Here is an animation (MPEG or Quicktime) illustrating the inflation of the ground caused by the formation of Volcano "X." There is no bulge when the magma is still very deep. The bulge begins and grows as the magma rises, reaching a maximum when the magma is near the surface. The bulge deforms the rock near the surface which often causes small cracks in and around it. The eruption begins when the main magma-filled cracks reaches the surface. As the magma flows out onto the surface as lava, the bulge will decrease somewhat, like a deflating balloon.

Measurement of the volume of a bulge is very important because it provides an indication of how large the later eruption will be, since the volume of the bulge on the surface is roughly equal to the volume of the new magma underground. For example, about one square mile of the north side of Mount St. Helens bulged outward about 450 feet (approximately one-tenth of a mile) before the May 18 eruption. Thus the volume of the bulge was about one square mile times about a tenth of a mile, or about a tenth of a cubic mile. By comparison, the volume of new lava expelled during the eruption was later estimated to be a few tenths of a cubic mile.

Let's estimate the volume of the new magma under the bulge of Volcano X. We will work with the maximum bulge shown in frame 4 of the animation. One way to measure the volume is to think of the bulge as a layer cake: the bottom layer is one centimeter (cm) thick, the next layer - given as 5 cm tall - is 5-1 = 4 cm thick, and the top layer - listed as 10 cm tall - is 10-5 = 5 cm thick. The volume of each layer may be found by measuring its area and multiplying by its thickness. The volume of the bulge is then found by adding the volumes of all three layers together. To illustrate a point, let's make a quick estimate, rather than an exact measurement of the volume. The bulge is nearly circular, except for the "tail" to the lower right, so we can get our rough estimate by approximating the outline of each layer as a circle. If we assume the diameter of the 1 cm contour is about 10 kilometers across, then the area of 1 cm layer is about 3.15x5² = 79 square kilometers (km²), the area of the 4 cm thick layer (about 7 km in diameter) is about 38 km², and the top 5 cm thick layer (about 3.5 km in diameter) is about 10 km². Multiplying each layer by its thickness and adding the products together (remembering that 1 km = 100,000 cm) give a volume of about 0.0028 cubic kilometers (km³). Remember, this is only an estimate because we approximated each layer as a simple circle.

A more exact volume can be found by using an image analysis program like NIH Image. [Directions for NIH Image: use File/Open to pull frame 4 into NIH Image. Use the measure tool (see measuring activity) to measure the vertical diameter of the 1 cm contour and Set Scale assuming this diameter is 10 km. Select Area under Analyze/Options Select Options/Density Slice and use the tool to narrow the red selection bar that appears on the LUT. Move the narrow selection bar up and down the LUT until one of the colored contours of the bulge is changed to red. This area is now "selected." Select Analyze/Measure. Repeat for each colored contour. Finally, select Analyze/Show Results to see your three measured areas.] Rather than treating the bulge like a layer cake, this approach measures the area of each thickness directly, so the volume of the bulge is obtained by multiplying each measured area by its total thickness and adding. Using NIH Image, we obtained 64.2 km² for the 1 cm area, 27.9 k m² for the 5 cm area, and 8.41 km² for the 10 cm area. Multiplying together and adding the volumes gives 0.00288 km³. This more accurate value is slightly larger than the estimate we first made, largely because we included the "tail" on the bulge. However, our first estimate was close enough for many purposes and shows that even without sophisticated image processing programs, useful measurements can be made by careful approximations.

Now that we have measured the volume of the bulge of Volcano X, let's consider what we have. By either technique, we obtain a rounded volume of 0.003 km³. Comparing this amount with the observed volumes of different sizes of eruptions in the table of Eruption Sizes shows that even if all of the new magma erupts onto the surface, this will still only be a small eruption. If the volume had turned out to be 10 or 100 km³, then the expected eruption would be much larger and destructive.

"Typical" Events This category includes a number of different types of observations, including avalanches and landslides triggered by inflation, releases of volcanic gas and steam, changes in the temperature or mineral content of springs on or near the volcano, and the formation of new fractures or faults. New cracks and faults form where the rocks are pushed out of shape and break. Faults typically form around the edges of a large bulge and indicate the area over which rock failure might occur during the coming eruption.

For example, large fractures formed around the edges of the bulge on Mount St. Helens. Virtually all of the heavily fractured zone broke and fell off the mountain during the May 18 eruption.

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