When the Iapetus Ocean closed between 480 and 390 million years ago the continents on either side collided and were forced upwards to build an enormous mountain chain. Between 450 and 350 million years ago rocks melting in the roots of this Caledonian mountain chain due to the heat and pressure of collision generated a magma that forced its way up into the colder crust. In some areas pull-apart faulting within the mountain chain created ‘sedimentary basins’ where the Earth’s crust was thin enough for the magma to force its way to the surface and erupt as volcanos like the one at Eshaness.
As eruption followed eruption, layers of lava and pyroclastic rock built up rapidly to form a steep and unstable cone around a central vent. Volcanoes like this are called stratocone volcanoes. The cone at Eshaness eroded away long ago and the cliffs cut right through the flank of volcano, revealing layer upon layer of lava and volcanic ash.
Many volcanoes occur near the edges of the Earth’s tectonic plates – rigid slabs of Earth’s crust that float on the molten mantle beneath. The plates move around on top of the mantle at the rate of several centimetres a year. Sometimes they collide, causing one plate to sink beneath another. When this happens the sinking plate melts. Molten rock rises up through the crust above and collects in a magma chamber. Eventually, pressure builds up in the chamber and the magma erupts out onto the surface as lava. The edge of the Pacific Ocean where the Nazca plate is sinking beneath the South American Plate is one of the most volcanically active areas in the world. In Britain we are a long way from any geologically active zones and there have been no volcanoes here for about 60 million years.