<P> During the nine hours of vigorous eruptive activity, about 540,000,000 tons of ash fell over an area of more than 22,000 square miles (57,000 km). The total volume of the ash before its compaction by rainfall was about 0.3 cubic miles (1.3 km). The volume of the uncompacted ash is equivalent to about 0.05 cubic miles (210,000,000 m) of solid rock, or about 7% of the amount of material that slid off in the debris avalanche . By around 5: 30 p.m. on May 18, the vertical ash column declined in stature, but less severe outbursts continued through the next several days . </P> <P> Generally, given that the way airborne ash is deposited after an eruption is strongly influenced by the meteorological conditions, a certain variation of the ash type will occur, as a function of distance to the volcano or time elapsed from eruption . The ash from Mount St. Helens is no exception, and hence the ash properties have large variations . </P> <P> The bulk chemical composition of the ash has been found to be approximately 65% silicon dioxide, 18% aluminium oxide, 5% ferric oxide, 4% each calcium oxide and sodium oxide and 2% magnesium oxide . Trace chemicals were also detected, their concentrations varying as shown: 0.05--0.09% chlorine, 0.02--0.03% fluorine, and 0.09--0.3% sulphur . </P> <P> The index of refraction, a number used in physics to describe how light propagates through a particular substance, is an important property of volcanic ash . This number is complex having both real and imaginary parts, the real part indicating how light disperses and the imaginary part indicating how light is absorbed by the substance . </P>

When was the last time mt st helens erupted before 1980