The Birth of a Mountain Range in the Appalachians of Newfoundland
Editor’s Note: Ever wonder how the mountains you’ve come to love came to be? John Waldron provides an overview of our early ideas and an outline of plate tectonics. Learn how the large arms of the Gros Morne National Park fjords were formed and what ice recession means for the mountains in our future.
This article first appeared in the newly released 2019 State of the Mountains Report. We'll continue to publish articles exploring the science on our current state of Canada's alpine on our blog throughout the year. Find them all here.
Figure 1
(a) Map of present-day northern Australia and Paua New Guinea showing the complex plate boundaries and other faults (black) that mark the ongoing process of continent-arc collision³.
(b) Map of eastern North America showing major faults marking ancient plate boundaries within the Appalachian Orogen.
(c) Map showing possible of geography and plate configurations that existed in the nascent Appalachian Orogen during the Middle Ordovician Period, about 460 million years ago, based on rotated and reflected version of map (a)⁴.
Origin of mountains: History of ideas
In changing times, it’s easy to focus on changes in the mountain landscape that are caused by humans, but mountains have been growing and changing for billions of years. Only by understanding the slow, inexorable geological changes that mountains undergo, can we come to grips with the rapid pace of modern changes to the landscape.
The founders of geology were awed by mountains but had no easy way to explain their formation. Nineteenth century geologists observed that many mountains are made of enormous piles of sedimentary rock, and suggested that sediment-filled troughs, or “geosynclines,” thousands of kilometres long, must have somehow been squeezed to form mountain belts or orogens. Geosynclines were postulated along both the eastern and western margins of North America, to explain the Appalachians and the Cordillera.
However, geologists aspire to explain the ancient world in terms of present-day processes. Modern geosynclines proved quite elusive, so geologists struggled to find an explanation for the origin of mountains.
Plate tectonics: How mountains are made
Two developments changed this picture in the late twentieth century. First, geophysical techniques allowed researchers to image the crust below the continental shelf and continental slope. The thicknesses of strata were enormous; for example, off eastern Canada, about twelve kilometres of sedimentary strata have accumulated since the start of the Jurassic Period, about 200 million years ago.¹
The second development was the discovery of plate tectonics², and particularly the process of subduction, in which one plate of the Earth's outer shell, the lithosphere, slides beneath another and is eventually absorbed back into the mantle. In a subduction zone, the over-riding plate, the one that is not subducted, typically carries a curved chain of volcanoes called a “volcanic arc.” Along the edge of the over-riding plate, piles of sediment derived from the volcanoes are also enormously thick.
“The founders of geology were awed by mountains but had no easy way to explain their formation.”
One process whereby plate tectonics can initiate a new mountain belt is called “arc-continent collision.” This happens when a continent is dragged into a subduction zone. The continent is too buoyant to be subducted, so it gets telescoped against the volcanic arc, and eventually subduction stops. The process squeezes one huge pile of sedimentary strata, from the continental margin, against another pile of sedimentary and volcanic rock, from the arc, deforming both and producing a new orogen. We can see arc-continent collision in progress in Taiwan, and along the northern margin of the Australian Plate in Papua New Guinea (Figure 1a). These are orogens in the early stages of growth; eventually, much larger ranges (e.g. the Himalaya) may form by continent-continent collision.
Figure 2. Maps showing the possible positions of major continental blocks from Cambrian to Late Ordovician time⁵. Major plate boundaries are shown in black. At about 470 Ma (million years ago) a subduction zone collided with the eastern margin of ancient North America (Laurentia), initiating the building of the Appalachians⁵.
Mountain building in the Newfoundland Appalachians
The mountains of western Newfoundland form part of the Appalachian Orogen (Figure 1b), which extends from Alabama to the east coast of Newfoundland. The Appalachians are part of an even larger belt, that extended through Pangea as far as northernmost Norway and Greenland.
The Appalachians started to develop about 480 million years ago, around the end of the Cambrian Period, as the ancient Iapetus Ocean began to close by subduction. Around 470 million years ago, the precursor of the North American continent, known as Laurentia, felt the first effects of deformation as a subduction zone collided with its eastern margin (Figure 2). Evidence for this collision occurs throughout western Newfoundland, particularly in the spectacular “arms” (fjords) and peaks of the Bay of Islands (Figure 3a) and Gros Morne National Park.
“Millions of years of erosion have sculpted the landscape,
so that we can see the interior of the former mountain range.”
The old continental margin is represented by thick piles of limestone, deposited on a continental shelf. They contain reefs built by corals and sponges, and other fossils, that clearly show that Newfoundland lay in the tropics at the time. In the Middle Ordovician Period, the shelf started to subside as the edge of the continent approached a subduction zone. The limestones were overlain by sandstone, containing grains derived from the advancing volcanic arc.
Next, the continental margin was pulled beneath a deformed mass of sedimentary and igneous rock, known as the Humber Arm Allochthon (“allochthon” is from two Greek words meaning “from another foundation”). At the top of the pile was a slab of ocean floor, representing the collided volcanic arc; remnants of this make up the present-day highlands, including the Lewis Hills (the highest point on the island), Blow-Me-Down Mountain (Figure 3), and the famous Tablelands in Gros Morne National Park. By about 460 million years ago, ancient Newfoundland must have looked very much like a flipped around version of modern Papua New Guinea, as shown in Figure 1c. Mountain building was to continue for another hundred million years, as the supercontinent Pangea was assembled.
Figure 3 – View over the Bay of Islands from Blow-Me-Down Mountain, Newfoundland. The rusty orange rocks are peridotite, a rock rich in iron and magnesium, representing an overthrust slab of the Earth’s mantle. The low ground in the distance is occupied by softer rocks of the ancient margin of Laurentia.
Uplift and erosion - a continuing story
The spectacular structures we see in western Newfoundland and elsewhere in the Appalachian Orogen were formed by deformation deep within the mountain belt as it formed. Millions of years of erosion have sculpted the landscape, so that we can see the interior of the former mountain range. Some of the most rapid erosion was geologically very recent: in the last two million years, glaciers carved the deep valleys and fjords into the west coast of the island. As the ice receded, the continuing buoyancy of the continental lithosphere has allowed it to rise. As is true in mountains over the world, the present-day elevation of the peaks is the result of a delicate balance between uplift and erosion, which continue to compete with one another.
John Waldron is a professor in the Department of Earth and Atmospheric Sciences at the University of Alberta. He teaches structural geology, and researches the development of mountain belts, particularly in Eastern Canada and NW Europe, using evidence from deformed sedimentary rocks.
References
1. Keen, C. E. et al. Deep seismic reflection data from the Bay of Fundy and Gulf of Maine: tectonic implications for the northern Appalachians. Canadian Journal of Earth Sciences 28, 1096–1111 (1991).
2. Dewey, J. F. & Bird, J. M. Mountain belts and the new global tectonics. Journal of Geophysical Research 75, 2625–2647 (1970).
3. Amante, C. & Eakins, B. W. ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis. NESDIS NGDC-24, (National Geophysical Data Center, NOAA, 2009).
4. White, S. E., Waldron, J. W. F. & Harris, N. B. Anticosti Foreland Basin Offshore of Western Newfoundland: Concealed Record of Northern Appalachian Orogen Development. Basin Research in press, (2019).
5. Waldron, J. W. F., Schofield, D. I., Murphy, J. B. & Thomas, C. W. How was the Iapetus Ocean infected with subduction? Geology 42, 1095–1098 (2014).