Iliniza (Illiniza) is a 5,125 m double peaked compound stratovolcano located 50 km S of Quito, in the central segment of the Ecuadorian Volcanic Front.  The two peaks are separated by 1.8 km with a saddle in between.  While the combined structure is surrounded by a thick pyroclastic layer, that appears to have been produced by a single explosive phase.  Most of the activity here has been effusive, with dome building being the most recent type of eruption over the last 10 ka. 

The twin peaks are among the highest in Ecuador, with South Iliniza (Iliniza Sur) being 119 m higher than North Iliniza (Iliniza Norte).  The volcano is a popular climbing destination, though South Iliniza is considered the most difficult due to glacial coverage.  Climbers must plan for high altitude climbing.  There is a cabin available to climbers on the saddle between the peaks. 

Due to its proximity to cities in the neighboring valleys, there are a large number of people living close by, nearly 200,000 within 30 km, most of them S in Latacunga.  There are 3.9 million within 100 km, most of these in Quito 50 km N.

Latacunga is 90 minutes S from Quito on the Pan-American highway.  It has a railway station and international airport.  Even though it is in a local valley, it is still quite high, 2,860 m above sea level.  There are 1.4 million living within its metropolitan area.  The city was destroyed four times by earthquakes 1698 – 1798.  It is located 25 km from Cotopaxi which has repeatedly put ash on it.  There are neighboring ruins of an older town that may date from the Incan empire. 

The economy is based on agriculture, floriculture and commerce.  Pumice deposits are locally mined.  Local sparkling water is bottled under the San Felipe brand name. 

Climate is considered to be an oceanic climate, with average temperatures running between 19° – 7.6°C.  While dry, there is still enough rainfall to support agriculture, 52 cm/year.

Over the last 30 years, instruments have been placed at several Ecuadorian volcanoes as a cooperative international effort.  As far as I can find, Quilotoa is not among the volcanoes yet monitored.  The Servicio Nacional de Sismologia y Volcanologia is the organization monitoring volcanic activity.  While there have been multiple papers published on the growth of the operation, I have not yet found an active online presence.  Perhaps this will change in the years to come. 

Region

At least 84 recently active volcanoes have been identified in the Ecuadorian Andes.  There are two types of volcanoes in the Ecuadorian volcanic arc.  The long-lived volcanic complexes have multiple edifices and cone building stages, active for most of the past 1 Ma.  The short-lived compound volcanoes have brief cone-building stages a few tens of thousands of years long, mostly emplaced in relatively recent times.  Iliniza is one of the short-lived volcanoes.  Neighboring Santa Cruz is a long-lived system.  Why the difference in neighboring systems is a fascinating, though as yet unexplored question.  The bulk volume of Iliniza at 46 km3 is similar to other compound volcanoes in the Ecuadorian Andes (31 – 61 km3).  Large, long-lived volcanic complexes are much larger 110 – 160 km3. 

Volcanoes in the region produced multiple cone building stages, major sector collapses, highly explosive eruptions, directed blasts, and caldera formation.  Most activity ended with recurrent dome complexes following magma transitions from andesites to dacites.  Large urban areas like Quito are surrounded by a number of volcanoes. 

Due to the large number of volcanoes in the immediate vicinity, we will limit our review of the region in the N to volcanoes immediately surrounding Quito, 50 km N of iliniza and those within 50 km to the S.  Note that all distances are measured from the South Iliniza summit. 

Pululahua

Pululahua is a 3,356 m highly eroded recently active stratovolcano on the NE border of Quito, 82 km NNE from Iliniza.  The 5 km summit caldera is breached to the E and partly refilled with a group of dacite lava domes.  Older pre-caldera domes are found on the E, SE and S sides of the caldera.  Four post caldera domes rise nearly 500 m above the caldera floor.  The caldera was formed during a series of eruptions 2.6 ka.  The most recent eruption took place 330 AD from post caldera domes producing lavas and pyroclastic flows as the domes collapsed.  I have found nothing about flank collapse, debris flows, or directed blast for this system, though the shape is similar to other breached calderas in the region. 

Casitagua

Casitagua is a 3,501 m stratovolcano that appears to be extinct.  It is located 74 km NNE from Iliniza.  The northern part of Quito is built on its eastern foot.  The volcano is topped with a caldera and a number of post-caldera domes.  There were no eruptions from this volcano over the last 10 ka.

Guaga Pichincha

Guaga Pichincha is a 4,784 m stratovolcano topped with a caldera breached to the W.  It is located 55 km NNE from Iliniza.  Most of central Quito is built on its eastern foot.  The flank collapse took place around 50 ka.  Subsequent activity was explosive, with growth and destruction of successive lava domes.  Multiple minor eruptions took place since the mid-16^th Century.  The largest of these was in 1660 which put ash over a 1,000 km radius.  Pyroclastic flows and surges went mainly W and impacted local agriculture. 

Ilalo

Ilalo is a 3,185 m highly eroded stratovolcano immediately SE of Quito, 55 km NE from Iliniza.  A single sample was dated 1.62 Ma.  It is considered extinct. 

Atacazo

Atacazo is a 4,463 m andesitic stratovolcano located 25 km SW of Quito, 35 NNE from Iliniza.  It is topped by a relatively recent 6 km caldera breached to the SW.  The caldera is partly filled with recent dacitic dome complexes.  Several Plinian eruptions took place here over the last 10 ka.  The most recent of these was 2.3 ka, extruding domes, Plinian eruptions, and pyroclastic flows that traveled as far as 35 km down valleys to the W. 

Sincholagua

Volcan Sincholagua is a 4,893 m recently active volcano some 41 km NE from Iliniza.  It is part of a small volcanic group including Cotopaxi, Pasochoa, Corazon and Ruminahui all 30 – 40 km E from Iliniza. 

Pasochoa

Paschoa is a heavily eroded stratovolcano just S of Quito, 34 km NE from Iliniza.  It has a large flank collapse amphitheater open to the NW.  Part of S Quito is built on the debris field.  It tops out at 4,199 m.  The flank collapse appears to coincide with a hot eruption around 100 ka. 

Corazon

Volcan Corazon is located 16 km NE from Iliniza.  It is a 4,786 highly eroded volcano with a summit caldera breached to the W.  There are lavas erupted within the last 20 – 10 ka.

Ruminahui

Ruminahui is a highly eroded 4,712 m stratovolcano with a summit caldera breached to the W 25 ENE from Iliniza.  It is only 14 km NW from neighboring Cotopaxi. 

Cotopaxi

We covered the active Cotopaxi, some 31 km E from Iliniza, with a pair of posts in 2015, the first in June, the second in August. 

Chalupas

Volcan Chalupas is the largest caldera in the region, an 18 km wide caldera that erupted the 90 km3 Chalupas ignimbrite.  It is located 49 km SE from Iliniza, topping out at 4,812 m.  The resurgent, highly eroded Quilindana stratovolcano is constructed within the caldera.  The system has been referred to as a supervolcano as a result of the VEI 6.9 Chalupas ignimbrite eruption.  The youngest lava flows were not eroded by glaciation.

Putzalagua

Putzalagua is an eroded 3,512 m lava dome some 38 km SE from Iliniza.  It is eroded just SE from Latacunga. 

Quilotoa

Quilotoa is located 29 km SW from Iliniza.  We covered Quilotoa in an Aug 2023 post.

Santa Cruz

Santa Cruz is a highly eroded 3,978 m stratovolcano 10 km E of South Iliniza in the Inter-Andean valley.  The S flank of it is exposed, cut by wide, deep valleys.  There are highly eroded, altered dacite lava domes at the summit.  The N flank is mostly covered by erosion deposits of the valley, 300 m higher than the base elevation of the S flank.  An andesitic lava flow on the S ridge dates 702 ka, thought to be an intermediate age of the system.

Activity resumed at Santa Cruz after a 620 ka period of quiet with extrusion of the Loma Saquigua dome 79 ka.  The upper structure has even younger andesitic lava flows, 60 ka.  The evolution of Iliniza is closely related to that of neighboring Santa Cruz volcano. 

Iliniza

The twin peaks of Iliniza are located on the E edge of the Western Cordillera, about 1,100 m above the neighboring valley.  The edifice covers 127 km2, with an estimated volume of 46 km3.  Eruptive activity was interrupted by long-term periods of quiet between relatively compressed construction stages.

Neighboring Santa Cruz was constructed in the Inter-Andean valley 10 km E from Iliniza.  Its activity was mostly effusive until around 702 ka, erupting dacites and dacite domes on the summit area.  There was a break in local activity until initial activity at Iliniza extruded the Pilongo lava dome 353 ka.

After a break in activity, construction of North Iliniza (Tioniza) began some 124 ka.  The volcano has an oval base about 10 km in diameter, steeper on the Inter Andean Valley side.  The SE flank is eroded.  There is a narrow, steep amphitheater likely carved by glacial erosion.  North Iliniza lavas partly cover Huayrapungo Peak (4,585 m), 2.5 km NAE from the summit.

Activity then shifted a couple kilometers S, building South Iliniza through the S flank of North Iliniza.  It has a well-defined semicircular shape 14 km in diameter.  Its summit is located just 1.5 SE of North Iliniza summit.  The base of South Iliniza is 3,300 – 3,600 m.  Final activity extruded the Pilongo and Tishiguchi lava domes on the N and S flanks of Iliniza, 5.2 km from the summit. 

The volcano edifice was eroded by glaciation, with most erosion on the older, North Iliniza section.  The NW flank of Huayrapungo peak has a series of lateral moraines creating a glacial fan at the bottom of the volcano.  Erosion on South Iliniza cut narrow, steep glacial valleys.  Moraine sequences have not been well studied but appear similar to glacial deposits from the last glacial maximum 35 – 20 ka and previous glacial cycles.

There is an active hydrothermal system on South Iliniza complete with hot springs and a sulfur-encrusted hot waterfall popular with tourists. 

The next several sections will present a more detailed review of the growth of Iliniza since activity began there. 

Pilongo lava dome

The oldest feature on the edifice is the Pilongo lava dome on the NW flank.  It is 600 m in diameter, 250 m above the valley floor, 0.02 km3 in volume, and was formed before activity at North Iliniza began.  Due to its low altitude, it has not been substantially eroded by glaciers.  Erosion by flowing water created steep slopes on its E and S flanks.  The N and W flanks are surrounded by a gently sloping plain that is part of the N flank of Iliniza.  The dome is made of rhyodacites.

North Iliniza (NI)

After a quiet period of 230 ka, activity resumed, building the North Iliniza cone.  Two successive lava sequences erupted after the glacial maximum.  The Huayrapango satellite peak is connected to the lower lava sequence.  Glacial erosion before 49 ka carved wide radial glacial valleys around the summit and scoured the Pilongo and Providencia Pongo alleys. 

The second stage of activity built the North Iliniza cone.  It has a semicircular base, concave shape, to its 5,200 m summit.  The symmetry axis is located 200 m SW of the present-day summit and narrow amphitheater on the E flank.  The 28 km3 volume is relatively small compared to the volcano’s 1,700 m height above the neighboring valley.  North Iliniza was active 124 – 116 ka.  There is a lava flow from this phase into neighboring Pilongo valley.  Reconstruction of the E sector of the North Iliniza cone 45 ka is difficult as it has been covered by South Iliniza deposits. 

North Iliniza is constructed from two thick piles of andesitic to dacitic lava flows, and breccias.  Pyroclastics here are scarce.  The upper part of the cone has widespread hydrothermal alteration.  Lavas from the Huayrapungo satellite peak are layered between the two main sequences.

The Lower NI stage is a pile of breccias, 5 – 30 m lava flows, and a few pyroclastics.  They are typically andesites and dacites.  The sequence is exposed in glacial ridges around the North Iliniza edifice, especially on the W Cordillera side.  Maximum thickness here is 200 – 300 m; 700 – 800 m on the Inter Andean Valley side.  Blocks in the breccia layers on the N flank date 123 – 122 ka.

The Huyrapungo stage (NI-H) built what is now an eroded peak with voluminous andesitic lavas.  They crop out in the N NI flank.  Lavas are banded with columnar jointing close to the summit.  These lavas are at least 250 m thick, covering pyroclastic layers probably related to the Lower NI stage.  This is considered to be a NI satellite vent and is dated 121 ka.

The Upper NI stage is a 600 – 700 m thick pile of viscous andesite to dacite lava flows, typically 30 – 60 m thick shaping the upper structure of the North Iliniza above 4,400 m.  There is a voluminous 100 m thick lava flow exposed in the summit area that flowed 1,500 m NW.  Erosion exposed a secondary vent located at 4,800 m.  This lava is dated 116 ka.  There is a 90 m thick lava on the W flank in the Quillotoro Hill that covers the NI-L pile.  This lava was cut by a glacial valley near its top.  Emplacement age of this similar to the 116 ka flow. 

South Iliniza (SI)

South Iliniza was built on top of the partly eroded North Ilniza.  There is a structural discontinuity between the two growth phases of South Iliniza.  The earliest phase erupted thick dacites and pyroclastics around 45 ka, forming the base of the new cone. 

Construction of South Iliniza also took place in three stages:  lower cone building, upper cone building, and terminal effusive eruptions.  Building the 18 km3 cone took place 46 – 25 ka.  Eroded volume is estimated at 6 km3.  The lower flanks are covered by a thick pyroclastic layer associated with the second stage.

The lower SI stage (SI-L) is the early cone built on top of the NI layers and remnants of the Santa Cruz volcano.  It is a 1,100 m pile of massive dacite lavas, sparse breccia layers, and pyroclastic flow deposits.  Outcrops near the center show moderate hydrothermal alteration.  Lavas on the N and E flanks date 46 – 45 ka.  Glacial cuts are preserved on the N lava flow, confirming its pre-glacial (35 – 20 ka) age.  The base stratovolcano is disrupted above 4,700 m, forming a truncated cone.  The slope change is particularly noticeable on the S flank, where the saddle shaped Cantarilla marks a structural unconformity. 

The Jatuncama ignimbrite (SI-Ji) is a widespread pyroclastic layered deposit covering the lower flanks of the SI edifice to the SW of the combined volcano.  It dates around 34 ka and is thought to be due to dome building activity.  The explosion formed a 1.5 – 2.0 km crater with a minimum of VEI 5 eruption sequence.  The Cantarilla disconformity is the inferred crater rim created during the Jatuncama eruption(s).  

The deposits cover semi-consolidated pyroclastic deposits from the Lower SI stage to the E and soils to the S.  The ignimbrite are multiple thick bedded block and ash flows overlain by pumice and ash flow deposits.  These are continuous, graded to massive, poorly sorted, and gray to pinkish in color.  There are no paleosol layers in the sequence.  Low density pumice lapillis are scarce.  Average thickness 30 – 40 m maximizes to 120 m in the Jatuncama Valley.  The flows cover around 60 km2 with an average thickness of 35 m, giving a volume of 2.1 km3.  As yet, no tephra has been found associated with these pyroclastic flow deposits, meaning the estimated volume is a minimum.  The layered deposit is covered by a thick sequence of paleosoils, tephras from Cotopaxi and other recent materials.  The Cotopaxi rhyolitic tephras date 7.7 ka.  There are minor landslide scarps on the E and SE flanks.

The Summit Lava Dome Complex (SI-Usdc) is built on top of the remnants of the Lower SI edifice 4,710 – 5,050 m reaching 1.7 km in diameter.  The vent is located slightly N of the base cone.  Multiple lava lobes and remnants of earlier talus are exposed on the S flank.  The summit domes are dacitic and date around 34 ka.  The higher sections of South Iliniza are covered by reworked dacite pyroclastic flow breccias related to dome forming activity

The terminal SI stage (SI-T) erupted several massive lava flows and breccias that descend along the W flank into neighboring Providencia Valley.  These andesitic lavas shape the summit above 5,000 m and are up to 250 m thick.  Two large Termanchuo lava flows outcrop on the summit W flank and can be traced 3 km downslope into the upper Providencia valley.  The lower 70 m thick lava flow is dacite.  The upper 50 m thick flow is andesitic.  A third andesitic lava flow is located on the S SI flank.  It is partly covered by a glacial moraine.  These flows date 31 – 25 ka. 

Recent satellite lavas

The Tishigcuchi dome coulee and Pongo lava flow are the youngest eruptive products from Iliniza.  While undated, the dome age can be estimated by what is above and below it, at 20 – 8 ka.  This doesn’t mean that the extrusion took place for the entire 12 ka, rather it took place somewhere in that range.  The 6 ka age of the Pongo lava flow is the youngest activity on Iliniza. 

Tishigcuchi (T) is a satellite dome coulee on the S flank of South Iliniza.  It is a 1,100 m wide lava dome (4,185 m) and at least three lava flows up to 90 m thick and 1.8 km long.  The lava flowed over a slope created by the Tiliche lava and Jatuncama pyroclastics.  The lava and coulee is andesitic.  There are no collapse or explosion crater structures and no pyroclastics.  There is no glacial erosion.  The dome is covered by soils layered with Cotopaxi’s rhyolitic lapilli and ash fall deposits.  This dome is as yet undated.

Pongo (Pg) is a large lava forming a 4 km terrace between the Pongo and Providencia rivers in the Jatuncama valley.  Erosion by the Pongo River exposed a 60 m cliff with columnar jointing on the SW edge of the lava.  Erosion also exposed the Jatuncama ignimbrite next to the Pongo lava.  The lava is andesitic.  Sampling once again was difficult here, though an age of 6 ka was sampled near the base of the columnar jointing.  This flow is thought to originate from a satellite vent on the SW base of South Iliniza. 

Hazard assessment

Activity at Iliniza over the last 10 ka has erupted mainly andesitic lavas.  The most probable future activity would erupt lava flows, extrude domes, and produce pyroclastic flows, block and ash flows, and regional tephras.  As the eruptive centers migrated S over time, South Iliniza is thought to be most likely to erupt.  An eruption on the steeper slope would tend to destabilize that slope.  These events would impact the 200,000 residents within 20 km of the volcano.

Although there was an explosive phase at Iliniza, this was unique throughout its eruptive history, and in turn unlikely.  There is no evidence of a major flank collapse from the system.  The volcano is located above an active strike-slip fault system, and it does have active weathering, erosion and hydrothermal alteration of rock, all of which increase the risk of a potential collapse caused by a tectonic event. 

Tectonics

Regional tectonics are similar to those we discussed in our previous Quilotoa post.  I will reprise a shortened version of that discussion below. 

Major plate reorganization of the Farallon Plate around 28 Ma gave birth to the Nazca and Cocos Plates.  The Nazca Plate started subducting beneath the South American Plate some 2 – 9 Ma.  There are three tectonically distinct ocean-floor domains in the Nazca Plate W of Columbia and Ecuador.  The northernmost subducts under Columbia at 35°, producing a single volcanic chain.  The central section contains the Carnegie Ridge, which has been subducting under South America for the last 2 Ma.  The subduction angle reduced to 25° due to the buoyancy of the ridge.  This created the wider volcanic arc of the Ecuadorian Andes.  In southernmost Ecuador, the Nazca Plate shallows to a 14° dip, effectively shutting down volcanism in southern Ecuador and much of northern and middle Peru.

The most significant tectonic change in Ecuadorian volcanism in recent years has been the subduction of the Carnegie Ridge (undersea ridge built by the Galapagos hot spot beneath South America.  Estimated dates for the initial collision range from 1 – 15 Ma.  The age of the eastern end of the Carnegie Ridge may be as old as 20 – 25 Ma, the onset of activity from the Galapagos hot spot and the breakup of the Farallon Plate into the Cocos and Nazca Plates.  Based on these ages, the eastern end of the Carnegie Ridge entered subduction 6 -8 Ma.  The Galapagos hot spot has moved around 750 km E relatively to the South American Trench over the last 15 Ma. 

The buoyancy of the subducting Carnegie Ridge has uplifted most of Ecuador.  Coastal zones are uplifted at least 200 – 300 Ma.  The Coastal Cordillera is located some 120 km E of the trench.  The highest points of this range are 600 – 700 m above sea level and this uplift is relatively recent at 1 – 1.5 Ma.  The Carnegie Ridge has forced the subducting Nazca Plate into relatively shallow subduction for the first 300 km from the trench underneath Ecuador.  At this point, the plate bends down more steeply.  This bend is located a bit more than 100 km below the surface and is located beneath the two Cordilleras (Occidental and Real) in central Ecuador.  The bend is also the source of melts powering volcanoes of the Ecuadoran Andes. 

Conclusions

Iliniza is located in an incredibly busy volcanic region.  Its neighbors are multiple calderas, breached calderas, amphitheaters, flank collapses, and at least one massive VEI 6.9 eruption producing a 90 km3 ignimbrite (Chalupas).  Due to the volcanic debris, the surrounding valleys, though relatively high altitude (3,500 – 4,000 m) are fertile, supporting agriculture and large populations.  The most active volcano in the region, Cotopaxi, is only 31 m E.  Activity at Iliniza was most recently effusive with andesite lavas and dome building / collapse.  Future eruptions are expected to be effusive.  There are a lot of people in the immediate region, with over 3.1 million in Quito to the immediate N. 

Additional information

Smithsonian GVP – Iliniza

Iliniza – Wiki

Illiniza Norte – Summit Post, B Andrean

Geochronological evolution of the potentially active Iliniza volcano (Ecuador) based on new K-Ar ages, Santamaria, et al, Apr 2022

I. Notes on the microscopic structure of some rocks from the andes of Ecuador collected by E Whymper.  No. V (conclusion). Altar, illiniza, sincholagua, cotocachi, sara-urcu, &c, TG Bonney, Jan 1884

How tectonic, volcanic and climatic processes in Andean ‘sky islands’ shaped the diversification of endemic ground beetles, Murienne, et al, Oct 2021

CCVG-IAVCEI 13^th Gas Workshop, Sept 24 – Oct 3, 2017

Adakitic magmas in the Ecuadorian volcanic front:  petrogenesis of the Iliniza volcanic complex (Ecuador), Hidalgo, et al, Jan 2007