Abstract

Cotopaxi volcano, situated in the Eastern Cordillera of the Ecuadorian Andes, is one of the most active volcanoes on Earth. The volcano is well known for the magnificence of its almost perfectly symmetrical cone topped by ice and snow and for the destructive power of its large-scale, syneruptive lahars. This paper presents a stratigraphic study of the post–twelfth-century eruptive products that reveals the existence of 21 continuous tephra beds. Most of them were characterized from both a physical (dispersal areas, deposit volumes, peak Mass Discharge Rate [MDR] of the eruptions) and compositional point of view. New 14C dates, linked with a new examination of historical chronicles, allow us to create a new chronostratigraphic scheme for this period of activity, which is bracketed by the emplacement of a regional tephra marker (A.D. 1140 ash bed from Quilotoa volcano) and the present day. The first period (A.D. 1150–1742) included only two moderate-intensity explosive eruptions, the oldest being possibly related to a dome disruption. In contrast, the period A.D. 1742–1880 started with two high-intensity, Plinian eruptions (maximum column heights of 25 and 29 km), followed by several short-lived but sustained, convective episodes. Deposits of pyroclastic surges and scoria flows were emplaced during some of these short-lived events and may have been related to column collapse and boiling over activity, respectively. Post-1880 activity, reported in 1904, 1906, and 1912, likely consisted of minor explosions that affected only the crater area.

Our study of recent activity at Cotopaxi shows that high dispersive power (peak mass discharge rates from 1.1 to 9.3 × 107 kg/s) is associated with the eruption of only moderate amounts of magma (1.1 × 1010–6.0 × 1011 kg, or ∼0.005–0.2 km3, Dense Rock Equivalent [DRE]). Additionally, al-though the past 2000 yr of activity at Cotopaxi have been interpreted to reflect a fairly uniform magma supply rate, detailed analysis of the past centuries, and a reanalysis of data from the past 2000 yr show that Cotopaxi's eruptive activity is characterized by clusters of eruptive events that are separated by periods of long quiescence punctuated by isolated eruptions, often of slightly more evolved magma.

No systematic variations in composition emerge in the time sequence. Although new magmatic phases commonly start with the eruption of mafic magma, this is not always observed. Additionally, eruption clusters may show either compositional trends of increasing SiO2 content or abrupt compositional changes within a cluster. We interpret the temporal and compositional variations in eruptive activity to reflect the complex interplay of deep versus shallow magmatic processes. An important result from the perspective of volcanic hazards is our conclusion that, over the studied period, no clear relation exists among repose time, eruption magnitude, and magma composition. This conclusion contrasts with the periodic eruptive behavior that has been postulated at many central volcanoes worldwide, thus inviting a reexamination of other intermediate-composition volcanic systems and a reassessment of the assumption of periodic activity.

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