Curious Nature: Where will gravity strike next?
May 15, 2011
Gravity always wins out. Mountains and civilizations will rise, but the force of gravity pulls at them constantly, wearing them away stone by stone, brick by brick, and eventually, they all fall. The good news is that most of them won’t fall quickly, and they won’t fall in our lifetimes. Occasionally, though, we hear about places where the land gives way to the forces of gravity, and a landslide occurs, pulling down rocks, mud, and water in a giant cascading torrent rushing across the land. On a good day, these landslides happen out in the wilderness, where we might stumble upon a debris pile, but unfortunately, they also sometimes happen across our roads, homes, and communities, causing destruction as they rampage downward.
Landslides in all their many forms, mudslides, sinkholes, debris flow, slumps, etc., occur relatively often in the Rocky Mountains. The reason why might seem obvious, related to the steep slopes where it’s easy to see that things have a tendency to roll downhill. As usual, though, the story behind these natural disasters runs deeper than we might think. We know that water plays an important role, carrying away support structures one grain at a time until the last support is removed and the land falls. But the final piece to this puzzle is one that dates back millions of years, to the time when the ground we stand upon was formed and shaped.
Several structural components in the mountains themselves contribute to its inherent instability. Much of the rock we see throughout our valley is sedimentary in nature, formed during the eons when shallow seas covered the land. Shale makes up a large portion of the rock around us, and it can be seen in the dark gray layers on the south side of the highway at Dowd Junction. Shale was formed in deep ocean water, where tiny particles of mud collected on the ocean bottom, cementing together from the combination of pressure and mineral cement dissolved in the water. Perhaps you held a piece of shale in your hand once, maybe to skim it across the serene surface of a pond, or maybe just to peel it back, watching the flat sheets flake off in your fingers. It is this crumbly texture, combined with its nature to break off in flat sheets, which makes shale such an unstable member of our supporting bedrock.
The famous rock, gypsum, is another agent responsible for local movements of earth. Gypsum is an evaporite deposit, formed when ancient oceans evaporated, leaving behind the once-dissolved minerals. Made from a chemical compound of silicon and oxygen, gypsum can act something like silly putty. This means that weight building up on top of it eventually causes it to give way, oozing like a giant field of quicksand through underground cracks, and causing the land above it to settle down to fill the void. This can sometimes happen suddenly, creating sinkholes or slides; or it can happen very slowly, causing the land to slowly “creep” downward.
When we think about the dangers associated with living in the Rocky Mountains, we typically think about things related to extreme weather or terrain. Exposure, hypothermia, heat exhaustion, lightning, and maybe hitting a tree while skiing, are some of the biggest dangers we think about. However, other dangers may well be lurking. As we approach the June 1 anniversary of the I-70 sinkhole on Vail Pass, and as the rivers rush past brimming with spring runoff, we can only wonder where the forces of gravity will strike next.
Jaymee Squires is the director of graduate studies at Walking Mountains Science Center. Jaymee lives in Eagle with her family and she loves a chance to get outside and unearth a little geology.