Volcanoes are referred to as 'active', 'dormant', or 'extinct'. In general, an active volcano is one that has erupted within historic time. In regions where the historic record is very short (such as Canada), an active volcano is defined as one that has erupted within the last 200 years. In most cases, these volcanoes are expected to erupt again. A volcano that is actually erupting is said to be 'active' or 'in eruption'. A dormant volcano is one that has not erupted within historic time, but is expected to erupt again. An extinct volcano is not considered likely to erupt again.

Figure 28. Mount St. Helens Photograph of a small explosive eruption of Mount St. Helens that occurred in October 1980. (Photograph by P. Hickson) larger image [JPEG, 21.0 kb, 255 X 390, notice]

When magma comes out of a volcano, either as lava flows or pyroclastic debris, the volcano is said to be 'erupting'. Volcanic eruptions can last from minutes to days. Commonly, a volcano will undergo several eruptions closely spaced in time. These closely spaced eruptions are part of the same eruptive episode or cycle. Such cycles can be passive (dome building or very small and local ejections of volcanic bombs referred to as 'fire fountaining') or explosive. For example, the most recent eruptive episode at Mount St. Helens (in southwestern Washington State; (Figure 28) included numerous minor explosions, six major explosive eruptions, and passive extrusion of lava to form a dome above the volcano's vent. All this occurred over six years.

Mount St. Helens provides a good example of how a volcanic eruption can start and how the style of eruption can vary over time - even though the magma remains chemically similar. After over 100 years of dormancy, hundreds of small earthquakes occurred very closely spaced in time (an 'earthquake swarm'), starting 20 March 1980, heralding renewed activity at Mount St. Helens. By 27 March, small steam explosions had opened two small craters on top of the volcano. Steam explosions, some with small amounts of ash, continued intermittently until the catastrophic eruption began at 08:32 h on 18 May 1980. The gas-rich, dacitic magma exploded in an eruption that sent a column of tephra over 24 km into the atmosphere. Five more large explosive eruptions occurred through mid-October 1980. After that, as the volatile content of the dacitic magma decreased, activity became limited to the extrusion of lava and the formation of a lava dome over the vent.

Figure 29. Growing lava dome The growing lava dome in the crater of Mt. St. Helens, shortly after the end of explosive activity in 1980. The dome is about 50 m across and is so viscous that it cannot flow far from the vent. As a result it piles above the vent into a steep, dome-shaped form. The steam rising from the crater floor is emanating from a series of radial fractures. (Photo by D. Swanson (U.S. Geological Survey)) larger image [JPEG, 21.6 kb, 400 X 257, notice]

Figure 30. Lava spine A lava spine growing from the Mt. St. Helens lava dome in 1981. At times extruding lava is so viscous that it cannot flow at all, and so is pushed almost vertically upward as a rod of lava. The spine shown here is about 15 m tall. (Photo by D. Swanson (U.S. Geological Survey)) larger image [JPEG, 26.2 kb, 400 X 254, notice]

Volcanic eruptions are commonly described by comparing the styles of eruption to the styles at well known volcanoes. Thus, Hawaiian-type eruptions are characterized by fire fountaining and the effusion of fluid lava flows. They are commonly not very hazardous. At the other extreme, Plinian-type eruptions, named for Pliny the Younger who first described them while watching the eruption of Mount Vesuvius in AD 79, are characterized by very large, violent, and very dangerous explosions. The 18 May 1980, eruption of Mount St. Helens was a Plinian eruption. In between these two extremes of danger and explosiveness are a variety of other types of eruptions with names such as 'surtseyan', 'vulcanian', 'strombolian', and 'pelean'. However, it is important to remember that an erupting volcano may exhibit more than one type of behaviour, and it is not uncommon for both passive and explosive phases to occur in a single eruptive cycle.