Washington: home to dangerous volcanoes

Living along the U.S. Gulf coast or East Coast puts you in the path of hurricanes. If you want to avoid that inconvenience, you can move to the West Coast, where earthquakes and volcanoes will keep you awake at night. The U.S. Geological Survey National Volcanic Threat assessment was updated in 2018, and Washington State has the two most dangerous volcanoes in the Continental USA: Mount St. Helens and Mount Rainier.

What makes a dangerous volcano?

The danger a volcano poses is directly related to how many people it will affect. So, a big, active volcano in the middle of nowhere is less dangerous that one that has been sluggish for a while but has a significant population in its hazard zone. This threat describes Mount Rainier where 80,000 people are under threat from the volcano.

Mount Rainier is a landmark that towers almost 3 miles (14,410) over the Seattle area. It last erupted in 1894-95 with some small summit explosions. This volcano is classified as dangerous.

Volcanic flows

Gravity is not kind to those who live at the base of volcanoes. Anything coming off the top of the mountain naturally wants to make its way down to the bottom. Most people immediately think of lava slowly flowing down the slopes, burning away trees and houses. This does happen sometimes, but it is the least problematic of three types of dangerous volcanic flows.

Pyroclastic flows come next in the pecking order of hazards. These flows are composed of a slurry of molten ash, rock, and hot volcanic gases, and they travel at speeds of up to 100 miles per hour. If we could revive any of the unfortunate citizens of Pompeii, then we could get a good description of why you don’t want to be in the path of a pyroclastic flow.

While lava and pyroclastic flows can travel up to 10 miles from the volcano, the real threat to Seattle comes from the third type of flow, lahars. If you take volcanic ash and mix it with water, you get a muddy slurry that can travel at speeds of up to 50 miles per hour and reach distances 30 to 40 miles from the volcano. When a lahar comes to rest, the slurry quickly solidifies into hard, cement-like rock.

So those attractive homes, built in the scenic valleys surrounding Seattle, are the reason Mount Rainier has captured its coveted position at the top of the dangerous volcano list. Those valleys are where gravity will take the next big lahar that peels off the slopes of Mount Rainier.