Shanaka L. de Silva

Global influence of the AD 1600 eruption of Huaynaputina, Peru

It has long been estabished that gas and fine ash from large equatorial explosive eruptions can spread globally, and that the sulphuric acid that is consequently produced in the stratosphere can cause a small, but statistically significant, cooling of global temperatures,. Central to revealing the ancient volcano–climate connection have been studies linking single eruptions to features of climate-proxy records such as found in ice-core and tree-ring chronologies. Such records also suggest that the known inventory of eruptions is incomplete, and that the climatic significance of unreported or poorly understood eruptions remains to be revealed. The AD 1600 eruption of Huaynaputina, in southern Peru, has been speculated to be one of the largest eruptions of the past 500 years; acidity spikes from Greenland and Antarctica ice, tree-ring chronologies, along with records of atmospheric perturbations in early seventeenth-century Europe and China,, implicate an eruption of similar or greater magnitude than that of Krakatau in 1883. Here we use tephra deposits to estimate the volume of the AD 1600 Huaynaputina eruption, revealing that it was indeed one of the largest eruptions in historic times. The chemical characteristics of the glass from juvenile tephra allow a firm cause–effect link to be established with glass from the Antarctic ice, and thus improve on estimates of the stratospheric loading of the eruption.

Late Cenozoic magmatism of the Bolivian Altiplano

Small basalt to dacite volcanic centers are distributed sparsely over the Bolivian Altiplano, behind the Andean volcanic front. Most are Pliocene to Recent in age, and are characterized by textural and mineralogical disequilibrium with abundant xenoliths and xenocrysts. True phenocrysts are rare in the more mafic samples. Compared with Recent volcanic rocks from Andean stratovolcanoes, the Bolivian centers overlap in major element trends. Incompatible element contents tend to be higher, particularly in the eastern Altiplano. The ranges of isotopic compositions reflect ubiquitous crustal contamination. Pb isotope compositions are dominated by Pb from isotopically heterogeneous basement, resulting in a wide scatter of data lying between inferred crustal compositions and showing little overlap with possible mantle sources in the region. Rocks sampled from the Bolivian Altiplano were probably derived from asthenospheric mantle and subjected to extensive open system differentiation during ascent through the 70 km thick crust of the region. Major element trends are largely controlled by the fractionating phase assemblage (olivine, clinopyroxene and amphibole). Trace element and isotope systematics, however, defy realistic attempts at modeling due to the geographic scatter of samples, the uniformity of compositions at a given center, and the heterogeneity of the contaminant. Nevertheless, there are first order systematic trace element variations that appear to relate to the geometry of the subduction zone. In particular, LIFE/HFSE (exemplified by Ba/Nb), and Zr/Nb ratios decrease from the arc front eastward into the Altiplano. These variations are not easily reconciled with control by crustal contamination alone. A model is proposed in which the asthenospheric source is fluxed by high Ba/Nb slab-derived fluid to induce melting. Beneath the arc, high fluid flux increases the Ba/Nb ratio of the asthenosphre and leads to high degrees of partial melting (high Zr/Nb). To the east, lower or no fluid flux leads to low Ba/Nb and low degrees of partial melting (low Zr/Nb). Melting in the back arc region of the Altiplano may be facilitated by lithospheric delamination that leads to decompression melting of counter-flowing asthenosphere. There is no unequivocal evidence that requires a significant role for the lithospheric mantle.

Hydro‐isostatic deflection and tectonic tilting in the central Andes: Initial results of a GPS survey of Lake Minchin shorelines

Sufficiently large lake loads provide a means of probing rheological stratification of the crust and upper mantle. Lake Minchin was the largest of the late Pleistocene pluvial lakes in the central Andes. Prominent shorelines, which formed during temporary still-stands in the climatically driven lake level history, preserve records of lateral variations in subsequent net vertical motions. At its maximum extent the lake was 140 m deep and spanned 400 km N-S and 200 km E-W. The load of surficial water contained in Lake Minchin was sufficient to depress the crust and underlying mantle by 20–40 m, depending on the subjacent rheology. Any other differential vertical motions will also be recorded as departures from horizontality of the shorelines. We recently conducted a survey of shoreline elevations of Lake Minchin with the express intent of monitoring the hydro-isostatic deflection and tectonic tilting. Using real-time differential GPS, we measured topographic profiles across suites of shorelines at 15 widely separated locations throughout the basin. Horizontal and vertical accuracies attained are roughly 30 and 70 cm, respectively. Geomorphic evidence suggests that the highest shoreline was occupied only briefly (probably less than 200 years) and radiocarbon dates on gastropod shells found in association with the shore deposits constrain the age to roughly 17 kyr. The basin-wide pattern of elevations of the highest shoreline is composed of two distinct signals: (27±1) m of hydro-isostatic deflection due to the lake load, and a planar tilt with east and north components of (6.8±0.4) 10−5 and (−5.3±0.3) 10−5. This rate of tilting is too high to be plausibly attributed to steady tectonism, and presumably reflects some unresolved combination of tectonism plus the effects of oceanic and lacustrine loads on a laterally heterogeneous substrate. The history of lake level fluctuations is still inadequately known to allow detailed inferences of crust and mantle rheology. However, it is already clear that the effective elastic plate thickness is closer to 40 km than the 60–70 km crustal thickness in the central Andes and the effective viscosity is less than 5 1020 Pa s.

Volcanic rocks from the Bolivian Altiplano: Insights into crustal structure, contamination, and magma genesis in the central Andes

Lavas from Quaternary minor volcanic centers behind the Central Andean volcanic arc in Bolivia form a compositional continuum with rocks from the arc front, extending to more primitive compositions. The lavas show petrographic and geochemical evidence for deep crustal contamination during differentiation. Sr, Nd, and Pb isotopic data from centers within 30 km of each other encompass the entire range found in the Central Volcanic zone of the Andes. The most mafic rocks are largely typical of arc magmas elsewhere, and may represent parental magmas to the porphyritic andesites and dacites erupted from stratovolcanoes along the arc to the west. Andean arc magmas therefore undergo considerable contamination during ascent through the thick crust. The only basalts lacking petrographic evidence for contamination are picrobasalts from Chiar Kkollu that form the most primitive end member of the compositional spectrum. These rocks resemble ocean-island basalts in some respects, indicating that variations in crustal contamination across the strike of the arc may be superimposed on variations in the mantle source composition.

Magmas in collision: Rethinking chemical zonation in silicic magmas: Comment and Reply: COMMENT

Eichelberger et al. suggest that chamber recharge should replace fractional crystallization as the dominant mechanism producing chemically heterogeneous eruptions. I find their arguments to be based on a subjective reading of the literature on zoned silicic magma systems. Among the flaws in their

Global change: Eruptions linked to El Niño

Statistical validation of a relationship between explosive volcanic eruptions and the El Niño/Southern Oscillation is a step forward in understanding the effects of such eruptions on climate.Statistical validation of a relationship between explosive volcanic eruptions and the El Nino/Southern Oscillation is a step forward in understanding the effects of such eruptions on climate.

Global change: Eruptions linked to El Ni|[ntilde]|o

Statistical validation of a relationship between explosive volcanic eruptions and the El Niño/Southern Oscillation is a step forward in understanding the effects of such eruptions on climate.Statistical validation of a relationship between explosive volcanic eruptions and the El Nino/Southern Oscillation is a step forward in understanding the effects of such eruptions on climate.

Eruptions linked to El Niño: Global change

Statistical validation of a relationship between explosive volcanic eruptions and the El Niño/Southern Oscillation is a step forward in understanding the effects of such eruptions on climate.Statistical validation of a relationship between explosive volcanic eruptions and the El Nino/Southern Oscillation is a step forward in understanding the effects of such eruptions on climate.