The Mountains Come Down – Breaking Down the Rocks
Key Topics
Introduction
As we look upwards, it's hard to imagine the
mountains almost twice their present size – however in their infancy they were
almost that large. Slowly, through the processes of weathering and erosion, the
mountains have been reduced to a fraction of their original size. The material
removed from these mountains has been used to help fill in the many mountain
lakes in the area. This will all lead to a very different terrain in a few dozen
million years from now.
Beginning upslope, a number of processes have
conspired to slowly wear the mountains down. Frost heaving is one of the most
critical erosional process. During the day, water flows into cracks and crevices
in the rocks and, as the temperature drops with the onset of evening, freezes.
Since water expands when it freezes, it forms a powerful wedge which can quarry
large pieces of rock. Most of the loose rock or scree slopes so common in the
mountains have been formed in this way.
Other processes include the slow carving by
glaciers and rivers, both of which tend to wear the rocks down to smaller and
smaller particles until they reach the consistency of flour. This rock flour, as
well as coarser material, flows downstream (or down-glacier since glaciers are
little more than a river of ice) and further helps to carve the valley bottoms.
It does this by acting like a giant abrasive, wearing down the soft rocks of the
Rockies. This adds more and more material to the river.
One might ask where all of this material ends
up? It finds its way into the lakes and oceans downstream. Water moving at a
high speed, such as in a steep river, can carry huge amounts of material – Lake
Sunwapta at the base of the Athabasca Glacier receives 570 tonnes of material
every day during summer runoff. When these fast flowing streams enter any large
body of water, their speed drops rapidly and restricts their ability to carry
sediment. As a result, the material is unceremoniously dumped at the entrance to
the lake, subsequently resulting in the formation of deltas. Lake Sunwapta will
likely disappear within the next 10 years. Peyto Lake, on the Icefields Parkway,
has a huge delta growing outward from its source. Almost 1/3 of the lake has
been filled.
Slowly, the forces of nature are wearing the
mountains down from the summits, and filling in the lakes from their stream
entrances outwards. This implies a natural preference for flat terrain. If
everything stayed consistent for all time, the world would be a rather flat and
uninteresting place. However, thanks to constantly changing climates and large
scale geological movements, like mountain building, we are treated to unending
diversity. As nature slowly sculpts the landscape and works to flatten it,
occasionally rapid (geologically), large scale disruptions, like the uplifting
of mountains, force it to begin anew.
Landslides in the Rockies
Landslides are one of natures most powerful
events. Suddenly, without warning, huge amounts of material may break off a
formerly quiet hillside and come thundering down the valley wall. Moments later,
the peace is restored, but beneath the dust is a scene of power and devastation.
In 1903, in the Crowsnest Pass, the sleeping town of Frank was buried beneath
hundreds of tonnes of debris when part of Turtle Mountain released the Frank
Slide. The magnitude of this event was sufficient to inspire an impressive
interpretive centre designed to showcase this historic event.
Despite their potential devastation, large
landslides are as rare as they are spectacular. Most of our mountain slopes are
made up of rocks that were originally deposited horizontally, and later thrust
up at steep angles to the horizontal. In some cases, the boundary between two
layers may provide an area of weakness that may catastrophically release without
warning. To complicate matters worse, glaciers cut deeply into the sides of most
of our valleys, leaving many of these slabs unsupported. This has occurred along
the Icefields Parkway just north of Tangle Falls. The fracture can be clearly
seen in the thick layers of pink quartzite which now litter both sides of the
highway.
Moraine Lake lies behind a dam of debris
caused from a rockslide that came off the Tower of Babel. Some of this debris
may have fallen on glacial ice and been moved a short distance before being
dumped at its present location.
Numerous other mountain lakes have been
similarly formed. Medicine Lake in Jasper National is caused by a large slide
from the Queen Elizabeth Range, and Maligne Lake from a slide off the Opal Hills
and surrounding slopes.
Agents of Change - How are the Mountains Slowly Shrinking
Chemical
Weathering
Many of the rocks that make up the mountains,
in particular limestone, are susceptible to chemical reactions with materials
carried in the air and water. In some cases, a chemical reaction to the water
changes the material. In others, the water may merely dissolve the substance.
Most of our caves are formed by limestone dissolved by the action of water.
Finally, the oxygen in the air around us may oxidize minerals in the rock. The
reddish colour of the rocks making up the exterior walls of the Banff Springs
Hotel is the result of iron in the shales slowly oxidizing once exposed to air.
Freeze and
Thaw
Water is unique. As it freezes, it expands in
volume. In the mountains, snow melts during the day, and the resulting water
flows into cracks in rocks. Overnight, as temperatures drop, the water expands
and freezes. This expansion provides an incredible wedging force and slowly
chips away at the rock face. Most of the local scree or loose rock slopes were
formed by the slow action of ice wedging. As water freezes, it expands 9%,
providing the sheering force of dropping a shot put 3 m. onto the rock.
Glaciation
Glaciers take the landscape and sculpt it.
Sharp v-shaped valleys are smoothed into rounded u-shaped valleys. Glaciers near
the summits create sharp horn peaks like Mount Assiniboine. In other cases
glaciers on two sides of a ridge can create a sharp knife-blade ridge like
Nihahi Ridge in the eastern part of Kananaskis Country.
Avalanches
As snow accumulates high in the mountains,
eventually the excess is released in the form of avalanches. Since these
releases will follow the most efficient route, they normally follow the same
path year after year. Some years may widen the path if snowfalls are large,
other years may allow tree growth to slowly try to reclaim the path. The signs
are always evident, and these paths provide opportunities for lush summer
growth.
Running Water
Water is the king of erosion. Nothing carves
up a landscape like the action of water. Even glaciers are simply another form
of this versatile material. As water begins to move, it picks up material in the
form of gravel and debris. As these sediments are carried within the water, they
provide the abrasive needed to begin carving river channels. As the water
continues to move, it picks up increasing amounts of water and sediment, and the
process continues.
Soil Creep
Very subtle, creep can be almost
undetectable. It is normally found on weathered mountain slopes. Under the force
of gravity, the material slowly begins to slide downhill, and eventually the
signs of creep become evident. Tree trunks curve upward as the creep had caused
them to tilt downhill, and they in turn corrected for this. Fence posts and
hydro poles may also begin to lean downslope as the effects of creep increase.
Slumping
In many situations, a steep hillside will
show a spoon-shaped depression within which the material has begun to slide
downhill. This slump, as it is known, occurs along a distinct fracture zone,
often within materials like clay, that once released, may move quite rapidly
downhill. In some cases the slump is caused by water beneath the slope weakening
it. In many cases it is simply the result of poor engineering along highways
where it is a regular occurrence.
Rockslides
Sometimes, as layers of rock are steeply
uplifted, the bonding of one layer to another may be weakened by the action of
water or other agents of erosion. As the force of gravity constantly pulls down
on the surface of a steep slope, there may eventually be a failure. As a
fracture occurs at the top, a layer may slide down suddenly creating a large
rockslide.
Wind
In the Rockies, wind tends to play a minor
erosional role. In area’s where the wind constantly blasts, as in the Bow
Valley, the wind may add to the effects of other forces.
Biological
Action
The orange and green lichens that coat many of our rocks are slowly breaking them down. With the ability to slowly break down their limestone home, they begin the lengthy process of soil creation. On a larger scale, the trees that seem able to grab a tenuous roothold on the steepest of slopes , do so by jamming their roots down any available crack. This creates a classic wedging effect which may break off large pieces of rock
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All Material © Ward Cameron 2005