The volcanic rocks of the Lake District:

An Introduction

 

The central Lake District is dominated by rugged mountains made up of hard rocks, often dark green in colour, which were originally deposited as lava flows and ash beds around volcanoes. Geologists refer to them as the Borrowdale Volcanic Group.

It's hard to imagine it now, as you look out across Coniston Water, but 450 million years ago, during the Ordovician period, a massive explosive volcano was centred near the Scafell mountain range, with others nearby, and the rocks of Coniston Old Man started life as outpourings from these volcanoes. The eruptions that occurred here were highly explosive, comparable to those seen in modern times around the Pacific (Pinatubo, Krakatoa and Mount St. Helens) or the West Indies (Montserrat).

Explosive volcanoes develop where tectonic plates move together, and at that time the British Isles was the site of a plate boundary. A deep ocean, known as Iapetus, had existed here for many millions of years, and thick piles of muddy sediment had accumulated in it. As the plates began to converge the oceanic plate was subducted (pushed under) the continental plate causing melting in the Earth's mantle, with the molten rock (magma) rising to form volcanoes. A line of volcanoes would have formed a volcanic arc marking the margin of the continent. The volcanic arc extended across Cumbria, and built up land out of the ocean. The earliest eruptions were mainly lavas but later eruptions became extremely explosive and large amounts of volcanic ash and debris were ejected as pyroclastic flows; some remains in the original deposits but elsewhere ash was eroded and redeposited by rivers. Some molten rock was also intruded as sills (magma injected between existing layers). The volcanic deposits were in places up to 8 kilometres thick, which is large even by the standards of the most active volcanoes we know today. After this explosive interlude, the volcanoes sank beneath the waves and marine sediments were deposited again. The classic Lakeland Green Slates are fine grained volcanic ash from the Borrowdale Volcanic Group, that was redeposited in rivers and shallow lakes.

The eventual closure of the Iapetus Ocean as the tectonic plates came together squeezed the deeply buried sediments and volcanic rocks, turning them into slates. Mountain ranges developed across the northern part of the British Isles. This mountain building episode is called the Caledonian Orogeny and culminated about 400 million years ago, at the beginning of the Devonian period. At this time the landscape would have looked more like the Alps or Himalayas, except that only the most primitive land plants existed back then. Beneath, the deep crust began to melt and the molten rock or magma rose up to form a large igneous intrusion or batholith about 10 kilometres below the surface. This magma cooled very slowly to form coarse-grained granite, which we now see at the surface in the eastern Skiddaw Fells and at Shap. The slates into which the granite was intruded were of course heated very strongly and recrystallised to a metamorphic rock known as hornfels, which was the first Lakeland rock to be used for musical instruments.

For more information follow the link below

The Volcanic rocks of the Lake District

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Created by Ruskin Rocks Team, August 2010
Last updated: Rebecca Hildyard, 1 September 2010