When volcanoes erupt, they may send out flows of scorching lava. They may explosively eject debris high into the air instead, or may eject a combination of flowing lava and exploding debris. Tephra is a volcanologist’s term for the solid materials ejected from volcanoes.
Viscosity is the opposite of fluidity; it is resistance to flow. Eruptions featuring lava flows happen when magma has low viscosity and the gases it contains are able to escape. When magma has high viscosity, though, gases are trapped. High volumes of gases held in magma by high pressures underground expand explosively as they rise towards the surface. They blast viscous lava into the ash, lapilli, blocks, and bombs of tephra.
Lingering in the atmosphere, small particles of tephra cause acid rain. Spread on the ground in successive layers, tephra fields predict the movement of ash clouds from future eruptions. Incorporated into sedimentary rocks like tuff, volcanic tephra retains information about a volcano’s geologic past.
Tephra in the atmosphere
Volcanic ash often has an acid coating. Incorporated into rainfall, it corrodes metal and pollutes surface and underground water supplies, sometimes killing plants and animals.
Particles of ash can hang in the atmosphere for years, coloring sunsets far from the eruption site as they cool the earth. The year without a summer in 1816 occurred because ashes from the 1815 eruption of the Tambora volcano in Indonesia circled the earth. In New England, farmers experienced frost in every month of summer.
Tephra on the surface
While volcanic ash from some eruptions circles the earth, tephra also takes heavier forms. Lapilli are small rounded volcanic stones that fall to earth while still somewhat molten. Volcanic blocks are larger, angular fragments that were solid when ejected. Bombs are rounded aerodynamic shapes formed of ejected molten rock.
Tephra fields give geologists clues to the probable movement of ash clouds in future eruptions. They are also used to map and analyze past eruptions.
Tuff is a sedimentary rock that includes igneous material. To a volcanologist, the tephra may be the most important component of a rock’s sediments. Water and pressure press loose tephra into rock in a process called cementation.
Tephra that melts together where it falls is called welded tuff. It contains chunks of material bonded to finer components. Agglomerates incorporate rounded volcanic bombs into rock. Volcanic breccias incorporate angular blocks.
The slow saga of the earth’s evolution is recorded in its rocks. Rocks containing tephra illuminate the explosive history of earth’s volcanoes.