Basaltic-volcano systems GEORGE P. L. WALKER Hawaii Center for Volcanology, Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA Abstract: This review and personal account of basaltic volcanism concentrates on the geological aspects and physical controls, unlike most others which concentrate on geo­ chemistry. It serves as an update to reviews by Wentworth & Macdonald 20 to 40 years ago. The concept of volcanic systems is developed, a system including the plumbing, intrusions, and other accoutrements of volcanism, as well as the volcanic edifice. Five types of systems are described, namely lava-shield volcanoes, stratovolcanoes, flood-basalt fields, mono­ genetic-volcano fields, and central volcanoes (having silicic volcanics as well as basaltic). It is postulated that the system-type depends on (a) the magma-input rate and (b) the frequency with which magma-batches enter the system (or modulation frequency). These controls determine whether a hot pathway is maintained from the source to the high-level magma chamber, and hence if eruptions are concentrated in the central-vent system. Reviews are given on many aspects: the different kinds of rift zones that volcanoes exhibit, and their controls; the mostly small intrusions, including coherent-intrusion complexes, that occur in and under the volcanic edifices; the great sill swarms that are an alternative to flood basalts; the origin of the craters and calderas of basaltic volcanoes; high-level magma chambers, their locations, and cumulate prisms; the positive Bouguer gravity anomalies that are attributed to cumulates and intrusion complexes; the structures of basaltic lava flows; the characteristics ofbasaltic explosive volcanism and the consequences ofparticipation by non­ volcanic water; the products of underwater volcanism; the origin and significance ofjoints, formed either by contraction or expansion, in volcanic rocks; and the distribution ofvesicles in basaltic rocks. A fairly extensive bibliography is included. This is in part a review; it takes stock of changes Basaltic magma is derived by incongruent par­ in concepts since the publications by Wentworth tial melting of mantle peridotite, favoured in & Macdonald (1953) and Macdonald (1967), tectonic settings (e.g. hotspots and rifts) where which were models of clear description of basal­ mantle rock rises adiabatically to relatively shal­ tic volcanics, and since the publication 12 years low levels, or in subduction-zone settings where ago of the landmark Basaltic Volcanism Study volatiles decrease the melting temperature of Project (1981). It also incorporates new ideas, mantle rock. new interpretations, and new ways of looking at Magma properties are basic parameters in the subject. The scope of basaltic volcanism is volcanology: too broad to cover all aspects, and petrology and • magma density relative to lithosphere density geochemistry are specifically omitted. makes volcanism possible and helps deter­ More than half of the world's volcanoes are mine the positions of magma chambers and basaltic or include basalt among their products; intrusions; and about one third of known eruptions, involv­ ing about 20 volcanoes per year erupt basaltic • viscosity and yield strength determine the geo­ metry and structures of lava flows and in­ magma. Basaltic volcanoes occur in all tectonic trusions; settings. Basaltic volcanism is associated with both divergent and convergent plate boundaries; • gas content promotes eruptions and deter­ mines their explosivity; and it characterizes spreading ridges mostly con­ cealed beneath the ocean; it characterizes hot­ • gas content combined with viscosity and rheology controls the explosive violence of spot volcanism which, in oceanic settings, is eruptions by determining the ease with which almost exclusively basaltic and, in continental gases escape from magmas. settings, is commonly bimodal (basaltic plus rhyolitic); and it is widespread associated with Little mention is made of these basic magma andesitic and more silicic magmas in subduc­ properties, because their values tend to be rather tion-zone settings, particularly where oceanic uniform amongst different basaltic volcanoes. lithosphere is subducted below oceanic or thin They are, thus, the unifying features of basaltic continental lithosphere, as in island arcs (e.g. the volcanism. More attention is therefore directed Mariana and Kurile arcs). at the influence of non-magmatic variables (e.g. From Prichard, H. M., Alabaster, T., Harris, N. B. W. & Neary, C. R. (eds), 1993, 3 Magmatic Processes and Plate Tectonics, Geological Society Special Publication No. 76, 3-38. G. P. L. WALKER (d) MOST VENTS OCCUR WITHIN THIS AREA PC 50 km Flood­ basalt field (e) cinder cone \;~ Central volcano (b) ~'O"6 .-, (c) 3 kg/s HIGH ,km.;:.3.:.../Y"-- k_m_+'yl/year 5 10 DECCAN FB LS KILAUEA LS W MAUNA LOA LS ETNA SV VESUVIUS SV TAUPO CV 2 3 10 IZU.QSHIMA SV 10- C HARRAT RAHAT FB MV FB AUCKLAND MV LOW -1110.000 10-1 HONOLULU MV LOW TIme-averaged HIGH years output rate Fig. 1. Volcano types and volcano systems. (a) The five types of basaltic-volcano systems, illustrated schematically by block diagrams. b: basaltic vents; c: caldera; d: dyke; Is: lava shield of scutulum type; m: magma chamber; rz: rift zone; r: rhyolitic lava dome; s: sill or intrusive sheet; u: cumulates. (b) Time-averaged output rates of selected volcanic systems. W: equivalent world energy consumption by man (excluding food); C: equivalent energy consumption (including food) by city having a population of 1 million. (c) Inferred fields for the five volcanic-system types on a plot of modulation frequency against time-averaged output rate. (d) Sketch map of a moderate-sized flood-basalt field: Harrat Rahat in Saudi Arabia (Camp & Roobol 1989). Solid black: lavas of 10-1.7 Ma old; open diagonal shading with dots: lavas of 1.7-0.6 Ma old and their cinder cones; close diagonal shading: lavas of <0.6 Ma old, including those of AD 641 and 1256; PC: Precambrian basement rocks; Qal: Quaternary sediments near Red Sea coast. (e) Sketch map of the Auckland monogenetic-volcano field, New Zealand, simplified by omitting the coastline (after Searle 1964). BASALTIC-VOLCANO SYSTEMS 5 magma-supply rate and involvement of non­ polygenetic structures commonly exceeding magmatic water) in causing most of the diver­ 1000 km3 in volume, and are not to be confused sity. with small monogenetic lava shields categorized by Noe-Nygaard (1968) as of scutulum type (Latin: scutulus, diminutive of scutus - 'shield') Volcano types and volcanic systems 3 which have volumes of 0.1-15 km • Five volcano types are distinguished (Fig. la): lava-shield volcanoes, stratovolcanoes, flood Examples of lava-shield volcanoes Mauna Loa basalts, monogenetic volcanoes, and central vol­ in Hawaii is the largest. It has an estimated canoes, this last having a significant proportion volume of 40000 km3, rises to 9-10 km above of silicic products in addition to basalt. Flood the surrounding ocean floor and has a keel that basalt vents, as well as monogenetic volcanoes, projects well below it. Subaerial slopes are erupt once, and once only. Lava shields, strato­ mostly 3-6° except in the scars of old landslides. volcanoes and central volcanoes are polygenetic Pico in the Azores and Fogo in the Cape Verdes structures that erupt more than once. have steep (> 30°) cones atop the shield. Shields There is much merit in regarding volcanoes as in the Galapagos consist of an upwardly-convex parts of magmatic or volcanic systems. A system dome with slopes as steep as 30° rising above the may embrace the intrusions, magma chambers, low-angle shield, and some of the eruptive conduits, magma source and accompanying fissures that occur on a narrow plateau around geothermal fields as well as the volcano itself. their large summit calderas are annular. Some The concept of volcanic systems acknowledges lava shields, notably Kilauea and perhaps also that the visible edifice of a volcano is only one Pico, consist mainly of pahoehoe. Others, such part of a bigger entity. as Mauna Loa, consist of roughly equal pro­ A volcanic system may be compared with a portions of pahoehoe and aa. Others again, for system of civilization such as a city, with the example Madeira, Tutuila (Samoa) and the extensive infra-structure oftransportation, water Galapagos volcanoes are composed mainly of and power supply, waste disposal and communi­ aa. cations on which a city depends. Just as a vol­ canic system is sustained by a supply of energy (in the form of hot magma), so a city system is Stratovolcanoes sustained by a supply of energy (in the form of These consist of a stratified succession of lava fossil fuels, electric power, and food). Very flows and interbedded pyroclastic deposits and roughly, a city of I million people requires about tend to have a conical form with a slope that the same power input as a volcanic system generally steepens upward until approximating having a magma input of I million m3 per year. the repose angle (about 33-36°) of loose debris. Basaltic systems have a source in the mantle Most subduction-related basaltic volcanoes from which magma ascends, mainly because of conform with this type. Commonly the cone is its positive buoyancy but sometimes aided by truncated by a caldera. Rift zones tend to be tectonic forces, toward the surface. They have diffuse and ill-defined and grade into radial vent one or more conduits by which the magma systems, and cinder cones tend to be conspi­ ascends. Polygenetic volcano systems generally cuous features on the volcano flanks. In many possess a high-level magma chamber, situated at arc volcanoes basaltic andesite and more silicic a neutral buoyancy level, which stores magma types accompany basalt. and modulates its delivery to the volcano and to sub-volcanic intrusions. Deep storage reservoirs Examples of stratovolcanoes The Japanese may also exist. island volcano of Izu-Oshima is representative of the arc-type basaltic cones.