DISTRIBUTION OF ACTIVE VOLCANOES

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2 DISTRIBUTION OF ACTIVE VOLCANOES The earth is a dynamic planet. Its rigid outer surface layer is broken into several tectonic plates which are in constant motion relative to one another. As demonstrated in the world map below, most of the ~550 active volcanoes on earth are located along the margins of adjacent plates.

3 DISTRIBUTION OF ACTIVE VOLCANOES World map showing plate boundaries (blue lines), the distribution of recent earthquakes (yellow dots) and active volcanoes (red triangles). Courtesy of NASA.

4 PLATE MOTION, MANTLE CONVECTION, AND MAGMA GENERATION Tectonic plates are composed of lithosphere, the rigid outer portion of the earth. With a thickness of about 100 km, the lithosphere is composed of an upper layer of crust (~7 km thick under the oceans, and ~35 km thick under the continents) and a lower, denser layer of the earth's upper mantle. The lithosphere is underlain by the asthenosphere, a hot, mobile layer of partially molten rock lying within the earth's upper mantle. (For detailed information, click the Earth's Interior.)

5 PLATE MOTION, MANTLE CONVECTION, AND MAGMA GENERATION The rigid lithospheric plates are driven by convection within the mobile asthenosphere. Hot mantle rises beneath mid-oceanic ridges, and cold, denser mantle descends at oceanic trenches. Lateral motion of the lithospheric plates above these circular convection cells is analogous to rigid blocks riding above a rotating conveyor belt. Images modified from USGS.

6 How volcanoes work? Volcanic eruptions above these lithospheric plates are driven by the ascent of magma (molten rock) from deep beneath the surface. The various magma types are described in Physicochemical Controls on Eruption Style. They vary from mafic, intermediate, to felsic as their silica (SiO2) content increases. Mafic (basaltic) magmas are generated directly from the mantle, either within the asthenosphere or within the overlying mantle lithosphere. Many mafic-to-intermediate (basaltic-toandesitic) magmas appear to be derived from the melting of hydrated lithospheric mantle. More differentiated, intermediate-to-felsic magmas, on the other hand, are partly derived from the melting of continental crust by hot, mafic magmas that either pond at the crust-mantle boundary, or intrude into the overlying continents where they reside in magma chambers located at various crustal levels.

7 How volcanoes work? Volcanism is typically widespread along plate boundaries. Although volcanism in the interior of plates is less common, these intraplate regions can also generate voluminous eruptive products. The regional volcano-tectonic processes associated with plate-boundary environments and intraplate environments are described in more detail below.

8 VOLCANISM AT PLATE TECTONIC BOUNDARIES Plate boundaries mark the sites where two plates are either moving away from one another, moving toward one another, or sliding past one another. Adjacent plates are delineated by three types of boundaries defined by this relative motion: Divergent plate boundaries -- Plates diverge from one another at the site of thermally buoyant mid-oceanic ridges. Oceanic crust is created at divergent plate boundaries. Convergent plate boundaries -- Plates converge on one another at the site of deep oceanic trenches. Oceanic crust is destroyed at convergent plate boundaries. Transform plate boundaries -- Plates slide past one another.

9 VOLCANISM AT PLATE TECTONIC BOUNDARIES Although volcanism is abundant at divergent and convergent plate boundaries, there is a distinct lack of significant volcanism associated with transform plate boundaries. Spreading center volcanism occurs at divergent plate margins, and subduction zone volcanism occurs at convergent plate margins. Intraplate volcanism describes volcanic eruptions within tectonic plates. Each of these three volcano-tectonic environments is depicted in the following diagram:

10 VOLCANISM AT PLATE TECTONIC BOUNDARIES

11 VOLCANO TYPES A volcanic vent is an opening exposed on the earth's surface where volcanic material is emitted. All volcanoes contain a central vent underlying the summit crater of the volcano. The volcano's cone-shaped structure, or edifice, is built by the more-or-less symmetrical accumulation of lava and/or pyroclastic material around this central vent system. The central vent is connected at depth to a magma chamber, which is the main storage area for the eruptive material. Because volcano flanks are inherently unstable, they often contain fractures that descend downward toward the central vent, or toward a shallow-level magma chamber. Such fractures may occasionally tap the magma source and act as conduits for flank eruptions along the sides of the volcanic edifice. These eruptions can generate cone-shaped accumulations of volcanic material, called parasitic cones. Fractures can also act as conduits for escaping volcanic gases, which are released at the surface through vent openings called fumaroles.

12 VOLCANO TYPES Fumarole Summit Crater Parasitic Cones

13 MAIN VOLCANO TYPES Although every volcano has a unique eruptive history, most can be grouped into three main types based largely on their eruptive patterns and their general forms. The form and composition of the three main volcano types are summarized here:

14 MAIN VOLCANO TYPES VOLCANO TYPE VOLCANO SHAPE COMPOSITION ERUPTION TYPE SCORIA CONE Straight sides with steep slopes; large summit crater Basalt tephra; occasionally andesitic Strombolian SHIELD VOLCANO Very gentle slopes; convex upward Basalt lava flows Hawaiian STRATO VOLCANO Gentle lower slopes, but steep upper slopes; concave upward; small summit crater Highly variable; alternating basaltic to rhyolitic lavas and tephra with an overall andesite composition Plinian

15 SUBORDINATE VOLCANO TYPES Lava and tephra can erupt from vents other than these three main volcano types. A fissure eruption, for example, can generate huge volumes of basalt lava; however, this type of eruption is not associated with the construction of a volcanic edifice around a single central vent system. Although point-source eruptions can generate such features as spatter cones and hornitos, these volcanic edifices are typically small, localized, and/or associated with rootless eruptions (i.e., eruptions above the surface of an active lavaflow, unconnected to an overlying magma chamber). Vent types related to hydrovolcanic processes generate unique volcanic structures, discussed separately under hydrovolcanic eruptions

16 WHEN IS A VOLCANO CONSIDERED ACTIVE, DORMANT, OR EXTINCT? Classifying a volcano as active, dormant, or extinct is a subjective and inexact exercise. A volcano is generally considered active if it has erupted in historic time. This definition, however, is rather ambiguous, because recorded history varies from thousands of years in Europe and the Middle East, to only a few hundred years in other regions of the world, like the Pacific Northwest of the United States. Scientists generally consider a volcano active if it is currently erupting, or exhibiting unrest through earthquakes, uplift, and/or new gas emissions. The Smithsonian Institution's catalog of active volcanoes, recognizes 539 volcanoes with historic eruptions. In addition, there are 529 volcanoes that have not erupted in historic times, but which exhibit clear evidence of eruption in the past 10,000 years. These latter volcanoes are probably best considered "dormant," since they have the potential to erupt again.

17 WHEN IS A VOLCANO CONSIDERED ACTIVE, DORMANT, OR EXTINCT? Whether or not inactive volcanoes are considered truly extinct, or just dormant, depends partly on the average repose interval between eruptions. As noted in eruptive variability, explosive eruptions like those at Toba and Yellowstone have repose intervals of hundreds of thousands of years, whereas non-explosive eruptions have very short repose intervals. Thus, the Yellowstone region, which has not experienced an eruption for 70,000 years, can not be considered extinct. In fact, many scientists consider Yellowstone to be active because of high uplift rates, frequent earthquakes, and a very active geothermal system. Many inactive scoria cones, on the other hand, may be viewed as extinct shortly after they erupt, because such volcanoes are typically monogenetic and only erupt once.

18 SCORIA CONES Scoria cones, also known as cinder cones, are the most common type of volcano. They are also the smallest type, with heights generally less than 300 meters. They can occur as discrete volcanoes on basaltic lava fields, or as parasitic cones generated by flank eruptions on shield volcanoes and stratovolcanoes. Scoria cones are composed almost wholly of ejected basaltic tephra. The tephra is most commonly of lapilli size, although bomb-size fragments and lava spatter may also be present. The tephra fragments typically contain abundant gas bubbles (vesicles), giving the lapilli and bombs a cindery (or scoriaceous) appearance. The tephra accumulates as scoria-fall deposits which build up around the vent to form the volcanic edifice. The edifice has very steep slopes, up to 35 degrees, although older eroded scoria cones typically have gentler slopes, from 15 to 20 degrees. Unlike the other two main volcano types, scoria cones have straight sides and very large summit craters, with respect to their relatively small edifices. They are often symmetric, although many are asymmetric due to (1) the build up of tephra on the downwind flank of the edifice, (2) elongation of the volcano above an eruptive fissure, or (3) partial rafting of an outer wall of the volcano due to basalt lava oozing outward from beneath the volcano edifice. Where scoria cones have been breached, they typically reveal red-oxidized interiors.

19 SCORIA CONES Sunset Crater scoria cone Scoria cone on Mauna Kea La Poruna scoria cone

20 SCORIA CONES Scoria cones are generated by Strombolian eruptions, which produce eruptive columns of basalt tephra generally only a few hundred meters high. Many scoria cones are monogenetic in that they only erupt once, in contrast to shield volcanoes and stratovolcanoes. An exception is the Cerro Negro volcano in Nicaragua, which is the Earth's most historically active scoria cone. It is one of several parasitic cones on the northwest flank of Las Pilas volcano. Cerro Negro has erupted more than twenty times since it was born in Its most recent eruptions were in 1992 and

21 SHIELD VOLCANOES Shield volcanoes are broad, low-profile features with basal diameters that vary from a few kilometers to over 100 kilometers (e.g., the Mauna Loa volcano, Hawaii). Their heights are typically about 1/20th of their widths. The lower slopes are often gentle (2-3 degrees), but the middle slopes become steeper (~10 degrees) and then flatten at the summit. This gives shield volcanoes a flank morphology that is convex in an upward direction. Their overall broad shapes result from the extrusion of very fluid (low viscosity) basalt lava that spreads outward from the summit area, in contrast to the vertical accumulation of airfall tephra around scoria-cone vents, and the build-up of viscous lava and tephra around stratovolcanoes. Cross-sections through shield volcanoes reveal numerous thin flow units of pahoehoe basalt, typically < 1 m thick. Pyroclastic deposits are minor (< 1%) and of limited dispersal, generally from flank eruptions associated with parasitic scoria cones, or from rare, localized hydrovolcanic eruptions..

22 SHIELD VOLCANOES Very thin pahoehoe flow units (< 0.5 m thick) on the flank of a shield volcano in western Saudi Arabia. Photo by Vic Camp. The Mauna Loa volcano on the Big Island of Hawaii, seen here in both a oblique and satellite views, is the world's largest shield volcano: Mauna Loa from the southeast Mauna Loa satellite view

23 SHIELD VOLCANOES Shield volcanoes are generated by Hawaiian eruptions. However, there is some variability in their eruptive style, which translates into variations in shield morphology and size. The almost perfect symmetry and small volume (~15 km3) of Icelandic shields, for example, stands in marked contrast to the elongation and huge volume (thousands of km3) of Hawaiian shields. These variations are largely attributed to the monogenetic, small-volume, centralized summit eruptions, typical of icelandic shields, and the polygenetic, large-volume, linear fissure eruptions, typical of most hawaiian shields. Still different are the symmetrical Galapagos shields, shown below, which have steep middle slopes (>10 degrees) and flat tops occupied by large and very deep calderas. These shield types appear to be generated by ring-fracture eruptions, which delineate the sides of the caldera and mark the site of caldera collapse.

24 SHIELD VOLCANOES Coalesced shield volcanoes of the Galapagos Islands Three-dimensional image of the Alcedo shield volcano on Isabella Island, Galapagos

25 STRATOVOLCANOES Stratovolcanoes, also known as composite cones, are the most picturesque and the most deadly of the volcano types. Their lower slopes are gentle, but they rise steeply near the summit to produce an overall morphology that is concave in an upward direction. The summit area typically contains a surprisingly small summit crater. This classic stratovolcano shape is exemplified by many well-known stratovolcanoes, such as Mt. Fuji in Japan, Mt. Mayon in the Philippines, and Mt. Agua in Guatemala.

26 STRATOVOLCANOES Mt. Mayon Mt. Agua In detail, however, stratovolcano shapes are more variable than these classic examples, primarily because of wide variations in eruptive style and composition. Some may contain several eruptive centers, a caldera, or perhaps an amphitheater as the result of a lateral blast (e.g., Mt. St. Helens).

27 STRATOVOLCANOES Typically, as shown in the image to the left, stratovolcanoes have a layered or stratified appearance with alternating lava flows, airfall tephra, pyroclastic flows, volcanic mudflows (lahars), and/or debris flows. The compositional spectrum of these rock types may vary from basalt to rhyolite in a single volcano; however, the overall average composition of stratovolcanoes is andesitic. Many oceanic stratovolcanoes tend to be more mafic than their continental counterparts. The variability of stratovolcanoes is evident when examining the eruptive history of individual volcanoes. Mt. Fuji and Mt. Etna, for example, are dominanted by basaltic lava flows, whereas Mt. Rainier is dominated by andesitic lava, Mt. St. Helens by andesitic-to-dacitic pyroclastic material, and Mt. Lassen by dacitic lava domes.

28 STRATOVOLCANOES Stratovolcanoes typically form at convergent plate margins, where one plate descends beneath an adjacent plate at the site of a subduction zone. Examples of subductionrelated stratovolcanoes can be found in many places in the world, but they are particularly abundant along the rim of the Pacific Ocean, a region known as Ring of Fire. In the Americas, the Ring of Fire includes stratovolcanoes forming the Aleutian islands in Alaska, the crest of the Cascade Mountains in the Pacific Northwest, and the high peaks of the Andes Mounains in South America. A satellite view of three stratovolcanoes from the Andes is shown here:

29 STRATOVOLCANOES Three Andean stratovolcanoes in northern Ecaudor

30 STRATOVOLCANOES The eruptive history of most stratovolcanoes is delineated by highly explosive Plinian eruptions. These dangerous eruptions are often associated with deadly pyroclastic flows composed of hot volcanic fragments and toxic gases that advance down slopes at hurricane-force speeds. Like shield volcanoes, stratovolcanoes are polygenetic; however, they differ from shield volcanoes in that they erupt infrequently, with typical repose intervals of hundreds of years between eruptions. Most active stratovolcanoes worldwide appear to be < 100,000 years old, although some, like Mt. Rainier, may be more than 1 million years old.