Eshaness volcanics

Significance: the Eshaness peninsula is formed from a sequence of volcanic rocks within the Old Red Sandstone Melby basin. The Melby Basin lies west of the Melby (St. Magnus Bay) Fault that cuts off the Eshaness volcanic sequence, crosses St. Magnus Bay to cut across the Melby peninsula at the south-west corner of the bay. The volcanic rocks of Eshaness, Melby and the island of Papa Stour thus all may well belong to the same volcanic province. The magma generation that gave rise to the vulcanism may have been due to subduction beneath Shetland (and Scotland) of a remaining fraction of the crust of the Iapetus Ocean.

Now folded into a gentle NNE-SSW syncline, the Eshaness sequence plunges to the SW in the southern part of the area and NE in the northern. The Eshaness volcanics are separated from the younger granophyre of the Northmaven plutonic complex by the Melby (St. Magnus Bay) Fault. Although the three lowest divisions of the sequence appear similar to the middle Devonian volcanic rocks on the island of Papa Stour to the SW they have been shown not to correlate on geochemical grounds. An ignimbrite from the Eshaness sequence has been dated as upper Devonian (365Ma).

Localities 1. HU 205 785. Agglomerate and tuff. Blowhole. (Parasitic cone?).

            The top of the cliffs to the south of the lighthouse is mostly clear of vegetation and provides excellent vertical sections through beds of tephra which have built up on top of the andesite lavas.

            Note the thicker dykes cutting the lavas at the Kirn O' Slettans. The Kirn O' Slettans is a deep blowhole in the cliffs possibly formed by the collapse of a lava conduit; during storms jets of water are forced high in the air through the blowhole.

Here the lava beneath the pyroclastics shows flow banding and is ‘ponded’ around pyroclastic clasts and lava blocks, (could this area be the remnants of a parasitic cone on the flank of the main volcano?). 

Locality 2. Blackhead of Breigeo. HU 211 799. Lava flows. Cliff collapse.

The cliffs here are made up of a series of relatively thin lava flows that are separated from the underlying thick flow by a thick band of soft, pale green material.  This soft material is a zone of weakness that has allowed weathering and wave action to undermine and collapse the cliff face to form an amphitheatre on the south side of the headland. The headland is formed from a more massive flow (rhyolite?) that in places displays flow banding.

            The band of soft green material would appear to be a product of intense weathering; (perhaps this was a blocky lava flow or pyroclastic deposit that experienced a period of intense weathering before being buried beneath later flows).

Locality 3. Grind of the Navir. HU 213 804. Ignimbrite. Storm beach.

            The Grind of the Navir (Gate of the Borer) is a large amphitheatre hewn out of ignimbrite. At the seaward side, part of the cliff has collapsed forming a gap 10m wide and 13m high some 12m above the sea. Storm wave action, the power of which is concentrated by this narrow opening, has caused the rock to split along joints into large blocks. Some blocks measure 2.5 x 1.2 x 1.2m. These were then carried over 100m inland by the force of the waves to form an impressive storm 'beach' of imbricate blocks at the head of the amphitheatre.

            The rock is seen to have the eutaxitic texture of a welded rhyolitic ignimbrite with abundant clasts and shards of flattened and welded (now devitrified) pumice. This ignimbrite formed as a result of deposition by nuees-ardentes that swept down a steep volcanic slope. Extensive ignimbrite sheets are commonly associated with calderas, however the bounding faults of caldera basins have not been identified in Shetland.