Estimated to be less than 3,000 years old, Arenal Volcano is a geologically young volcano. During the various cycles of its life activity has been characterized by an alternating of an explosive, effusive phase with one of weak, explosive activity. Major events in recent history begin with the 1968 killer eruption.
July 29, 1968
At 7:20 AM "Cerro" Arenal--as it was called by local residents--began explosive activity that literally blew the west side off the volcano. Two villages at the foot of the volcano-- Pueblo Nuevo and Tabacón--were completely destroyed and 78 people died. Between July 29 and 31 three new craters were formed on the western flank of the volcano and a fifteen-square kilometer area (5.8 square miles) was devastated.
During this month a period of explosive activity resulted in four strong explosions that blew large amounts of ash into the sky. The ash was carried as far away as Tilarán, 26 kilometers (16 miles) distant. There were also several burning avalanches originating in the top crater that were channeled by the valley of the Tabacón River on the northwestern flank of the volcano.
After a period of high effusive activity of intermittent lava flows, a new explosive phase began with 3 to 20 explosions per day of low to moderate magnitude. These explosions of steam, water, gases, and ash reached altitudes of up to 5 kilometers where winds again carried the material across the Arenal reservoir (Lake Arenal) and over the town of Tilarán.
This month saw expulsions of pyroclastic flows (ash and rock of no more than one centimeter that appear when the rock breaks apart). Some 500 of the larger, one-centimeter size fell near the thermal pools of Tabacón Resort.
Since this month the volcano has produced regular lava flows, accompanied by intermittent explosions of gases. Until May 5, 1998 this was the regular activity of the volcano.
May 5, 1998
(This information is based on reports in the Costa Rica daily La Nación after the Tuesday eruption) Arenal Volcano experienced a series of large eruptions on Tuesday afternoon, May 5, 1998. The first was recorded at 1:05 PM local time (US Central Standard Time). In that explosion. part of the northwest wall of the crater fell away. Large amounts of lava, rock and ash issued from the volcano in this explosion. Another eruption took place at 2:20 PM local time with material falling from the same area of the volcano.
A specialist with the Costa Rica Volcanic and Seismic Observatory explained that what occurred Tuesday is relatively frequent with this volcano. On this occasion the amount of lava within the crater was great and therefore more material left the crater. Also, this time there was a landslide (avalanche) as part of the crater wall fell away on the northwest side. This volcano produces similar phenomena periodically, although this time it was much larger than usual.
As a normal precaution, authorities declared a red alert, closed the road between La Fortuna and Tilarán which runs around the north side of the volcano, and evacuated approximately 450 people (mostly tourists) from the immediate area which includes several hotels and tourism oriented businesses. There were no reports of injuries from the volcanic activity.
At 5:20 PM on Tuesday the volcano was still discharging material, but activity had decreased significantly.
May 7 update
Tuesday's eruptions damaged two square kilometers and burned a 400 by 100- meter area of green forest on the side of Arenal. A fissure in the wall of the crater 500 meters long by 10 meters deep was also opened up. All this material slid down the side of the volcano. Experts say there is currently a risk of avalanches of this loose, accumulated material, especially when the heavy rains begin.
During Tuesday's episode there were a total of 23 eruptions between 1:05 PM and 7 PM local time, after which the volcano returned to its normal state. During this time there were only two major eruptions of gases.
The area around Arenal Volcano is a high-risk zone--this volcano has been active for the past 30 years with regular eruptions providing tourists excellent displays of nature's awesome power. The volcano has become an important tourist attraction for Costa Rica. Several hotels, restaurants and other tourist-oriented ventures have sprung up very close to Arenal in recent years.
Referring to this explosion of tourism infrastructure, volcanologist Jorge Barquero of the Costa Rica Volcanic and Seismic Observatory stated that Arenal's activity on Tuesday is normal in an active volcano like this one, but what is not normal is that man has developed construction in such a high-risk zone. Barquero also commented that in this incident the volcano was telling us that just like this eruption, there could be others of greater magnitude at any moment, possibly with regrettable consequences.
May 31 addendum
After the eruption of May 5, Arenal returned to its typical occasional rumblings. Authorities reported no unusual behavior and the national park was reopened the same week. Local volcanologists continue to keep a close watch on the volcano and park rangers continue to vigorously enforce the safety perimeter.
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