Volcanoes erupt in many different ways, from big to small, long-lasting to quick, explosive to non-explosive. Additionally, many times a single eruption can produce different activity at different times or even at the same time in different places. Obviously, knowing what type of eruption is going on is important if you are worried about volcanic hazards.
There have been various attempts made to classify volcanic eruptions. As usual of course, these are human classifications - there is no "law" that says nature has to fit all eruptions into our definitions! The most widely used eruption classification scheme was published by Dr. Gordon Macdonald in his book "Volcanoes". This scheme attempts to arrange eruptions in a spectrum of increasing explosivity (due to increasing gas content and increasing viscosity). Note, however, that it is not quite a continuous spectrum, with Peleean eruptions being a notable exception.
This classification scheme has been used widely for many years but it is less than satisfactory, mainly because there are no clear distinctions between the different classifications. Note for example, that hawaiian magma is "fluid" whereas Strombolian magma is "moderately fluid", hawaiian explosive activity is "weak..." whereas Strombolian explosive activity is "weak to violent...", and so on. These are such subjective descriptions that it would be difficult to say that one person's "weak" is the same as another's, especially when trying to explain all this to inquisitive students!
The answer has been provided by Lionel Wilson, George
Walker, and others, who decided to look at different eruptions
from the point of view of understanding the actual processes that
occur during eruptions. This effort has led to clearer distinctions
between types of eruptive activity and slowly but slowly it is
being recognized and adopted. The main factors that enter into
this new classification include those of the old scheme (behavior of gas
and viscosity),
with the important additional consideration of the rate at which
magma is supplied to the vent. The differences between the eruption
types are now exact descriptions of volcanic processes rather
than subjective boundaries along a continuous spectrum.
PLINIAN ERUPTIONS
VULCANIAN ERUPTIONS
PYROCLASTIC FLOWS
PHREATOMAGMATIC ERUPTIONS