How long does an eruption usually last?
John Lim
Hi John,
There really isn't a typical eruption duration. You can find examples of
eruptions that lasted only a few hours to eruptions that have lasted
decades to even centuries. This is using the broadest definition of the
term "eruption", meaning any time that lava is being erupted onto the
surface. Some of the long-duration eruptions are quite un-spectacular
most of the time, and often the distinction between sudden increases in
activity and reports of new "eruptions" is unclear.
Some examples: When westerners first observed Kilauea in Hawaii, there
was an active lava lake in Halema'uma'u crater within the summit caldera.
This counts as an eruption since active lava was at the surface.
Furthermore, this lava lake lasted almost uninterupted until the early
1920's. You would say, therefore, that this was an eruption that lasted
almost 100 years. But what about the time before westerners arrived? The
Hawaiians kept an oral history and various interpretations of it indicate
that the lava lake existed for many years prior to 1823. To complicate
things further, during this time there were a few eruptions out on the
flank of Kilauea. These events were more typical as "eruptions".
Lava lakes occur at a few volcanoes around the world, including Kilauea
(although not at present), Nyaragongo in Zaire (also not at present),
Masaya in Nicaragua, and Erta Ale in Ethiopia. All of these are basaltic
shield volcanoes and it probably is a requirement that the lava be fluid
(like basalt) for a lava lake to develop.
Another type of long-duration eruption is lava dome growth. At a number
of strato volcanoes lava domes will grow for decades. This is presently
occurring at Unzen (Japan) and Merapi (Indonesia) to name a couple of
examples. These domes may grow so slowly that delicate instruments are
required to see the motion. Occasionally the dome may grow to the point
that its edges are oversteepened and they collapse, generating what are
called block-and-ash flows, a type of pyroclastic flow. These events are
often considered "eruptions", particularly by the press, but actually they
are only events during a longer-term eruption.
Finally, there are volcanoes such as Stromboli in Italy. It has been the
"Lighthouse of the Mediterranean" for at least 2000 years. It is the
original example of "strombolian" activity (makes sense), which consists
of numerous separate small explosions of incandescent usually-basaltic
lava. Each explosion generates very little material onto the flank of the
volcano but the eruptions go on for very long periods of time.
Occasionally a few of the explosions will be bigger than the previous ones
and lots of people (usually the press again) consider these to be "new
eruptions!!" when again, they are only events during a longer term eruption.
By now you probably consider this answer to be a long-term eruption so
I'll stop now. You asked a good question.
Sincerely,
Scott Rowland, University of Hawaii
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