The origin of the Borrowdale Volcanic rocks lies deep within the Earth’s mantle, many miles beneath the surface, where the violent death throes of the Iapetus Ocean were creating a furnace and the minerals within the rocks were beginning to melt. Quartz, feldspar and pale mica were the first to succumb: pools of red-hot molten minerals started to coalesce and little streams of thick, sticky magma began once again to creep along cracks and fissures, causing the Earth to tremble as they forced their way up into the heart of the Skiddaw Slates. There, a mile or so beneath the surface, the magma collected, simmering restlessly.
The young Skiddaw Slates arched and bulged as they struggled to contain the seething cauldron of magma. At the surface, the rocks began to crack open, smoking with poisonous gases, and the land began to rumble and shake. Suddenly the magma surged upwards and the Skiddaw Slates were ripped apart with a volley of colossal explosions. Huge lumps of shattered rock were hurled into the air and came crashing back to the ground onto a fast-growing pile of smoking rubble. Billowing plumes of incandescent ash and pulverised rock rose in a towering, swirling column, swept aloft by the rush of escaping gases, until the choking clouds of dust were caught by the wind and spread far and wide, casting a veil of darkness over the land. |
Boiling lava gushed out of the gaping hole left by the explosion, and the bare rocks of the Skiddaw Slates were inundated in a flood of molten rock, churning and bubbling with trapped gases. Out in the open now, the red-hot lava began to cool and a dark crust formed on the surface, only to be torn into fragments and swept along by the fast-flowing lava, sinking down into the hot liquid. But as it fled further from its fiery cauldron, the flow slowed to an ooze and the lava began to crumple up into contorted folds, held back by the thickening crust, bursting free here and there to flow a few more feet, glowing red-hot, then finally cooling to grey, hardening to black.
The volcano poured out its heart for many days and many nights. When finally it fell quiet, the Skiddaw Slates lay buried beneath a thick, crumpled, contorted blanket of smoking lava. A few pockets still glowed red or orange, but the lava was cooling rapidly now, so rapidly that there was little time for crystals to grow within it, and so it turned to glass. Onto its blackening surface fell a steady hail of cinders and shattered rock, until at last this, too, slackened off to a sprinkling of fine powder which drifted gently down, bringing an eerie peace to this scene of devastation.
But in the aftermath of the eruption came violent thunderstorms. Torrential rain poured over the steaming piles of rubble around the volcano and over the crumpled blanket of lava, washing away the fine powdering of ash, the loose cinders, and the broken, grey fragments of Skiddaw Slate. A slurry of mud and boulders swept downhill, to be smeared over the rocks beyond the cooling lava when the flood subsided.
This chaotic residue from the first eruption of the Borrowdale Volcano is known to geologists as “volcaniclastic mud”, but it is far more beautiful than this name might suggest. It can be found in and around Cat Ghyll, on the eastern shore of Derwent Water, sandwiched between the youngest layers of the Skiddaw Slates and the oldest lavas of the Borrowdale Volcanics. The rocks hereabouts are stained red and bruised purple, not, it seems, by the trauma of the volcanic eruption, but by iron seeping through them at some later date. |
There would be many more outpourings of lava from the Borrowdale volcano: tongues of red-hot liquid spreading out from vents and fissures, snaking across the barren landscape; cascades of molten rock pouring over cliffs into old eruption craters; roaring fountains of fire forming graceful curves against the night sky. Sometimes the lava flowed for months, sometimes for just a few hours. Sometimes it spread for miles, thinning out as it flowed; sometimes it would be contained within a valley or hollow. |
The rocks it formed as it cooled are tough and resilient, but they carry to this day the marks of the violence and chaos that attended their birth. Some show bands created by the flowing lava, or the folds and contortions that formed as it cooled. Some contain a jumble of broken fragments, suspended in petrified swirls. Some are pock-marked with holes where gas was trapped in the cooling lava; others contain the shattered casings of burst bubbles, where the gas escaped from the viscous liquid. Many cracked as they cooled and shrank, and intricate engravings or proud, angular structures emerged from the molten mass. Millennia later, some of these cracks became conduits for hot fluids, which crystallized as they cooled to form delicate networks of mineral veins.
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But perhaps most fabulous of all, draped across the path in Grizedale lies an exquisite patchwork quilt, unrivalled in its antiquity, preserving 460 million years of memories.
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Each eruption added a new layer of rock, thickest around the vent, thinning into the distance, to form a gently sloping cone. Eventually the first vent fell quiet, its feeder pipes choked with solidified magma, but by then others had already appeared, smoking fissures and yawning craters scattered across the landscape. These, too, developed cones which gradually merged, forming a broad, uneven plateau, over which rivers of lava continued to flow sporadically.
Yet there were long periods, many thousands of years perhaps, in which the volcano fell completely silent, biding its time, gathering its forces. When the breach finally, inevitably came, instead of the usual outpouring of lava, pent-up gases, suddenly released, might blast their way out of the hot magma, shattering it into tiny droplets and blobs of froth, hurling them high into the air where within seconds they cooled to ash and were carried aloft in dark, towering, billowing clouds. |
Great lumps of rock, dislodged by the force of the exploding gases, fell close to the vent, dropping like bombs into the loose debris that had already accumulated there. A harsh, dark rain of ash and cinders cascaded down onto the uneven slopes of the Borrowdale Volcano, but the finest ash particles could ride on the wind and might travel long distances before they drifted down to earth.
Violent ejection from the mouth of the volcano was often only the first stage of a long and hazardous journey. Sometimes the fine ash dust climbed into the upper atmosphere only to find itself in the midst of a gathering rain- or hailstorm. Sticking to the surface of the raindrops or hailstones as they tumbled about in the wind currents, the dust collected around them in layers to form little pellets called accretionary lapilli, until their weight brought them down to the ground. These concentric layers may still be seen under certain conditions, earning the rocks in which they are found the name of bird’s-eye tuff. (Confusingly, rainspot slate owes its spots not to the rain but merely to the different sizes of the ash fragments of which it is made.) |
On the ground, the loose piles of ash and shattered rock were easily disturbed. Precariously placed fragments tumbled off the piles, causing minor avalanches as they fell. Earth tremors triggered by the eruption caused whole piles to shudder and collapse. In the torrential rainstorms caused by the vast quantities of steam escaping from the volcano, slurries of hot debris slumped downslope, merging to form fast-moving, steaming mudflows that swept across the landscape until they finally came to rest in some hollow or depression, a chaotic mass of ash, cinders, rock and lava, fragments of all shapes and sizes caught in a sea of mud. Further eruptions might bring more debris raining down to add to the chaos, or a lava flow to mask it, or a tongue of magma might force its way into one of the piles without ever reaching the surface, melting the ash as it advanced. Who knows what havoc was caused by this sudden intrusion of boiling hot magma? As it slowly cooled, insulated by the surrounding layers of ash, tiny crystals formed within it while the ash baked in its heat. |
As the volcano subsided and the land relaxed once more into a peaceful slumber, the finest ash continued to drift softly down, laying a shroud of fine powder that extended for many miles. Streams and rivers began to eat away at the edges of the debris piles, carving out channels and carrying away the fire-shattered fragments on helpless, halting, fitful journeys, rolling and tumbling along the river bed, abandoned for the summer on some shingle bank, only to be swept further downstream in the winter floods, until finally they settled thankfully to rest in the tranquility of a lake or lagoon. Whatever their individual story, whatever adventures or traumas they experienced, these pyroclasts, these fire-shattered fragments, would eventually become rock. What once was ash became tuff, a dull name perhaps for a family of rocks of immense character and diversity: boulder-strewn block tuff close to the vent; coarse tuff, rough and knobbly; lapilli tuff, scattered with little stones; bedded tuff laid down in layers by a series of eruptions; graded tuff, the fragments getting smaller as the eruption lost its power. The finest tuffs are easily mistaken for lava, so tiny are the dust particles of which they are made. |
Those great sculptors, the wind and the rain, have worked on the tuffs for many millennia. Sometimes they mask the detail, concealing it beneath a dull, grey crust, but often they chip painstakingly away at weaknesses in the rock, rounding the edges and revealing the breathtaking beauty of its structure. |
And so the years went by: brief outbursts of violence and chaos followed by centuries of peace and tranquillity in which the lava and ash cooled, settled and turned to rock. The Borrowdale Volcano grew to a great height, a broad plateau extending for many miles, its uneven surface fractured with smoking fissures. The far-off days of its youth are unimaginably distant now, yet we can still see, stretching in an arc from Ullswater in the north-east, across Thirlmere, Borrowdale and Wastwater to the Duddon Valley, remnants of the layers of ash and lava that made up this once great volcano.
And yet the eruptions we have seen thus far were mere childish tantrums: the Borrowdale Volcano was slowly maturing and its power would eventually reach truly terrifying proportions.
And yet the eruptions we have seen thus far were mere childish tantrums: the Borrowdale Volcano was slowly maturing and its power would eventually reach truly terrifying proportions.