Submarine Mountains Off Eastern Canada
Offshore from Canada’s Atlantic Provinces, in the North Atlantic Ocean (Figure 1), are mountains every bit as majestic and varied as those of the Western Cordillera. They form chains of seamounts, together with larger ridges and plateaus rising one to three thousand metres above the deep ocean floor.[1] They are important sites of marine biodiversity. Their distribution and morphology are known only in general terms, mostly from echo-sounder profiles collected over the past century. Only relatively small areas have been mapped using modern multibeam sounders, which provide bathymetric maps typically gridded at fifty metres. As Roger Revelle famously wrote some 60 years ago, “we still know more about the Moon’s backside than about the ocean’s bottom.”
The Canadian Pisces IV submersible (1973-1986) held a pilot and two scientists, but its maximum depth rating of 2000 m meant that it could be used only on the crests of a few larger seamounts. Here it is being recovered after a dive on Laurentian Fan that included author David Piper.
Few people have directly seen these submarine mountains. The deep ocean is dark, and the lights on manned submersibles or remotely operated vehicles (ROVs), equipped with cameras, normally illuminate only a few metres. In the late twentieth century, manned submersibles were used for close-up investigation of deep-sea mountains, but for safety reasons, ROVs have been preferred this century.[2] A few rock samples have been collected by ROV, but most samples have been recovered by the crude technique of dredging: dragging a chain basket with toothed jaws up steep slopes, in the hope of snagging something important, and not just boulders dropped by icebergs.
Figure 1. Bathymetric map of the North Atlantic Ocean off eastern Canada. (Source: GEBCO). Based largely on conventional echosounder data.
The geological origin of these submarine mountains is varied. Orphan Knoll (Figure 1), off northeastern Newfoundland, is a fragment of the North American continent that separated from Canada 100 million years ago, during the stretching of the continental crust prior to the separation of Ireland from Newfoundland by the North Atlantic Ocean. The flat-topped plateau lies at around 1,800 metres below sea level and is about the area of Banff National Park. The precipitous fault-bound northeastern slope is almost 2,000 metres high and eroded by large rockfalls and landslides. The southern and western slopes have a cover of younger sediment that has been sculpted by the powerful deep currents that sweep down the western edge of the Labrador Sea. To the south, Flemish Cap is an analogous fragment of continental crust, with a plateau 130 to 1,000 metres deep and steep fault-bound cliffs to the east and south. ROV observations on the 3,000-metre-high southern slope show a series of rock cliffs and terraces, with some granite walls that would be a climber’s delight if they were not under the sea.[2]
Three seamount chains or clusters lie partly within the seabed in the Atlantic Ocean claimed by Canada under the United Nations Convention on the Law of the Sea: the New England, Fogo, and Newfoundland seamounts, each including more than a dozen individual volcanic cones (Figure 1). In addition, there are several individual seamounts within Canadian territory, the best known being Orphan Seamount (Figure 2), just southeast of Orphan Knoll, which was investigated by ROV in 2010.[3] Orphan Seamount rises 1,600 metres above the surrounding ocean floor and is cone shaped with an average gradient of about twenty degrees.
Figure 2. Contoured multibeam bathymetric map of Orphan Seamount.3 Inset shows basalt lava near the base of the seamount.
Of the seamount chains, the Fogo Seamounts are the best known.[4] More than forty conical seamounts are recognised, the highest more than 1,000 metres above the regional sea floor. The Fogo Seamounts range in age from 150 to 125 million years, and are partly buried by younger sediment on the continental slope. Some related volcanoes on the Grand Banks were drilled by exploratory petroleum wells in the 1970s. Some of the Fogo Seamounts have flat eroded tops, at present depths of 3,000 to 4,500 metres, suggesting that they originally formed volcanic islands, and were eroded off at sea level, before gradually subsiding to their present depth.
The Newfoundland Seamounts are younger, about 100 million years old, but higher, rising up to 2,000 metres above the regional sea floor. They form a more linear chain in comparison to the scattered Fogo Seamounts (Figure 1). The New England Seamounts also form a linear chain and range in age from 88 to 123 million years. The largest rise 3,000 metres above the ocean floor, and their flanks were investigated by a scientific drill ship in 1975, as well as by manned submersible. All of our seamounts have subsided several kilometres, together with the surrounding ocean floor, due to cooling of the underlying crust and mantle since their formation.
Author David Piper with a damaged coring device used to recover samples from the deep sea bed. Aboard CSS Hudson in 1985.
The J-anomaly Ridge and the SE Newfoundland Ridge (Figure 1), extending southwest and southeast from the southern tip of the Grand Banks, are also of volcanic origin. Both originally formed about 130 million years ago, as the Grand Banks were rifting away from Spain and Portugal, allowing the Atlantic Ocean to start propagating northward. The J-anomaly ridge formed by unusually voluminous volcanism at the northeastern tip of the oceanic spreading ridge of the central Atlantic Ocean between Nova Scotia and Morocco. The SE Newfoundland Ridge formed by coalescing volcanoes along the Newfoundland-Azores-Gibraltar Fracture Zone that marked the termination of the central Atlantic Ocean at that time.
A dredge haul from one of the Newfoundland Seamounts coming aboard the CSS Hudson in 1973.
More generally, the origins of the plateaus, escarpments, volcanic ridges and seamounts are all linked to the progressive opening of the North Atlantic Ocean between eastern Canada and western Europe. This rifting apart of the continents started about 160 million years ago, but was completed only about 100 million years later, when continental rifting gave way to oceanic spreading between Greenland and both northeastern Canada and northwestern Europe. The rifting was intimately linked to convection and heat distribution in the Earth’s mantle, which softened the crust and produced basaltic magma by partially melting the upper mantle. The details of these processes and the role played by the magma that built the seamounts and volcanic ridges are a matter of continuing debate.[5]
David J.W. Piper is an Emeritus Scientist with the Geological Survey of Canada (Atlantic). His first research cruise to the Newfoundland Seamounts was in 1974, and he has been fascinated with the underwater mountains offshore eastern Canada ever since.
Georgia Pe-Piper is Professor Emerita of Geology at Saint Mary’s University in Halifax. She has worked over the last 40 years on the geochemistry and origin of ancient volcanic rocks offshore.
References
[1] Normandeau, A., et al. 2020. The seafloor of southeastern Canada. Chapter 20 of O. Slaymaker and N. Catto (eds.), Landscapes and Landforms of Eastern Canada, World Geomorphological Landscapes (Springer, 2020) https://doi.org/10.1007/978-3-030-35137-3_20
[2] Hudson 029 2010. http://hudson0292010.blogspot.com/
[3] Pe-Piper, G., Meredyk, S., Zhang, Y.Y., Piper, D.J.W., and Edinger, E. Petrology and tectonic significance of seamounts within transitional crust east of Orphan Knoll, offshore eastern Canada. Geo-Marine Letters 33, 433B47 (2013)
[4] Pe-Piper, G., Piper, D.J.W., and Jansa, L.F. Early Cretaceous opening of the North Atlantic Ocean: implications of the petrology and tectonic setting of the Fogo Seamounts off the SW Grand Banks. Geological Society of America Bulletin 119, 712–24 (2007).
[5] Bronner, A., Sauter, D., Manatschal, G., Peron-Pinvidic, G., and Munschy, M. Magmatic breakup as an explanation for magnetic anomalies at magma-poor rifted margins. Nature Geoscience 4, 549–53 (2011)