Geo-Trips :: A Geological Field Trip to the Ballantrae Ophiolite - Day 2

Ayrshire

A walk from Girvan, Sun, 27 Sep 2020 by Anne Burgess

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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, but may serve as background for some of the rocks seen.

I have classified this as a walk, because there is no option for a mixed driving and walking trip. We used cars to get to various locations from which we then walked to the outcrops.

Click the blue circles to see a photograph taken from that spot and read further information about the location. The blue lines indicate the direction of view. There is also a slideshow of this trip. ( )

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NX1894 : Byne Hill by Anne Burgess
Byne Hill
Looking towards the summit from the track above Brochneil Farm.

NX1894 : Quarry at Brochneil by Anne Burgess
Quarry at Brochneil
This small quarry exposes some of the ultramafic rocks belonging to the base of... more

NX1894 : Drystane Dyke by Anne Burgess
Drystane Dyke
An old drystane dyke separates the improved grassland from the rougher part of Byne Hill.

NX1894 : View from Byne Hill by Anne Burgess
View from Byne Hill
Most of this view is in square, including Balaclava Wood, which is the conifer... more

NX1894 : Magmatic Breccia by Anne Burgess
Magmatic Breccia
The slightly paler band through the dark gabbro is a magmatic breccia. Breccia is... more

NX1894 : Colourful Moss by Anne Burgess
Colourful Moss
I wonder if the red moss might be some sort of sphagnum, but I have no idea what... more

NX1894 : Bluebell (Campanula rotundifolia) by Anne Burgess
Bluebell (Campanula rotundifolia)
This is the Scottish Bluebell, called Harebell in England. The flower which in... more

NX1894 : Marsh Thistle (Cirsium palustre) by Anne Burgess
Marsh Thistle (Cirsium palustre)
Probably the commonest species of thistle in wet or damp ground.

NX1894 : Slickensides by Anne Burgess
Slickensides
The linear fabric sloping gently down from left to right as an example of... more

NX1794 : Byne Hill Summit by Anne Burgess
Byne Hill Summit
You might expect a triangulation pillar here, but there is one of those just a... more

NX1794 : View from Byne Hill by Anne Burgess
View from Byne Hill
The foreground is the summit area of Byne Hill. The low green ridge is Mains Hill... more

NX1794 : Summit Path by Anne Burgess
Summit Path
This is the path to the summit from the north-east. Stiles have been provided to... more

NX1794 : Plagiogranite by Anne Burgess
Plagiogranite
This unobtrusive rock tells geologists a lot about the evolution of the Ballantrae... more

NX1794 : View from Byne Hill by Anne Burgess
View from Byne Hill
A 'panorama' taking in the burgh of Girvan on the right, the Isle of Arran in the... more

NX1894 : Mushroom or Toadstool? by Anne Burgess
Mushroom or Toadstool?
I wondered if this was a Parasol Mushroom but would welcome expert help with... more

NX1894 : Mushroom or Toadstool? by Anne Burgess
Mushroom or Toadstool?
This fungus was lying like this close to [[[6626434]]].

NX1894 : Unseen Unconformity by Anne Burgess
Unseen Unconformity
This is looking towards the summit of Byne Hill from close to a subsidia ry... more

NX1894 : Conglomerate on Byne Hill by Anne Burgess
Conglomerate on Byne Hill
This outcrop is of the conglomerate that lies on top of the volcanic ocean crust... more

NX1894 : Caldron Glen by Anne Burgess
Caldron Glen
The reservoir at Glendrissaig is in this grid square, but the turbines on Laggan... more

NX1894 : Slender St John's Wort (Hypericum pulchrum) by Anne Burgess
Slender St John's Wort (Hypericum pulchrum)
A late-flowering spike of the most delicate spacies of St John's Wort.

NX1593 : A77 at Kennedy's Pass by Anne Burgess
A77 at Kennedy's Pass
Unfortunately this is a fairly typical view of much of the main trunk road between... more

NX1493 : Basal Conglomerate by Anne Burgess
Basal Conglomerate
These rocks are part of the first sediments to form on top of the uplifted... more

NX1493 : Roadside Litter by Anne Burgess
Roadside Litter
Why do people do this? Would it really be so hard to stop at a recognised dump or... more

NX1493 : Bedded Conglomerate by Anne Burgess
Bedded Conglomerate
This conglomerate contains small clasts, in a fine-grained matrix. The beds have... more

NX1493 : Conglomerate Beds by Anne Burgess
Conglomerate Beds
As well as being tilted to nearly vertical, these beds of conglomerate have been... more

NX1593 : Folded Turbidites by Anne Burgess
Folded Turbidites
Turbidites are deposited under water from debris flows, which are like underwater... more

NX1593 : Folds in Turbidite by Anne Burgess
Folds in Turbidite
Turbidites are deposited under water from debris flows, which are like underwater... more

NX1593 : Fault Line by Anne Burgess
Fault Line
Turbidites are deposited under water from debris flows, which are like underwater... more

NX1189 : Sand Ripples by Anne Burgess
Sand Ripples
A complex pattern of ripples left as the tide receded round the boulders on the beach.

NX1189 : Beach Boulders and Big Isle by Anne Burgess
Beach Boulders and Big Isle
For reasonas that are unclear, the rocky spit on the beach here is called Big Isle.

NX1189 : Whilk Isle by Anne Burgess
Whilk Isle
At this stage of the tide the tidal islet of Whilk Isle is just beginning to... more

NX1289 : Roadside Flowers by Anne Burgess
Roadside Flowers
The roadside verge seems to have been planted with flowers - Corn Marigold,... more

NX1289 : Cornflowers (Centaurea cyanus) by Anne Burgess
Cornflowers (Centaurea cyanus)
Cornflowers are commoner further south, and when found here are often garden... more

NX1289 : Carleton Fishery by Anne Burgess
Carleton Fishery
A row of cottages at the fishing station at Carleton, south of Lendalfoot.

NX1289 : Fishermen's Huts by Anne Burgess
Fishermen's Huts
The rather shabby huts on the promontory at Carleton are fishermen's huts. In the... more

NX1289 : Carleton Bay by Anne Burgess
Carleton Bay
Looking north along the bay to Lendalfoot and Pinbain Hill.

NX1289 : Dolerite Intrusion by Anne Burgess
Dolerite Intrusion
The rocks making up the headland at Carleton are ultramafic rocks brought up from... more

NX1289 : Carleton Bay by Anne Burgess
Carleton Bay
Looking across the bay past Lendalfoot from the promontory at Carleton Fishery.... more

NX1391 : Bonney's Dyke by Anne Burgess
Bonney's Dyke
The pale rocks in the foreground are serpentinised ultramafic rocks, named... more

NX1391 : Pectolite by Anne Burgess
Pectolite
The whitish lump is of pectolite, which is a hydrated aluminosilicate, in other... more

NX1391 : Bonney's Dyke by Anne Burgess
Bonney's Dyke
The dyke, named in 1932 in honour of the 19th century geologist whose studies... more

NX1390 : Beach North of Lendalfoot by Anne Burgess
Beach North of Lendalfoot
Lendalfoot lies at the south-west end of the bay, with Balsalloch Hill rising... more

NX1390 : Intrusions by Anne Burgess
Intrusions
In this field on the raised beach is a series of rocky fingers, each of which is... more

NX1390 : Intrusive Rock by Anne Burgess
Intrusive Rock
This is the largest, and the most southerly, of the series of igneous intrusions... more

NX1391 : Igneous Dykes by Anne Burgess
Igneous Dykes
These dykes jutting up in the middle of the field are of gabbro, intruded into the... more

NX1391 : A Complicated Rock by Anne Burgess
A Complicated Rock
This exposure has several different types of rock, and has been much deformed and... more

NX1391 : Ultramafic Breccia by Anne Burgess
Ultramafic Breccia
The greenish rocks here are oceanic crust obducted as part of the Ballantrae... more

NX1391 : Brecciated Pillow Lava by Anne Burgess
Brecciated Pillow Lava
These dark lumps are recognised as pillow lavas, erupted originally at the sea... more

NX1391 : 'Macadam' Breccia by Anne Burgess
'Macadam' Breccia
The name is strictly unofficial, and is applied to these lava pillows which have... more

NX1391 : Rocky Gully by Anne Burgess
Rocky Gully
A gap on the foreshore among the lavas of the Ballantrae ophiolite.

NX1391 : Sea Stacks by Anne Burgess
Sea Stacks
This embayment was cut by the sea when sea level was higher than it is now,... more

NX1391 : Faulted Contact by Anne Burgess
Faulted Contact
The cliff on the left is of sandstone, and to the right is serpentinite. The gully... more

All images © Anne Burgess and available under a Creative Commons licence external link.

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