Deadly mudslides can unfold in any of the 50 U.S. states, but a combination of geologic factors makes the West Coast especially vulnerable to the type of destructive flow that pummeled northwest Washington on Saturday (March 22), geologists say.
Mudslides generally form when a massive layer of unconsolidated rock becomes waterlogged and slips under the force of gravity. The basic ingredients for a mudslide include large areas of unconsolidated rock, steep mountain slopes, and areas with shallow water tables that quickly become saturated with rain or snow water, particularly during short but intense spurts of precipitation, Noah Finnegan, a geologist at the University of California, Santa Cruz, told Live Science. [See Photos of Washington Landslide's Destruction]
"The reason the West tends to have more landslides than the East is related to all three of these factors," Finnegan said. "In the West, active tectonics make for steeper slopes, the regional climate ensures that moisture is delivered over a relatively smaller portion of the year, and the rocks are often much weaker on the West Coast."
1. Weaker rocks
West Coast rocks are generally weaker than those in the East, because Western rocks are younger by millions of years. Through geologic time, the older formations out East have been exposed to extreme pressures and temperatures within the Earth's mantle that have made them harder and more compact than 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 billions of years old and have been deeply buried in their history, and have been cooked and hardened," O'Connor told Live Science. "Whereas, not everywhere, but a lot 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 like the rocks on the East Coast."
2. Thicker sediments
During the last ice age, glaciers on both coasts grinded against rocks and formed a loose sediment layer that sits beneath the top soil. Since the West Coast rock is younger and softer, the glaciers produced larger quantities of loose sand and stones and left behind thicker deposits, whereas the harder East Coast rocks did not give way as easily and did not produce as much loose material. The sediment layer that overlies West Coast terrain is therefore generally thicker and looser than that of the East, and more susceptible to losing grip and giving way to gravity, O'Connor said.