Physical volcanology of the post-twelfth-century activity at Cotopaxi volcano, Ecuador: Behavior of an andesitic central volcano

Resumen

Cotopaxi volcano, situated in the Eastern
\nCordillera of the Ecuadorian Andes, is one of
\nthe most active volcanoes on Earth. The volcano
\nis well known for the magnifi cence of its
\nalmost perfectly symmetrical cone topped by
\nice and snow and for the destructive power
\nof its large-scale, syneruptive lahars. This
\npaper presents a stratigraphic study of the
\npost–twelfth-century eruptive products that
\nreveals the existence of 21 continuous tephra
\nbeds. Most of them were characterized from
\nboth a physical (dispersal areas, deposit volumes,
\npeak Mass Discharge Rate [MDR] of
\nthe eruptions) and compositional point of
\nview. New 14C dates, linked with a new examination
\nof historical chronicles, allow us to
\ncreate a new chronostratigraphic scheme for
\nthis period of activity, which is bracketed by
\nthe emplacement of a regional tephra marker
\n(A.D. 1140 ash bed from Quilotoa volcano)
\nand the present day. The fi rst period (A.D.
\n1150–1742) included only two moderateintensity
\nexplosive eruptions, the oldest being
\npossibly related to a dome disruption. In
\ncontrast, the period A.D. 1742–1880 started
\nwith two high-intensity, Plinian eruptions
\n(maximum column heights of 25 and 29 km),
\nfollowed by several short-lived but sustained,
\nconvective episodes. Deposits of pyroclastic
\nsurges and scoria fl ows were emplaced during
\nsome of these short-lived events and may
\nhave been related to column collapse and
\nboiling over activity, respectively. Post-1880
\nactivity, reported in 1904, 1906, and 1912,
\nlikely consisted of minor explosions that
\naffected only the crater area.
\nOur study of recent activity at Cotopaxi
\nshows that high dispersive power (peak
\nmass discharge rates from 1.1 to 9.3 ×
\n107 kg/s) is associated with the eruption of
\nonly moderate amounts of magma (1.1 ×
\n1010–6.0 × 1011 kg, or ~0.005–0.2 km3, Dense
\nRock Equivalent [DRE]). Additionally, although
\nthe past 2000 yr of activity at Cotopaxi
\nhave been interpreted to refl ect a fairly
\nuniform magma supply rate, detailed analysis
\nof the past centuries, and a reanalysis of
\ndata from the past 2000 yr show that Cotopaxi’s
\neruptive activity is characterized by
\nclusters of eruptive events that are separated
\nby periods of long quiescence punctuated
\nby isolated eruptions, often of slightly
\nmore evolved magma.
\nNo systematic variations in composition
\nemerge in the time sequence. Although new
\nmagmatic phases commonly start with the
\neruption of mafi c magma, this is not always
\nobserved. Additionally, eruption clusters
\nmay show either compositional trends of
\nincreasing SiO2 content or abrupt compositional
\nchanges within a cluster. We
\ninterpret the temporal and compositional
\nvariations in eruptive activity to refl ect the
\ncomplex interplay of deep versus shallow
\nmagmatic processes. An important result
\nfrom the perspective of volcanic hazards is
\nour conclusion that, over the studied period,
\nno clear relation exists among repose time,
\neruption magnitude, and magma composition.
\nThis conclusion contrasts with
\nthe periodic eruptive behavior that has
\nbeen postulated at many central volcanoes
\nworldwide, thus inviting a reexamination
\nof other intermediate-composition volcanic
\nsystems and a reassessment of the assumption
\nof periodic activity.