Minard L. Hall

Tungurahua Volcano, Ecuador : structure, eruptive history and hazards

Abstract Tungurahua, one of Ecuadors most active volcanoes, is made up of three volcanic edifices. Tungurahua I was a 14-km-wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series. Tungurahua II, mainly composed of acid andesite lava flows younger than 14,000 years BP, was partly destroyed by the last collapse event, 2955±90 years ago, which left a large amphitheater and produced a ∼8-km3 debris deposit. The avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for ∼15 km. A large lahar formed during this event, which was followed in turn by dacite extrusion. Southwestward, the damming of the Chambo valley by the avalanche deposit resulted in a ∼10-km-long lake, which was subsequently breached, generating another catastrophic debris flow. The eruptive activity of the present volcano (Tungurahua III) has rebuilt the cone to about 50% of its pre-collapse size by the emission of ∼3 km3 of volcanic products. Two periods of construction are recognized in Tungurahuas III history. From ∼2300 to ∼1400 years BP, high rates of lava extrusion and pyroclastic flows occurred. During this period, the magma composition did not evolve significantly, remaining essentially basic andesite. During the last ∼1300 years, eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied composition (andesite+dacite), and end with more basic andesite lava flows or crater plugs. This pattern is observed in the three historic eruptions of 1773, 1886 and 1916–1918. Given good age control and volumetric considerations, Tungurahua III growths rate is estimated at ∼1.5×106 m3/year over the last 2300 years. Although an infrequent event, a sector collapse and associated lahars constitute a strong hazard of this volcano. Given the ∼3000 m relief and steep slopes of the present cone, a future collapse, even of small volume, could cover an area similar to that affected by the ∼3000-year-old avalanche. The more frequent eruptive episodes of each century, characterized by pyroclastic flows, lavas, lahars, as well as tephra falls, directly threaten 25,000 people and the Agoyan hydroelectric dam located at the foot of the volcano.

Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America)

A geochemical and isotopic study of lavas from Pichincha, Antisana and Sumaco volcanoes in the Northern Volcanic Zone (NVZ) in Ecuador shows their magma genesis to be strongly influenced by slab melts. Pichincha lavas (in fore arc position) display all the characteristics of adakites (or slab melts) and were found in association with magnesian andesites. In the main arc, adakite-like lavas from Antisana volcano could be produced by the destabilization of pargasite in a garnet-rich mantle. In the back arc, high-niobium basalts found at Sumaco volcano could be produced in a phlogopite-rich mantle. The strikingly homogeneous isotopic signatures of all the lavas suggest that continental crust assimilation is limited and confirm that magmas from the three volcanic centers are closely related. The following magma genesis model is proposed in the NVZ in Ecuador: in fore arc position beneath Pichincha volcano, oceanic crust is able to melt and produces adakites. En route to the surface, part of these magmas metasomatize the mantle wedge inducing the crystallization of pargasite, phlogopite and garnet. In counterpart, they are enriched in magnesium and are placed at the surface as magnesian andesites. Dragged down by convection, the modified mantle undergoes a first partial melting event by the destabilization of pargasite and produces the adakite-like lavas from Antisana volcano. Lastly, dragged down deeper beneath the Sumaco volcano, the mantle melts a second time by the destabilization of phlogopite and produces high-niobium basalts. The obvious variation in spatial distribution (and geochemical characteristics) of the volcanism in the NVZ between Colombia and Ecuador clearly indicates that the subduction of the Carnegie Ridge beneath the Ecuadorian margin strongly influences the subduction-related volcanism. It is proposed that the flattening of the subducted slab induced by the recent subduction (<5 Ma?) of the Carnegie Ridge has permitted the progressive warming of the oceanic crust and its partial melting since ca. 1.5 Ma. Since then, the production of adakites in fore arc position has deeply transformed the magma genesis in the overall arc changing from ‘typical’ calc-alkaline magmatism induced by hydrous fluid metasomatism, to the space- and time-associated lithology adakite/high-Mg andesite/adakite-like andesite/high-Nb basalts characteristic of slab melt metasomatism.

Subduction controls on the compositions of lavas from the Ecuadorian Andes

Three volcanoes of the Ecuadorian Andes, Atacazo, Antisana, and Sumaco, lie in a transect perpendicular to the trench and the main trend of the Andean arc. Each of the volcanoes lies on crust of substantially different age, composition, and thickness. Few compositional or isotopic features correspond in a straightforward way to the type of the crust through which the magmas have passed. Isotopic data limit assimilation to <15% at each of the volcanoes. Instead, a systematic relationship exists between the compositions of the lavas and the depth to the Benioff zone, suggesting that subduction imparts the principal control on the compositions of the magmas. Atacazos lavas have low concentrations of the incompatible trace elements and very large LIL/HFS ratios. Sumacos lavas are strongly enriched in the incompatible trace elements and have small LIL/HFS ratios. Antisanas lavas are intermediate in almost every respect. These features are consistent with devolatilization of the subducted slab controlling the extent of partial melting of a depleted mantle source. A mixing and melting model suggests the volcanic front magmas are made by large extent of partial melting (∼15%) and include a large slab input (1.1% added to the depleted mantle). The magmas of the middle belt of volcanoes are made by smaller extent of partial melting (3%), induced by moderate amounts of slab-derived fluid (0.06%). The back arc magmas result from small degrees of melting (2%) and small slab input.

Volcano-tectonic segmentation of the northern Andes

Data from LANDSAT images, published geologic and geophysical reports, and field observations are used to document eight major transverse boundaries that divide the northern Andes into segments about 125 km long. Most of the segment boundaries have no apparent relation to structure on the Nazca plate being subducted, and may instead represent fundamental and long-lasting structures of the continent. Subduction of the Carnegie Ridge results in increased uplift of the Andes, widespread and chemically diverse volcanism, and major faulting and seismicity. Subduction of a highly fractured zone in southern Ecuador has led to the formation of a separate platelet with a distinct orientation and to a notable change in volcanic style—from andesitic stratovolcanoes to rhyolitic ash-flow sheets. Recognition of the segmented structure of the northern Andes provides a framework for interpreting details of the regional geology and illustrates that not all segment boundaries are related to Subduction.

Sangay volcano, Ecuador: structural development, present activity and petrology

Abstract Sangay (5230 m), the southernmost active volcano of the Andean Northern Volcanic Zone (NVZ), sits ∼130 km above a >32-Ma-old slab, close to a major tear that separates two distinct subducting oceanic crusts. Southwards, Quaternary volcanism is absent along a 1600-km-long segment of the Andes. Three successive edifices of decreasing volume have formed the Sangay volcanic complex during the last 500 ka. Two former cones (Sangay I and II) have been largely destroyed by sector collapses that resulted in large debris avalanches that flowed out upon the Amazon plain. Sangay III, being constructed within the last avalanche amphitheater, has been active at least since 14 ka BP. Only the largest eruptions with unusually high Plinian columns are likely to represent a major hazard for the inhabited areas located 30 to 100 km west of the volcano. However, given the volcanos relief and unbuttressed eastern side, a future collapse must be considered, that would seriously affect an area of present-day colonization in the Amazon plain, ∼30 km east of the summit. Andesites greatly predominate at Sangay, there being few dacites and basalts. In order to explain the unusual characteristics of the Sangay suite—highest content of incompatible elements (except Y and HREE) of any NVZ suite, low Y and HREE values in the andesites and dacites, and high Nb/La of the only basalt found—a preliminary five-step model is proposed: (1) an enriched mantle (in comparison with an MORB source), or maybe a variably enriched mantle, at the site of the Sangay, prior to Quaternary volcanism; (2) metasomatism of this mantle by important volumes of slab-derived fluids enriched in soluble incompatible elements, due to the subduction of major oceanic fracture zones; (3) partial melting of this metasomatized mantle and generation of primitive basaltic melts with Nb/La values typical of the NVZ, which are parental to the entire Sangay suite but apparently never reach the surface and subordinate production of high Nb/La basaltic melts, maybe by lower degrees of melting at the periphery of the main site of magma formation, that only infrequently reach the surface; (4) AFC processes at the base of a 50-km-thick crust, where parental melts pond and fractionate while assimilating remelts of similar basaltic material previously underplated, producing andesites with low Y and HREE contents, due to garnet stability at this depth; (5) low-pressure fractionation and mixing processes higher in the crust. Both an enriched mantle under Sangay prior to volcanism and an important slab-derived input of fluids enriched in soluble incompatible elements, two parameters certainly related to the unique setting of the volcano at the southern termination of the NVZ, apparently account for the exceptionally high contents of incompatible elements of the Sangay suite. In addition, the low Cr/Ni values of the entire suite—another unique characteristic of the NVZ—also requires unusual fractionation processes involving Cr-spinel and/or clinopyroxene, either in the upper mantle or at the base of the crust.

Estimating rates of decompression from textures of erupted ash particles produced by 1999–2006 eruptions of Tungurahua volcano, Ecuador

Persistent low- to moderate-level eruptive activity of andesitic volcanoes is difficult to monitor because small changes in magma supply rates may cause abrupt transitions in eruptive style. As direct measurement of magma supply is not possible, robust techniques for indirect measurements must be developed. Here we demonstrate that crystal textures of ash particles from 1999 to 2006 Vulcanian and Strombolian eruptions of Tungurahua volcano, Ecuador, provide quantitative information about the dynamics of magma ascent and eruption that is difficult to obtain from other monitoring approaches. We show that the crystallinity of erupted ash particles is controlled by the magma supply rate (MSR); ash erupted during periods of high magma supply is substantially less crystalline than during periods of low magma supply. This correlation is most easily explained by efficient degassing at very low pressures (<<50 MPa) and degassing-driven crystallization controlled by the time available prior to eruption. Our data also suggest that the observed transition from intermittent Vulcanian explosions at low MSR to more continuous periods of Strombolian eruptions and lava fountains at high MSR can be explained by the rise of bubbles through (Strombolian) or trapping of bubbles beneath (Vulcanian) vent-capping, variably viscous (and crystalline) magma.

Time‐averaged field at the equator: Results from Ecuador

Seventy sites were collected from Pliocene to recent lavas in Ecuador between 0.19° and 1.1°S latitude. Lightning affected many of the sites sampled, and the final data set consisted of 51 sites with α95 of 10° or less; 21 flows yielded normal directions of magnetization (declination = 354.6°, inclination = −5°, α95 = 7.2°) and 30 yielded reverse (declination = 183.5°, inclination = 5.6°, α95 = 5°). These sites pass a reversal test at a high level. The combined data give a direction of declination = 359.9°, inclination = −5.4°, and α95 = 4.2°, which is just significantly different from the GAD field but is in agreement with a GAD field plus 5% quadrupole field. Mean VGPs are not significantly different from the geographic poles. The ASD for the combined sites is 13.3°, which is in agreement with model “G.” Paleointensity measurements yield values of VADM, which range from 1.39 × 1022 to 22.6 × 1022 Am2. The mean value for the reverse field is 3.2 × 1022 Am2, while for the normal field it is 10.2 × 1022 Am2. The 40Ar/39Ar dates on selected sites show that they are 2.6 Ma and younger. The age dates and magnetic polarity indicate that the Chacana structure is Brunhes/Matuyama in age.

Origin of espanola island and the age of terrestrial life on the galapagos islands.

Geological studies of Espa�ola (Hood) Island, Gal�pagos, Ecuador, indicate that the island had a subaerial rather than a submarine origin. Because the younger lava flows are dated at 3 million years, Espa�ola has apparently existed as an island for at least that long. Thus terrestrial life may have existed or arrived on the Gal�pagos Islands at least 3 million years ago, more than twice as long as had been assumed.

Volcanic eruptions with little warning:: the case of Volcán Reventador's Surprise November 3, 2002 Eruption, Ecuador

Successful mitigation of a possible volcanic disaster depends upon the early detection of renewed volcanic activity. With considerable optimism, volcano observatories instrument dangerous volcanoes, with the hope of an early recognition of the reactivation of a volcano. Reventador volcanos November 3, 2002 eruption came with little warning and had a tremendous socio-economic impact. Reventador volcano, a young andesitic cone in the Eastern Cordillera of Ecuador, has had, at least, 16 eruptions between 1541 and 2002. These eruptions were characterized by small pyroclastic flows, blocky lava flows, debris flows, and limited ash falls. With the exception of a M=4 earthquake near the volcano one month earlier, only seismic activity related to the eruption onset was registered. Following only 7 hours of seismic tremor, the paroxysmal eruption began at 0912 h on November 3, 2002 with a sustained column that ascended to 17 km and five pyroclastic flows, that traveled as much as 9 km to the east. By mid-afternoon ash falls of 1-10 mm thickness began blanketing the Interandean Valley near Quito. The economic impact was significant, including severe damage to the principal petroleum pipelines, closure of schools, businesses, and Quitos airport for 10 days. It is important to conclude that for those volcanoes that are characterized by low silica, volatile-rich, fluid magmas, magma ascent can be aseismic, rapid, and without much warning. This event should serve as a clear reminder to scientists, civil defense, and authorities of the rapid onset of the eruptions of some volcanoes.

Enhancing volcano‐monitoring capabilities in Ecuador

Ecuador has 55 active volcanoes in the northern half of the Ecuadorian Andes. There, consequences of active volcanism include ashfalls, pyroclastic flows (fast moving fluidized material of hot gas, ash, and rock), and lahars (mudflows), which result in serious damage locally and regionally and thus are of major concern to Ecuadorians. In particular, Tungurahua (elevation, 5023 meters) and Cotopaxi (elevation, 5876 meters) are high-risk volcanoes. Since 1999, eruption activity at Tungurahua has continued and has produced ashfalls and lahars that damage towns and villages on the flanks of the volcano. More than 20,000 people live on these flanks.