Does the size of the volcano determine the height of the eruption?

J.D. Hauge

Dear J.D.,

Good question. First lets do the short and tall. The biggest volcanoes are shields like Hawaii. Although they are giants (six miles tall), their fire fountains shot tephra only a couple thousand feet into the air (except during rare explosive eruptions). At the other extreme are big caldera systems like Yellowstone (see the bottom of the page that describes Hot Spots). Their ash deposits can extend a thousand miles from the volcano. The eruption cloud must have been many tens of thousands of feet tall. But big calderas tend to be relatively flat or low-relief features. Stratovolcanoes are small compared to shield volcanoes and calderas. Yet their eruption columns can reach 30-40 km above the volcano. The key factor is the velocity of the tephra as it leaves the volcano. The velocity is influenced by the amount of gas dissolved in the magma and the radius of the vent. Gas escapes from basaltic magmas before it reaches the surface so they cannot produce big eruption columns. Silica-rich magmas (andesite and rhyolite) have more dissolved gas and therefore produce taller columns.

Steve Mattox, University of North Dakota

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