Hope Slide

The debris field from Hope Slide - you can see the slip face in the background.

The word "slide" has many different connotations; depending on who you are the term will elicit many different meanings.  A child would think of playground equipment, a musician of a trombone.  A mechanic may visualize the greased movement of a piston while a geologist cogitates on mass movement.  It is this last one which I have in my thoughts today.

A landslide is a terrifying event to behold.  Mass movement, also called mass wasting, of any kind, is natures way of leveling the playing field, literally.  What is up comes down.  As I live in BC we hear about these all the time in the form of avalanches.  Made of snow, trees, and some rock, they are a terrible threat to any who might be in their path.  Mud flows are another form, as are slumps and a few other geological oddities.  It is the landslide though that really holds destructive power.

Landslides occur here frequently, most of them relatively small.  Have a look at this website which mentions twelve local recent ones that have affected humans.  Large ones are relatively rare though.  There are two significant ones which have occurred in Western Canada in the last 125 years.  The largest of these is Hope Slide, which took place in 1965, and Frank Slide, which predates it by 62 years.  Both were devastating.  Washington state had a landslide occur on Mount St. Helens in 1980 releasing a pyroclastic explosion estimated to be equivalent to 15,000 Hiroshima bombs.  Explore the volcanic eruption in this video.

A landslide can be caused by a large number of factors.  Human activities including building mines, roads, and dams all increase the chance of inducing one.  Stimuli in nature include massive rainfall, earthquakes, and weathering.  Hope slide may have been caused by natural faults and expansion related to freezing in the exposed sections.  Two minor earthquakes occurred in the area as well, although seismologists aren't sure if they precipitated the mass movement or if they were forebearers of things to come. 

Fifty million tons of rock slipped from the mountainside into the valley below.  A lake was completely inundated; you can see on the opposite side of the valley the place where the lake waters tsunamied into the forest.  Four people were killed, and the rebuilt highway sits some 55 meters above the previous one.  The rock, once it starts moving, builds up a pressure wave of air underneath it which allows the material to travel far further and faster than what may be expected.  If you are in the path of a landslide of epic proportions, there is no escaping it.

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