2020

NX1796 : Horse Rock

taken 4 years ago, near to Girvan, South Ayrshire, Scotland

This is 1 of 3 images, with title Horse Rock in this square
Horse Rock
Horse Rock
Horse Rock is one of two geologically interesting rocks at the south end of Girvan beach. Both are made up of conglomerate resting on top of the Ballantrae Ophiolite, but they are distinctly different from one another. This one is much more irregular and chaotic, and it contains large rounded granite boulders as well as lavas from the ophiolite.
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 limestones and turbidites.

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


Creative Commons Licence [Some Rights Reserved]   © Copyright Anne Burgess and licensed for reuse under this Creative Commons Licence.
This photo is linked from: Automatic Clusters: · South [4] · Beach [2] · Conglomerate [2] Title Clusters: · Horse Rock [3] ·
1:50,000 Modern Day Landranger(TM) Map © Crown Copyright
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1:50,000 Modern Day Landranger(TM) Map © Crown Copyright
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Grid Square
NX1796, 16 images   (more nearby 🔍)
Photographer
Anne Burgess   (more nearby)
Date Taken
Saturday, 26 September, 2020   (more nearby)
Submitted
Saturday, 3 October, 2020
Subject Location
OSGB36: geotagged! NX 1793 9612 [10m precision]
WGS84: 55:13.5374N 4:51.8521W
Camera Location
OSGB36: geotagged! NX 1795 9612
View Direction
WEST (about 270 degrees)
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Image Type (about): geograph 
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