Who knows? Probably the place where it blew up.
A "bulge" developed on the north side of Mount St. Helens as magma pushed up within the peak.
Hawaii's volcanos are quite compared to Mt St Helens because Mt St Helens blew up from a earthquake of two plates shifting making the volcano vary voilent and loud which blew one half of the volcano
The famous Mount St. Helens blast that took 57 lives occurred at 8:31 AM on May 18, 1980. The volcano re-awoke with a significant ash puff on Oct 4, 2004 and a major puff on March 8th, 2005. See photos and details of the blast at http://www.mountsthelens.com/history-1.html
Different artifacts made from the ash of the volcano Mt. Saint Helens can be worth up to $150. Mt. Saint Helens ash salt and pepper shakers are currently selling on Etsy for as little as $8.00.
It rumbles approximately 376 to 482 times a day. That's not actually true I'm just making this up. You should not listen to any of this answer, I am a lier and I know absolutely nothing about Mt. St. Helens or whatever you asked.
Mount fuji blow up 1707
I'm sure someone can provide a better answer, but pressure built up in the magma under the volcano. When the pressure gets too great: eruption.
Mount St Helens is made up of andesitic and rhyolitic pyroclastic materials.
The plates of earth rest on liquid lava (the mantel). As they move pressure builds up under the volcano and when its too much it explodes.
700 miles away but it was not heard in surrounding areas because the sound waves went state up and bounced off the atmosphere.
Mt. St. Helens cost Washington state a total of $2.74 billion dollars.