The Geology of Arthur's Seat and Holyrood Park, Edinburgh

Jun 15, 2010
Jennifer Young

Former Volcano, Arthur's Seat, Edinburgh - Eileen Henderson

Arthur's Seat in Edinburgh is part of a volcanic landscape which formed around 340 million years ago and has been extensively altered over time.

Edinburgh’s skyline is dominated by the Castle Rock, the steep hill of Arthur’s seat and the cliffs of the neighbouring Salisbury Crags; while the surrounding area is noted for its hills, including those of Calton, Craiglockhart and Corstorphine. These striking natural landmarks are the remnants of a former volcanic landscape which was active many millions of years ago.

The Geological Context of Arthur’s Seat

From the late Devonian period until the early Carboniferous, the land which we now know as central Scotland appeared very different. Alternating between low-lying coastal plain and shallow seas as relative sea level rose and fell, the rocks which formed during this period were predominantly sandstones and limestones.

As continental plates moved together, the pressure they created generated heat and pressure which melted rock. The resulting magma, migrating upwards, fed volcanic activity whose remnants in the form of lava flows and volcanic hills can be seen throughout the whole of the Midland Valley of Scotland (McKirdy et al). Arthur’s Seat, perhaps the best known example, is worthy of further consideration.

Arthur’s Seat: a Volcanic Complex

Volcanic activity began in the Lothians around 340 million years ago, continuing for millions of years (Upton). Molten rock, or magma, being hotter and more buoyant than the surrounding rock, forced its way upwards and exploited weaknesses in the surrounding rocks. In places the magma reached the surface, erupting in the form of volcanoes.

The evidence suggests that the Edinburgh area was characterised by many relatively small volcanic vents and that four of them were probably involved in the formation of the Arthur’s Seat complex (Upton). Of these, the vents known today as the Lion’s Head and the Lion’s Haunch are most clearly visible.

The volcanoes erupted extensively but not continuously, forming between 13 and 19 separate lava layers of different thicknesses. These layers are interleaved with thin layers of sedimentary rocks formed in shallow seas during periods of quiescence (Edinburgh University). They are visible as the steep cliffs which characterise the surrounds of Arthur’s Seat.

Many other volcanic formations are visible on and around Arthur’s Seat, including the remnants of the volcanic cone (forming the top of Arthur’s seat itself) the various lava flows and ash flows and, at Samson’s Ribs beside the road through the park, there is a good example of hexagonal jointing - evidence of slow cooling of a thick lava flow (Edinburgh Geological Society).

Salisbury Crags: A Volcanic Sill

The complexity of the lava flows, coupled with the problems of later large-scale modifications to the landscape, mean that it’s difficult to say when the eruptions ceased. The steep cliffs of Salisbury Crags are of later date and are thought to date from around 315-330 million years ago, implying that active volcanism lasted for less than ten million years.

Sills are formed when magma doesn’t reach the surface but forces its way between layers of sedimentary rocks, cooling slowly and generally displaying a different structure and grain size to a rock which has reached the surface, even where the chemical composition is the same. Salisbury Crags at up to 40 metres thick, is an outstanding example.

The crag is important in the development of geological knowledge, being the location where James Hutton (the so-called Father of Modern Geology) found evidence of the intrusion of molten rock which he used to demonstrate his ideas about the constantly changing and evolving nature of the Earth (Edinburgh University).

Later Modifications to the Volcanic Landscape

The landscape which is visible today bears little resemblance to the scene in the early Carboniferous. As Hutton demonstrated, even large-scale features can change: the exposure of Salisbury Crags and the tilting to the east of the whole volcanic complex (along with the rest of the surrounding area) as a result of tectonic movements are ample demonstration of that.

In addition to large scale tectonic movements, erosion has played a significant part. Millions of years of exposure to wind and rain have stripped away vast quantities of rock so that what we see are not volcanic cones but the more solid and resistant rocks which formed in the vents of the volcanoes.

The extensive glaciations of the Ice Age has further modified the landscape.

Arthur’s Seat is a city centre park and offers many easily accessible opportunities for visitors to see clear examples of geological landforms. An excursion guide, Discovering Edinburgh’s Volcano, is available from the Edinburgh Geological Society and occasional guided walks are run during the year, especially as part of Scottish Geology Week which takes place annually during the month of September.