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Volcanology (火山学)

Mount Usu / Sarobetsu post-mined peatland
From left: Crater basin in 1986 and 2006. Cottongrass / Daylily

[world, each volcano, volcanic materials (tephra, lava), volcanic geomorphology]

Volcanology (vulcanology)

The study of volcanoes, including lava, magma, and their related geological, geophysical and geochemical phenomena. Also, it includes volcano ecology (s.l.).

Plutonic theory

unstratifled rocks were formed by fusion in fire

Vesvio Volcano, (Rittmann, 1936)

Magma (マグマ)

A mixture of molten or semi-molten rock, volatiles and solids found beneath the surface of the earth
→ a complex fluid substance, of which temperature is high, i.e., ranging from 700°C to 1300°C, rarely becomingg 600°C and 1600°C
Fig. 3-3. Schematic illustration of the probable minimum percentage water solubility in granitic liquids formed during water-under- saturated melting processes together with suggested water-pressure isobars (Brown 1970)
The structure of volcanoes
estimated by magma movement

The rise of mantle


Fig. 3. Marsh's model illustrating generation of diapiric magma conduits and evenly spaced volcanic centers in an island arc (Marsh 1939)

[ the respective volcanoes ]

Volcanoes in the world (分布)





World volcanoes
Fig. Distribution of active volcanoes in the world
Concentration areas
1. hotspot (e.g., Hawaii)
2. island arc (ark)
3. great rift valley
[ volcanoes : the characteristics of eadh volcano or volcanic group]
Table 2. Classification and characteristic features of volcanic regions in the world based on the plate tectonics model (after Nakamura 1974; Miyashiro 1975; Ui 1975; Fujii 1975).
  • Region: Tectonic setting (Eruption rate km3/year) - Major volcanic rock series
  • Island arcs and continental margins: Consuming plate boundaries (subduction) (0.75) - Calc-alkalic andesite-rhyolite, tholeiitic basalt, alkalic rocks)
  • Oceanic ridges: Accreting plate boundaries (4-6) - Tholeiitic basalt (K2O < 0.4%, ΣFeO/MgO < 0.2)
  • Rifts: Intra-continental plate (segmentation) (0.006) - Alkalic basalt and differentiates
  • Marginal seas: Behind island arcs (0.1) - Tholeiitic basalt
  • Oceanic Islands and seamounts: Intra-oceanic plate (< 1) - Tholeiitic and alkalic basalt
  • Plateau basalts: Intra-continental plate (< 0.1) - Tholeiitic basalt
Table 1. Populations of volcano types and morphologic explosion indeces for each geological region in the world (Suzuki 1975). Data based on the list of world active volcanoes (Katsui 1971)
Volcano typeRegion
Island arcs
and continental
Rift and
Stratovolcano (+ caldera)4321971729504
Shieldvolcano (+ caldera)1462265098
Caldera volcano47121152
Monogenetic volcano221023845
Morphologic explosion index977782413885

Eruption (噴火)

Magmatic eruption (マグマ性噴火)

Phreatomagmatic explosion (マグマ水蒸気爆発)

Phreatic explosion (水蒸気爆発)

Eruption types (噴火様式)

Terrestrial volcanoes (陸上火山)

0. Defined by the present land form
not considering the factors and mechanisms → dead words
1. Named by the previously-erupted, famous volcanoes
easy to understand ↔ various classifications
Table 1. The types of eruptions and the examples. The classical terms derived from the specific volcanoes (Macdonald 1972) modified by the additions of pyroclastic flow and marine eruptions
Icelandic (basaltic flood) (Laki 1783, suggesting the volcanic activities on oceanic ridges)
Hawaiian (ハワイ式): high fluidity, less-gas lava

Hualalai, Kilauea, Lohoi, Mauna Loa (1942), Mahukona, Kahoolawe, Kohala, Mauna Kea, Maui, Oahu

Strombolian (ストロンボリ式): basalt, basaltic, andesite, lower viscosity than Hawaiian (relatively low), repeated small explosions with least gas

Stromboli, Mihara (三原山)

Vulcanian (ブルカノ式): large explosions with long period of dormancy

Vulcano 1888-90

Plinian (プリニー式): alkali laterite, foamed lava

Vesuvius AD 79

Eruption accompanying pyroclastic flow

Nuée ardente

Peleean type (プレー式) - Pelee 1902: lava dome, small-scaled pyroclastic flow
Merapi type (メラピ式) - Merapi 1930-31
St. Vincent type - Souiriere 1902

Intermediate type - Asama 1783
Pumice and ash flow (rhyolitic flood) - Koma 1929, Krakatau 1883, Katmai 1912

Phreatic eruption (Ultravulcanian) - Vulcano 1888 (initial phase)
Volcanic mudflow (火山泥流) - Kelut 1919
Subaqueous eruption

Shallow water: Hydromagmatic - Taal 1965

(occurrence of base-surge)
Surtseyan (basaltic) - Surtsey 1963

Sub-glacial - Grimsvotn
Deep sea - not observed

* exceptionally violent Vulcanian
2. Classified by the producitons of volcanic smoke, pumice, ect.
a) Combined with the sizes of volcanic ejecta, physical properties, etc.
Factors defining kinds of volcanic eruptions (Aramaki 1979)
  1. Physical properties of ejecta:
    a) gas, liquid or solid
    b) temperature
    c) viscosity
    d) size (and shape)
  2. Quantity of ejecta
  3. Explosivity:
    a) intensity (gas pressure)
    b) magnitude (kinetic energy)
    c) spatial configuration
  4. Rate of ejection (time sequence)
  5. Type of vent: central, lateral, or eccentric pipe or fissure
  6. Mode of transportation and emplacement: projection-free fall, gas jet, gravity flow (turbulent, laminar)
  7. Surface environment: subaerial, subaqueous (water depth), sub-glacial
b) physical properties of volcanic ejecta
i) Table 6. Types of volcanic eruption as defined by the kind of ejecta.
1. Gas emission from:

a. fumarole, solfatara, etc.
b. Open vent, crater

2. Emission of aqueous solutions from

a. hot and cold springs
b. temporary vent, crater

3. Emission of gas + solid (including semi-liquid) particles

a. gas + accessory and accidental fragments → phreatic
b. gas + essential (± accessory) fragments → Peleean, Vulcanian, Plinian
c. gas + liquid particles → Strombolian, fountaining

4. Emission of liquid magma (little gas)

a. lava lake
b. lava flow
c. endogenous lava dome
d. plug dome, spine

ii) The height of eruptions
Table 4. Classification of simple central volcanoes (Rittmann 1962)
Quality of magmaQuantity of magma producedTypes of activity
Small →→ Great
Fluid, very hot, basic
Individual lava flows (hornitos)Exogenous domes in flat countryIcelandic typeHawaiian typeEffusive

Viscosity, gas content and silica percentage
Cinder (scoria) cones, with lava flowsLava flows predominant stratovolcanoes Pyroclastic material predominantWith parasitic cinder cones and lateral lava streams

Loose pyroclastic cones with thick lava flowsRuptured dome with thick lava flowCinder cone with endogenous domeWith parasitic cinder cones and lateral lava streamsMixed

Viscous, relatively cool, acid
Maare, with thin covering of pyroclastsMaare with pyroclasitc ring-walls (ramparts)Craters with ring-wall and covering of pyroclastisCaldera with covering of pyroclasts

Extremely viscous with abundant crystalsGas Maare (diatremes)Explosion cratersExplosion calderaVolcano-tectonic sinksExplosive; gasnous only
Table 5. Classification of fissure volcanoes (Rittmann 1962)
Quality magmaQuality of magma producedType of activity
Very hot and fluid basicLava flows from fissures(Basalt) lava plateau; eruptive fissures mainly coveredEffusive
Lava flows with banks of scoriae along fissure(Basalt) lava plateau with rows of cinder cones and banks of scoriae
Hot, moderately fluid basicBanks of pyroclasts along fissure; lava flowsStratovolcanic ridges, with rows of cratersMixed
Viscous, cooler, intermediateEndogenous (pressure) ridges with banks of scoriae (pumice)Unknown
Extremely viscous, with abundant crystalsExplosion trenches with banks of scoriaeIgnimbrite sheets, often with volcano-tectonic sinksExplosive
Entries used for description of the kind of volcanic activity in the Catalogue of the Active Volcanoes (IAVCEI) (Simkin & Siebert 1994)
central crater, parasitic crater, radical fissure, regional fissure, lava lake, crater lake, normal explosion, nuee ardente, lava low, lava dome, spine, solfatara fields (vapors), phreatic explosion, mud eruption, mud flow, subglacial, submarine, islet formation, tidal wave (tsunami)
destruction of arable land, casualties

Conventional symbols used in the Catalogue and the Bulletin of Volcanic Eruptions: obtaining generalization by using symbols to express eruption types
Ex. ENTA: ○↑(m) ∞↑(m) →(m)
○: eruption in the central crater, ∞: eruption in a parasitic crater, ○: eruption in a radial fissure, =: eruption in a regional fissure, ↑: normal explosions, →: eruptions producing nuées ardentes (including ash flow, pumice flow, etc.), →: lava flows, etc.

Marine volcanoes (海洋火山)

marine volcano
Fig. 2. Water and K2O contents of deep-sea basalts from Hawaiian region. Revillagigedo Islands region, and Juan de Fuca Ridge. Symbols represent H2O+ content and right-hand end of line is total H2O content. (Moore 1970)
Marine eruptions rarely occur where the sea depth becomes more than 2000 m.
marine volcano
Fig. 3. Weight percent of F, Cl, P2O5, and H2O+ in deep-sea basalt from Revillagidedo Islands region, Hawaiian region, and Juan de Fuca Ridge. (Moore 1970)
Deep sea volcanism
Deep sea basaltic volcanism
→ fluidial magma

→ quiet (minimal lava-water interaction)

→ massive basalt, pillow lava (similar to pahoehoe)

→ viscous magma

→ hyaloclastic (strong lava-water interaction)

→ fragmentation and hydration of the lava

→ palagonite grains (smectites)
→ (gradual alteration on the ocean floor)
→ zeolites (phillipsite) smectites

→ chemical leaching from lava fragments

→ Fe-Mn oxides

Eruption scale (噴火階級)

Determined by intensity (強度) and magnitue (規模)
Volcanic explosivity index (火山爆発指数), VEI
ranging from 0 to 8
indicating a relative explosiveness of volcanic eruptions, based on volume of products, eruption cloud height, and qualitative observations

EVI8 = Yellowstone (640,000 BC), Toba (74,000 BC), Taupo (24,500 BC), La Garita Caldera (26.3 Ma)
EVI6 = Pinatubo (1991), Krakatoa (1883)
EVI5 = Mount St. Helens (1980)

→ differences in density and vesicularity (gas bubbling) of the volcanic products are not considered
Table IX. Intensity of volcanic activity, classified according to the volume of solid ejecta (Tsuya 1955). Types: A: essentially lava-flows, B: fragmentary ejecta only, D: essentially ejected old detritus. I: intensity. L: limit of volume of volcanic ejecta (km3)
Grade  Scale         Volcano                      Year  Ejecta (km3)
                                                        Type Volume

IX    > 100          Tambora, Soembawa            1815    B     150
VIII  100-10         Krakatau, Sunda Str.         1833    B      18
VII   10-1           Kljutschewskaja Sopka,       1820    A     3.7
                     Shtyubelya, Kamchatka        1907    B     3.0
                     Sakurajima, Japan            1914    AB  > 1.5
                     Bandaisan, Japan             1888    D     1.2
VI    1-0.1          Fujisan, Japan               1707    B     0.8
                     Taal, Luzon                  1911    D     0.5
                     Komagatake, Hokkaido, Japan  1929    B     0.1
                     Asamayama, Japan             1783    AB    0.2
                     Sakurajima, Japan            1946    A     0.1
V     0.1-0.01       Torishima, Japan             1939    AB   0.09
                                                  1902    B    0.03
                     Oshima, Izu, Japan           1950-51 A    0.03
                     Miyakezima, Izu, Japan       1940    AB   0.02
                     Lemongan, Java               1877    A    0.01
IV    0.01-0.001     Goentoer, Java               1843    B   0.008
                     Adatarasan, Japan            1900    B   0.004
                     Keloet, Java                 1901    B   0.002
III   0.001-0.0001   Azumasan, Japan              1893    B  0.0005
II    0.0001-0.00001 Oshima, Izu, Japan           1912    A 0.00002
                     Tokachidake, Japan           1926    D 0.00001
I     0.00001 >      Yakeyama, Niigata, Japan     1949    D   small
                     Kusatsu-Shiranesan, Japan    1932    D   small
0     0              Volcanoes displaying                 0       0
                     fumarolic activity only

(Walker 1973)

Dispersal index (D, 分散指数)
surface area covered by an ash or tephra fall, where the thickness is equal or more than 1/100 of the thickness of the fall at the vent
Dense-rock equivalent, DRE
estimating volcanic eruption volume

Table 5. Estimation of volcanic energies on Japanese and Indonesian volcanoes (Yokoyama 1957) = energy equivalent based on volcanic eruptions

volume (cc)

Total energy

volume (cc)**

Tambora19851 × 1017B8.4 × 1026271017VIIIΟ↑
Sakurajima19142.1 × 1015AB4.6 × 1025261016-1015VIIΟ-↑≈>⊗
Krakatoa18835 × 1015B< 1025251016-1015VIIΟΟΟ↑∧
Asama17834.5 × 1014AB8.8 × 1024251015-1014VIΟ↑≈>⊗†
Fuji17078.5 × 1014B7.1 × 1024251015-1014VIΟΟ↑⊗
Sakurajima19468.3 × 1013A2.1 × 1024241014-1013VΟΟ↑≈>
Mihara1777-83.4 × 1013A1.0 × 1024241014-1013VΟ↑≈>
Torishima19393.1 × 1013AB9.7 × 1023241014-1013VΟ↑≈>⊗
Mihara1950-13.0 × 1013A9.4 × 1023241014-1013VΟ↑≈>
Koma19295.1 × 1013B5.6 × 1023241014-1013VΟ↑
Miyakejima19401.9 × 1013AB4.8 × 1023241014-1013VΟΟ≈>↑Ο
Bandaisan18881.2 × 1015D< 1023231016-1015VIIΟ↑~>⊗†
Guntur18437.8 × 1012B6.5 ×1022231013-1012IVΟ↑⊗
Asama19352.2 × 1012B4.8 × 1022231013-1012IVΟ↑
Pematang Bata19332.1 × 1011D4.5 × 1022231015-1014VI
Una Una18982.2 × 1012B1.8 × 1022221013-1012VI↑~>⊗
Mihara19544 × 1011A1.3 × 1022221012-1011IIIΟ↑≈>
Adatara19001.1 × 1012B6.4 ×1021221013-1012IVΟ↑↑
Asama19381.5 × 1011B4.0 × 1021221012-1011IIIΟ↑
Azuma18935 × 1011B< 1021211012-1011IIIΟ↑†
Mihara19122 × 1010A6.3 × 1020211011-1010IIΟ↑≈>
Tokachi19262.7 × 1010DB2.8 × 1020201013-1012IVΟ↑~>⊗†
Myoozin reef1952?B< 101919??∧↑↑
Kusatsu-Shirane19321.6 × 1010D1.6 × 1018181011-1010IIΟ↑

Types of ejecta: A, essential lavaflow. B, fragmentary ejecta only. D, essentially ejected old detritus. *: Tsuya's intensity scale. **: volume of ejecta

Volcanic ejecta (火山噴出物)

1. volcanic gas (火山ガス)

Table 1.1. Classification of volcanic gas based on the temperature of fumaroles (Iwasaki, et al. 1966)

Type: temperature __ °C __ chemical components except water vapor
I ___ 1200-800 __HCl, SO2, CO2, H2 >> H2S, N2
IIA __ 800-100 __ HCl, SO2, H2S, CO2 >> N2, H2
IIB __ 800-100 __ SO2, H2S, CO2 >> N2 > HCl, H2
III ___ 100-60 ___ H2S, CO2 > N2 > SO2 >> H2
IV __ < 60 _____ CO2 > N2 > H2S

2. lava (溶岩)

molten rock (magma) formed of basalt, andesite, dacite, or rhyolite, rocks (in order of increasing silicate content)
Fig. 301. Idealized diagram illustrating the classification of lava surfaces (Jones 1943)
a. Pahoehoe lava (パホイホイ溶岩)
= ropy lava (縄状溶岩)
basalt forming smooth, glossy surfaces; low in silicates
b. A'a lava (or Aa) (アア溶岩)
basalt forming a jumble of irregular cinder blocks; low in silicates
c. Block lava ((塊状溶岩)
formed of large, smooth-sided blocks; may form domes; usually andesitic

Ex. Sakurajima (桜島)

Lava surface
Fig. 301. Idealized diagram illustrating the classification of lava surfaces (Jones 1943) Lava dome
Fig. 355. Diagram showing features produced by a lava flow overwhelming a forest.
Underwater lava (水中溶岩)
Underwater lava
Fig. 5.6. Diagrammatic cross section of a mass of pahoehoe toes (A) and pillow lava (B), showing characteristic structures. The dashed lines represent the inner margin of the glassy skin on the pillows, the stippled areas represent sedimentary material filling interstices between the pillows, and the black areas are open spaces. (Macdonald 1967) Underwater lava
Fig. 5.8. Cross section of a hyaloclasite flow formed in a lake in the Columbia River Plain of Oregon, showing bedding of the hyaloclasite and elongated pillow. (Macdonald 1972)
d. Domes (円頂丘)
viscous intrusions found in craters, commonly of dacite or rhyolite
A. Bulbous type________________B. Linear type
Fig. 323. Diagrams showing formation of "squeeze-ups". A. Bulbous squeeze-ups formed by successive extrusion of lava. When a bulbous mass is sealed off, the lava escape along one of the sides and forms another. B. Linear squeeze-ups formed along cracks in the crust of a flow. Squeeze-ups are small surface features generally less than a meter in height.
Lava dome
Fig. 329. Diagram showing formation of pressure ridge and its medial wedge. The crust of the flow is cracked by arching, whereupon molten lava forces its way upward between the tilted blocks. At one point the lava has broken through to the surface with enough mobility to flow a short distance down the slope of one of the tilted blocks.

3. Pyroclastic materials (火山砕屑物)

Table 1. Definitions of major types of volcanic materials. Materials which have little direct impact on volcanic succession (火山遷移) are not listed. (Francis 1993).

Bombs produced from
Oshima-Ooshima (collected
in September 1979) (at
Hakodate Observatory on
June 6 2015)

  • pyroclastic rocks: material ejected in solid fragments
    • pyroclastic flows (火砕流): an eruption cloud consisting of gas and very hot solids driven by gravity and hugging the ground; include surges and nuees ardentes (incandescent mix of ash and large materials); form deposits called ignimbrites
    • pyroclastic falls: any solid material returning to earth after eruption into the air, called tephra
      • ash (火山灰): tephra less than 4 mm in size (little stones
      • lapilli (ラピリ): tephra between 4 and 32 mm in size
      • blocks (bombs) (火山弾): tephra greater than 32 mm in size
      Ex. Mount Tokachi: westerlies → aerial differentiation
    • scoria (スコリア): not containing much glass → becoming rock from a basaltic lava (fragmented, cindery textured, whether from falls or flows of any sort)
    • pumice (軽石): contaning much silica and gas → frothy, low density pyroclastic rock deposited by flows or falls (becoming light and light-colored)
  • lahar (mudflow) (泥流): all water-transported debris flows, regardless of origin, with > 50% water (old definition)
    Fig. 7. Stress trajectories around a rock circle. Note the isotropic point (IP). Fig. 8. Open spaces (black) adjacent to an inclusion in partly welded tuff. Fig. 9. Folded foliation adjacent to an inclusion (Schminska & Swanson 1967).
  • debris flow: a large, wet mass of material falling under the force of gravity, with < 50% water (old definition)
  • avalanche: a mass of nearly dry material falling under the force of gravity
Table 7. Mode of transportation and emplacement of eruption products.
1. Gas

A. streaming

→ free atmospheric dispersal

2. Gas + solid (-liquid) particles

A. streaming (jet)

→ free fall (± wind)
→ turbulent, fragmental gravity flow (pyroclastic flow)

B. propulsion by gas explosion

→ free fall (± wind)
→ turbulent, fragmental gravity flow (pyroclastic flow)

3. Liquid (-semi-solid) A. viscous gravity flow (lava flow, lava dome)

B. gravity slope movement (talus, slop deposit)

4. Water + solid particles

A. gravity flow (mud flow, flood, subaqueous sedimentation)


Tephra (テフラ)

All air-fall pyroclastic materials produced by volcanic eruptions

An example on the classificaiton of tephra by particle size (mm in diameter)

Ash: < 2 (or 4)
Lapilli (volcanic cinders): 2 to 64 (or 4 to 32)
Volcanic blocks: > 64 (or > 32)

soil (土壌), tephra in Hokkaido (in JPN)

Lava (溶岩)

Molten rock expelled from volcanoes during eruptions

Lava flow (溶岩流)

A moving outpouring of lava, created during a non-explosive effusive eruption

Mishima lava (三島溶岩), lava flow on Rangitoto Island in NZ

Table. Classification of lava flows (Jones 1947). Ordered smooth surfaces to rough disordered surfaces

Weak loose structure to solid structure
Glazed surfaces found inside caves, and vents along with drip pendants and flow-lines Massive pahoehoe of very thick cross-section (10 + feet) with hummocks or tumuli, and with cracks at infrequent intervals Ropy lava (on surfaces, slabs and blocks) fine smooth ropes to coarse rough ropes (1 to 6 inches) formed by folding of viscous surfaces due to the drag of underlying mobile lava (grades to furroughed aa)
Scaly pahoehoe thin (one-quarter to one foot) small flow-units overlapping like scales, but solid, may show pillows and toes Aa-lava in place

Fine-aa: 0.1 to 1 cm, < 1/2 inch
Medium-aa: 1 to 10 cm, 1/2 to 4 inch
Gross-aa: 10 to 100 cm, 4 to 40 inch

Shelly pahoehoe thin bubbles of weak structure (one half foot) that break into slabs and plates of loose structure = slab-lava Aa-rubble of fragmental scoriaceous character, few fractured surfaces (grades from bubble to aa-clinker)
Block-lava usually four to five fractured sides (grades in size from one-half foot to 10 + feet)

[geography (地形)]

Volcanic geomorphology (火山地形学)

Ritman 1961 Rittman

Geological fissure eruption - ex. Iceland
Central eruption

d > h → dome
d < h → spine

Lava topography (溶岩地形)

Horst and Graben
Graben and Horst

References (* comprehensive textbook)

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  • Francis P. 1993. Volcanoes: a planetary perspective. Oxford University Press, Oxford, 443 pp.
  • Macdonald GA 1972. Volcanoes. Printice-Hall, Englewood Cliffs, NJ. pp 510*
  • Marsh BD, Carmichael ISE. 1974. Benioff zone magmatism. Journal of Geophysical Research 79: 1196-1206
  • Minakami T, Ishikawa T, Yagi K 1951. The 1944 eruptions of volcano Usu in Hokkaido, Japan. Bull. Volc., Ser. II 11: 45-157
  • Minato M, Hashimoto S, Fujiwara Y, Kumano S, Okada S. 1972. Stratigraphy of the Quaternary ash and pumiceous products in southwestern Hokkaido, N. Japan. Journal of Faculty of Science, Hokkaido University, Ser. IV 15: 697-736
  • Moore JG. 1970. Water content of basalt erupted on the ocean floor. Contributions to Mineralogy and Petrology 28: 272-279
  • Rittmann A 1960. Vulkane u., ihre Tatigkeit. (2nd edn.)*
  • Simkin T, Siebert L. 1994. Volcanoes of the World (2nd edn). Geoscience Press, Tucson
  • Tilling RI. 1989. Introduction and overview. Volcanic hazards (Tilling RI ed.) American Geophysical Union 1-8
  • Walker GPL. 1973. Lengths of lava flows. Phil. Trans. Roy. Soc. London A274: 107-118
  • Williams H 1954. Problem and progress in volcanology. QJGS, London
  • Williams H, McBirney AR 1979. Volcanology*