Glaciology - Glaciovolcanic Hazards

Describe the main glacier-related hazards and their processes in Parque Nacional los Nevado, Colombia.

Fiona Fang, Trinity Hall, 03/2025, 1473 words

Parque Nacional Natural Los Nevados is a high-altitude protected area in the Colombian Andes, home to a chain of glaciated stratovolcanoes, including Nevado del Ruiz, Nevado del Tolima, and Nevado de Santa Isabel. The interaction between volcanic activity and glacier dynamics drives some of the region’s most significant natural hazards, the most destructive being lahars—fast-moving mudflows composed of pyroclastic material, rocky debris, and water. This was tragically demonstrated by the 1985 Nevado del Ruiz eruption (VEI = 3), where lahars engulfed the town of Armero, causing approximately 23,000 fatalities (Naranjo et al., 1986). The ideal conditions for lahar formation and long-distance transport include (1) a substantial water source; (2) an abundance of loose debris and sediment; and (3) steep topography such as gorges (Lowe et al., 1986), which will be discussed in this essay. I will also assess how different volcano-glacier interactions (eg tephra, lava flow, geothermal) contribute to lahar development, with a primary focus on the 1985 Nevado del Ruiz event, yet the processes discussed apply broadly to similar stratovolcanoes in the region. At the end of the essay, I will highlight that although lahars in this region are primarily triggered by volcanic activity, non-volcanic processes—including lake outbursts, heavy rainfall, and earthquakes—can also initiate or compounds mass movements. Given the shrinking glaciers and increasing populations in this region, understanding and analyzing these hazards and their underlying processes is critical for future risk assessment and mitigation.

To generate a lahar hazard, a significant volume of meltwater must be available. Major and Newhall (1989) identified three main mechanisms by which eruptions can generate glacial meltwater. The first is tephra deposition onto ice and snow. For the 1985 Nevado del Ruiz eruption, however, this contribution was likely minor due to the limited tephra volume and small glacier surface area (20 km²) (Naranjo et al., 1986). The second mechanism is basal melting from subglacial eruptions or geothermal activity. For instance, a hydrothermal reservoir significantly contributed meltwater during the 2007 Nevado del Huila eruption. In comparison, although Nevado del Ruiz has similar conditions, no direct evidence suggests this process played a major role in the 1985 lahar (Naranjo et al., 1986). The third mechanism is surface melting due to lava flows, molten rock outpourings from a vent during an effusive volcanic eruption. This process may have occured on Nevado de Santa Isabel, where evidence of lava flows were found and dated to 2800, 5500, 6750, and 7450 years BP (Méndez, 1997). However, its impact is proposed to be limited because (a) heat transfer efficiency is low, (b) melt extent depends on lava effusion rates, (c) melting may not occur if the glacier is insulated by snow or debris (Barr et al., 2018), and (d) lava flows on ice travel only 500–700m, and therefore unlikely to generate significant avalanches or meltwater (Huggel et al., 2017).

The fourth mechanism is the mechanical erosion and melting by pyroclastic density currents. This is the mechanism that has contributed to the most severe hazards in this area and therefore warrant significant concern. Pyroclastic density currents are hot, gravity-driven mixtures of volcanic debris and gas that move rapidly downslope (Druitt,). These currents can reach considerable speeds (30–40 m/s; Myers and Brantley, 1995) and transport hot debris over large distances (Branney and Kokelaar, 2002), and therefore cause significant ice loss through melting, abrasion, and erosion (Julio-Miranda et al., 2005; Waythomas et al., 2013). During the 1985 Nevado del Ruiz eruption, the pyroclastic density currents were small but capable for melting and entraining snow and ice, leaving numerous grooves and channels in the snow and firn, as well as pressions up to 2 m in diameter and depth over the glacier (Thouret, 1990). Steep ice surfaces were smoothed by abrasion. Toothed sérac fields were flattened. In the steeper margins of the glacier where there are topographically confined sections of 1998ice, or cross steep and fractured icefalls, the pyroclastic flow can be even more destructive. In Nereidas, Azufrado and Lagunillas Glaciers, they generated gullies over 100 m long and 1-4 m wide (Thouret, 1990; Pierson et al., 1990). Pyroclastic density currents also triggered ice and rock avalanches that led to major ice losses from glaciers in the Azufrado, Lagunillas and Farallon-Guali basins, destroying the 10–15 m thick crevassed terminus of Lagunillas Glacier and the hanging glaciers on the headwall of the Azufrado valley (Pierson et al., 1990). Altogether, ~ 10% of the ice volume (0.06 km3) was removed during the 1985 event (Huggel et al. 2007; Granados et al., 2021), which significantly contribute to the formation of lahars as discussed below. This demonstrates that a single moderate volcanic eruption can have an appreciable impact on the mass balance of ice on Andean stratovolcanoes, especially given low ice accumulation rates on tropical glaciers (Ceballos et al. 2006). This means that while glaciers are shrinking under climate change, a larger eruption could still generate significant meltwater and even more destructive lahars (Huggel et al., 2017). Furthermore, research suggests that glacier retreat may increase explosive volcanic activity (Barr et al., 2018), making this hazard even more pressing.

Sediment and topography also play crucial role in driving lahar hazards. Meltwater scours loose sediment along the steep valleys it traverses (Lowe et al., 1985). As sediment concentration increases, so does the yield strength of the flow. Once it reaches a critical threshold where large particles remain suspended indefinitely, the hyperconcentrated flow transitions into a debris flow (Pierson, 2005). During the 1985 Nevado del Ruiz eruption, solid material made up 35–65% of the lahar’s volume, with a total flow volume of 90 × 10⁶ m³ and mean peak velocities ranging from 5 to 15 m/s (Thouret, 1990). In addition, the distribution and destructive potential of lahars were strongly controlled by topography: Lahars were particularly erosive when confined within canyons and steep valleys, cutting down to bedrock and leaving a trimline 20–30 meters above normal river level (Naranjo et al., 1986). Four primary lahar pathways were observed (Figure 1). The two most destructive lahars flowed eastward down the Río Lagunilla and its tributary, the Río Azufrado, toward Armero; a third moved northeast down the Río Gualí toward Mariquita; and a fourth flowed northwest via the Río Molinos into the Río Claro and then the Río Chinchiná toward Chinchiná (Lowe et al., 1986). The Lagunillas-Azufrado lahar maintained an average velocity of 30 km/h and a thickness of 5 meters when it reached Armero, 70 km from the volcano, engulfing the entire town and causing 23,000 fatalities (Naranjo et al., 1986). Many victims suffered chemical burns, not from heat, but due to the high acidity of the lahar, particularly its elevated sulfate content (Lowe et al., 1986).

Figure 1. Distribution of tephra fall and lahar deposits from the 13 November 1985 eruption of the Nevado del Ruiz volcano. Lahars are black. Isopachs of the tephra fall are in millimeters and are shown by dashed and solid lines (source: Naranjo et al., 1986)

Notably, non-volcanic processes can also initiate or compounds mass movements and should be considered in hazard assessments. The 1845 Nevado del Ruiz lahar, with a significant volume of 300 million m³, is believed to have been enlarged by a flood triggered by the breach of a dam formed by a contemporaneous debris avalanche (Thouret et al., 1990). Meanwhile, earthquakes are frequent in this region and can trigger mass movements, particularly when combined with heavy rainfall. In 1994, a magnitude 6.4 earthquake (Richter scale) struck Nevado del Huila following a period of intense rainfall, triggering 3,000 shallow landslides and debris avalanches. These coalesced into a ~320 million m³ debris flow that descended the Páez River, reaching inundation heights of 40 m and maximum velocities of 25 m/s. The flow traveled over 150 km, causing 1,000 fatalities and widespread destruction (Scott et al., 2001; Pulgarín, 2003). Although Nevado del Huila lies outside Parque Nacional Natural Los Nevados, it serves as a warning for other stratovolcanoes in the park, particularly Nevado del Tolima, which features a highly crevassed structure and steep glacier outlet slopes (Huggel et al., 2017).

To summarize, this essay focuses on lahars, the most destructive glacier-related hazard in Parque Nacional Natural Los Nevados. It illustrate that pyroclastic density currents contribute significantly to the generation of meltwater, particularly compared to other processes such as tephra fallout and lava flow. Sediment abundance and geomorphology also key control factors on the features of lahars. These lahars can lead to devastating damages to local communities. Beyond volcanic activity, non-volcanic triggers—including lake outbursts, rainfall, and earthquakes—can amplify lahars or initiate similar mass movements. Given the ongoing retreat of glaciers and rising populations in downstream communities, future eruptions could result in even more severe impacts, highlighting the need for integrated hazard monitoring and risk mitigation strategies. 

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