Pebbles at Kilmurrin Cove – Part 2

Wet beach stones of volcanic rock

Here are some more examples of the pebbles at Kilmurrin Cove on the Copper Coast in County Waterford, Southern Ireland (see the previous post). Again, most of them seem to be volcanic rocks of Ordovician age, derived from the underlying bedrock of the beach and the outcrops in the surrounding cliffs, and including rhyolite in its solidified lava form and also as consolidated volcanic ash and breccias. Some stones may be of different geological types and origins, having been brought from much further afield and deposited by melting ice sheets.

This time most of the beach stones were photographed at the western end of the beach where  the stream or small river, that is dammed-up behind the pebble bank, surfaces through the stones and makes a break for the sea via jagged outcropping bedrock. The water is stained tea-colour by the peat through which it has flowed down the glaciated valley.

Click on the pictures to enlarge them and see the description.

Pebbles underwater in a beach stream

Dry beach stones with frond of seaweed

Wet beach stones and pebbles on the beach

Wet beach stones in a cove rimmed by cliffs of Ordovician rock topped by glacial deposits

Kilmurrin Cove pebbles on the Copper Coast in Ireland

Dry beach stones with washed up kelp

Pebbles with spots and stripes at Kimurrin Cove

Wet beach stones of mainly Ordovician volcanic origin

Pebbles underwater in a beach stream

Wet beach stones at Kilmurrin Cove

Wet beach stones at Kilmurrin Cove

Wet pebbles with sea foam bubbles at Kilmurrin Cove

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

 

Pebbles at Kilmurrin Cove – Part 1

Spotty rhyolite pebble with phenocrysts

Continuing with my Irish pebble and rock theme, trying to understand the phenomena I encountered on my travels, I noticed lots of patterned pebbles on the next shore along the Copper Coast at Kilmurrin Cove in County Waterford. Many of the beach stones had natural patterns based on either spots or stripes, or a combination of the two. As far as I can make out, the spot pattern is due to a phenomenon where thick viscous lava from a volcano is extruded and cools quickly trapping many gas bubbles. The shape of the bubbles is preserved in the solidified lava. Over time, the gas is replaced by minerals such as clear quartz, salmon-coloured K-feldspar, off-white sodium-rich plagioclase, or natural glass, which crystallise in the spaces initially created by the gases. Sometimes the bubble shapes have merged in the lava giving odd shaped spaces for the new minerals to fill.

Rhyolite is one of the rocks in which this happens.  It has the same chemical composition as granite but because it is an extrusive rock and cooled quickly, the crystals of the matrix of the rock cannot be distinguished. [Whereas granite is an intrusive rock that cooled slowly giving rise to a rock composed entirely of large visible crystals]. The crystals that make up the solidified lava form of rhyolite are so small that they cannot be seen with the naked eye or even with a hand lens. However, the minerals that have percolated into the bubble spaces are macroscopic, they can easily be seen. I think the types of crystals like those shown in image 1.1 are called spherulites; these  spherulites are rounded bodies, often coalescing, comprising radial aggregates of needles, usually of quartz or feldspar. Spherulites are generally less than 0.5 cm in diameter, but they may reach a metre or more across – though not in this part of the world as far as I know. These relatively regular structures in the rock can be compared with isolated large crystal inclusions that are known as phenocrysts.  Rhyolite with phenocrysts is called porphyritic rhyolite.

A number of the pebbles have parallel lines or swirling layers defined by varying colour or granularity – maybe with spherulites as well. These rhyolite pebbles may be showing flow banding that appears like linear or striped patterns when seen in cross-sections of the rock. The lines have been described as being similar to tree rings. This type of rock is called banded rhyolite and forms from slow flowing lava in which bubble- and crystal-rich layers form on the cooling surface. Multiple flows build up one on top of the other to create the multiple lines. At least that is what I think is shown in these striped pebbles. I am open, as always, to correction. I suppose I can’t rule out that some of the lines I noticed might be Liesegang rings.

Not all rhyolite rocks are solid forms of lava. Rhyolites are mostly tuffs and breccias rather than lavas. Rhyolitic Tuffs are rocks consisting of consolidated volcanic ash ejected from vents during a volcanic explosion, while rhyolitic breccias are composed of larger angular fragments thrown out by the explosions. I’ll talk more about this subject later when I write about my visit to Bunmahon Geological Garden further along the Copper Coast.

As usual, click on the pictures to enlarge them and see the description for the image.

Natural patterns, shapes and textures in pebbles on the beach

Beach stone with stripes and spots

View of a pebble beach on a Copper Coast in Southern Ireland

Pebble bank on the shore at Kilmurrin Cove

Dry beach stones on the Copper Coast in Ireland

Natural patterns, shapes and textures in pebbles on the beach

Natural patterns, shapes and textures in pebbles on the beach

Natural pattern of banding in a rhyolite pebble

View of the west end of Kilmurrin Cove showing river dammed by large pebble bank.

View of the west end of Kilmurrin Cove with stream crossing exposed Ordovician rocks

Natural patterns, shapes and textures in pebbles on the beach

Dry beach stones on the Copper Coast in Ireland

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

A Copper Coast Geopark Sculpture

 

We swerved off and parked a while to get a closer look at this fantastic sculpture as we drove along the coast road between Annestown and Bunmahon in southern Ireland. The monumental block of limestone with brightly coloured mosaic inlays is called “Ice, Fire and Water” has been carved by Colette O’Brien. It stands overlooking Dunabrattin Head near Boatstrand, and symbolises the elemental forces that gave rise to this spectacular stretch of rocky shoreline, and celebrates the establishment of the Copper Coast Geopark of which it is a part.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Rock Textures at Annestown

Close-up of Ordovician volcanic rock texture in cliffs at Annestown

The cliffs at Annestown Beach in County Waterford, which are part of the 25 kilometre stretch of the Copper Coast Geopark, are Ordovician in age. This is a very early period of land formation dating from between 460 to 450 million years ago when a volcano on the seabed near to the South Pole erupted with a violent explosion that forced a mixture of ashes, rock fragments and debris through the water to the surface of the ocean and this material fell back on the water to settle on the seabed. Molten lava in the form of rhyolite was also extruded to join this layer of volcanic debris. Through almost unimaginable periods of time, this layer consolidated to form rock, was moved along with the tectonic plate on which it rested to its present position in the northern hemisphere, and was raised to the surface. Quite a journey and a fantastic story, don’t you think?

Close-up of Ordovician volcanic rock texture in cliffs at Annestown

Close-up of Ordovician volcanic rock texture in cliffs at Annestown

Close-up of Ordovician volcanic rock texture in cliffs at Annestown

Close-up of Ordovician volcanic rock texture in cliffs at Annestown

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Pebbles at Annestown

Pebbles on a Copper Coast beach

The first beach that I visited on my recent trip to Ireland was near the small village of Annestown in County Waterford. It is part of the Copper Coast Geopark, and I wish I had known at the time that the Geopark website offers informative trail guides and an audio podcast to guide visitors on a walk around this particular area, starting at that very beach.

I was immediately struck by how different the rocks in the cliff are from anything I have seen before, and the pebbles on the windy and surf-washed shore have their own unique character. A sign-board in the car-park explains that the rocks in this location are extremely old, mostly dating from the Ordovician Period, resulting from ocean-bed volcanic eruptions at a time when the land which is now Ireland was formed near the South Pole between 460 and 450 million years ago. Movements of the earth’s crustal plates over vast eons of time have caused the land to gradually migrate northwards to its current position.

In amongst the pebbles of volcanic origin and Ordovician age are others from sources further along the coast and also, no doubt, pebbles derived from the deposits of clay, boulders, and sand that were dumped over the land surface at the end of the Ice Age 12,000 years ago as the ice melted, and which can be seen today as a yellow-brownish layer on top of the cliffs.

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

At Dogs Bay

Dogs Bay in Connemara has a wonderful white sandy beach composed of the tiny shells of microscopic one-celled creatures that live mostly on the mud of the ocean bed. These animals are called Foraminifera. When they die, millions upon millions of their calcium skeletons, bearing many chambers and holes, and not visible to the naked eye, wash ashore to form this unusual sand. This is such a rare occurrence that Dogs Bay beach is the only one composed of foraminifera in the northern hemisphere.

The bedrock of the land around this wonderful white sandy shore is made up of volcanic rocks including granite that has many different colour forms and patterns due to the different mineral crystals that it contains – if you get up really close to see it. The granite outcrops on the shores often have a rounded surface where ice sheets or glaciers passing over them have ground them smooth. The waterside rocks form attachments for a variety of seaweeds, along with many seashore creatures, particularly gastropod molluscs like periwinkles and limpets, whose brightly-coloured empty shells accumulate at the base of boulders low down in the intertidal zone.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

On The Burren

Natural features and man-made structures amongst the karstic erosional surface of the Carboniferous Limestone landscape in The Burren in County Clare, Ireland. These photographs show the famous Neolithic portal tomb known as the Poulnabrone Dolmen and the humble, often lichen and moss-covered, stone walls. Typical limestone erosion features include the limestone pavements with their deeply weathered clints and grikes; and solution hollows or pans also called kamenitzas. Isolated large boulders, standing out incongruously on the flat bare rock platforms, are glacial erratics dumped by receding ice sheets.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

On Garrettstown Strand

Rocks on an Irish beach

Rocks on an Irish beach

Rocks on an Irish beach

Rocks on an Irish beach

Patterns in rock on an Irish beach

Again, simply a few pictures, as a “taster” of the many photographs I have been taking, to show the incredible geology I have been witnessing on my current travels in Ireland. I will write up full descriptions and identifications with more images (as best I can) when I return home. These rock patterns and structures were captured on Garrettstown Strand, also known as White Strand or Garrylucas Beach, near Old Kinsale Head in County Cork.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Red Point Rocks on Grand Manan

Off the coast of Canadian New Brunswick, in the Bay of Fundy, lies an island that is split geologically in two. A fault runs from north to south in Grand Manan Island and it can be clearly seen only at Red Point in the Anchorage Provincial Park. It is visible as a diagonal line in the cliff. There are distinctly different rocks on each side of the fault.

To the west of the Red Point Fault are volcanic rocks known as the Southwest Head Member of the Dark Harbour Basalt – part the Late Triassic Flood Basalt dating from about 201 million years ago. Photographs of this type of rock showing textures and fracture patterns are shown above. The Red Point Fault itself is shown below. It is visible as a diagonal line in the low cliff structure with a stream flowing out of the fault across the shore. This is “a rare exposure of a major displacement intra-basin fault” (McHone).

The geological fault between basalt on the left and meta-siltstone on the right  at Red Point on Grand Manan Island.

To the east of the fault, the rocks are metamorphic and not volcanic. They are laminated meta-siltstones of the Long Pond Bay Formation of the Cambrian-Ediacaran Formation dating from 539 million years ago. Photographs of the colours, textures, layers, and fracture patterns in these rocks are shown below.

The Mesozoic basalts of the western two-thirds of the island conceal the ancient Ediacaran (Neoproterozoic) and Cambrian (Palaeozoic) metamorphic rocks lying below them. However, in the eastern third of the island, the rocks have all moved upward along the Red Point Fault, with the more recent strata of the Mesozoic basalts gradually being removed by erosion so that the ancient metamorphic layers are now exposed at the surface. “At least 300 meters (and possibly much more) of vertical offset is required for this juxtaposition” (McHone) … of the Dark Harbour Basalt with the Long Pond Bay Formation meta-siltstones.

Click here for a geological map of Grand Manan. The map is one of many invaluable downloads about the geology of Grand Manan made available on the website earth2geologists run by J. Gregory McHone who is a professional geologist personally responsible for much of  the mapping and research on the island, and who runs the Grand Manan Museum.

McHone, J. Gregory Mesozoic Geology of Grand Manan, Graduate Liberal Studies Program, Wesleyan University, Middletown, CT.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved