Ash: What is Volcanic Ash

Volcanic ash is a fine-grained material composed of rock, mineral, and volcanic glass fragments produced during explosive volcanic eruptions. It is typically less than 2 millimeters in diameter, but can range in size from microscopic particles to pebbles. Volcanic ash is often light and gray in color, but can also be black, red, or brown depending on its composition.

Volcanic Ash

Tiny particles of jagged rock, minerals, and volcanic glass, created during volcanic eruptions, characterized by their small size, being less than 2 mm (0.08 inches) in diameter. These particles are lightweight, can travel long distances on wind currents, and pose various hazards to aviation, health, agriculture, and infrastructure.

A plume of volcanic ash rises from the erupting Mount Pinatubo in 1991.

Formation of Volcanic Ash

Volcanic ash is formed when dissolved gases in magma expand and escape violently into the atmosphere. The force of the gases shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. The smaller the particles, the farther they can be carried by wind. Ash clouds can travel hundreds or even thousands of kilometers from their source, and can have a significant impact on the environment.

The term volcanic ash is also often loosely used to refer to all explosive eruption products (correctly referred to as tephra), including particles larger than 2mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere.

The force of the escaping gas shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometers away.

Volcanic ash is formed during explosive volcanic eruptions, phreatomagmatic eruptions and during transport in pyroclastic density currents.

Explosive eruptions occur when magma decompresses as it rises, allowing dissolved volatiles (dominantly water and carbon dioxide) to exsolve into gas bubbles. As more bubbles nucleate a foam is produced, which decreases the density of the magma, accelerating it up the conduit. Fragmentation occurs when bubbles occupy ~70-80 vol% of the erupting mixture.

When fragmentation occurs, violently expanding bubbles tear the magma apart into fragments which are ejected into the atmosphere where they solidify into ash particles. Fragmentation is a very efficient process of ash formation and is capable of generating very fine ash even without the addition of water.

Tuff, a type of rock formed from compressed volcanic ash

Summary Formation of Volcanic Ash

Volcanic ash forms during explosive eruptions, where magma is violently fragmented. The formation process includes:

Gas Expansion: Dissolved gases in magma expand as pressure decreases near the surface.
Fragmentation: Rapid expansion shatters the magma into tiny pieces.
Ejection: These fragments are expelled into the atmosphere along with gases and steam.
Solidification: The particles cool rapidly in the air, becoming solidified glass or crystal fragments.

Ash plumes can rise tens of kilometers into the atmosphere, where prevailing winds can transport them great distances.

Composition of Volcanic Ash

Volcanic ash is composed primarily of:

Silicate minerals: These minerals are abundant in volcanic material and are mostly composed of silica (SiO₂), including minerals such as quartz, feldspar, and mica.

Volcanic glass shards: These form when the molten lava cools and solidifies rapidly during the eruption.

Crystalline fragments: Tiny fragments of pre-existing rocks and crystals, such as feldspar, pyroxene, or olivine, can also be found in volcanic ash.

Gases: Volcanic ash particles may carry absorbed gases like sulfur dioxide, carbon dioxide, and hydrogen chloride, which can cause additional environmental and health effects.

A street covered in a layer of volcanic ash

Volcanic Ash Properties

Particle size: Volcanic ash particles are typically smaller than 2 mm in diameter, but they can be as fine as 0.001 mm. The fine nature of ash allows it to stay airborne for extended periods.

Color: Ash can vary in color from white to grey, brown, or black, depending on the composition of the magma and the eruption conditions.

Texture: It is gritty and abrasive, and the tiny glass fragments can scratch surfaces, damage machinery, and irritate the skin, eyes, and respiratory system.

Volcanic ash can range in color from light gray to dark brown or black.

Hazards

Volcanic ash poses significant risks to health, infrastructure, aviation, and the environment:

Health effects: Inhaling volcanic ash can lead to respiratory issues, eye irritation, and skin problems. People with pre-existing respiratory conditions, such as asthma, are particularly vulnerable.

Damage to infrastructure: Ash accumulation can cause roofs to collapse due to the added weight, clog drainage systems, and damage power lines and water supplies.

Impact on aviation: Ash clouds are a serious hazard for aircraft. Volcanic ash can damage jet engines, causing engine failure, and reduce visibility for pilots.

Environmental effects: Ashfall can cover large areas, affecting agriculture by damaging crops and contaminating water sources. It can also alter ecosystems, especially if large amounts of ash enter waterways.

Examples of Significant Ash Events

Mount St. Helens (1980): The eruption in Washington, USA, sent ash into the stratosphere, affecting areas as far away as the central United States.

Eyjafjallajökull (2010): The Icelandic volcano's ash cloud caused widespread disruption to air travel across Europe for several weeks.

Mount Pinatubo (1991): The eruption in the Philippines generated a massive ash cloud that spread globally, impacting climate by lowering temperatures.