Effusive Volcanism

Volcanic RockThe term effusive volcanism refers to the non-explosive extrusion of magma at the surface, and thus includes all eruptions of lava flows, coulees and domes. Effusive volcanism exhibits a remarkable variety of eruptive styles, environments, and deposits. Ranging from subaerial eruptions to those occurring at great depths in the sea or under vast glaciers, effusive eruptions are ubiquitous on the earth's surface, as well as on some of the terrestrial planets. Although effusive volcanism is by definition non-explosive, it is clearly related to explosive activity in many instances.

The chapter Basaltic Volcanoes and Volcanic Systems introduces the different types of basaltic volcanoes whose eruptions produce mainly lava flows. These volcanoes should be more properly thought of as volcanic or magmatic systems, since beneath each volcano is a complex system of magma generation, transport, and storage. Monogenetic volcanoes erupt only once, while polygenetic volcanoes erupt repeatedly. Lava shields are low-angle volcanoes built by rapid effusions of basaltic lava and intrusions of magma at shallow depths. Stratovolcanoes are steeper edifices constructed of pyroclastic material as well as lavas; these volcanoes are normally associated with subduction zones, with magmas that tend to be rich in gas. Flood basalts are vast lava flow fields which are erupted rapidly from fissure systems located in areas of broad crustal extension. Central volcanoes comprise an outer zone, in part of basaltic composition, and an inner zone with more silica-rich compositions; these volcanoes may have central calderas.

The different types of lava flows are treated in the chapter Lava Flows and Flow Fields. Interestingly, lava is the single most common feature on the surface of the terrestrial planets. Three main types of lava crusts are observed on Earth: pahoehoe, aa, and blocky. Pahoehoe is a smooth, continuous basaltic lava crust, aa is also basaltic but broken and irregular, while blocky lava is fragmental but less irregular and more silica-rich than aa. Most lava flows do not travel very fast, but some are extremely rapid. The development of a lava flow depends on the effusion rate at its source, its physical properties, and its environment, such as the local topography, subaerial, submarine, and so forth. The potential lengths of lava flows depend on several factors; chief among which is the effusion rate.

When magmas become more silica-rich than basalt, lava domes may form instead of lava flows. Lava Domes and Coulees examines different types of lava domes and why they form. Sometimes a lava dome is transitional to a lava flow, in which case it is called a coulee. Since the outer skin of a lava dome may cool and form a crust, buildup of pressure may occur within the dome's interior, leading to explosive activity such as vulcanian eruptions, pyroclastic flows, and pyroclastic surges. Indeed, cyclic dome growth and destruction have been observed at various volcanoes. If a dome grows rapidly, it has an increased tendency to collapse, frequently resulting in pyroclastic flows.

Fire fountains and spatter-fed lavas are discussed in the next chapter, Lava Fountains and Their Products. Fire fountaining is generally characteristic of basaltic magma which is both fluid and rich in dissolved gas; the reader will no doubt have a good image of the spectacular curtains of fragmental material thrown into the air by fire fountaining to many tens or even hundreds of meters. The fountains can occur along eruptive fissures or from central vents. The combination of rapid magma ascent and decompressive degassing and vesiculation forms the fountains. As the fragments fall to the ground, they may cool and remain in place, perhaps adhering to each other if they are still sufficiently hot and fluid. If the accumulation rate of particles is rapid, then the fragments may actually coalesce and flow as lava. This flow may not show any evidence of its initially fragmental origin.

The following chapter is Basaltic Volcanic Fields. Such fields are favored when the magma production rate is comparatively low while the crustal extension rate is high. The fields comprise a diverse variety of volcanic centers such as cones, maars, tuff rings, small shield volcanoes, and lava domes. The centers are frequently monogenetic, a good example being Parìcutin in Mexico, which erupted in the 1940's. The various volcanic centers may show structural control such as linear alignments, suggesting control along fault systems and the presence of dikes beneath the vents. The volcanic centers also may show clustering; they are found frequently on the flanks of larger volcanoes, as well as inside and outside the margins of calderas.

The following chapter is Flood Basalt Provinces. In contrast to basaltic fields where the magma production rate is low, flood basalt provinces are characterized by huge volumes of lava erupted rapidly over only one to two million years. Due to the high extrusion rates, basalt plateaus are formed instead of conical volcanic edifices. The magmas appear to come from great depths and are clearly associated with mantle plumes (hot spots), thinned lithosphere, and the breakup and separation of tectonic plates. Individual lava flows can be enormous in terms of their volume, area, and length; and the flows may be turbulent.

Submarine lava flows are the most abundant igneous rock type on the Earth's surface; the chapter Submarine Lavas and Hyaloclastite details the remarkable deposits found in this environment. At great depths in the ocean, exsolution of volatiles is hindered by the weight of the ocean mass, favoring lava flows at the expense of pyroclastic rocks. At shallower levels where the hydrostatic pressure is diminished, explosions are more feasible. The lavas are characterized by rapid cooling by seawater, resulting in pillow lavas, lobate lavas, and sheet flows, in order of increasing effusion rate. The rapid cooling also can thermally shock the lava, causing fragmentation and forming glassy fragments called hyaloclastite.

As submarine volcanoes evolve, they may progress to build seamounts or oceanic islands, the focus of the chapter Seamounts and Island Building. Seamounts may occur in clusters or chains, with the chains exhibiting a systematic age progression from one end to the other. The sizes of seamounts vary considerably; seamounts found in intraplate settings are generally much larger than those situated on or near mid-ocean ridges. Seamounts record a constructional stage which depends on the water depth, magma composition, eruption rate, and surrounding topography. As a seamount approaches the surface of the ocean, an increasing proportion of fragmental rocks is observed compared to lavas, since explosive activity is facilitated at shallower depths. Seamounts also record valuable information on mantle upwelling, lithospheric subsidence, and sea level changes.

Finally, Subglacial Eruptions closes this section on effusive volcanism by looking at eruptions which occur beneath glaciers. The chapter provides a useful bridge to the next section on explosive volcanism, since subglacial eruptions can be both effusive and explosive in nature. Large volumes of water are generated rapidly by melting of ice by the eruption; these volumes are sometimes drained catastrophically, resulting in huge floods called jökulhlaups. The large volumes of water also result in magma-water interactions which influence eruptive activity, such as hydrovolcanic eruptions.

John Stix
Université de Montréal

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