Wednesday, 11 June 2014

Climate or Climb It? How changing weather builds mountains

It is easy to think of mountains as permanent features of a landscape. Don’t do it. That is what they want you to do. Let your guard down for one second and those craggy jerks will be on you like white on rice. Mountains change and they do it constantly. We all know about plate tectonics and how ancient sea floors are now some of the loftiest places on the planet. But did you know that climate can also shape mountains?

  
Recent research from the University of California at Berkley has revealed that California’s already substantial Sierra Nevada Mountains have risen 10 millimeters in the past 7 years. You might expect that this is due to things like earthquakes and tectonics plates crashing into each other, but that is only partly the case. The researchers have also suggested that California’s ongoing and increasingly severe drought has been causing the peaks to shoot upward.


The thing about the Earth’s crust is that it’s a lot more like a mattress than it seems. Put some weight on it and it will dent pretty easily. Take the weight off and watch it spring back up (if you have a few decades to spare). Much of North America’s crust is still rebounding upward in response to the removal of the weight of glaciers from the last ice age, a process called isostatic rebound. In California, glaciers are generally hard to come by, even during an ice age. The uplifting there is the result of water loss in the soil.


It may not seem like it it, but water is massively heavy stuff. It is even heavier than ice, which is why the "rocks" in your scotch float. A one meter by one meter by one meter cube of water weighs a ton. That means the bed of an average pickup truck could hold several thousand pounds of water. Imagine how much the rain water that falls on a mountain range in a year weighs and you can begin to understand why a long running drought might cause mountains to rise.


As water fluctuates and the Earth’s crust reacts, we can get into some pretty hairy situations. A growing body of research is providing evidence that changing rainfall and ice-melt patterns associated with climate change might even cause volcanoes to become more active (Capra, 2006; Deeming et al., 2010). Your classic stratovolcano is just a mountain with a lot of internal pressure. As the amount of ice or water on the overlying mountain changes, the magma chamber underneath can become unstable. When enough weight has been removed the effect is like taking the cap off a shaken up bottle of coke.


Even if the magma chamber doesn’t blow its top, melting ice can destabilize the soil in a slope and cause landslides. The massive landslide in Washington State on March 22, 2014 that killed 41 people came after a period of intense rain that weakened the slope which eventually failed.

Mountains are pretty uncool in that way. They can sit there for millions of years looking all rock-solid and majestic. They watch and wait as we build towns at their bases so we can enjoy the view, going about their natural processes of erosion and uplift at a pace that the human eye just can’t observe. Then one day, either because it has rained too much or not rained enough they are capable of kicking things into overdrive. The lesson in all of this? It’s okay to make friends with a mountain. You can even hang out from time to time. But don’t for a second think you can trust them. They’re more lively than they seem.


References:

Capra, L. (2006). Abrupt climatic changes as triggering mechanisms of massive volcanic collapses RID C-2371-2011. Journal of Volcanology and Geothermal Research, 155(3-4), 329-333. doi:10.1016/j.jvolgeores.2006.04.009


Deeming, K. R., McGuire, B., & Harrop, P. (2010). Climate forcing of volcano lateral collapse: Evidence from Mount Etna, Sicily. Philosophical Transactions of the Royal Society A-Mathematical Physical and Engineering Sciences, 368(1919), 2559-2577. doi:10.1098/rsta.2010.0054

1 comment:

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