Author’s Note: This part of a weekly series on geology for a class I am taking this semester at BYU-Idaho. This week’s prompt required students to find a recent mass wasting or flood event in their state. Other posts from this assignment can be found under the “Geology 111” category.
Okay, so no, Union Gap isn’t in Oregon, but as we’ll be able to see Washington from our house once we move in a few weeks, I feel like it’s fair game. Plus, Union Gap is closer than other landslide events in the Portland-area.
For a few weeks in January and February it seemed like news media in the Pacific Northwest became briefly fascinated by a slope failure above a quarry south of Union Gap, but then all reporting kinda just stopped just as quickly as it began. Well, the slope is still sliding as it has been since at least October.
The fascinating Rattlesnake Hills landslide in Washington State:- https://t.co/fbvdgtGfwC pic.twitter.com/1Uf9wEEosK
— Dave Petley (@davepetley) January 17, 2018
Several news articles reported on the event. Since I have to include a link to a news article about it for the assignment, here are a few:
- Geologist warns Yakima-area landslide could be worse than officials expect (Seattle Times) (archive)
- Slow-moving landslide near Yakima has residents on high alert (KOMO, Seattle) (archive)
- Rattlesnake Ridge landslide: Yakima County declares disaster (KING, Seattle) (archive)
- Yakima County, Union Gap declare a disaster due to landslide (KIMA, Yakima) (archive)
- Watching a Ridge Slide in Slow Motion, a Town Braces for Disaster (New York Times) (archive)
- Report: Rattlesnake Ridge landslide could last years (Spokesman Review, Spokane) (archive)
Since these articles were written, attention has focused on other events around the nation and the world while the slope continues to slip approximately 1.6 feet every week. The hill, which is adjacent to Interstate 82 and the Yakima River, has forced the closure of Thorp Road. This road was closed back in December and remains closed indefinitely. Not only is it a safety hazard to drive on the road, but authorities have also placed large barriers on the road hoping that they will protect the freeway from falling rocks and boulders.
Despite these precautions to protect the thousands of people who use that stretch of freeway daily, reports show that geologists expect to see the slide continue to slowly fall into the quarry rather than to impact I-82. The same report, (archive) ordered by Governor Inslee, states that it is “very improbable” that the slide will dam the Yakima River.
Unlike many landslides in Washington state, such as the Oso Landslide in 2014, water does not appear to be a contributing factor in this event. The New York Times reports (archive) that the complete cause of the slide has yet to be determined, though it’s not a stretch of the imagination to consider the nearby quarry when thinking about it (but that’s the only assumption I’ll make).
The mass of the slide is basalt, laid down several million years ago when massive lava flows covered much of eastern Washington with deep lava. Since then, the Columbia Basin has experienced north-to-south compression creating several fault lines that are responsible for the growth of ranges such as the Rattlesnake Hills.
The thick slabs of basalt are found in layers, separated by small layers of silt. It is on one of these silt layers that the slab of basalt is sliding into the quarry.
In order to monitor the slope, several agencies have placed a massive amount of instruments on and around the sliding area. These include GPS stations to monitor movement, seismographs to monitor minor ground shaking caused by the movement and webcams to watch the situation.
It is possible that the slide could continue slowly for years, or even stop altogether.
“It could easily just stop moving, and then we’re sort of at ‘now what?'”
– Stephen Slaughter, Washington Department of Natural Resources
Quoted by the New York Times (archive)
That’s actually one of the biggest concerns that emergency managers have with this event – that it will continue slowly or stop completely. The reason for that is that such a situation could provide a false sense of security for those in the path of the slide. Furthermore, if the slide stops that doesn’t mean the slope is suddenly stable. It could resume at a later time by various triggers, such as an earthquake.
For now, though, it looks like the slide will keep moving along slowly. It could have an abrupt failure anytime – sometime today or sometime years from now. It could also never abruptly fail and just slowly slide into the quarry. At any rate, emergency managers are monitoring and preparing for the worst.
For more information on the slide, visit the following:
