Tephra is the collective term for all fragmented material ejected from a volcano during an explosive eruption. This unconsolidated debris consists of pulverized rock, mineral crystals, and volcanic glass shards. Tephra ranges from the finest dust to meter-sized boulders. Analyzing the distribution and characteristics of these deposits provides volcanologists with a record of a volcano’s past activity and helps assess potential hazards.
The Eruptive Origin of Tephra
Tephra is generated through the explosive fragmentation of magma and pre-existing rock, driven by the rapid expansion of gases. Magma contains dissolved volatile substances that remain dissolved under high pressure deep within the Earth. As magma ascends, the confining pressure decreases, causing these volatiles to form bubbles and expand dramatically. This internal pressure shatters the molten rock into fragments as it exits the vent, defining the magmatic eruption style.
An alternative process is a phreatomagmatic eruption, where magma interacts violently with external water sources. When molten rock meets water, the water instantly flashes to steam, creating a powerful thermal explosion that pulverizes the magma. The fragmented material is injected into the atmosphere, forming a towering eruption column, or plume. Wind then carries the smaller particles away from the volcano, determining the distribution pattern of the tephra fall.
Classifying Tephra by Particle Size
The geological classification of tephra is based on the size of the individual fragments, known as pyroclasts. This categorization helps predict how far the material will travel from the vent and the impact it will have upon landing.
Volcanic Ash and Lapilli
The smallest category is volcanic ash, defined as fragments measuring less than two millimeters in diameter. This material is composed of abrasive, pulverized rock and glass shards that can travel for thousands of kilometers. The next size category is lapilli, which includes fragments ranging from two to 64 millimeters in diameter. Lapilli can be composed of fresh pumice, scoria, or fragments of older rock, often accumulating near the vent.
Blocks and Bombs
Volcanic blocks and bombs are the largest fragments, both exceeding 64 millimeters in diameter. Blocks are angular pieces of solid rock torn from the volcano’s conduit walls. In contrast, volcanic bombs are ejected as semi-molten lava fragments that cool while spinning through the air, often giving them streamlined shapes. Due to their mass, blocks and bombs follow a ballistic trajectory and typically land within a few kilometers of the vent, posing a localized threat.
Immediate Environmental and Infrastructural Effects
The deposition of tephra, particularly the widespread ash component, creates immediate and significant hazards. A pressing concern is the impact on human and animal health, as fine ash particles are easily inhaled. The abrasive nature of the fragments can irritate the respiratory system and may contain crystalline silica, posing a long-term threat to lung health.
Accumulation of ash also causes considerable infrastructural damage. Even a relatively thin layer of wet ash can become extremely heavy, leading to the collapse of roofs. Transportation networks are severely disrupted. Fine airborne ash can be ingested by jet engines, causing engine failure and resulting in the immediate closure of airspace.
Ground transportation is also affected, as ash reduces visibility, clogs vehicle air filters, and makes roads hazardous. Electrical and water infrastructure is highly vulnerable. The fine particles contaminate water reservoirs and settle on power lines and insulators, especially when wet, leading to short circuits, power outages, and widespread disruption.