Fuego
Fuego (fire in Spanish) has been the most active volcano in Guatemala in historic time. Its eruptions are commonly brief and highly explosive, with an eruption column maintained for several hours (Plinian style). Fuego also has less explosive eruptions and lava flows. A period of robust eruptions began in 1932, just before the last major thrust earthquake in this area (1940, M8.2). Eruptions ceased in 1977 just after the Motagua-Mixco earthquake of 1976 (M7.3). This earthquake occurred on the Caribbean-North American Plate boundary. Fuego may be a sensitive monitor of strain changes associated with major earthquakes. The volcano appears to be in a near critical state (ready to erupt) and changes in the strain field cause waxing and waning activity at Fuego.
The Plinian eruption in 1974 was small (by Plinian standards) and basaltic, rather than silicic, but the eruption had the essential Plinian characteristics: a long duration (hours) of continuous, gas-streaming eruption and a convectively rising ash cloud. The height of such ash clouds is roughly proportional to mass eruption rate or rate of heat (Q) release or dQ/dt.
Plinian columns develop when a volatile rich magma is tapped. Gas (dominated by H2O in Central America) exsolves from rising magma as magma cracks its way to the surface. The gas nucleates as little bubbles that then grow. As the bubbles grow, the magma gets less dense, accellerating its rise. Near the surface, the bubbles are packed tight together and possibly overpressured, making an explosive froth. At contact with the atmosphere, the froth shatters into a variety of sizes. Hunks of froth make pumice or, more formally, pumice lapilli.. Most froth breaks into ash, fragments of bubble wall, usually planar bits or angular fragments that look like parts of a honeycomb cell.
Explosively expanding gases thrust the froth fragments into the atmosphere, where the tiny, red hot ash particles efficiently exchange heat with air. The hot mixture is so expanded that it is less dense than air and it rises. Air is drawn in from all around the plume. As it rushes in, it is deflected left by the rotation of the earth (Coriolus force) and the rising column develops a corkscrew appearance.
Later in the eruption, magma from deeper in the chamber gets to the vent and the explosiveness of the eruption decreases and this is bad news. The magma from deeper in the chamber has less water and other gasses. Therefore, fewer bubbles and denser froth (the bubbles are loosely packed). The low gas content and denser froth lead to less explosiveness and less efficient framentation (more big pieces). This leads to less mixing with the atmosphere and slower transfer of heat. At some point, the ash-air mixture becomes heavier than air and column collapse occurs. The result is a ground hugging pyroclastric flow, a turbulent, fluidized bed, accellerated by gravity to speeds of 100 or more km/hr. These eruption clouds have a fairly dense base or core. Jetting in front is a hurricane cloud or surge. The surge does most of the killing as it arrives first and has greatest extent. It leaves a thin deposit of well-sorted, cross-bedded, sand-size debris, often mostly mineral fragments. The pyroclastic flow deposit is thicker and has minimal sorting and layering.