Thursday, August 10, 2017

What Could be Worse than the Crater Lake Eruption? A look at Smith Rock State Park in Oregon


Standing on the rim of the Crater Lake caldera, as we did in our last post, it is hard to imagine the scale of the catastrophe. In that event just 7,700 years ago, 15 cubic miles of ash was blown into the atmosphere, covering much of western North America with volcanic dust. A similar-sized event at Tambora in 1815 caused the deaths directly of tens of thousands, and worldwide, possibly hundreds of thousands (from climate-induced famine). How could it be any worse? The answer is found not all that far away. On our Pacific Northwest journey last June, we stopped at Smith Rock State Park between the towns of Bend and Madras in Oregon. The two sites are about 120 miles apart.
Smith Rock State Park is small as such things go, only about a square mile, but the setting, as can be seen in these pictures, is rather spectacular. The 600 foot high tan-colored cliffs are popular with climbers, while a flat plateau (on the right side in the picture above) provides flatlands for parking and camping. The Crooked River flows through the park. How did these odd rocks come about?

The flat plateau is perhaps the easiest to explain. Newberry Crater is a massive basaltic shield volcano located about forty miles to the south. About 400,000 years ago, a basalt flow emanating from Newberry flowed north until it was stopped by the cliffs of Smith Rock. The Crooked River then eroded a channel between the contrasting rock types.

It is the tan cliffs that really tell the story of catastrophe. It was a disaster so huge that its dimensions were not recognized until fairly recent times. The cliffs of Smith Rock are part of the northwest corner of the Crooked River caldera, a sunken crater that is 25 miles long and 15 miles wide. Crater Lake's eruption produced around 15 cubic miles of ash. The Crooked River eruption produced around 200 cubic miles. Imagine a dozen Crater Lake eruptions happening at once and you start to get an idea. The eruption rivals some of the worst of the disasters at Yellowstone or Long Valley in eastern California. The only saving grace here is that the eruption took place around 29.5 million years ago. The magma chambers that fed the event have long since cooled.
As the hot ash landed, some parts remelted and cooled to form solid welded tuff. Other parts hardened as hot gases and steam coursed through gaps and openings called fumaroles. The cooling mass contracted and fractured into numerous joints. Differential erosion produced the various pinnacles and spires seen at the park.

Modern human beings have never experienced an eruption of this magnitude. The last one of this size worldwide, at Toba in Indonesia about 75,000 years ago, may have almost done in the human race (a controversial idea, but plausible). It involved around 470 cubic miles of ash.

I notice that Smith Rock sits at the south edge of totality during the coming Solar eclipse. If you are lucky enough to get to the park as a setting for this once in a lifetime event, I hope you'll spend a bit of time pondering the incredible history of these rocks as well.

For some detailed information about the history of the Crooked River Caldera, check this link.
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