Tag Archives: igneous rocks

Rock Texture & Pattern at Dog’s Bay

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The rocks at Dog’s Bay in Connemara, Ireland, are part of the Galway Batholith. In particular they are composed of the Errisbeg Townland Granite riven by faults and many dikes containing other younger intrusive igneous rocks. The juxtaposition of the different rock types is a phenomenon marked by contrasting, colours, textures, and patterns. The whole rocky terrain has been levelled off and smoothed by ice sheets and reflects many glaciation features.

Feely, M. Leake, B.E., Baxter, S. Hunt, J. and Mohr, P. A Geological Guide to the Granites of the Galway Batholith, Connemara, western Ireland.Geological Survey of Ireland, 2006.

Beach Stones at Corney Brook

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There were no other visitors on the dull day that we turned off the Cabot Trail to look at the Corney Brook shore in the western Cape Breton Highlands. There was very low cloud cover, and it began to rain after a while, but there were treasures to be found – at least if you are like me and are fascinated by beach stones. Three main rock types are found at Corney Brook. The oldest are Neoproterozoic-Ordovician granitic pluton rocks of the Bras D’Or Terrane. Ordovician-Silurian metasedimentary rocks of the Aspy Terrane are slightly younger. And red sandstones and conglomerates belonging to the Horton Group come from the Devonian to Carboniferous Period.

The stones on the beach include all three types and probably a lot more due to the glaciation of the area. I wish I could identify and tell you the exact composition of each photographed pebble, stone or boulder – but that is tough for an amateur to determine. There is a great variety of colour, pattern, and texture to the stones which look dull when dry but amazing when wet. They include igneous and metamorphosed rocks like granite, gneiss, schist as well as sedimentary rocks like sandstone. It is possible to see just how difficult it is to not only understand the texts but also to convert into straight forward language for the non-specialist reader from the following detailed description that I discovered about the Corney Brook schist by Jamieson et al. (1987).

Comey Brook schist (unit 3d)
Medium- to high-grade pelitic and semi-pelitic schists, with minor marble and psammite, occurring on the Cheticamp River, Corney Brook, the northeastern end of Jumping Brook, and the central highlands near Calumruadh and Coinneach brooks, are referred to here as the Corney Brook schist. This unit is equivalent to the “medium grade belt” of Craw (1984). Pelitic and semi-pelitic members of the unit characteristically contain coarse staurolite, biotite, and garnet porphyroblasts, with kyanite at the highest grade, in a phyllitic to schistose matrix. Medium- to high-grade marbles, quartzites, albite schists and hornblendite recognized in the Corney Brook area (Plint et al., 1986) have not yet been identified south of the Cheticamp River. Centimetre- to metre-scale compositional layering, folded by tight to isoclinal folds, is interpreted as transposed bedding. Based on bulk compositions and rare relict primary textures, the Corney Brook schist is interpreted to have formed as a suite of clastic sediments interlayerd with felsic tuffs and minor basic flows – that is, it appears to represent the higher grade equivalents of units 3a-3c.

The softer sandstone cliffs are being eroded back by the sea. This has implications for the ground higher up and the roadway further back from the shore. For this reason a sea defence structure has been emplaced to protect the base of the cliffs. This is a gabion made of wire cages full of large beach stones and boulders that are stacked up into a wall, positioned at the most vulnerable part of the shore.

Rock Texture & Pattern at Black Brook Cove

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Patterns of dykes in granite in the cliffs at Black Brook Cove

Black Brook Cove along the Cabot Trail in Cape Breton Island, Nova Scotia, gets its name from the dark colour of the river water which flows into it. On the southern edge of the cove, the upper banks of the estuary are piled high with large bleached driftwood lying on a bed of boulders and pebbles. Curving banks of pebbles on the main body of the beach give way to smooth waterworn rock outcrops; and spectacular jagged cliffs surmounted by pines form the northern arm of the cove.

The rocks at Black Brook Cove are part of the Devonian Black Brook Granitic Suite formed about 375 million years ago. They are igneous plutonic rocks. The magma from which they formed was created by the melting and recrystallization of meta-sedimentary rocks that were sub-ducted during the collision of the ancient land masses called Ganderia and Avalonia.

The remarkable feature of the rocky outcrops at Black Brook Cove, and at Green Cove just a little further south, is the number of criss-crossing dykes or veins of contrasting colour that create abstract angular patterns on the rock surfaces. These patterns and colours are accentuated when the rock is wet. The whole beachscape is captivating on a bright sunny afternoon but the area must look its best after a heavy downpour of rain.

The main rock is a grey granite with small black flakes of biotite. Earth movements and increased pressures on numerous occasions subsequent to its emplacement have cracked the rock and opened up fissures into which certain minerals that were squeezed out of the mother rock have entered and recrystallized. Mostly the veins formed in this way are composed of aplite or pegmatite. Both are pink-orange in colour Aplite is made of quartz and feldspar and is fine-grained with a smooth sugary texture. Pegmatite is darker and coarser with large visible individual crystals of quartz, feldspar and mica in both the black biotite and clear muscovite forms.

REFERENCES

Anoiyothin, W.Y. and Barr, S.M. (1991) Petrology of the Black Brook Granitic Suite, Cape Breton Island, Nova Scotia. Canadian Minerologist, Vol. 29, pp. 499-515.

Barr, S.M. and Pride, C.R. (1986) Petrogenesis of two contrasting Devonian Granitic Plutons, Cape Breton Island, Nova Scotia. Canadian Minerologist, Vol.. 24, pp. 137-146.

Donohoe, H. V. Jnr, White, C. E., Raeside, R. P. and Fisher, B. E, (2005) Geological Highway Map of Nova Scotia, Third Edition. Atlantic Geoscience Society Special Publication #1.

Hickman Hild, M. and Barr, S. M. (2015) Geology of Nova Scotia, A Field Guide, Touring through time at 48 scenic sites, Boulder Publications, Portugal Cove-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8, pp. 94-97.

Atlantic Geoscience Society (2001) The Last Billion Years – A Geological History of the Maritime Provinces of Canada, Atlantic Geoscience Society Special Publication No. 15, Nimbus Publishing, ISBN 1-55109-351-0.

Rock Texture & Pattern at Main a Dieu

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The wooden boardwalk from the Coastal Discovery Centre at Main á Dieu on the southeast coast of Cape Breton Island, in Nova Scotia, Canada, leads to a look-out platform that is built on top of a rocky outcrop. The rock is a basalt volcanic lava flow dating from the Neoproterozoic Period around 560 million years ago. The basalt is characterised by many interesting natural fracture patterns; veins and weathered surfaces of contrasting colours; and different textures depending on exposure to aerial or aquatic erosional elements.

[We stayed at the most excellent Louisbourg Harbour Inn while we explored this part of Cape Breton Island.]

REFERENCES

Atlantic Geoscience Society (2001) The Last Billion Years – A Geological History of the Maritime Provinces of Canada, Atlantic Geoscience Society Special Publication No. 15, Nimbus Publishing, ISBN 1-55109-351-0.

Barr, S.M. (1993) Geochemistry and tectonic setting of late Precambrian volcanic and plutonic rocks in southeastern Cape Breton Island, Nova Scotia. Can. J. Earth Sci. 30, pp. 1147-1154.

Donohoe, H. V. Jnr, White, C. E., Raeside, R. P. and Fisher, B. E, (2005) Geological Highway Map of Nova Scotia, Third Edition. Atlantic Geoscience Society Special Publication #1.

Hickman Hild, M. and Barr, S. M. (2015) Geology of Nova Scotia, A Field Guide, Touring through time at 48 scenic sites, Boulder Publications, Portugal Cove-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8, pp. 66-69.

Keppie, J.D., Dostal, J. and Murphy, J.B. (1979) Petrology of the late Precambrian Fourchu Group in the Louisbourg Area, Cape Breton Island. Paper 79-1, Nova Scotia Department of Mines and Energy.

Herm Granodiorite with Xenoliths

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Macro photograph of crystals in Herm Granodiorite with a xenolith

The delightful small island of Herm lies just a short boat ride away from Guernsey in the Channel islands. The entire island is made of the Herm Granodiorite (de Pommerai and Robinson 1994). This is an intrusive igneous rock that formed below the surface of the earth, probably during the later part of the Cadomian age which lasted from about 550 to about 700 million years ago. The southern part of the island is higher than the northern part. In the south, the rock comprises a plateau with a height around 60 metres. The rock has been extensively quarried and exported in the past. The stone is particularly hard and ideal for structures like kerbstones; examples of these still feature on the Thames Embankment in London.

To the north, the area is covered by wind blown sand that hides an old wave-cut platform, glimpses of which can be seen as jagged low-lying reefs offshore.. The sand is wholly composed of shells with not only fragments but also a high proportion of undamaged miniature molluscs and sea urchin tests. The underlying Herm Granodiorite is similar to some varieties of the Bordeaux Diorite occurring in Guernsey – typically made of feldspar and quartz with some biotite and hornblende crystals. One of the main characteristics of the Herm Granodiorite is the inclusion of many contrasting lumps of other igneous rock types known as xenoliths. There is a good exposure of this rock type on Mouisonniere Beach near a stone obelisk on the marram covered dunes. [The obelisk is a navigation marker that has been constructed on the site of an earlier Neolithic standing stone (dolmen) which was taken away by quarrymen in the 19th century].

The rocky outcrop on the sandy beach is full of xenoliths. It is of special interest to geologists because of the variety in the composition and shape of the xenoliths indicating a series of different processes were involved. Some are dark, angular and made of diorite. Others are paler and more rounded; often they have a rim of darker material. The combination of crystal types and sizes varies in each type of xenolith compared with the rock in which it is embedded. The causes and possible circumstances that led to the formation of these different sorts of xenoliths are the subject of much discussion among the experts.

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, pp 56 – 62.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 1 – The St Peter Port Gabbro, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pp 4 -5.

Rocks at Cobo Bay

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Quarried surface of Cobo Granite

The beautiful and popular beach of Cobo Bay on the north coast of the Channel Island of Guernsey marks a transition between two igneous rock types: the Cobo Granite and the Bordeaux Diorite Complex. The character of the rocks that punctuate the stretches of clean sand and clear blue water changes as you walk from one end of the bay to the other.

In the southeast, near Le Guet Quarry and  Albecq the orange-pink Cobo Granite is at its most even-textured and pure with coarse grained crystals of pink potassium-rich orthoclase feldspar, light grey plagioclase feldspar, glassy quartz grains, and small black shiny crystals of biotite mica. The colours are best seen in freshly broken rock but are often obscured or muted by weathering and encrustations (lichens on dry land; algae and barnacles between the tides). The pebbles at this end of the beach are mostly brightly coloured water-worn remnants of the Cobo Granite.

The Bordeaux Diorite Complex rocks are superficially grey, composed of mostly black and white crystals  with grey plagioclase feldspar, black biotite, with minor minerals such as hornblende, pyroxene, and quartz. The Cobo Granite is younger than the Bordeaux Diorite. Deep beneath the earth’s crust, the molten granite intruded into the diorite before it was fully solidified. This led to a mixing of the two types of magma, and also a breaking-off of pieces of semi-solidified diorite that became enfolded in the granite magma before cooling. Small dark grey pieces of diorite (xenoliths) can be seen in the granite to the west below the Le Guet quarry. The numbers of xenoliths increase as you walk north east. Veins of pink aplite also run through the rocky outcrops. Whole areas of rock on the beach below the Rockmount Hotel are greyish in colour where the two rock types have melded together.

Walking northeast in the direction of Port Soif, large patches of greenish-grey inclusion rock can be seen ever more frequently embedded in the granite. I think these kinds of rocks are called granite-diorite marginal facies. By the time you reach the end of the beach, the grey diorite is more evident in outcrops and boulders. An attractive stone slipway demonstrates the contrasting colours and textures of the two rock types, the dark grey of the diorite and the orange of the granite. The angular beach stones and rounded pebbles at this point  also show the two rock types but with the grey diorite dominating, in contrast to the mostly orange granite pebbles at the other end of the beach.

This is just a very simple description of the geology at Cobo Bay and is intended only as a general guide to the rock features. My apologies for any inaccuracies. The expert explanation is much more complex and can be found by consulting the references given below.

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, pp 48-51.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 1 – The St Peter Port Gabbro, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pp 66-70.

Gabbro at Spur Bay

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The rocks at Spur Bay in Guernsey, Channel Islands, are composed of igneous St Peter Port gabbro which was formed from 500 – 550 million years ago in the Cadomian phase of activity. This dark grey rock has characteristic large hornblende crystals that often occur in layers alternating with feldspar at this particular location. The distinct dark crystals also give this rock the common name of  “bird’s eye” gabbro. In fact the gabbro has quite a variable composition, texture and patterning within a relatively small area. Some of the natural patterns have been caused by the infill of later-forming cracks and fractures by different molten rocks such as aplite, and the inclusion of rock fragments within the aplite.

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, pp 40-42.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 1 – The St Peter Port Gabbro, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pp 45-52.

A Vogesite Dyke at L’Eree

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Detail of a vogesite dyke at L'Eree in the Channel Island of Guernsey

Dykes formed when molten lava flowed into deep cracks and fissures in pre-existing rocks millennia ago. The lava set in a sheet form within the other rock but frequently dykes are seen on the surface today, after many years of erosion and earth movements, as lines of contrasting rock type. Dykes are composed of many different mineral combinations. When I visited Guernsey in the Channel Islands last year I discovered three types of dyke cutting through the predominantly igneous and metamorphosed rocks. I have already shown some pictures of dolerite, albite dolerite, and lamprophyre dykes, all of which seem to be quite common on the island. However, on an expedition to L’Eree on the north-west coast I spotted a dyke with a very different texture cutting east-west across the other  north-south dykes. This proved to be a vogesite dyke.

Vogesite has a very characteristic texture made up of rounded mineral inclusions – ‘large euhedral amphibole phenocrysts’  – set in a fine grained groundmass of plagioclase feldspar, alkali feldspar and quartz (Roach et al. 1991).

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, 30-34.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, p 22.

Pink Aplite Veins in L’Eree Granite

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Broad pink aplitic vein in L'Eree Granite

Numerous pink veins ranging in width from 10 cm to 1 metre in thickness pass through the northern part of the outcrop of L’Erée Granite in the Channel Island of Guernsey (De Pomerai & Robinson 1994). These are made of aplite. When the hot magma had nearly completely cooled and crystallised to form what we know today as the L’Eree Granite, “residual fluids escaped along cracks in the rock, depositing their dissolved load as they cooled down”.

Aplite is defined as:

a light coloured, fine-grained, equigranular igneous rock composed of subhedral to anhedral grains of quartz and alkali feldspar, and found as late-stage veins in granite bodies. The quartz-alkali feldspar composition corresponds to the lowest temperature melts in granite magma systems, suggesting that they are residual melts formed by the differentiation of granite magma. The lack of any hydrous minerals and the fine grain size points to the aplites crystallising from dry residual melts.

(Oxford Dictionary of Earth Sciences)

REFERENCES

Allaby, M. (2008) A Dictionary of Earth Sciences, Oxford Paperback Reference, Oxford University Press, 3rd Edition, ISBM 978-0-19-921194-4.

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, page 32.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, . Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pages 11 & 75.

L’Eree Granite

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Close-up of L'Eree Granite with pink megacrysts of feldspar

The special thing about the L’Erée Granite is the presence of distinct large pink/orange crystals. I mean big. Some of them are a up to 4 centimetres across. They are magacrysts of feldspar which are thought to have grown very slowly deep within the earth’s crust when the bulk of the granite had already crystallised (Pomerai & Robinson 1994). The action of hot gases fed these feldspar crystals that sometimes have concentric rings of dark inclusions (as you can see in some of the photographs) marking stages in their growth between 646 + 25 million years ago in the early Cadomian Age.

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, pages 30 – 32.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, . Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pages 11 – 12, & 75 – 78.