Skip to main content

About this free course

Download this course

Share this free course

Volcanic hazards
Volcanic hazards

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

1.2 Volcanic Ash

At the time of writing, the most recent VEI 6 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] eruption is that of Mount Pinatubo in the Philippines in 1991 (Figure 4), notable for a plinian eruption column, 35 km high, which led to a heavy airfall as well as column-collapse pyroclastic flows.

The threat posed to aircraft by volcanic ash clouds is another matter. However, it is worth emphasising here that the disposition of ash clouds and the airfall that is deposited below them are controlled by the wind direction(s) during the eruption. Figure 5 maps the pattern of airfall distribution from the 18 May 1980 eruption of Mount St Helens showing clearly the effect of wind blowing from the west. Please see Box 1 for an explanation of the contoured information on maps such as Figure 5.

Figure 4 The VEI 6 plinian eruption of Mount Pinatubo in the Philippines, 12 June 1991 (a slightly different view from TYVET Figure 5.11).
Figure 4 The VEI 6 plinian eruption of Mount Pinatubo in the Philippines, 12 June 1991 (a slightly different view from TYVET Figure 5.11).
Figure 5 Thickness distribution of airfall from the 18 May 1980 eruption of Mount St Helens (1 inch ≈ 2.5 cm).
Figure 5 Thickness distribution of airfall from the 18 May 1980 eruption of Mount St Helens (1 inch ≈ 2.5 cm).

Box 1 Contoured information on maps

On maps, and on many kinds of two-dimensional plots where the value of a quantity changes from place to place, it is common to show lines (contour lines) that link places of equal value. You may be familiar with topographic maps on which contour lines link points of equal height above sea level. In Figure 5, contour lines have been drawn linking all points of equal thickness of the airfall deposited by a particular eruption. The outer contour marks the limits of where ash was found. The next contour in encloses the area where more than half an inch of ash was deposited, and the innermost contour encloses the area where more than 2 inches of ash were deposited. The thickness of the deposit between one contour line and the next must be a value somewhere between the values on the lines. In this example, the thickness range is indicated by colour coding (according to the key on the map) rather than by labelling the contour lines.