Rocks at Cape Tribulation

The striped and layered rocks that underlie the promontory at Cape Tribulation look like sedimentary strata. They were just that at one time in the past but they have been  altered and partly metamorphosed into metasedimentary rocks. They belong to a group known as the Hodkingson Formation and are the result of transformations brought about as two tectonic plates of the earth’s crust collided, with one plate then sliding under the other (subduction), generating tremendous heat and pressure that altered existing sedimentary rocks, promoted magma movements below the crust (leading to intrusive igneous rocks), and stimulated surface volcanic activity (making extrusive igneous rocks).

Rock Texture & Pattern at Dog’s Bay

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.

Volcanic Tuff near Louisbourg Lighthouse – Part 5

Not all the rock exposed at Louisbourg Lighthouse is composed of tuff. Molten lavas intruded the tuff at later stages forming harder bands of igneous rock with a contrasting greenish colour and distinct fracture patterns. The textures of the two kinds of rock are very different.

Volcanic Tuff near Louisbourg Lighthouse – Part 4

More rock textures from the compacted ash in tuff deposited from explosive volcanic eruptions during the Neoproterozoic period at Louisbourg in Cape Breton, Nova Scotia, Canada.

Volcanic Tuff near Louisbourg Lighthouse – Part 3

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

On a whole range of scales, there are variations to the simple layering of the tuff (which is made of volcanic ash) and constitutes swathes of faintly striped and banded rock on the shoreline at Louisbourg on Cape Breton Island in Nova Scotia. Subsequent to the deposition and consolidation of the volcanic ash into tuff rock, the build-up of great pressures from earth movements at different times during geological history has caused both minor and major fractures in the rock. Small cracks sometimes filled up with dissolved minerals that crystallised to form veins of contrasting coloured material. In other places, intrusive molten lava squeezed its way into weak areas between or across the layers forming large-scale dikes. The igneous rock type of the dikes may be a greenish colour, and often cracks upon weathering in a characteristic way giving it distinct fracture patterns that are not present in the tuff.

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

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

Rock colour, pattern, and texture in Main-a-Dieu sequence volcanics near Louisbourg

Rock colour, pattern, and texture in the Main-a-Dieu sequence on Cape Breton

Volcanic Tuff near Louisbourg Lighthouse – Part 2

 

Coastal exposure of volcanic tuff rocks in Cape Breton

The volcanic ash deposits or tuff found in coastal rocks around Louisbourg Lighthouse in Cape Breton show subtle colour banding. Originally, ash from volcanic eruptions fell into lakes occupying the valleys around the volcanoes, and accumulated in horizontal layers, each representing an individual eruption event. The colours of the ash layers differed slightly according to the content and the temperature. When ash remained very hot on its journey through the air from the volcanic vent, the particles often melted together on landing, forming welded tuff. Welded tuff has a purple colour instead of the more normal shades of grey. We can see the layers as colour bands because we now see the layers of consolidated ash in cross-section. The layers were originally deposited in horizontal beds in water. Over the great period of time that has elapsed since deposition (575 million years) earth movements have brought the layers into an almost vertical orientation so that they are now viewed end on.

The textures are varied but in a quiet way with combinations of different sized fragments and changes of hue in the finer ash and small pyroclastic rock pieces. One of the images below shows an example of a volcanic bomb. This was in the first instance a glob of molten lava that was spewed from the vent along with the ash, becoming rounded in shape as it fell through the air, and then landing and forming a depression in the soft ash surface. Subsequent ash falls buried it.

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

Volcanic bomb embedded in tuff

Detail of texture in volcanic tuff rock dating from the Neoproterozoic 575 million years ago

Texture of tuff - a rock made of volcanic ash

Texture of tuff - a rock made of volcanic ash

Volcanic Tuff near Louisbourg Lighthouse – Part 1

Rocks composed of volcanic ash (tuff) near Louisbourg Lighthouse in Cape Breton, Nova Scotia.

The rocks around Louisbourg Lighthouse on Cape Breton Island in Nova Scotia belong to the Lighthouse Point Member of the Main-a-Dieu sequence (formerly assigned to the Fouchu Group). They were deposited 575 million years ago, following a series of explosive volcanic eruptions in a subduction zone at the edge of a continental plate. They are the youngest rocks from the Coastal Belt and represent the end of an era of violent volcanic eruptions in this part of Avalonia. The pyroclastic deposits around Louisbourg Lighthouse are made of compacted pumiceous ash in ignimbritic units, and the resulting rock is called tuff. Layers within the tuff, distinguished by varying shades of grey, represent a series of separate eruption events. The grey layers transition into a purple layer of welded tuff where the ash and debris remained hot enough to melt the individual particles together as they landed.

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