What is the process by which volcanic ash forms?

It begins as a typical afternoon near a volcano, with people going about their daily routines, exchanging jokes and working. Suddenly, everything changes. The ground trembles with a deafening eruption. Moments later, the sky turns dark, and thick clouds of volcanic ash descend, blanketing the area in a heavy layer of gray dust.

Although the ash may seem like an inconvenience, it’s important to recognize the incredible destructive force of a pyroclastic flow. This fast-moving mixture of ash, gas, and rocks can travel at speeds of up to 125 mph (200 kph). With temperatures ranging from 752 to 1,472 degrees F (400 to 800 degrees C), it can scorch anything in its path.

Archaeologists investigating the catastrophic eruption of Mount Vesuvius in 79 A.D. often found skulls with fractures among the victims of Pompeii, indicating a swift and violent demise. The intense heat of the pyroclastic flow had effectively boiled their brains, causing the pressure to fracture their skulls like fragile eggshells. Pyroclastic flows can inflict immense devastation, but they are only associated with highly explosive eruptions, unlike the nonexplosive eruptions typical of Hawaii.

Luckily, most people today are not faced with the civilization-wrecking, skull-shattering consequences of volcanic eruptions. With an enhanced understanding of how volcanoes work, we have learned to recognize warning signs, allowing us to prevent the widespread loss of life that occurred in previous centuries due to volcanic activity.

However, ash fall still poses a threat to large areas. Winds can carry these particles over vast distances, endangering plants and animals wherever they settle. After the 1980 eruption of Mount St. Helens in Washington, one of the largest in North America, ash spread all the way to Montana.

But how is volcanic ash created, and what becomes of it after the eruption? In this article, we'll delve into these questions and uncover the true nature of this material.

The Formation of Volcanic Ash

If you've ever watched a campfire, you may have noticed smoke and drifting particles of ash rising from the flames. These are the direct result of combustion: a rapid chemical reaction that generates both heat and light. In contrast, the smoke produced by an erupting volcano is made primarily of minuscule mineral particles, created by the explosive expulsion of gases.

Plate tectonics beneath the Earth's crust generate pockets of molten rock (known as magma). This magma is packed with pressurized gases. When the magma's pressure decreases or the gas pressure increases, the magma bursts out to the surface.

Volcanoes are akin to a shaken bottle of soda. When gas bubbles break free from the bottle, they often carry soda with them. In a volcanic eruption, escaping gases propel magma into the air. The force of the eruption shreds the magma into tiny pieces, similar to how a sneeze sends droplets of moisture into the air. These tiny magma fragments solidify while floating through the sky, forming volcanic ash. The flowing magma on the Earth's surface is referred to as lava.

Volcanic ash is made up of small, sharp-edged particles of rock, minerals, and volcanic glass. These fragments can vary in size, ranging from 0.00004 inches (0.001 mm) to 0.08 inches (2 mm), which is about the size of a grain of rice. Volcanoes may expel larger pieces, but these are categorized as cinders, blocks, or bombs. Volcanic ash is hard and rough, and its jagged particles give it an abrasive texture. The specific mineral composition of the ash depends on the minerals present in the magma.

After volcanic ash is released into the air, three primary factors determine how far it will travel before it falls back to Earth:

Some volcanic ash particles remain suspended in the air, where they interact with other dust particles to form condensation nuclei, which are sites where water vapor condenses to form clouds. In some cases, intense eruptions can add so much volcanic ash to the upper atmosphere that it can lower global temperatures by several degrees. For example, the 1883 eruption of Krakatoa caused a global temperature drop of 2.2 degrees F (1.2 degrees C) for a year [source: The Independent].

Is the ash falling from the sky?

The impact of volcanic ash falling back to Earth varies, from mild to catastrophic. The severity depends on the volume of ash ejected and the proximity to the eruption site. Ash can blanket an area for a few minutes or linger for days, coating everything with millions of tons of dense ash. The event can be unsettling, terrifying, and potentially fatal.

In large amounts, volcanic ash can be disastrous for the environment. Similar to how a tarp can suffocate grass underneath, a heavy ash fall can deprive plants of sunlight and oxygen, harming the ecosystem. It can also destroy microorganisms in the soil and break the branches of large trees, similar to the damage caused by an ice storm. Additionally, the ash may carry toxic chemicals from the eruption, and its acidity can alter the soil's composition, preventing certain plants from surviving.

Animals are not immune to the dangers of volcanic ash. Insects, especially those with pollen-collecting hairs, are hindered by the weight of the ash. Larger animals experience skin and eye irritation, and when the ash particles are small enough (less than 10 microns), they become respirable, causing respiratory issues. Livestock is particularly at risk, as the ash often contains harmful chemicals like fluoride, which not only coats the animals but also contaminates their food and water supply.

While volcanic ash causes significant immediate destruction, over time it can enhance the soil and ocean floors. This process can take anywhere from weeks to millennia, depending on the ash's composition. Eventually, these ash particles, originating from erupted magma, enter our ecosystems, supplying essential organic carbon and nitrogen to plants. Volcanic soils are also highly effective at retaining water, improving irrigation for vegetation.

Over time, large accumulations of ash and pyroclastic material may solidify into rock formations. Hot ash layers often fuse together to form welded tuff sheets. Much of the remaining material gradually integrates into the landscape, leaving traces of volcanic ash fall throughout the environment.

Resourceful humans have discovered various applications for volcanic ash, using it in ceramics, construction materials, industrial abrasives, and even in toothpaste.