How do scientists classify volcanoes and their eruptions? There is no easy answer to this question, as scientists classify volcanoes in several different ways, including size, shape, explosivity, lava type, and tectonic occurrence. Furthermore, these different classifications often correlate. A volcano that has very effusive eruptions, for example, is unlikely to form a stratovolcano.
Let’s take a look at five of the most common ways of classifying volcanoes.
Active, Dormant, or Extinct?
One of the simplest ways to classify volcanoes is by their recent eruptive history and potential for future eruptions. For this, scientists use the terms “active,” “dormant,” and “extinct.”
Each term may mean different things to different people. In general, an active volcano is one that has erupted in recorded history—remember, this differs from region to region—or is showing signs (gas emissions or unusual seismic activity) of erupting in the near future. A dormant volcano is not active but is expected to erupt again, while an extinct volcano has not erupted within the Holocene epoch (past ~11,000 years) and is not expected to do so in the future.
Determining whether a volcano is active, dormant, or extinct isn’t easy, and volcanologists don’t always get it right. It is, after all, a human way of classifying nature, which is wildly unpredictable. Fourpeaked Mountain, in Alaska, had been dormant for over 10,000 years before erupting in 2006.
Around 90 percent of volcanoes occur at convergent and divergent (but not transform) plate boundaries. At convergent boundaries, a slab of crust sinks below another in a process known as subduction. When this occurs at oceanic-continental plate boundaries, the denser oceanic plate sinks below the continental plate, bringing surface water and hydrated minerals with it. The subducted oceanic plate encounters progressively higher temperatures and pressures as it descends, and the water it carries lowers the melting temperature of the surrounding mantle. This causes the mantle to melt and form buoyant magma chambers that slowly ascend into the crust above them. At oceanic-oceanic plate boundaries, this process produces volcanic island arcs.
Divergent boundaries occur when tectonic plates pull apart from each other; when this occurs underwater, it is known as seafloor spreading. As the plates split apart and form fissures, molten material from the mantle melts and quickly rises upward to fill in the space. Upon reaching the surface, the magma cools quickly, forming new land. Thus, older rocks are found farther away, while younger rocks are located at or near the divergent plate boundary. The discovery of divergent boundaries (and dating of the surrounding rock) played a huge role in the development of the theories of continental drift and plate tectonics.
Hotspot volcanoes are a completely different beast—they often occur intraplate, rather than at plate boundaries. The mechanism by which this happens is not completely understood. The original concept, developed by renowned geologist John Tuzo Wilson in 1963, postulated that hotspots occur from plate movement over a deeper, hotter portion of Earth. It was later theorized that these hotter, sub-crust sections were mantle plumes—deep, narrow streams of molten rock that rise from the core and mantle due to convection. This theory, however, is still the source of contentious debate within the Earth science community.
Examples of each:
Students are usually taught three main types of volcanoes: cinder cones, shield volcanoes, and stratovolcanoes.
- Cinder cones are small, steep, conical piles of volcanic ash and rock that have built up around explosive volcanic vents. They often occur on the outer flanks of shield volcanoes or stratovolcanoes. The material that comprises cinder cones, usually scoria and ash, is so light and loose that it does not allow magma to build up within. Instead, lava may ooze out of the sides and bottom.
- Shield volcanoes are large, often many miles wide, and have a gentle slope. They are the result of fluid basaltic lava flows and are often associated with hotspot volcanoes.
- Stratovolcanoes, also known as composite volcanoes, are the result of many layers of lava and pyroclastics. Stratovolcano eruptions are normally more explosive than shield eruptions, and its higher viscosity lava has less time to travel before cooling, resulting in steeper slopes. Stratovolcanoes may reach upwards of 20,000 feet.
Type of Eruption
The two predominant types of volcanic eruptions, explosive and effusive, dictate what volcano types are formed. In effusive eruptions, less viscous (“runny”) magma rises to the surface and allows potentially explosive gasses to easily escape. The runny lava flows downhill easily, forming shield volcanoes. Explosive volcanoes occur when less viscous magma reaches the surface with its dissolved gasses still intact. Pressure then builds up until explosions send lava and pyroclastics into the troposphere.
Volcanic eruptions are described using the qualitative terms “Strombolian,” “Vulcanian,” “Vesuvian,” “Plinian,” and “Hawaiian,” amongst others. These terms refer to specific explosions, and the plume height, material ejected, and magnitude associated with them.
Volcanic Explosivity Index (VEI)
Developed in 1982, the Volcanic Explosivity Index is a 0 to 8 scale used to describe the size and magnitude of an eruption. In its simplest form, the VEI is based on total volume ejected, with each successive interval representing a ten-fold increase from the previous. For example, a VEI 4 volcanic eruption ejects at least .1 cubic kilometers of material, while a VEI 5 ejects a minimum of 1 cubic kilometer. The index does, however, take other factors into account, like plume height, duration, frequency, and qualitative descriptions.