
These huge pyroclastic surges and flows were slow to cool, and steam continued to rise from their surface many years after the eruption had ceased. Today they form pale, flinty rocks which have been given the name of ignimbrites. They occur in abundance on the middle and upper slopes of the central fells and you will see them if you climb towards the summit of Crinkle Crags, Bowfell or the Langdale Pikes. Rich in silica, their surface often weathers to white, within which the dark streaks of the fiammes are easily seen. |
Even as the nuée ardente poured its shroud of devastation over the land, dark, swirling clouds of fine ash rose from the mouth of the volcano and climbed towards the heavens. Caught by the wind, this suffocating smog spread swiftly across the sky, blotting out the sun until the scene was lit only by the firey glow of the erupting volcano. When, after many dark and terrible days, the force of the eruption finally began to wane, this billowing tower collapsed to the ground, burying the scorching ignimbrite beneath yet another layer of ash.
It is believed that the Scafell region lies at what was the epicentre of this colossal eruption, which left the Borrowdale Volcano severely weakened: the roof of the now empty magma chamber soon began to cave in, and in the years that followed the great mountain slowly collapsed into the gaping void. Huge slabs of rock crashed down into the ever-deepening crater, where they were buried by landslides of loose debris. Sporadic eruptions of ash and lava added to the chaos and confusion, while here and there, tongues of lava forced their way directly into the loose piles of ash, where they hardened to form solid sills. At the bottom of the steep-sided caldera, lakes began to form in the hollows and depressions of its broken surface until eventually it sank below the level of the sea and was completely submerged.
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The ancient scar-line followed by Dunmail Raise marks perhaps its eastern edge, while near to the summit of Bow Fell, above the ignimbrites at Three Tarns, you will find spectacular cliffs of bedded tuffs whose complex and beautiful structures were formed when ash from a long series of eruptions fell into the crater lake and slowly settled in layers on the lake bed. Like the Skiddaw Slates, these sedimentary rocks tell the story of their own creation in minute detail: the colour, texture and thickness of each layer of ash records the strength and duration of the eruption which produced it; each shudder and tremor of the collapsing volcano shows as a distortion or dislocation in the delicate layers of ash as they slowly hardened on the bed of the lake; even the waves lapping on the shores of the shallow crater lake have left their mark as ripples in the ash layers. |
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