I’m watching the local evening news right now. They’re covering the USGS’s monitoring of Mt Baker, just north of the Greater Seattle area. One of the things they mentioned monitoring is gravity. It’s increasing in the caldera.
I know a little bit about volcanos, but not that much.
First, how is gravity measured?
Second, why and how does gravity increase as volcanic activity increases?
Does the increase predict an eruption?
Baker has been muttering to itself since 1975. In fact, more attention was directed toward Baker when St Helens went blooey.
Thanks.
Here is what seems to be a typical gravity measuring instrument. It appeares to be based on the idea of precisely measuring the time it takes a known object to fall a known distance in a vacuum chamber.
When magma fills a volcano’s “magma chamber,” local gravity will increase. This is associated with an increased probability of an eruption.
It’s hard to tell from that picture, but I think most such devices actually use a pendulum. Dropping a mass might seem more direct, but you can only do one drop at a time. A pendulum, on the other hand, you can let go for a few thousand swings, to minimize random measurement error. And you’d want it to swing through only a very small angle, since a pendulum isn’t actually a true simple harmonic oscillator, so the vacuum housing would be very tall and narrow, as is seen in that picture.
My house is 50 miles west of Mt. Baker. I’m going to have to get back up there. 
Which Mount St. Helens blooey? The 1980 eruption or the eruption that has been ongoing for the past two years?
The 1980 one. I took some tourist-friends to the Seattle Science Center a few weeks before and they had all sorts of equipment monitoring Baker, but nothing about St Helens.
Johnny L.A., it doesn’t look any different. It’s been rumbling for 31 years now. I guess this gravity thing is new. I don’t think your house is in danger, but you really should move back, just in case. 
Xema, but why does the magma chamber filling increase gravity? I guess what I’m asking is the physics of gravity.
In short, the magma is moving into a previously empty space, so there is more mass under your feet, increasing the downward pull that you feel. That is, you feel an increased gravitational force at that location.
Hmmm.
I’ve got a nice view of Baker from my deck. Looking forward to a good show. IIRC, the big worry is flooding down the Skagit river if the reservoir is taken out by an eruption.
Of course, I remember when Baker was steaming, back in the 80’s. Had everyone worried back then, too.
According to the map on this page it looks as if my house is safe from lahars.
What Dervorin said. If someone filled your basement with granite, the gravity in your house would be slightly greater.
Thanks to Dervorin & Xema. That makes such plain sense, I’m :smack:
** Johnny L.A.**, I was just trying to lure you home. 