How many volcanic eruptions occur every year?On average, there are about 50-70 volcanoes that erupt every year. Some of them erupt multiple times, while others only have one eruption.
The typical number of individual eruptions per year is more in the range of about 60-80.
Eruptive phases vs eruptions
In many cases, it is not easy to define the start and end of an eruption and whether long-lasting eruptions should be considered in such statistics or not. Generally, volcanologist prefer to talk about eruptive phases of a volcano that belong to one single eruption: two or several eruptive events separated by a pause in activity, which can span hours to even weeks.
However, it is often impossible to give a final answer to whether two eruptive events at the same volcano should be considered two eruptions or two phases of the same eruption.
Eruptions in 2015 exampleThe most widely accepted scientific database for volcanic eruption data is Simithsonian's Global Volcanism Program (GVP). For example, 43 confirmed and one uncertain eruptions are listed here for 2015, a rather typical (if not below average) year of volcanic activity:
Masaya, Nicaragua (2015 Dec 11)
Etna, Italy (2015 Dec 2)
Momotombo, Nicaragua (2015 Dec 1)
Kanlaon, Central Philippines (2015 Nov 23)
Marapi, Sumatra (2015 Nov 14)
Tengger Caldera, Java (2015 Nov 12)
Lascar, Northern Chile, Bolivia and Argentina (2015 Oct 30)
Rinjani, Lesser Sunda Islands (2015 Oct 25)
Turrialba, Costa Rica (2015 Oct 23)
Copahue, Central Chile and Argentina (2015 Oct 6) - VEI 2
Alaid, Kuril Islands (2015 Oct 2)
Michael, Antarctica and South Sandwich Islands (2015 Sep 30)
Telica, Nicaragua (2015 Sep 23)
Asosan, Ryukyu Islands and Kyushu (2015 Sep 3)
Lokon-Empung, Sulawesi (2015 Aug 30)
Klyuchevskoy, Kamchatka Peninsula (2015 Aug 28)
Cotopaxi, Ecuador (2015 Aug 15)
Kick 'em Jenny, West Indies (2015 Jul 24)
Gamalama, Halmahera (2015 Jul 16)
Sabancaya, Peru (2015 Jul 9)
Sirung, Lesser Sunda Islands (2015 Jul 7)
Hakoneyama, Honshu (2015 Jun 29)
Cereme, Java - Uncertain Eruption - (2015 Jun 24)
Asamayama, Honshu (2015 Jun 16)
Cleveland, Aleutian Islands (2015 Jun 14)
San Cristobal, Nicaragua (2015 Jun 6)
Kuchinoerabujima, Ryukyu Islands and Kyushu (2015 May 29)
Wolf, Galapagos Islands (2015 May 25)
Lokon-Empung, Sulawesi (2015 May 20)
Telica, Nicaragua (2015 May 11)
Bulusan, Luzon (2015 May 1)
Axial Seamount, Pacific Ocean (northern) (2015 Apr 23)
Calbuco, Southern Chile and Argentina (2015 Apr 22)
Ruiz, Nevado del, Colombia (2015 Apr 22)
Tungurahua, Ecuador (2015 Apr 6)
Ubinas, Peru (2015 Mar 17)
Turrialba, Costa Rica (2015 Mar 8)
Chikurachki, Kuril Islands (2015 Feb 16)
Fournaise, Piton de la, Indian Ocean (western) (2015 Feb 4)
Raung, Java (2015 Feb 1)
San Miguel, El Salvador and Honduras (2015 Jan 26)
Karymsky, Kamchatka Peninsula (2015 Jan 19)
Soputan, Sulawesi (2015 Jan 6)
Klyuchevskoy, Kamchatka Peninsula (2015 Jan 1)
For example, Klyuchevskoy and Lokon are listed with two eruptions each, so the number of volcanoes with confirmed eruptions would only be 42 according to the above list.
However, we quickly realize that some of the most known active volcanoes are missing: Kilauea, Stromboli, Yasur and many others. The reason is that in addition to the above list there are about 20 volcanoes world-wide in near-constant (persistent) activity, which are not listed because the start of their eruptions is back in the past and not considered a new eruption in this list.
Kilauea has been in ongoing eruption that started in January 1983. Stromboli has been in eruption since at least 2000 years (when first reports are available), Yasur on Vanuatu has been erupting ever since its first sighting by a European (J. Cook) in 1774 and so on.