are destructive but:
- generally notlife-threatening because:
- they normally advance so slowly that people can walk or run away from them.
Extremely viscous dacitic lava:
does not flow easily;
it tends to form steep-sided lava domes overvents
Similar explosions could occur
-without warning in the future
- but become progressively less likely with continued cooling of the domes hot interior
-and the subsurface conduit that supplied the magma for dome growth.
A lateral blast is a
-they are gravity-controlled flows that are channeled into valleys
-as they move downhill
-they can flow about twice as fast as water in channels of similar depth and slope.
Lahars triggered at Mount St.Helens in 1980 were 3 to 15 m (10 to 50 ft) d
It updates previous assessment:
taking into account both recent experience aterupting:
geologic changes initiated at Mount St. Helensby the 1980 eruptions.
These changes include
(1)beheading of the summit, forming a trunca
Explosive Ejection of Ballistic Projectiles
can impel rock fragments on ballistic trajectories that may be counter oroblique to the
Such events may occur:
either during or independently from an ongoing magmatic eruptio
Neither a large debris avalanche nor a major lateral blast like those of May 18, 1980is
likely now that a deep, open crater has formed.
Sufficient time has elapsed since the lastdome-building eruption in October 1986 for
magmain the conduit beneath the do
Ash suspended in the air from a large eruption can be a major source of aggravation
even hundreds of kilometers (a few hundred miles)downwind from its source
-both during its initialaccumulation and later as fine dry ash is remobilized by wind
(The May 18, 1980 tephra plume rose about 25 km [15 mi] in less than 30 min).
As the energy required to keep them in suspension diminishes:
the particles begin to fall out of the plume under the influence of gravity.
fall back to earth cl
Addition of a 50 percent safety factor to the 10-km (6-mi) range identified for ballistic
-from the Sugar bowl blasts suggests that hazard from rock projectiles might extend 15
km (9 mi) northward from the crater
-in laterally directed explosion
Pyroclastic flows from the May 18, 1980eruption ran out only about 8 km (5 mi) from
As they impinged on Johnston Ridge, theywere deflected westward downvalley and
eastwardto Spirit Lake.
During the past 4,000 years:
during which time the volcanos