PNW volcanoes

Factual Questions
1

I’ve heard it said many times, “It Mt. Rainier blows, Seattle is toast.”

Of course we all remember (or at least heard about) Mt. St. Helens. Up near the northern border, there is Mt. Baker. Is Mt. Hood a volcano?

Anyway, how “dangerous” are these volcanoes? Will Seattle really be taken out if Mt. Rainier blows? What if it blows on the east side? Everyone seems to believe the eruption will be “pointed” at Puget Sound. Is there reason to believe that it was erupt toward the west and not toward the east?

What about Mt. Baker? Is it “pointed” toward Vancouver? How would we know? Is Mt. St. Helens still a threat? ISTR that the deaths it caused were of people who did not evacuate the area in time; and that while it caused numerous problems, they were not particularly life-threatening.

And on a related note, has anyone heard anything about Mammoth Mountain recently?

2

Most of the Cascade Ranges volcanoes are potential threats within our lifetimes. That said, some have more active recent histories than others.

The USGS has the straight dope on current activity. Compared to living in L.A. with earthquakes, you have nothing to worry about.

The actual eruption is one thing. Another is all “stuff” on the slopes of a volcano that will come downslope as well. The danger with Mt. Rainier is glaicer melt combined with rocks and mud. Called a lahar it’s a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows. They form in a variety of ways, chiefly by the rapid melting of snow and ice by pyroclastic flows, intense rainfall on loose volcanic rock deposits, breakout of a lake dammed by volcanic deposits, and as a consequence of debris avalanches.

I cannot find anything specific about Mt. Baker.

Mount St. Helens is still classified as an active volcano. However, barring something totally unforseen by volcanologists, nothing to worry about here. Damn! :slight_smile:

Mt. Hood is another story. Yep, it’s a volcano and there is potential for an eruption within 30-50 years. It had an earthquake swarm in June 2002, along with a 4.5 quake, the strongest in decades. The media had a field day with the event, then they went away. If you are betting man make it even money between Mt. Rainier and Mt. Hood.

One of the big unknowns is Three Sisteres south of Mt. Hood. There has been a gradual uplift in the general area, thought to be caused by magma pooling under the volcano. Again, no real hard evidence to support any potential eruption.

The USGS Observatory for Long Valley has current information about Mammoth Mountain. Basically, a contuining release of carbon dioxide is killing trees there.

3

Oh, I’m not worried. Just curious. If humans had centuries-long lifespans, one of the things I’d be is a geologist. (Incidentally, my house – in which I may be living soon – is about 50 miles from Mt. Baker foothills.)

I’d forgotten about the lahars. I was thinking of they pyroclastic flow. But yeah, the lahars caused a lot of death and destruction in Central America a number of years ago.

4

Major Cascades volcanoes include, from south to north:

[ul][li]Lassen Peak, Shasta Co., CA[/li][li]Mt. Shasta, Siskiyou Co., CA[/li][li]Mt. Mazama, Klamath Co., OR[/li][li]Mt. Jefferson, Linn & Jefferson Cos., OR[/li][li]Mt. Hood, Hood River Co., OR[/li][li]Mt. St. Helens, Skamania Co., WA[/li][li]Mt. Rainier, Pierce Co., WA[/li][li]Mt. Baker, Whatcom Co., WA[/ul][/li]
I believe there are a couple in B.C. as well, but my atlases don’t do western Canada well. There are undoubtedly other volcanoes in the Cascades, but these seem to be the largest and most significant mountains.

The idea is that an eruption of any major size would be destructive to a city within a short distance, regardless of “which side of the mountain blows” – a side-collapse eruption such as St. Helens had not being the norm though not uncommon.

The problem with Seattle (and even more its southern suburbs and Tacoma) in a Rainier eruption would not be so much the pyroclastic effects, which would probably die down within a ten-mile range, but the fact that it’s a Flippin’ High Mountain right above the Seattle-Tacoma-Olympia Metro. Area, and the two main channels downhill from it are the Pullayup and White Rivers, which descend to Tacoma and the Kent Valley respectively. Both gas and lava would follow these two paths downhill.

IMHO, all of the Cascades volcanoes are a threat to some degree, with Shasta (and Mazama) perhaps the least likely to blow. The danger falls into two categories – threat to life from those near the volcano who do not evacuate in time, and threats to property in the path of eruption products (solid, liquid, and gaseous). The violence of an eruption influences what is “too near” and what “the path of eruption products” might be. There are buried ash beds from Pliocene and Pleistocene eruptions extending across the inland Northwest (south Idaho, southwest Montana, western Wyoming) from eruptions orders of magnitude above those known to history. Such violent eruptions are unlikely, of course – but not at all impossible. IIRC, St. Helens ranks with the most violent eruptions in history, somewhere about sixth or seventh.

5

I live in rural King County just east of Kent and Auburn. The area is considered high ground and is considered a moderate threat if Mt. Rainier blows. Visit any of the valley areas within 50 miles of the mountain and eruption evacuation route signs are common. My sister lives near Orting and to the so called experts, she would have about 30 minutes to evacuate if Rainier erupts.

According to my handy dandy emergency resourse guide (came in a Seattle newspaper a couple years ago), the biggest threat to Seattle is the ash cloud after Rainier erupts. Threats listed as low include wind blast, flooding related to mud flows, and debris fallout. Tacoma, being about 30 miles closer, has a good chance of suffering severe damage. The area considered to be most vulnerable is the Yakima area, it is 25 miles due east of Rainier and has very little in the way of natural geography to redirect wind blasts and mudflows. As a life long resident of the area, an eruption of Mt. Rainier is way down on my list of things that I worry about that could ruin my day. I consider the earthquake threat to be a much bigger cause of concern.

6

This is all great stuff! :slight_smile:

BTW: My dad got a little capsule (like in a gumball machine) of Mt. St. Helens ash. I think I put it with his personal effects.

7

There are a lot more volcanoes in the PNW than just the snow capped peaks. I mean a lot more. [The ones with glaciers are of course double dangers since the meltwater from a glacier suddenly heating up (a la St. Helens) can do a lot of damage a long way off. If the north side of Ranier blows, you can kiss a lot of Tacoma goodbye.]

My favorite little volcano: Mt. Tabor inside the city of Portland. There’s a nifty little natural theater in the crater.

So lookout below.

8

Yep, they extend up through BC. Lots of info on volcanoes on the Canadian side of the border can be found here. I haven’t been through the entire site, but most of them haven’t erupted since the Pleistocene.

9

Yup, but chances are we bottled that little vial right here in North Idaho, and just had to scoop it out of the yard to fill thousands of the vile things.

10

Collectively, these volcanoes are the effects of the Cascadia subduction zone.

There is evidence of a huge earthquake sometime in the 1700’s, from things like submerged and raised tree stumps which have been dated by dendrochronologists fairly reliably.

If this ever goes off, it will make any of thos volcanic eruptions seem like a small diaster, predictions of earthquakes go up to 9+ on the Richter scale.

From here

http://www.metrokc.gov/prepare/hiva/earthquakes.htm

enjoy.