NX1391 : Brecciated Pillow Lava

taken 1 month ago, near to Lendalfoot, South Ayrshire, Great Britain

Brecciated Pillow Lava
Brecciated Pillow Lava
These dark lumps are recognised as pillow lavas, erupted originally at the sea bed, then broken up and re-lithified as breccia, but still retaining the general shape of pillows.
The Ballantrae Ophiolite

The Ballantrae Ophiolite is one of the classic areas of geology, not just in Scotland but worldwide.

An ophiolite is basically a slice of oceanic crust that has been obducted, that is, uplifted by tectonic movement and is now exposed on land at the surface of the Earth. The principal components of ocean crust are peridotite, layered gabbro, sheeted dykes and pillow lavas.

Peridotite is rock from the mantle, below the crust. It is ultramafic, which means that it contains high levels of iron and magnesium, contained in minerals such as olivine and pyroxene.

When mantle rocks melt, the resulting magma preferentially contains the minerals that are less heat-resistant, and the more heat-resistant minerals, for example olivine, are left behind in the peridotite while the magma cools to form gabbro, a mafic rock type. During cooling the denser minerals in the magma sink to the floor of the magma chamber, so the chemical composition of the rock changes. New injections of melt lead to alternating layers of denser and less dense material, termed a layered gabbro.

We have all seen videos of red lava erupting in water, cooling quickly to smooth dark grey sausage shapes from which more tongues of molten lava break out and solidify in their turn. These are termed pillow lavas, and they are a sure indication that they formed in water.

The lava that forms pillows rises from the mantle in the form of dykes that exploit weaknesses in the crustal rocks. In ocean crust repeated injections of magma result in many dykes together forming a sheeted dyke complex.

When ultramafic mantle rock is obducted, its constituent minerals are generally altered by hydrothermal fluids to serpentine. Asbestos is one variety of serpentine, and talc is another. The resulting rock is called serpentinite.

Ophiolites also feature sedimentary rocks overlying the obducted rocks, for example conglomerates, sandstones and turbidites.

This is all of course a huge oversimplification of a complex subject.

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NX1391, 83 images   (more nearby search)
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Date Taken
Sunday, 27 September, 2020   (more nearby)
Sunday, 4 October, 2020
Geographical Context
Coastal  Geological interest  Rocks, Scree, Cliffs 
Subject Location
OSGB36: geotagged! NX 1370 9143 [10m precision]
WGS84: 55:10.9174N 4:55.6513W
Camera Location
OSGB36: geotagged! NX 1369 9143
View Direction
EAST (about 90 degrees)
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Image Type (about): geograph 
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