Volcanic ash: What it is and how it forms

Conference · Fri Sep 13 04:00:00 EDT 1991

OSTI ID:6138320

Heiken, G

There are four basic eruption processes that produce volcanic ash: (1) decompression of rising magma, gas bubble growth, and fragmentation of the foamy magma in the volcanic vent (magmatic), (2) explosive mixing of magma with ground or surface water (hydrovolcanic), (3) fragmentation of country rock during rapid expansion of steam and/or hot water (phreatic), and (4) breakup of lava fragments during rapid transport from the vent. Variations in eruption style and the characteristics of volcanic ashes produced during explosive eruptions depend on many factors, including magmatic temperature, gas content, viscosity and crystal content of the magma before eruption, the ratio of magma to ground or surface water, and physical properties of the rock enclosing the vent. Volcanic ash is composed of rock and mineral fragments, and glass shards, which is less than 2 mm in diameter. Glass shard shapes and sizes depend upon size and shape of gas bubbles present within the magma immediately before eruption and the processes responsible for fragmentation of the magma. Shards range from slightly curved, thin glass plates, which were broken from large, thin-walled spherical bubble walls, to hollow needles broken from pumiceous melts containing gas bubbles stretched by magma flow within the volcanic vent. Pumice fragments make up the coarser-grained portions of the glass fraction. Particle sizes range from meters for large blocks expelled near the volcanic vent to nanometers for fine ash and aerosol droplets within well-dispersed eruption plumes. 18 refs., 6 figs., 1 tab.

• Research Organization: Los Alamos National Lab., NM (United States)
• Sponsoring Organization: DOE; USDOE, Washington, DC (United States)
• DOE Contract Number: W-7405-ENG-36
• OSTI ID: 6138320
• Report Number(s): LA-UR-91-3267; CONF-9107185--1; ON: DE92002451
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
540210 -- Environment
Terrestrial-- Basic Studies-- (1990-)
58 GEOSCIENCES
580000* -- Geosciences
ASHES
COMBUSTION PRODUCTS
ERUPTION
FRAGMENTATION
GEOLOGY
GEOMORPHOLOGY
GEOTHERMAL SYSTEMS
GROUND WATER
Geothermal Legacy
HYDROGEN COMPOUNDS
IGNEOUS ROCKS
LAVA
MAGMA
MAGMA SYSTEMS
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
PUMICE
RESIDUES
ROCKS
VOLCANIC ROCKS
VOLCANISM
VOLCANOES
WATER