What is a volcano?
One of the most difficult problems of standardization has been the varying usage of the word "volcano." The definition that we start from is that a volcano is an accumulation of explosively or effusively erupted materials originating from single or multiple vents or fissures at the surface of the Earth or other planets. Clearly, looking at the variety of options in that definition, the real world does not lend itself to simple categories. So how do we convert the complexity of geology into an understandable framework to manage in a database? Due to some of the issues expanded upon below, we don't follow strict rules for what is included as a distinct volcano with it's own Volcano Number in the database. Criteria varies somewhat depending on the amount of research available, the known eruptive history, and the practical needs of local observatories or government agencies to distinguish hazards at different locations.
Our definition includes "single or multiple vents or fissures" which relate to geographic extent. Such features range from individual vents (measured in meters) through volcanic edifices (measured in kilometers or tens of kilometers) to volcanic fields (measured in hundreds of kilometers). In a database compilation, the disadvantage of the narrowest definition is not so much the multiplicity of names introduced, but the dividing of a single volcanic plumbing system's history (both eruptive and intrusive) into apparently unrelated separate records. Locations provided for volcanic fields are often not at a specific feature, but close to the center of the known vents. Summit locations are typically given in cases with a distinct primary edifice, even if that is not the current location of eruptive activity.
The problem is particularly difficult in Iceland, where eruptions separated by many tens of kilometers along a single rift may share the same magmatic system. A volcanic field such as Mexico's Michoacán-Guanajuato field (comprising nearly 1,400 cinder cones, maars, and shield volcanoes derived from a single magmatic system, dotting a 200 x 250 km area) may be counted the same as a single volcanic edifice. The interiors of ancient volcanoes, now eroded and exposed for geologic study, show us that most subsurface magma chambers--the suppliers of lavas to overlying volcanoes--are at least several kilometers in diameter. We also know that many contemporary volcanoes grow by additions from countless flank vents as well as activity at a central crater. Consequently, we have tended to group closely-spaced volcanic features such as the historical vents of the Canary Islands by the major volcanic edifice on which they are found. Unless there are clear scientific reasons to indicate distinct systems, volcanoes listed here are rarely closer than 10 km to their nearest neighbor, and are commonly separated by at least 20 km.
Simply identifying volcanoes can be a problem. Prominent, steaming cones are easy to recognize, but water, ice, erosion, collapse processes, or dense vegetation can mask very dangerous volcanoes. For example, Lake Taupo, in the center of New Zealand's North Island, is beautifully tranquil, with no obvious features alerting non-geologists to its particularly violent history. In the Alaskan summer of 1975, two volcanologists traced an ever-thickening ash layer to a vent now covered by the Hayes Glacier, and a "new" volcano was added to the NE end of the Aleutian arc. Also in Alaska, five decades passed before the true source of a large (VEI 6) eruption was recognized: subsurface magma connections led to prominent collapse of Mount Katmai in 1912, and this was assumed to be the eruption's source until careful fieldwork showed it to be Katmai's inconspicuous neighbor, Novarupta. These examples illustrate why the contents of this database must be recognized as incomplete. Inclusion may depend on thoroughness of mapping or other research--quite variable through the world's volcanic regions--and some dangerous volcanoes not be yet recognized or included on the Holocene list.
Although it could be argued that mud volcanoes meet the definition given above, we do not include such phenomena. Volcanoes identified by GVP are exclusively driven by magmatic activity.
Global Volcanism Program, 2013. Volcanoes of the World, v. 4.7.6. Venzke, E (ed.). Smithsonian Institution. Downloaded 16 Feb 2019. https://doi.org/10.5479/si.GVP.VOTW4-2013