On March 22, 2014, a deadly landslide struck in Washington State, about 50 miles of Seattle on the banks of the Stillaguamish River north.
(Image: © King County Sheriff’s Office — Air Support Unit)
Deadly mudslides can unfold in virtually any of the 50 U.S. states, but a mixture of geologic factors makes the West Coast especially susceptible to the kind of destructive flow that pummeled northwest Washington on Saturday (March 22), geologists say.
Mudslides generally form whenever a massive layer of unconsolidated rock becomes waterlogged and slips beneath the force of gravity. The essential ingredients for a mudslide include large regions of unconsolidated rock, steep mountain slopes, and areas with shallow water tables that swiftly become saturated with rain or snow water, during short but extreme spurts of precipitation particularly, Noah Finnegan, a geologist at the University of California, Santa Cruz, told Live Science. [See Photos of Washington Landslide’s Destruction]
«The reason why the West will have significantly more landslides compared to the East relates to all three of the factors,» Finnegan said. «In the West, active tectonics lead to steeper slopes, of the entire year the regional climate means that moisture is delivered over a comparatively smaller portion, and the rocks are much weaker on the West Coast often.»
1. Weaker rocks
West Coast rocks are usually weaker than those in the East, because Western rocks are younger by an incredible number of years. Through geologic time, the older formations out East have already been subjected to extreme pressures and temperatures within the Earth’s mantle which have made them harder and smaller sized compared to the more crumbly rocks out West, according to Jim O’Connor, a hydrologist with the U.S. Geological Survey in Oregon.
«The rocks back East are vast amounts of years old and also have been deeply buried within their history, and also have been hardened and cooked,» O’Connor told Live Science. «Whereas, not everywhere, but most of the rocks on the West Coast are younger sedimentary and volcanic rocks that just aren’t deeply buried and haven’t been hardened up just like the rocks on the East Coast.»
2. Thicker sediments
Over the last ice age, glaciers on both coasts grinded against rocks and formed a loose sediment layer that sits under the top soil. Because the West Coast rock is younger and softer, the glaciers produced larger levels of loose stones and sand and left out thicker deposits, whereas the harder East Coast rocks didn’t cave in as easily and didn’t produce as much loose material. The sediment layer that overlies West Coast terrain is generally thicker and looser than that of the East therefore, and more vunerable to losing grip and giving way to gravity, O’Connor said.
3. Steeper mountains
The peaks of old East Coast mountains aren’t as steep than those out west, largely because they have already been subjected to millions more years of erosion. Just as river stones are more rounded and less jagged as time passes as the force of water and other rocks smooth them down, mountains cave in to the components and be less steep through time. Not merely have West Coast mountains experienced less long-term erosion, however, many are also still tectonically active today, generating fresh, steep cliff faces that continue steadily to grow upward.
4. More extreme wet season
The National Weather Service has reported that, previously 45 days, the spot where the Washington mudslide occurred has experienced double its normal rainfall because of this season, today according to USA.
Such rapid delivery of precipitation will not allow time for water to flow deep underground, and causes sediment to be saturated considerably faster than it could if rainfall were distributed less intensely over a wider timeframe, as is more prevalent on the East Coast.
«Rainfall is key,» Finnegan told Live Science, explaining that large slides, just like the one in Washington, react to rainfall over weeks to months, whereas small slides respond over hours to days.
5. Tectonically active
While heavy rainfall alone can trigger mudslides, earthquakes also instigate the flow often, and tend to be common on the more vigorous West Coast compared to the East Coast tectonically. Officials in Snohomish County, Wash., week where in fact the mudslide occurred last, reported a 1.1 magnitude earthquake occurred about fourteen days prior to the slide; so even, the U.S. Geological Survey has mentioned that the earthquake didn’t cause this event. Much more likely, recent soil and rain saturation triggered the lethal landslide, the USGS said.
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