The 1,670 m conical Volcan Arenal (Sand Volcano) is the youngest stratovolcano in Costa Rica.  It is one of the most active volcanoes in Costa Rica, erupting mostly andesites, though there are basaltic andesite, basalts and some dacite.  The earliest known eruptions from it took place 7.0 ka.  It was concurrently active with neighboring Volcan Chato until 3.5 ka, when activity at Chato stopped.  There are periodic major explosive eruptions at several hundred-year intervals and periods of lava flows.  The most recent of these was 1968 – 2010.  Being young, Arenal has not yet suffered a massive flank collapse, though part of the cone is slumping, which may be a precursor. 

The area around Arenal is sparsely populated with over 96,000 living within 30 km.  However, over 3 million live within 100 km, most of them in San Jose, 76 km SE.  Arenal sits at the E end of Lake Arenal and is surrounded by Parque Nacional Volcan Arenal (121 km2).  This is quite a large area, connected to the Children’s Eternal Rainforest (230 km2) private land trust and preserve, and the Monteverde Cloud Forest Reserve (105 km2) biological reserve to the S. 

The three parks and reserves are popular with tourists.  Neighboring La Fortuna is one of the most popular jumping off places.  Tourists are treated to incredible biodiversity of plants, wildlife and birds.  Pictures     que Arenal looms above both the forests and neighboring Lake Arenal.  Hiking, canopy tours, white water rafting, kayaking on Lake Arenal, fishing, mountain biking, waterfalls and wind surfing are all popular pursuits.  There are hot springs and resorts associated with those hot springs in the region. 

Volcano tours and climbing Arenal are also very popular.  Tours used to take tourists to the crater rim.  Tourists were frequently injured by falling ash and fumes.  The government created a 4 km exclusion zone in 1998 that restricted access to park officials and scientists.  Visitors and tour guides are ever creative, finding alternate routes to access the top of Arenal much to the dismay of park and government officials.  These tours have grown in popularity since Arenal stopped erupting in 2010.   

The closest main town to Arenal is La Fortuna (formerly El Borio), 6 km E.  It has a population of just over 15,000 and is known as the gateway to the parks surrounding Arenal.  There is a local rumor that the town was renamed to La Fortuna because it escaped most of the eruptive effects of Arenal in 1968.  The name changed was already in process before the eruptions began.  Makes a nice story, though.  La Fortuna (The Fortune) was renamed due to its fertile lands and tourist attractions. 

Climate here is considered tropical.  While there are formally four seasons, in reality there are only two – wet and dry.  Nearly 500 cm of rain falls on parts of Arenal yearly.  Jun – Oct are wet parts of the year.  Average temperatures range 20° – 26°C. 

Volcano and seismic monitoring and reporting in Costa Rica is done by the Volcanological and Seismological Observatory of Costa Rica.  The site is entirely in Spanish.  It has real time seismograph output for the entire country (OV-VACR and OV-VAVL are the active seismograph for Arenal) and 6 active webcams on 3 active volcanoes (Turrialba, Poas, Rincon de la Vieja).  There are multiple webcams trained on Arenal.  The Costa Rica Volcano Cam in La Fortuna is probably as good a place to start as any.   

Region

Arenal is one of seven historically active volcanoes in Costa Rica.  Others include Poas, Irazu, Miravalles, Orosi, Rincon de la Veja and Turrialba.  We published multiple posts on many of these systems, Poas in 2015 and 2017, Rincon de la Veja in 2018, and Turrialba in 2016.  Our 2023 Platanar post also touches on neighboring volcanic systems along the Costa Rican volcanic front.  Any of these would be good places to review for additional information about volcanoes in Costa Rica.  Every time we cover a volcano, we tend to look at it a little differently, looking at the same thing with different eyes (experience or education) picking up on things either missed or ignored based on what we learned over time. 

Orosi

The 1,659 m Orosi is one of a cluster of four eroded, vegetated cones at the NW corner of Costa Rica, 101 km NW from Arenal.  The complex includes Orosi, Orosilito, Volcan Pedregal and Cacao.  Cacao has a pair of flank collapse amphitheaters breached to the SW and E.  While there were a pair of reported eruptions from Orosi in 1844 and 1849, the volcano is completely overgrown.  The reports may have been from neighboring Rincon de la Vieja.  The most recent documented activity produced mudflows around 3.5 ka.  The mudflows are visible and dated while the volcanic activity (if any) that triggered them is unknown. 

RIncon de la Vieja

We covered Rincon de la Vieja in a 2018 post.  It is located 79 km NW from Arenal.

Cerro las Mercedes

Cerro las Mercedes (Cono Mercedes) is a 190 m cinder cone on the Caribbean coastal plain near the border with Nicaragua.  Basaltic lapilli and bombs cover around 1 km2.  It is located 68 km NE from Arenal.  There are no records of eruptions here for the last 11 ka.

Miravalles

Miravalles is a 2,028 m andesitic stratovolcano located some 58 km NW from Arenal.  It is one of five post-caldera cones along a NE-trending line in the 15 x 20km Guayabo (Miravalles) caldera.  The caldera was formed during at least two major eruptions.  The most recent of these was a 580 ka VEI 7.7 that ejected a surprisingly small 5.5 km3 of material.  The oldest of these was a 1.46 Ma VEI 6.0 that ejected 30 km3. 

Post-caldera activity covered most of the E and S rims.  These stratovolcanoes also collapsed, producing major debris avalanche deposits to the SE.  Youthful lava flows cover the W and SW flanks of the Miravalles complex.  Most recent activity was a small steam explosion in 1946 on the SW flank.  The most recent earthquake swarm was Oct 1997.  High heat flow supports a large developed geothermal field.  This field has been seismically monitored since 1977. 

Tenorio

Tenorio is a cluster of densely forested volcanic cones oriented generally NNW – SSE, 41 NW from Arenal.  Overlapping lava flows from the 1,916 m andesitic / basaltic andesitic / basaltic peak blanket the NW – SW flanks, descending down the NE flank.  The central cone has three craters, is sparsely vegetated, and appears to be the most recently active.  Volcan Montezuma to the N has two craters.  The N of these was the source of a lava flow to the NE.  There are pyroclastic cones on the NE and SW of the central complex.  Bijagua lava domes were constructed on the N flank.  A major debris avalanche covered around 100 km2 below the S flank.  There is a legend of an 1816 eruption, but the volcano was heavily forested at the time of an 1864 visit.  No documented eruptions are known.  There are active fumaroles on the NE flank. 

Cerro Choppo

Cerro Choppo is a 402 m pyroclastic cone located some 40 km W of Arenal.  The basalt cone stands 250 m above the basement of local ignimbrites and andesite lava flows.  The crater is elongated to the NW and was the source of andesitic lava flows.  The cone is estimated at 40 – 10 ka.  It is located next to a well-traveled road.  The interior has been exposed by quarrying for road construction materials.  There is another small, isolated cinder cone of similar age along the Rio Corobici to the W.

Cerro Tilaran

Cerro Tilaran (Cerro Tovar) is a small andesitic shield volcano located some 30 km W of Arenal.  It tops out at 634 m.  While the shield looks young, it has been dated anywhere from 1.0 Ma to as young as 12 ka, with the younger estimate being more likely.  It is overlain by tephras from Arenal. 

Platanar

We covered Platanar in 2023.  It is located 41 km SE from Arenal. 

Cerro San Miguel

Cerro San Miguel is an isolated lava dome 49 km S from Arenal, on the SW flank of the Tilaran Range near the Gulf of Nicoya.  It is a dacite dome built nearly 50 km behind the volcanic front that dates no older than 700 ka.  The 9 km2 dome is fractured.

Poas

We covered Poas in a pair of posts in 2015 and 2017.  It is located 60 km SE from Arenal. 

Barva

Barva (Barba, Las Tres Marias), located 76 km SE from Arenal is a complex stratovolcano with multiple summit and flank vents just outside the capital city San Jose.  It erupted the 25 km3 Tiribi Tuff some 322 ka from what became a 10 km diameter caldera at the summit.  This is the most widespread and exposed ignimbrite in central Costa Rica.  Post caldera activity built at least four pyroclastic cones in the now 2 x 3 km summit caldera.  One of these cones has a crater lake.  There was a large Plinian eruption in the early Holocene.  Recent eruptions were reported around 1760 and 1867 along with a mudflow.  The volcano represents a clear and present danger to the 2.4 million living at its foot and flanks.

Volcan Arenal

Arenal is a small (1,100 m above the surrounding plains, 1,657 m above sea level) basaltic andesitic stratovolcano.  Its total volume is 15 km3.  The most notable thing about Arenal is its near continuous activity for a 42-year period 1968 – 2010.  It is also quite young, with early activity no older than 7.5 ka.  The striking conical shape was modified over the last half century with creation of three new craters, addition of a significant amount of new material mostly on its W flank, and a slumping of that newly expanded W flank.  The magma chamber is shallow less than 4 km below the volcano. 

Arenal is located about halfway between neighboring active volcanic groups along the Costa Rican volcanic front.  Activity at Arenal and neighboring Chato has migrated W and NW over time.  Prior to the 1968 eruption, it was not listed among active volcanoes in Costa Rica.  For the period of its continuous eruption, it was considered as one of the most active volcanoes in the world. 

There are several eruptive vents in the vicinity of Arenal.  These include neighboring Chato, Los Perdidos and Laguna Poco Sol.  All of these are associated with a lineament stretching SSE from Arenal.  While none of these are formally described as part of a larger Arenal volcanic complex, their close proximity and recent activity would argue otherwise, especially for Chato and its domes.  There are multiple hot springs in the region.

Volcan Chato is the closest of satellite volcanic systems associated with Arenal.  It is a 1,140 m stratovolcano 3 km SE.  Chato has two domes, Chatito and Espina on the SSE flank.  Chato, Los Perdidos and Laguna Poco Sol are roughly aligned along a NNW – SSE lineament extending S of Arenal.  It was initially active 38 ka, with its last eruption 3.5 ka.  Its activity overlapped with activity at Arenal for 4 ka.  The cone has a well-defined, forested 500 m crater that slopes steeply into an oval crater lake 340 m long.  Seismicity before the 1968 reawakening of Arenal had epicenters under Chato indicating a close relationship between the two plumbing systems. 

Los Perdidos an inactive system (caldera?) 7 km SSE from Arenal that also lies along the lineament between Laguna Poco Sol and Arenal.  Volcanic activity has migrated NW along this lineament toward the historically active Arenal volcanic complex.  There are several andesitic lava domes on the rim of the caldera that date 99 – 83 ka.  It is heavily forested and eroded, making it difficult to pick out from satellite photos. 

Laguna Poco Sol is a 200 m wide, lake filled depression (maar?) 13 km SSE from Arenal.  It is near the southern end of the volcanic lineament that extends NW to Arenal volcano, and is considered by some to be part of the greater Arenal volcanic complex.  The crater is either a phreatic explosion crater (maar) or a depression caused by a landslide.  There are lava flows, lahar deposits, tuffs and volcanic breccias.  The area has an active hydrothermal system with mud pots, hot springs, and sulfur deposits. 

Flank Collapse?

Visually, Arenal presents a striking profile, an isolated, tall, steep volcanic cone.  As it is both a stratovolcano and young, it has not yet suffered a flank collapse.  A 2010 analysis of 2005 – 2009 InSAR data by Ebmeier, et al discovered that the W flank of the cone is slumping, and doing so quite fast, perhaps 7 cm/year.  The plane of movement appears to be located between new material deposited 1968 – 2010 and older lavas and paleosols. 

Arenal shows several characteristics associated with shallow failure on other volcanoes.  These include a layered structure and steep conical shape.  The cone is constructed by interlayered tephras and lavas.  Lavas erupted since 1968 significantly altered the shape of the cone, extending and increasing the load on the W slopes.  The location of the cone migrated W during the 1968 – 2010 activity.  42 years of activity increased the volume of the cone by 4%, unusual among stratovolcanoes of similar size and composition. 

While the observed movement can best be explained by shallow movement along a slip plane, deformation of the base under the foot of the volcano is not impossible.  The presence of the strike – slip Danta fault below Arenal may also contribute to overall instability, though its rate of motion is not known.  The base of the 1968 lava lies on a layer of tephra from the 1968 lateral blast which is in turn on top of a loose paleosol surface, giving a likely sliding plane for a slow slope failure.  Total volume of new material above this plane is around 0.55 km3. 

The dangers of an unstable stratovolcano to the local area present a problem given local towns and the Lake Arenal reservoir 4 km W of the base of the volcano.  The reservoir is the major hydroelectric producer for Costa Rica, with the dam 6 km W of the summit, and the lake 4 km W.  A major flank collapse into the lake will move a lot of water, creating a seiche / tsunami. 

At least 20 major flank collapse events have taken place worldwide over the last 500 years.  A magmatic intrusion similar to the one thought to trigger the 1968 lateral blast or a large earthquake could trigger failure at Arenal.  While the magma chamber is thought to be in the middle to lower crust, it is possible that a flank collapse could depressurize a shallow conduit similar to what we saw on Mount St Helens.  The relatively constant sliding rate indicates the W flank is in steady state.  Any change should be taken as a precursor to collapse.  Future monitoring of movement of Arenal’s slopes is important for hazard abatement. 

Eruptions

Arenal is a very young system, no older than perhaps 7.5 ka.  The Smithsonian GVP lists at least 15 eruptions 7.1 – 2.2 ka.  At least 12 of them VEI 4.  That string continues after 0 AD, with another 8 VEI 4 eruptions 400 – 1400 AD.  The three eruptions 1440 (VEI 2), 1750 (VEI 0), 1915 (unknown intensity) and 1922 (VEI 2) were smaller.  Prevailing winds from the Caribbean tend to push most tephras and other ash fall deposits W or SW. 

Explosive eruptions began perhaps 7 ka, with tephras overlying regional tuffs older than 20 ka, older volcanic and sedimentary rocks.  There are four major known Plinian eruptions separated by 750 – 1080 years.  There are 8 known sub-Plinian eruptions and 7 violent Strombolian eruptions.  One of the most violent of these deposited 0.9 km3 of tephra.  10 other minor Vulcanian explosive eruptions are recognized.  Total tephras deposited are around 4.5 km3.  Effusive eruptions generally follow the Plinian eruptions. 

1968 – 2010 Eruption

The 42-year long near continuous eruption sequence 1968 – 2010 is collectively listed as VEI 3.  That sequence triggered at least 157 Bulletin Reports Jul 1968 – Nov 2012, by far the most I have come across researching these posts.  Activity during this sequence was a little bit of everything including explosions, significant ashfall, lava flows, night glow, crater lake ejected from the crater, formation of a new crater, blocky lava flows, block and ash flows, vigorous gas emissions, incandescent tephras, dome building, Strombolian activity, fountaining, avalanches from lava flow fronts, pyroclastic flows, lava bombs, seismicity, tremor, inflation, deflation, pyroclastic cone construction and acid rain.  Color photos of the action showed up Mar 1999.  Before that, most of the information was in verbiage, graphs and the occasional black and white drawing. 

We will take a closer look at some notable events during this sequence.

The most recent sequence started 29 Jul 1968, with an unexpected violent eruption after nearly 200 years of quiet.  These continued for several days, burying 15 km2 of neighboring countryside under rocks, lava and ash.  By the end of this sequence, it has killed 87, buried three villages, and dusted over 232 km2 of land, spoiling crops and killing large numbers of livestock.  At the height of the activity, it ejected large blocks weighing tons over a kilometer from the crater.  The explosions created three new active craters.  The three destroyed villages were on the W side of the volcano, where the directed blast, pyroclastic and block and ash flows were aimed.  The plume during this was at least 10 km.    

The 1968 eruption began with 10 hours of intense seismic activity.  The first of a series of large explosions took place 0730 29 Jul.  Most fatalities took place during this.  The last fatalities took place two days later when the last major explosion took place.  At least three new explosion craters formed during the initial 3 days of eruption, generally aligned E-W.  All three were opened by a lateral blast during the first major explosion.  Multi-ton blocks ejected by this eruption ended up as far as 5 km from the newly formed craters. 

The eruption was caused by shallow intrusion of a mass of andesitic magma, described by Alvarado, et al, 2006 as a possible cryptodome.  A N-S thrust fault was involved, as was an E-W dike in the conduit.  The new craters were formed along a radial fracture on the W flank.  After the initial vent-clearing explosion, the system depressurized creating a series of directed blasts on 29 Jul and block and ash flows the next two days. 

The explosive phase took place 29 – 31 Jul.  It was followed by relative quiet 31 Jul –3 Aug.  Considerable ash and vapor emissions 3 – 10 Aug.  Activity quieted to fumaroles 10 Aug –14 Sept.  Explosions resumed 14 – 19 Sept.  Lava flows were extruded Sept 19 Sept 1968 – 1973 mainly to the SW.  As the flow fronts of these lava flows collapsed, they created pyroclastic flows down the slopes ahead of them.

There was a 7-month break in lava effusion Aug 1973 – Mar 1974.  Fumarole activity continued from newly created Crater C on the flank. 

Activity shifted from Crater A (the lowest of the three new craters) to Crater C (highest), with almost continuous emission of blocky lava flows.  An explosion created a pyroclastic flow Jun 1975.  Other explosion phases took place Aug 1980 and May – Jun 1981. 

A new explosive phase began mid-1984.  Discharge rates for lava flows increased a bit.  There was continuous effusion of lava flows into 1987.  By 1987, lava accumulation on the lower flank crater was capped by a thick, stable crust, feeding longer down the flank.  The longest of these was 3.2 km by 1992. 

Crater wall (1993, 1998, 2000, 2001) and plume collapse (1987, 1998, 1989) created major pyroclastic flows.  There was also intense explosive activity, Strombolian activity, and low lava fountains.  Lava flow flanks were constrained by levees or debris.  The toes were blocks and lava protruding from the debris.  Smaller pyroclastic flows were created by collapses in the flow front of lava flows, particularly in 2001. 

The 28 Aug 1993 pyroclastic flow from the summit crater was due to collapse of the crater wall and drainage (discharge) of a lava pool.  The 3 km long pyroclastic flow was confined to narrow valleys on the slopes.  Thickness ranged 1 – 10 m.  Its deposits were similar to the 1975 flows. 

The next large eruption took place on 5 May 1998, collapsing the NW wall of the crater.  Large amounts of lava, rocks and ash were ejected during this explosion.  The amount of lava within the crater was significantly larger than normal, pouring down the flank.  The avalanche associated with the collapse of the crater wall was also large.  This sequence was several explosions over the course of a few hours, mostly tailing off by late afternoon.  Authorities closed a local road and evacuated 450 during this eruption.

Eruptions continued on 7 May 1998, damaging 2 km2 of forest near the volcano.  It also opened a fissure in the wall of the crater, putting debris down the flank.  23 explosions were reported over a 6-hour period, with the volcano calming down at the end. 

After 2000, lava discharge rate dropped with a decrease in seismicity.  The low-level emission of lava continued until 2010.  While there were occasional explosions during this 10-year span, including three in Feb 2005, the last explosion was 7 Oct 2010.  Lava effusion stopped by the end of Oct. 

Simplified tectonic framework of Costa Rica and location of Arenal volcano.  GVR = Guanacaste volcanic range.  TVR = Tilaran volcanic range.  CVR = Central volcanic range.  CCRDF = Central Costa Rica deformed belt.  PFZ = Panama fracture zone.  Np = Nazca Plate.  Black continuous and dotted lines are faults.  Triangles are reverse faulting pointing towards the overriding plate.  Image courtesy Mora, et al, Jun 2022

Tectonics

Given its proximity, we are reproducing a shortened version of the tectonic description of this part of the Costa Rica volcanic front published in our 2023 Platanar post. 

Costa Rica is located on the western end of the Caribbean Plate which is moving generally eastward.  It is being overtaken by the Cocos Plate to the west, with subduction of the Cocos beneath the Caribbean taking place at the Middle American Trench.  It is this subduction that provides melt powering its volcanoes.  The subduction also produces no small number of large megathrust earthquakes.

Subduction in Costa Rica is not simple, with the arrival of the buoyant Cocos Ridge and its subduction under the southern end of Costa Rica.  The Nazca Plate lies to the south of the Cocos Ridge and the spreading center between the Cocos and Nazca Plates.  This spreading center is generally abeam Panama, to the S of Costa Rica. 

Arenal topographic surface and main features.  Craters are labeled A, B, D, D, with D being the oldcrater.  A – C were created during initial blast of 29 – 31 jul 1968 eruption.  Main labeled faults are:  ARF = Aguacaliente River fault; DF = Danta fault.  Dotted lines are fault traces covered by lava fields.  Image courtesy Mora, et al, Jun 2022

The volcanic centers in central Costa Rica (Platanar, Poas, Barva, Irazu and Turrialba) form the Cordillera Central, a large structure of overlapping lava flows and pyroclastic rocks.  It is the SE-most segment of the Central American volcanic front associated with subduction of the Cocos Plate. 

The volcanic front in Costa Rica is complex and has not yet been comprehensively studied.  The best studied of these volcanoes is Arenal starting in the late 1980s.  It is small at 10 km3 and very young at less than 5,000 years.  The volcanic centers of Cordillera Central have volumes 250 – 750 km3 and ages more than 0.5 Ma.  So far, we only know an incomplete outline of the most recent history of the Cordillera Central and its volcanoes.

Conclusions

Volcan Arenal is the newest stratovolcano in Costa Rica.  It is located on the volcanic front with notable volcanic structures nearby.  It was nearly continuously active for 42 years, finally quieting down in 2010.  The region has multiple hot springs and other hydrothermal features indicating an ample magma supply.  Its newly expanding W flank is slumping, which may or may not be a precursor to a future flank collapse.  Finally, the longer the break in eruptions, the larger the next one is expected to be.  This is a dangerous system that should be treated with a great deal of respect. 

Additional information

The volatile content of magmas from Arenal volcano, Costa Rica, Wade, et al, Mar 2006

Analysis of seismic and acoustic observations at Arenal volcano, Costa Rica, 1995 – 1997, Hagerty, et al. Nov 1999

Eruptive history of Arenal volcano, Costa Rica, 7 ka to present, Soto & Alvarado, Mar 2006

Volume, energy and cyclicity of eruptions of Arenal volcano, Costa Rica, Melson & Saenz, 1973

Geochemical stratigraphy and magmatic evolution at Arenal volcano, Costa Rica, Mar 2006

Pyroclastic flow generated by crater-wall collapse and outpouring of the lava pool of Arenal volcano, Costa Rica, Alvarado & Soto, Sept 2001

New constraints on source processes of volcanic tremor at Arenal volcano, Costa Rica, using broadband seismic data, Benoit & McNutt, Feb 1997

A model of degassing and seismicity at Arenal volcano, Costa Rica, Williams-Jones, et al, 2001

Explosion mechanisms at Arenal volcano, Costa Rica:  an interpretation from integration of seismic and doppler radar data, Valade, et al, Jan 2012

Explosive activity and generation mechanisms of pyroclastic flows at Arenal volcano, Costa Rica between 1987 and 2001, Cole, et al, Jan 2005 

The 1968 andesitic lateral blast eruption at Arenal volcano, Costa Rica, Alvarado, et al, Sept 2006

Complex behavior and source model of the tremor at Arenal volcano, Costa Rica, Lesage, et al, Sept 2006

Closed to open system differentiation at Arenal volcano (1968 – 2003), Ryder, et al, Sept 2006

Shallow velocity structure and seismic site effects at Arenal volcano, Costa Rica, Mora, et al, Apr 2006

Evolution and dynamics of the open vent eruption at Arenal volcano (Costa Rica, 1968 – 2010):  what we learned and perspectives, Mora, et al, Jun 2022

Thin-skinned mass-wasting responsible for widespread deformation at Arenal volcano, Ebmeier, et al, Dec 2014

Steady downslope movement on the western flank of Arenal volcano, Costa Rica, Ebmeier, et al, Dec 2010