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Scourie Achmelvich Laxford Clachtoll Stoer Assynt Skiag Bridge Glencoul Knockan Borralan Ledmore

Volcanic mudflow deposit, Stoer Group (Torridonian)

From Stac Fada, Bay of Stoer


Outcrop

Volcanic mudflow deposit, Stoer Group (Torridonian), Bay of Stoer
This rock type occurs as a single sheet several metres thick. It can be seen in the left centre of this view forming vertical faces, purple where surfaces are clean, grey where covered in lichen The rock has no bedding in it, and the layer has an irregular bottom surface where in places it looks as if the rock has been injected horizontally into the bedding of the adjacent rocks, e.g. towards the right hand side of the picture. The best explanation for its texture and appearance is that it was a volcanic mudflow, a liquid slurry of water and suspended particles that rapidly and violently spread over the landscape during a major volcanic eruption. The layer can be traced out for 50 km along the west coast of Scotland: this must have been a very dramatic event!

Volcanic mudflow deposit, Stoer Group (Torridonian), Bay of Stoer
This close-up view of the rock shows angular fragments of greenish volcanic material suspended in purple muddy sandstone. Also suspended in the rock are a couple of fragments of red-brown siltstone. The hammer head is about 15 cm across.


Hand specimen

Volcanic mudflow deposit, Stoer Group (Torridonian), Bay of Stoer
This hand specimen consists of angular dark green rock fragments and sand grains (quartz and white feldspar) in a purplish-brown matrix. There is no layering - the rock has no bedding. The material caught up in it is a mixture of altered volcanic glass and recycled sedimentary grains.


Thin section

Volcanic mudflow deposit, Stoer Group (Torridonian), Bay of Stoer
At first sight in thin section, the rock looks like a sandstone. However, the grains are very angular and poorly sorted, and there is no bedding. The larger fragments are of a yellowish-green rock that is poorly crystalline and sometimes shows a banding: it is an altered glassy volcanic rock, and its banding was produced by flow just before it solidified. The smaller angular grains are cloudy feldspar and clear quartz: they will have been recycled from other Torridonian sediments.

Plane polarized light, field of view 7 mm across.

Volcanic mudflow deposit, Stoer Group (Torridonian), Bay of Stoer
This is a detailed view of one of the volcanic fragments. Volcanic glass (such as obsidian) will eventually begin to crystallize, even at low temperature. The new crystals are extremely small (the textures can be called microcrystalline or cryptocrystalline), and in some cases many crystals begin to grow at the same point and build a spherical radiating cluster of needle-like crystals. Most of this field of view is crystallized glass. The round objects are radiating aggregates of green chlorite (a hydrated iron-magnesium silicate), and the colourless material in between them is microcrystalline quartz and feldspar.

Plane polarized light, field of view 3 mm across


Scourie Achmelvich Laxford Clachtoll Stoer Assynt Skiag Bridge Glencoul Knockan Borralan Ledmore
Home Geological History Stratigraphy Area map Rock Index About

D.J. Waters, Department of Earth Sciences, May 2003