The Mount Sinabung eruption: Understanding volcanic jargon
This morning, the Mount Sinabung eruption shook Indonesia. When talking about calamities of this scale, it can help to understand the terms used.
[This originally appeared in The Focus]
Earlier this year, after 43 years of dormancy, the Philippine island of Luzon was shaken by the eruption of Taal Volcano. It was reported that its continuous eruption, over a period of only 5 hours, “generated a tall 10 to 15 kilometres steam-laden tephra column with frequent volcanic lightning that rained wet ash fall on the general north as far as Quezon City and Caloocan.”
This morning (August 10), according to CNA, Indonesia’s Mount Sinabung belched a “massive column of ash and smoke 5,000m into the air.” Authorities have so far reported zero casualties. However, residents have been alerted to the potential appearance of lava and lava flows.
Such events are calamitous for locals and dark curiosities for many of us living in distant, geologically safer lands. Volcanoes, the great givers of life, can also take it away. When reading about such disasters, it helps to understand the language used to describe them.
What is the difference between lava and magma?
Magma is molten rock, and exists beneath the Earth’s surface, in the crust. Lava is basically magma that has reached the surface through a volcano vent. So the short answer is that magma becomes lava once it reaches the surface.
To melt rock, one needs incredibly high temperatures, measured using a thermocouple, a metallic rod. Once the magma reaches the surface, it begins to crystallise and solidify as it loses heat. Lava flows slowly, because it is so viscous. You can outrun it. But if it catches up to you, you’d better be ready to move.
Volcanic lightning — apocalyptic displays
Sometimes you will read about, and see pictures of, volcanic lightning. This really is what it sounds like, and is quite awesome. Japan boasts some of the most spectacular examples of volcanic lightning.
Such electric displays occur when there is an abundance of very fine particles in the air. It is thought that the friction between particles and gaseous molecules results in potential differences and a build-up of electrical energy. Bolts travel in any and all directions: “broad bolts, St. Elmo’s fire (ball lightning) […] separate small sparks, [and] branching displays.” It may appear apocalyptic, but generally volcanic lightning is more anxiety-inducing than dangerous.
Solid materials — tephra and ash
All explosive volcanic eruptions generate tephra. According to the US Geological Survey website, tephra is composed of “fragments of rock that are produced when magma or rock is explosively ejected.” This ejected material varies greatly in size.
“The largest fragments, blocks and bombs (>64 mm, 2.5 inches diameter), can be expelled with great force but are deposited near the eruptive vent.” Smaller tephra fragments “can be carried upward within in a volcanic plume and downwind in a volcanic cloud.” Most of these fall to the ground as the eruption cloud cools.
The smallest pieces of ejected solid material, for example rock, mineral and glass particles less than 2mm in diameter, can float for days, weeks or even months on convection currents. This constitutes ash which, if it falls, is called ash fall (sometimes written as ‘ashfall’).
This is the real killer — the stuff that lingers, grounds planes for weeks at a time, blankets crops and roads in a thin film of black soot, and covers large areas for extended periods of time. Aviation and agriculture suffer tremendously from the effects of ash fall.
Fissure vents
These holes in the ground go by several names: fissure vents, volcanic or eruption fissures, or simply fissures. When underground magma finds its way into a small crack in the Earth’s crust, it can lead to the creation of larger fractures, or fissures. If there is sufficient pressure behind the magma, the fracture expands and reaches the surface.
The tensile pressure in this instance forms a linear volcanic vent, through which the magma flows. Lava erupts, and you’ve got yourself a volcano.
