A perilous beauty

Ever heard of the "Bonneville Slide?"

It sounds like some obscure country line dance, but the real story is more interesting, and it comes with a connection to a curious Native legend that turns out to refer to a real historical event.

The Klickitat People have lived for centuries on both sides of the Columbia River, up into what is now Skamania and Klickitat Counties, Washington, and down into Multnomah and Clackamas Counties, Oregon.  They tell the tale of Pahto and Wy'east, the two sons of the chief of all the gods, Tyhee Saghalie.  The two young men did not get along, and fought over who would rule over which parcel of land.  Their father shot one arrow south and the other north; Pahto was given the lands around where the northern arrow landed, and Wy'east the territory surrounding where the southern arrow fell to the ground.  Tyhee Saghalie then shook the Earth and created a great bridge across the Columbia River so the two could visit each other.

But soon trouble broke out again.  Pahto and Wy'east both fell in love with the same young woman, the beautiful Loowit, and began to fight, burning villages and destroying forests and crops.  Tyhee Saghalie tried to reason with them, but to no avail.  In the end he grew angry himself and shook the Earth again, destroying the bridge; the cataclysm created a flood that washed away whole forests.  He turned all three into mountains -- Wy'east became Mount Hood, Pahto Mount Adams, and the lovely Loowit Mount Saint Helens.  But even in mountain form they never forgot either their anger or their burning love, and all three still rumble and fume to this day.

What is fascinating is that this odd story actually appears to have some basis in fact.

In around 1450 C.E., an earthquake knocked loose about a cubic kilometer of rock, soil, and debris from Table Mountain and Greenleaf Peak.  The resulting landslide -- the Bonneville Slide  -- roared down the Columbia Gorge, creating a dam and what amounted to a natural bridge something like sixty meters high across one of the biggest rivers in the world.  

Greenleaf Peak today [Image licensed under the Creative Commons Eric Prado, Greenleaf Peak, Washington, CC BY-SA 4.0]

The dam couldn't last, however.  The Columbia River has a huge watershed, and the lake that built up behind the dam eventually overtopped the natural "Bridge of the Gods."  The whole thing collapsed -- probably during a second earthquake -- releasing all that pent-up river water in a giant flood.  It left behind geological evidence, both in the form of a layer of flood-damaged strata west of the slide, and the remains of drowned forests to the east, where trees had died as the dammed lake rose to fill the gorge.

Despite the reminder we got in 1980 -- with the eruption of Mount Saint Helens -- it's easy to forget how geologically active the Pacific Northwest is.  Not only is there the terrifying Cascadia Subduction Zone just offshore (about which I wrote two years ago), the other Cascade volcanoes, from Silverthrone Caldera (British Columbia) in the north to Lassen Peak (California) in the south, are still very much active.  Right in the middle is the massive Mount Rainier, visible from Seattle, Tacoma, and Olympia on clear days, which is one of the most potentially destructive volcanoes in the world.  Not only is it capable of producing lava and pyroclastic flows, it's capped by huge glaciers that would melt during an eruption and generate the catastrophic mudflows called lahars.  The remnants of two historical flows from Rainier -- the Osceola and Electron Lahars -- underlie the towns of Kent, Orting, Enumclaw, Puyallup, Auburn, Buckley, and Sumner, and in some places are twenty to thirty meters deep.

The Earth can be a scary, violent place, but somehow, humans manage to survive even catastrophic natural disasters.  And, in the case of the Bridge of the Gods, to incorporate them into our stories and legends.  Our determination to live in geologically-active areas is due to two things; volcanic soils tend to be highly fertile, and we have short memories.  Fortunately, though, we couple what seems like a foolhardy willingness to take risks with a deep resilience -- allowing us to live in places like the Cascades, which are bountiful, and filled with a perilous beauty.

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Mudslide

As part of our ongoing exploration of things that are big and scary and powerful and can kill you, today we have: underwater avalanches.

It's a topic I looked at a while back apropos of the Storegga Slide, which sounds like a bizarre mashup of Swedish folk music and a country line dance but isn't.  This was an undersea avalanche that occurred a bit over eight thousand years ago, a catastrophic slope failure between Iceland and Norway that displaced over three thousand cubic kilometers of debris and triggered a methane clathrate explosion -- resulting in a tsunami estimated at thirty meters in height which went on to inundate large parts of coastal northern and western Europe.

Underwater avalanches are a vastly understudied -- and therefore underestimated -- danger.  The reason it comes up today is a paper this week in Science Advances about a newly-discovered one that was on the same scale as Storegga, but in a different location.  This avalanche occurred an estimated sixty thousand years ago in Agadir Canyon, off the coast of Morocco.

The Agadir Canyon avalanche seems to have started small, possibly triggered by an earthquake.  But like snow avalanches in mountainous regions, once a bit of material starts to move, it causes other parts of the slope to fail, and pretty soon what you have is a monster.  From seafloor analysis of the sediment layers, what appears to have occurred is that the initial slide involved about 1.5 cubic kilometers of debris (itself not an inconsiderable amount), but by the time it peaked, the sediment flow was a hundred times that volume.

"What is so interesting is how the event grew from a relatively small start into a huge and devastating submarine avalanche reaching heights of two hundred meters as it moved at a speed of about 15 m/s, ripping out the sea floor and tearing everything out in its way," said Chris Stevenson, a sedimentary geologist from the University of Liverpool, who co-led the research, in an interview with Cosmos.  "To put it in perspective: that’s an avalanche the size of a skyscraper, moving at more than 64 km/h from Liverpool to London, which digs out a trench thirty meters deep and fifteen kilometers wide, destroying everything in its path.  Then it spreads across an area larger than the UK burying it under about a meter of sand and mud."

Yeah, that puts it in perspective, all right.

The path of the Agadir Canyon avalanche [Image credit: Christoph Bottner, Aarhus University]

The Agadir Canyon avalanche undoubtedly caused a massive tsunami, but given how long ago it occurred, it'd be hard to find evidence at this point.  Let's just say that it would have been a very bad time to live along the west coast of Africa or east coast of the Americas.

"We calculate the growth factor to be at least a hundred, which is much larger compared to snow avalanches or debris flows which only grow by about four to eight times," said Christoph Bottner of Aarhus University in Denmark, who also co-led the team.  "We have also seen this extreme growth in smaller submarine avalanches measured elsewhere, so we think this might be a specific behavior associated with underwater avalanches and is something we plan to investigate further."

The problem is, just about every continent is surrounded by a region of relatively shallow water (the continental shelf) with the abyssal regions just beyond its edge; at the boundary between the two is a very steep region called the continental slope, where the depth increases drastically over a relatively short horizontal distance.  These areas are prone to failure, and while most events are minor -- comparable to a small mudslide on a mountainside -- some of them, like Storegga and Agadir Canyon, can grow to colossal proportions.

And at the present, we don't know which areas are likely to be safe, and which are at significant risk.

So that's our unsettling science story of the day.  This kind of thing is why I always get a grim chuckle out of people who say how benevolent the Earth is, some even going so far as to describe the universe as "fine-tuned for our existence."  This ignores the inconvenient fact of how much of it is actively hostile -- and some of the most hostile bits are right below the seemingly tranquil surface of the ocean.

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