Re: Why did Mt. Vesuvius 'explode', rather than a lava eruption?

   The dynamics of erupting volcanoes can be fairly complex, but in general 
the composition of the magma (molten rock under the surface, often in a 
magma chamber) will determine the type of eruption.  Basaltic magma is low 
in silica content, and usually flows easily.  Rhyolitic magma has more 
silica, is "stickier," and doesn't flow as easily.  Rhyolitic magma is most 
often associated with violent eruptions (explosions, ash and rock falls, 
and pyroclastic flows), such as the eruptions of Mt. Vesuvius (79) and 
Mount St. Helens (1980).  The amount of water and gasses in the magma can also 
have a major affect on how a volcano erupts.  The gasses themselves can be 
hazardous as well, consisting of steam and compounds of sulfur, nitrogen, 
carbon, etc.
   Mt. Vesuvius erupted on the 24th and 25th of August, 79 A.D., inundating 
the towns of Pompei and Herculaneum.  It emitted roughly 4 cubic kilometers 
of material in the form of ash and pyroclasts (literally "fire-rock."  
Pyroclasts are hot rocks which are blown out of the volcano, often still so hot 
they glow after they hit the ground.  A pyroclastic flow is an 'avalanche' of 
glowing pyroclasts and ash).
   Recent research suggests that the victims of both Pompei and Herculaneum 
were killed by pyroclastic flows, and NOT by clouds of poisonous volcanic 
gasses.

I recommend the following website, The eruption of A.D. 79.  It 
has lots of information about the Vesuvius eruption, including recent scientific 
research.