The Cotacachi – Cuicocha Volcanic Complex, Ecuador

Cuicocha caldera with visible domes. Eroded Cotacachi volcano lies to the north. Image taken looking north. Image courtesy Patricio Ramon, 2003 via Smithsonian GVP

Cuicocha is a one of two caldera crater lakes in the Northern Andes of Ecuador. It is located at the foot of currently dormant (or perhaps extinct) Cotacachi volcano. The caldera measures 3 km in diameter and has at least four post caldera dacitic domes forming two steep-sided islands in the lake.

The Cuicocha – Cotacachi Volcanic Complex is another example of a paired volcanic system similar to the Santa Maria – Santiaguito system in Guatemala. Santiaguito is an active dome on the flanks of the larger Santa Maria stratovolcano. Cuicocha is a smallish caldera on the flank of the larger Cotacachi stratovolcano.

Domes in the middle of Cuicocha crater lake. Image courtesy Robinson Recalde, via Medium.com

The caldera names derive from indigenous language “Lago del Cuy”, or Guinea Pig Lake in English. The name refers both to the shape of the islands in the lake and a wild population of animals available locally that are used for food. They reproduce rapidly and do well in the high, dry country. They provide a high-protein food source for local populations.

The crater lake is located quite a distance above sea level in the Northern Andes, 3,246 m at its highest point. Its waters are highly alkaline and contain little life. The lake is quite deep at 143 m with steep slopes into it. These slopes limit zones that most water plants can grow other than algae. There is no known outlet for the lake.

Distribution of volcanoes in Ecuador. Cotacachi-Cuicocha Volcanic Complex is depicted in green in the northern part of the country. Screen capture from Garcia Peredes, 2020

The forested islands do support some wildlife, generally birds, small fish, frogs, snakes and insects. The caldera is located at the southern end of the Cotacachi – Cayapas Ecological Reserve.

There is a well-marked trail around the caldera rim that will take 5 – 7 hours to complete. Hikers should be ready to contend with the high altitude of the hike, 3,246 m. The trail is well marked and not technically difficult. Shorter hikes can be taken to and from the caldera rim.

Otavalo market with Imbabura volcano in the background. Image courtesy Gringos Abroad, 2019

The closest town is Otavalo, capital of Otavalo Canton, with around 40,000 inhabitants. The population is largely Otavalo indigenous people. It is in the valley floor between three surrounding volcanoes, Cuicocha – Cotacachi to the W, Cerro Imbabura immediately to the SE, and Mojanda immediately to the S. Both Cuicocha and Mojanda appear to have associated crater lakes.

Otavalo is famous for colorful woven wool textiles sold in local shops. The most famous of these is the combined Saturday market. At its peak, almost a third of the town is full of stalls selling various handmade goods, spices and foods.

Otavalo and surrounds map. Image courtesy Otavalo Tourism via Gringos Abroad, 2019

The town was originally a farming community due to the volcanic soils. Over the years, the popularity of the markets and handmade goods has grown to the point where tourism has become an important part of the local economy, catering to tourists and tours with local hotels, hostels, and tour operators. Neighboring towns specialize in other handmade goods such as leather and wood carving

178,000 live within 10 km of the lake, 364,000 within 30 km, and nearly 3.2 million within 100 km, most of them in the capital city of Quito 55 km to the S.

Cotacachi volcano from the west. Image courtesy Andreas Kay, Flickr, 2011

Volcano

The Cotacachi-Cuicocha Volcanic Complex has had a variety of effusive and explosive eruptive styles over its lifetime including lava flows, dome extrusion, flank collapses and debris avalanches, and at least one Plinian eruption. The Cuicocha volcano is currently emitting diffuse volcanic gasses and has an active hydrothermal system under the caldera.

Cotacachi is a dormant stratovolcano with three satellite domes on its flanks. These domes are named Loma Negra, Peribuela and Muyurcu. There are the remnants of the pre-Cuicocha caldera dome, the Cuicocha caldera itself, and four intra-caldera domes that have formed the islands of Teodoro Wolf and Yerovi in the lake.

Digital elevation map of Cotacachi – Cuicocha Volcanic complex. Caldera lake depicted is 3 km in diameter. Neighboring volcanoes Imbabura to the SE, Mojanda to the S. Image courtesy Gunkel & Beulker, 2009

Over its lifetime, Cotacachi suffered two large debris avalanches. Two central volcanic edifices were built during its lifetime. Cotacachi i was built 162,000 – 108,000 years before present (BP). It is constructed on a base of basaltic – andesitic lava flows. Andesitic lava flows and autobreccias were emplaced on the base. Some of these extended 18 km from the stratovolcano. These were strongly eroded by glaciation. There is a debris avalanche on NW flank of the volcano. It appears to have occurred at the end of the Cotacachi I construction.

Airborne photo showing Cotacachi I and II and Cuicocha caldera. Neighboring Imbabura also pictured. Image courtesy Marco Almdida, 2016

Cotacachi II was constructed next. It is mainly andesitic lava flows on its southern flank. This was followed by small dacitic lava domes and dikes. During its construction, a debris avalanche took place on its NE flank destroying part of the edifice. This is a quite active region, with the Cotacachi II debris avalanche emplaced on top of a debris avalanche from neighboring Cachimbiro and below one from neighboring Imbabura. This debris avalanche took place between 102,000 – 65,000 years BP.

The end of eruptive activity of Cotacachi II was a series of lava flows filling the crater of the volcano. There is a series of satellite lava domes on the edifice, including the NE debris avalanche deposit. These were built around 44,000 years BP. One of these was the original Cuicocha lava dome.

Muyurcu dome on the flanks of Cotacachi. Drawing depicts older collapse scarp the current dome was built upon. Image courtesy Marco Almdida, 2016

The andesitic Muyurcu dome is located on the SW flank of Cotacachi. It consists of 3 small domes covering 2 km2 build on top of a lava flow. It is around 138,000 years BP. The andesitic Loma Negra dome is on the SE flank of Cotacachi. It was built in three phases which included dome extrusion, collapse, various pyroclastic flows and ash flows. It is older than 40,000 years BP. The Peribuela dome is on the NE flank of Cotacachi. It is the largest of the domes covering over 4.5 km2. It is primarily built of dacites and dates some time after Loma Negra and before the pre-Cuicocha dome. It was built in two phases thich included pyroclastic flows and ash falls. It also has a significant flank collapse scar on its eastern flank.

Pre-Cuicocha dome on the flank of the caldera. Image courtesy Marco Almeida, 2016

The pre-Cuicocha caldera dome is located on the SW flank of Cotacachi and is part of the eastern edge of the Cuicocha caldera. It was originally 1.5 km in diameter and was built of andesites. Cuicocha began its activity with a series of eruptions 4,490 – 2,990 years BP. These eruptions produced ashfall and lava flows up to 150 m thick.

The caldera was created in VEI 5 eruption 3,100 years ago that ejected over 5 km3 of pyroclastic flow and fall deposits. 4.1 km3 of pyroclastics from that eruption are found primarily to the E of the volcano.

Post-caldera dacite domes in Cuacocha Caldera. Image taken from shoreline looking north. Image courtesy Marco Almeida, 2016

Following the caldera-forming eruption, a new dome was extruded and water accumulated in the newly formed caldera. A new explosive phase began some 110 years later which destroyed the newly emplaced dome. There was a smaller volume of juvenile material ejected than during the Plinian caldera-forming eruption. This stage erupted surge-dominated pyroclastic flows during phreatomagmatic eruptions. Most recent activity grew four dacitic intra-caldera domes building the two islands in the center of the caldera. The domes were built by subsequent eruptions 1,350 – 1,230 BP. There is no volcanic glass in fresh lava flows of the two islands, indicating that the lava had no contact with water at the time of eruption, with the lake filling sometime more recently.

Geologic map of Cotacachi – Cuicocha Volcanic Complex showing dome location. Image courtesy Marco Almeida, 2016

The lake began filling after the caldera was partly filled with slope debris from the crater walls and sediments from runoff clogged the fractures and fissures that allowed water to escape. This took place 1,000 – 500 years ago, making the crater lake a relatively new feature. The watershed feeding the lake is relatively small at 18 km2. The caldera walls and flanks of the islands are relatively steep, with 75% of the shoreline steeper than 45 degrees. The crater rim I,s very young with little consolidations. Frequent earthquakes trigger rockslides. The two basins of the lake are 148 m and 78 m at their maximum depths. As a very young lake, there has been little nutrient accumulation. Sediment deposition is also very low.

There was an earthquake swarm 2-3 Oct, 2018 with 62 volcano-tectonic events. Seismicity returned to normal on Oct 4. CO2 levels were normal and no deformation was observed.

Bathymetric map of Lake Ciucocha. WS = Water Sampling positions. WWS = Warm Water Springs. C = water inflow by Cascades. Black dots are areas with volcanic gas to the west of the island cluster. Image courtesy Gunkel, et al, 2008

The crater lake is fed by rainwater runoff. There is no outflow other than loss of water through various cracks and faults beneath and surrounding the lake. There is an active hydrothermal system under the lake. Given active heating and CO2 injection into the lake by the hydrothermal system, a limnic eruption (lake overturn and catastrophic CO2 release like Lake Monoun, Nyos or Kivu in Africa) was thought to be a sufficiently large hazard that a research team visited the lake in 2008. This visit was documented in a 2008 paper by Gunkel et al. Lake heating and the inflow of hydrothermal water was observed by divers, indicating an increase in activity of the volcano. This paper concluded that there is little risk of a limnic eruption at this time.

Sonar returns of rising volcanic gas bubbles from floor of Cuicocha western basin, 78 m depth. Screen capture courtesy Gunkel, et al, 2008

A second 2008 paper by Gunkel, et al described the volcanic hazards of lakes Quilotoa and Cuicocha in Ecuador. Cuicocha is a typical crater lake with one overturn yearly during the windy dry period. Degassing takes place during the overturn, but there is not a high level of CO2 observed. There are regular gas emissions into the lake mainly from a 500 x 600 m zone at 78 m depth in the western bay of the lake. CO2 and N2 gasses are of volcanic origin. The influx of water and gasses from this zone is sufficient to mobilize sediment from the floor of this zone.   This paper concludes that there is no risk of limnic eruption or CO2 gas cloud formation from this lake.

Water level dropped 6 m over the course of several weeks after the 1987 earthquake. Researchers took a look at sediments on the floor of the lake. Attempts to core the floor of the lake was not successful for a number of reasons, mostly because the lake floor is rocky with a thin layer of sediments. There are pores on the lake floor opened by rising gas bubbles and hydrothermal water.

Schematic of subduction of Nazca Plate under South America Plate. Image depicts subduction of Carnegie Ridge (center), Malpelo Ridge (center right) and Cocos Ridge (upper center into Central America. Image courtesy Marco Almeida, 2016

Tectonics

Volcanic activity in Ecuador is driven by the subduction of the Nazca Plate under the South American Plate, creating the Andes. This portion of the Northern Andes is segmented into 125 km-long chunks. Most of the segment boundaries appear to be unrelated to the structure of the subducted plate. They may be related to pre-existing structures of South America itself. Not all segment boundaries are related to subduction, though the mantle fluids erupted most certainly are.

The Carnegie Ridge is subducting under South America in this part of the collision zone. It has fed widespread and chemically diverse volcanic activity, major faulting and seismicity.

Subduction of a highly fractured zone in southern Ecuador created what has been described in at least one paper as a separate platelet with distinct orientation and change in eruptive style along its margins from andesitic stratovolcanoes to rhyolitic ash-flow sheets.

Earthquake fault plane solutions for subduction of Carnegie Ridge. Proposed locations of slab tears north and south of the ridge also depicted. Screen capture from Gutscher, et al, 1998

Volcanoes of the Northern Volcanic Zone generally lie along two parallel belts in Columbia and Ecuador. There are three separate segments in Columbia. There are four separate segments in Ecuador.

The 12 – 20 Ma Nazca Plate (in this region) is being subducted at an inbound angle of 31 – 45 degrees at a rate of 7 cm/yr. Dip angle in this region is 25 – 30 degrees. There is a break in subduction angle which decreases to the north. The southern part of this arc segment occurs where the Carnegie Ridge is subducting.

Distance from the subduction trench to volcanic front is also highly variable. The northern segment in Columbia is over 380 km E of the trench, and the volcanoes are 140 – 160 km above the subducted slab. Distance between the trench and volcanic front decreases to around 300 km in southern Ecuador, with the volcanoes only 80 – 100 km above the slab. The arc varies in thickness from a single chain of volcanoes in northern Columbia to a broad belt 80 – 120 km wide in Ecuador. Ecuador also has a line of alkaline back-arc centers another 50 km to the E. Crust thickness here varies from 40 – 55 km.

Schematic of slab tear associated with subduction of Carnegie Ridge under South America. Screen capture from Gutscher, et al, 1998

The Carnegie Ridge started subducting beneath South America for at least the last 2 and perhaps as long as the last 8 Ma. This subducted ridge may extend 110 – 500 km from the trench, meaning that subduction of it may have begun far earlier that currently believed. The Carnegie Ridge portion may be a local flat slab subduction regime similar to that of Peru. Modern seismicity in the region of the collision is similar to that of the San Andreas Fault in California, producing six Great Earthquakes greater than M 7.75 in the 20^th Century. The largest of these was a M 8.8 in 1906 with a rupture zone 500 km long.

There appears to be two tears in the subducting slab bounding the subducting Carnegie Ridge. The ridge tends to shallow out and subduct as a flat plate underneath South America. The portions of the Nazca Plate north and south of the tears subduct more steeply, at the 25 – 30-degree angle previously mentioned. The Carnegie Ridge thickness itself is thought to be larger than 17 km thick, which is in line with its source in the Galapagos Islands to the west. An alternate theory has the subducting portion of the Carnegie Ridge detaching and sinking into the mantle around 100 km from the trench, creating a slab window. This is supported by an observed seismic gap at intermediate depth. The 1998 Gutscher, et al paper this comes from concludes the two-tear description is the more likely description of what is actually going on.

Cuicocha caldera and intra-caldera lakes. Image courtesy Pintrest

Conclusions

Cuicocha is an active volcanic system with a relatively new crater lake. It has an active hydrothermal system, active volcanic gas emissions into the lake, and recent volcanic – tectonic earthquakes with a swarm in 2018. Eruptive products have been primarily andesites with dacites erupted as the magma evolved over time. There are at least four other recently active volcanic systems within a 15 km radius of the system, indicating a healthy supply of magma. There is no reason to believe activity at the Cotacachi – Cuicocha Volcanic Complex has abated, though repose times between periods of activity tends to be long.

Panoramic image of Cuicocha Caldera and intra-caldera domes / islands. Image courtesy Wiki

Additional information

Smithsonian GVP – Cuicocha

Wiki – Cuicocha

Metropolitan Touring – Cuicocha: An Impressive Lake in the Ecuadorian Andes

Lonely Planet – Otavalo

Gringos Abroad – Otavalo Tour: 11 things to know when visiting Otavalo market

Volcano Discovery – Chicocha volcano

Gutscher, et al, 1998, Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision

Stern, CR, 2004, Active Andean volcanism: its geologic and tectonic setting

Hall & Wood, 1985, Volcano-tectonic segmentation of the northern Andes

Gunkel & Buelker, 2009 – Limnology of the Crater lake Cuicocha, Ecuador, a cold water tropical lake

Gunkel & Buelker, 2008 – Hazards of volcanic lakes: Analysis of lakes Quilotoa and Cuicocha, Ecuador

Gunkel, et al, 2009 – Survey and assessment of post volcanic activities of a young caldera lake, Lake Cuicocha, Ecuador

Instituto Geofisico Escuela Politecnica Nacional, 20017 – Cotacachi – Cuicocha Volcanic Complex

ER Garcia Peredes, 2020 Thesis – Petrological study of the Cotacachi-Cuicocha Volcanic Complex, Ecuador: Understanding the eruptive dynamic and evolution of magma

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