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The following geological summary was written by Dr William Iams. Dr Iams participated in a field trip to Exploits Island in August of 2018

Geologic Aspects of Exploits Island and the Surgeon Cove Lighthouse peninsula

When hiking, paddling, sailing or cruising around the Exploits Islands, you will be looking at rocks that are hundreds of millions of years old and were originally formed deep under an ancient ocean’s surface.

The rocks of the Exploits Islands can best be understood in light of Plate Tectonics theory. This theory proposes that oceans are ephemeral features on the earth’s surface and, over hundreds of millions of years of earth history, they form, expand, contract and disappear only to be replaced by other ocean basins at other locations on the earth’s surface.  The ocean basins form from rifts in the earth’s crust which spread as molten rock (magma) from within the earth is expelled as lava and fills the rift as it widens to form an ocean basin.

The bedrock making up the cliffs and outcrops around the Surgeon Cove Lighthouse and, indeed, most of the bedrock of Exploits Islands, is basalt, a volcanic rock formed on an ancient ocean floor. Basalt is a dark, finely crystalline siliceous rock rich in iron and magnesium.    As described above, the rocks were formed as magma of basaltic composition  was extruded from the earth’s interior at deep sea ocean rifts (“spreading centres”) and formed vertical sheets of basaltic rock (sheeted dikes) as well as sub-horizontally layered basaltic lava flows which, in some places, display distinctive “pillow” structures .

The ocean basin in which the Exploits rocks formed existed during the Neoproterozoic to early Paleozoic time, some 650 to 450 million years ago, and is known as the Iapetus Ocean.  It’s sometimes referred to as the Proto-Atlantic Ocean, since it formed in somewhat the same relative position that the much later Atlantic is found.   The Iapetus Ocean closed and ceased to exist when the ancient continents of Laurentia, Baltica and Avalonia came together, collided to form the larger ancient continent of Laurasia, and squeezed the Iapetus Ocean out of existence.   During that tectonic event,  compression of the earth’s crust caused crumpling, folding and faulting of the sea floor rock which was then pushed up to become part of the developing Appalachian mountain chain.   The fractured and faulted rocks on Exploits Islands are evidence of this deformation.

Figure 1.   Fractured and weathered rocks below Surgeon Cove Lighthouse.   Stairs to lighthouse on the left and hoist rope on the right.   The rocks show fracturing and faulting that are a result of the early Paleozoic mountain building in the area (Photo courtesy Marie Iams)

Figure 2. The grey and rusty coloured rocks of the Exploits Islands are the result of chemical weathering of the basaltic bedrock that is a dark green colour on fresh surfaces. (Photo courtesy Marie Iams)

This orogenic (mountain building) activity occurred off and on for about 100 million years until a mountain range thought to be equivalent in heights to the modern day Rockies had formed from Newfoundland to the southeastern United States.   The present Appalachian chain, of which the Exploits rocks are a part, runs from Georgia in the US up through Newfoundland and continues as the Caledonian mountain system in Britain and northern Europe.  It is all that is left of that early Paleozoic mountain system due to hundreds of millions of years of weathering.

Such weathering is evidenced in the rocks on Exploits Islands in both the fracturing of the rock and in the colour of the rock surfaces.  Although the mafic rocks (e.g., basalts) are a dark green when seen on fresh surfaces, the weathered surfaces of the cliffs and shorelines are often a dull sooty grey or a rusty brown in colour.   This is due to a weathering process which oxidizes the iron-bearing minerals in the rocks giving rise to a dark or “rusty” iron-oxide coating on the rock surfaces.

Dr William J Iams

Associate Professor (retired)

Earth/Environmental Science

Sir Wilfred Grenfell Campus

Memorial Universiry of Newfoundland

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