The Formation of the Rocky Mountains

Key Topics

The Mountains Go Up – Mountain Building

To understand the formation of the Rockies, we need to understand Plate Tectonics. According to this theory, the surface of the Earth is made up of a series of plates, each of which move relative to the others. At one time, all the continents were joined into one large land mass known as Pangea. Slowly, this supercontinent began to break apart and the continents began to drift. Inevitably, the plates eventually began to collide with one another -- with mountainous consequences.

Periods of mountain building are known as orogenies and in this area, two have been responsible for the mountains we see today. Prior to these, the North American Plate had been moving in a westerly direction and the neighbouring Pacific Plate trending northward. The edge of the North American plate was located near to present-day Salmon Arm. Off the coast, sediments were deposited upon a basement of hard Canadian Shield rocks. As you moved into deeper and deeper waters, the layers of sedimentary rocks became increasingly deeper.

Contained as part of the Pacific Plate were chains of islands that became large land masses as the plate moved and literally bulldozed them together (think of it as a giant bulldozer traveling through the pacific piling all the islands into large accumulations). There were two such land masses in the Pacific and they were known as Terranes, more specifically the Intermontane and the Insular Terrane. For simplicity we'll call them the 1st and 2nd Terrane.

As the Pacific Plate moved north, the crust over which it moved was forced down by the North American Plate, back towards the Earth's core. However, as the plate closed in on the 1st Terrane, this land mass was too buoyant to be forced downward and so it was added onto the edge of the continent. This is where much of British Columbia joined North America. Along with this collision came intense forces compressing the already existing land mass. This brought on the first orogeny, known as the Columbia (it formed the Columbia Mountains made up of the Caribous, Selkirks, Purcells and the Monashees).

The collision causing the Columbia Orogeny occurred about 175 million years ago, and as the shock wave moved eastward, it forced huge masses of rock to crack and slide up over its neighbours. This is known as thrust faulting and was instrumental in the formation of the Rockies. The shock wave began piling up the western ranges, and then the main ranges, around 120 million years ago.

The 2nd Terrane collided around 85 million years ago setting off a whole new series of shock waves and beginning the Laramide Orogeny. The force behind this second collision provided the energy needed to form the front ranges and the foothills. Eventually the force died out as it approached Calgary and so the prairies were left undisturbed.

Mountain Types

Castellate

Typical of main range peaks, castellate mountains are distinctive of mountains composed of horizontal-lying layers. They often have vertical towers, and a step-like character resulting in a namesake resemblance to ancient castles. Castle Mountain, west of Banff townsite, represents a textbook example of a castellate mountain.

Mountains cut in dipping-layered rocks

Some mountains result from horizontal layers of rocks being thrust up at an angle of 50-60º. This results in a peak with one sweeping, smooth face, and one sharp, steep face where the edge of the uplifted layers are exposed. Mount Rundle provides a classic example of this type of summit.

Dogtooth Mountain - Mount Birdwood - click to learn moreDogtooth Mountains

When masses of almost vertical layers are eroded, layers of very hard rock may remain as an erosional remnant. These jagged peaks jut straight up into the sky and seem to defy the elements. Mount Louis in Banff National Park, and Mount Birdwood in Kananaskis Country are classic examples.

Sawtooth Mountains

When a long ridge of mountains is composed of almost vertical layers of rock, these layers may be eroded into a jagged ridge resembling the blade of a saw. In Banff, the Sawback Range exhibits classic sawtooth form.

Mount Assiniboine - Copyright Ward Cameron 2006Matterhorn Mountains

When glaciers scour four different sides of a summit, they may create a square-topped summit similar to the Matterhorn of Europe. Mount Assiniboine is the most photographed example in the Canadian Rockies.

Anticlinal Mountains

When rocks are compressed, they don’t always crack. Sometimes they are compressed into smooth domes (anticlines) or depressions (synclines). These structures can be preserved in the mountain form to create anticlinal mountains. Moose Mountain in the foothills, and several of the mountains of the Fairholme Range exhibit this character.

Synclinal Mountains

Conversely, mountains formed in dipping troughs, are known as synclinal mountains. Cirrus Mountain in Banff National Park, and Mount Kerkeslin in Jasper National Park are examples.

Complex Mountains

Some mountains defy classification. They may have a combination of upfolds and downfolds resulting in very complex structures. These mountain forms are common in the eastern portions of Banff and Jasper National Parks.  

Shifting Foundations – Plate Tectonics

There was a time when the continents were thought to be constant, that each had formed in the same location in which they now sit. Over time, scientists began to question the relationship between the various continents and they found reason to question the theories of the time. As early as 1858, a French scientist named Antonio Snider-Pellegrini began to speculate that all the continents had at one time been joined into a supercontinent he called Pangea. Even prior to Pellegrini’s theory, it had been noted that the continents could fit together like the pieces of a jigsaw puzzle. For instance South America fits almost perfectly against Africa.

In 1915, a German Meteorologist named Alfred Wegener published a book entitled The Origins of Continents and Oceans. He took the puzzle theory one step further. In theory, if two points were at one time joined, they should have a similar rock structure and fossil record. Wegener showed that fossils found in Brazil were identical to those found in adjacent area’s of Africa. The main problem with theories like Wegener’s came from the fact that no mechanism for the movement could be discovered. How could continents move?

The breakthrough came in the 1950’s when scientists began to carefully study the ocean floor. As they used sophisticated echo-sounding equipment to map the ocean floor, they discovered an immense ridge that completely dissected the ocean. It was approximately 65,000 km. in length. It also appeared that there was a valley at the top of this ridge that showed signs of splitting apart, as if the ocean floor was spreading at this seam. It was Geologist H.H. Hess, who in 1960 suggested that indeed the ocean floor must be spreading due to convection currents within the Earth’s molten mantle. The continents would merely be riding the wave of these convection currents as the seafloor spread.

The ocean soon came under the scrutiny of specialists specializing in paleomagnetism. It seems that the magnetic polarity of the Earth has not always been as it is today. The magnetic pole has not only moved, but at times the Earth’s magnetic polarity has completely reversed. The present situation of a magnetic north pole has only existed for approximately 700,000 years. Scientists now know that the poles tend to remain relatively stable for up to 3 million years, and then reverse. Over the past 4.5 million years there have been approximately nine reversals.

Basalt is one of the planets most common igneous (formerly molten) rocks. It retains a slight magnetic charge when it hardens, and thus also records the magnetic polarity at the time of its formation. As new sea floor is formed at the mid-oceanic ridge, basalt evenly spreads out in both directions from the centre. By studying the rocks, we can see parallel deposits of rock spreading from the ridge. By moving outward, we can determine the age of the rocks, and also study their magnetic polarity. This gives us a very clear idea of the timelines of the polarity reversals. Since the pattern on one side of the trench is mirrored on the opposite, this is further proof that the ocean floor is spreading and that the continents are moving.

Plate tectonics has now been recognized by most scientists, and has revolutionized the study of mountain landscapes.

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All Material © Ward Cameron 2005