Holyrood Park, Edinburgh: The volcano in the heart of Scotland’s capital city

The variety of tough igneous rocks that form this craggy, wild park in the heart of Edinburgh helped James Hutton explain deep time.

Arthur’s Seat. Image: Angus Miller.

The eruption of the Arthur’s Seat volcano and subsequent events created varied rock types that have been eroded into today’s spectacular landscape of cliffs, craggy summits and low, grassy valleys.

At the start of the Carboniferous Period, 360 million years ago, the area that is now Holyrood Park was a swampy, low-lying plain close to the sea, located just south of the Equator. The area slowly subsided and filled with sediment – pebbles, sand and mud – washed down from the mountains to the north. This formed sedimentary rock, which has provided the city of Edinburgh with building stone, coal, ironstone and lime.

About 342 million years ago, red-hot, liquid lava erupted at Arthur’s Seat and spread across the landscape. The lava flows cooled quickly to form black basalt full of gas bubbles. Over time, lava flows built up, forming a low cone reaching perhaps 200 metres above the plain. Magma broached the surface in different places, always finding the path of least resistance.

Sometimes the magma interacted with water, heating it to steam that expanded and forced the hot, gassy magma mix to the surface, where it exploded in clouds of black ash and flying lava blocks. This type of dramatic volcanic activity is responsible for the rough, red agglomerate rock on the high slopes of Arthur’s Seat.

Eventually the pulse of magma rising from deep below the surface slowed and stopped, and the magma solidified. All was quiet, with continued accumulation of sedimentary rock that buried the volcano. About 15 million years later, more magma forced its way upwards but was trapped underground, forced sideways through existing rock layers, before cooling slowly. This formed the crystalline igneous rock called dolerite that makes the dramatic cliffs of the Salisbury Crags.

Later tectonic movements caused uplift and folding, and the layers were tilted to the east. Then began 300 million years of slow erosion, stripping away hundreds of metres of overlying rock. In the last two million years, ice has been the dominant shaper of this landscape. Several times thick ice sheets have advanced from the west, grinding away the softer layers, exposing today’s crags and cliffs and leaving behind the varied topography of volcanic hills and flat, swampy lowlands that the first humans discovered a mere 10,000 years ago.

James Hutton found an interesting section of Salisbury Crags in the late 18th century, where he recognised magma had intruded into existing rock to form dolerite. He used this important exposure as evidence to support his idea that rocks were formed by natural processes over a lengthy time period.

Text contributed by Angus Miller

Find out more
Discovering Edinburgh’s Volcano leaflet: http://www.edinburghgeolsoc.org/publications/geological-excursion-guides/#discovering-edinburghs-volcano
Geowalks: http://geowalks.co.uk/
Dynamic Earth, located at the edge of Holyrood Park, is the UK’s only science centre dedicated to the story of our planet. Expect a fully immersive visitor experience! http://www.dynamicearth.co.uk
Investigating Holyrood Park – Site Guide for teachers. Edinburgh: Historic Environment Scotland. Available at: http://www.historicenvironment.scot/learn/learning-resources/teaching-resources/#site-guides_tab

Salisbury Crags with the summit of Arthur’s Seat behind. James Hutton explored the junction between the dolerite of Salisbury Crags and the underlying, older sandstone layers. Image: Angus Miller.

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