In general, the intensity and appearance of volcanic eruptions depend upon how fluid or gaseous the lava is. Shield volcanoes like Mauna Loa; produce fluid lava that come up slowly in the volcano, giving time for gas to escape. As a result, these volcanic eruptions are peaceful, but the lava has the ability to flow long distances.
Composite Volcanoes, like Mount Etna, have explosive eruptions because they tend to have thicker lava that trap gas bubbles at the surface and send up explosions of lava and ash.
Dome volcanoes, like Mount St. Helens, have the thickest lava, which looks like it has been squeezed out of the volcano. Sometimes, this process is so slow that the lava forms a volcanic plug. In this situation, the whole volcano might explode due to a build up of gases inside the volcano.
Cindercone - Galapagos Islands -
Mike Westerfield
Scientists use the Volcanic Explosive Index (VEI) which is based on a number of observable features, to help them assess the size and possible impact of a volcanic eruption. Although every year over 50 volcanoes erupt, you can see from the VEI that really violent eruptions of giant proportions don't happen that often - Phew!
| VEI | Description | Example | How
often |
| O | non-explosive Kilauea | daily |
| 1 | gentle | Stromboli | daily |
| 2 | explosive | Galeras, 1992 | weekly |
| 3 | severe | Ruiz, 1985 | yearly |
| 4 | cataclysmic | Galunggung, 1982 | 10's of years |
| 5 | paroxysmal | St Helens, 1981 | 100's of years |
| 6 | colossal | Krakatau, 1983 | 100's of years |
| 7 | super-colossal | Tambora, 1815 | 1000's of years |
| 8 | mega-colossal | Yellowstone, 2 Ma | 10,000's of years |
|
Volcanoes