Carlos Borrero

Perturbation and melting of snow and ice by the 13 November 1985 eruption of Nevado del Ruiz, Colombia, and consequent mobilization, flow and deposition of lahars

A complex sequence of pyroclastic flows and surges erupted by Nevado del Ruiz volcano on 13 November 1985 interacted with snow and ice on the summit ice cap to trigger catastrophic lahars (volcanic debris flows), which killed more than 23,000 people living at or beyond the base of the volcano. The rapid transfer of heat from the hot eruptive products to about 10 km2 of the snowpack, combined with seismic shaking, produced large volumes of meltwater that flowed downslope, liquefied some of the new volcanic deposits, and generated avalanches of saturated snow, ice and rock debris within minutes of the 21:08 (local time) eruption. About 2 × 107 m3 of water was discharged into the upper reaches of the Molinos, Nereidas, Guali, Azufrado and Lagunillas valleys, where rapid entrainment of valley-fill sediment transformed the dilute flows and avalanches to debris flows. Computed mean velocities of the lahars at peak flow ranged up to 17 m s−1. Flows were rapid in the steep, narrow upper canyons and slowed with distance away from the volcano as flow depth and channel slope diminished. Computed peak discharges ranged up to 48,000 m3 s−1 and were greatest in reaches 10 to 20 km downstream from the summit. A total of about 9 × 107 m3 of lahar slurry was transported to depositional areas up to 104 km from the source area. Initial volumes of individual lahars increased up to 4 times with distance away from the summit. The sedimentology and stratigraphy of the lahar deposits provide compelling evidence that: (1) multiple initial meltwater pulses tended to coalesce into single flood waves; (2) lahars remained fully developed debris flows until they reached confluences with major rivers; and (3) debris-flow slurry composition and rheology varied to produce gradationally density-stratified flows. Key lessons and reminders from the 1985 Nevado del Ruiz volcanic eruption are: (1) catastrophic lahars can be generated on ice- and snow-capped volcanoes by relatively small eruptions; (2) the surface area of snow on an ice cap can be more critical than total ice volume when considering lahar potential; (3) placement of hot rock debris on snow is insufficient to generate lahars; the two materials must be mechanically mixed together for sufficiently rapid head transfer; (4) lahars can increase their volumes significantly by entrainment of water and eroded sediment; and (5) valley-confined lahars can maintain relatively high velocities and can have catastrophic impacts as far as 100 km downstream.

Hetu Sheth: A Photographic Atlas of Flood Basalt Volcanism

Hetu Sheth, in his book A Photographic Atlas of Flood Basalt Volcanism (Springer International Publishing, 2018), continues the tradition of well-illustrated geology books, featuring 583 impressive photos with descriptive captions of geological formations from outcrop to landscape scale. Sheth’s Atlas boasts the additional virtue of resulting from a vast collaboration involving 89 researchers and non-geologists from 23 countries, who are listed at the beginning of the book. A Photographic Atlas of Flood Basalt Volcanism is a rich, colorful text, with photographs describing and illustrating, in considerable detail, the volcanological features of the main flood basalt provinces from small to large scale. These photographs also highlight geomorphologic, climatic, environmental, historical, archeological, and geographical aspects in some of these various scenarios around the world. Selected references for the illustrated features or landscapes are also included in the captions. The book is divided into 12 chapters, each one with its own overview. At the end, there is a glossary, a list of references with recommended websites, a flood basalt index with the location of the photographs by province, and an author index. In Chapter 1, the author introduces the framework of Continental Flood basalts and Oceanic Plateaus from a global perspective and summarizes the flood basalt literature. The author also discusses the reason for compiling a photographic field atlas. In Chapter 2, Flood Basalt Landscapes, the overview discusses the effect of climate on the development of the main provinces’ landscapes. Forty-seven photographs show exceptional flood basalt landscapes, which have mainly been sculpted by the climate, e.g., extreme temperatures, or other natural forces. Chapter 2 also illustrates erosional cliffs at sea level and impressive gorges related to glacial and riverine modeling as well as some landscapes whose geomorphological origins are unknown. Chapter 3, Morphology and Architecture of Flood Basalt Lava Flows and Sequences, discusses the significance of scale in terms of the voluminous nature of the lava flows and sequences. Eighty-eight photographs illustrate Bthe world^ of pāhoehoe lavas with lava morphotypes, inner and external structures, textures, and surfaces of compound and single Flood Basalt lava flows. The stratigraphy is shown in vertical sections with wonderful stacked lava flows. Editorial responsibility: K. Németh

Stratigraphic Transect of Northwestern Colombia: a Key to Understanding the Origin of the Panamanian Isthmus

We present new data on the stratigraphy of the Miocene in northwestern Colombia, in the Pacific Basin. The sedimentological and biostratigraphic study, based on an analysis of calcareous nannofossils, foraminifera, palynomorphs, and diatoms, has enabled a new framework to be constructed that allows the closing process of the Isthmus of Panama to be monitored and palaeoceanographic implications to be made.