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.
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
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.
Matterhorn 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