The diagram below demonstrates how the various eruption types can be expressed as a function of height of eruption column and explosiveness. These parameters can be quantified in ancient eruptions by measuring the dispersal and fragmentation of the erupted tephra deposits.
modified from Cas and Wright, 1988
Fissure eruptions are generated at several contemporaneous sites along a linear fracture, or along a parallel, but offset fracture system, such as that shown in the picture here.
Regional fracture systems can appear where the Earth's crust is broken and pulled apart by tensional forces. If these regions are underlain by reservoirs of basaltic magma, this low-viscosity melt will utilize the fractures and ascend through the crust to generate a fissure eruption. For example, Mid-oceanic ridges (divergent plate margins) typically extrude basaltic magma from fissure eruptions because these are areas where global-scale extension is coincident with the rise of par tially molten asthenosphere.
Southwest Rift Zone, Kilauea
Eruption Style: "The Curtain of Fire"
As fluid, gas-poor basaltic magma rises up through a fissure, it is extruded at the surface as a wall of incandescent, liquid-to-plastic fragments known as a curtain of fire. Two such eruptions are shown below from extrusive events on the Kilauea volcano, Hawaii. Fissure eruptions are quiescent, and the height of the airborne eruptive material is small, often only a few tens of meters. The basaltic fragments in the curtain of fire thus remain largely liquid when they hit the ground. These coherent lumps of hot, fluid lava are called spatter. When they land, they can be hot and fluid enough to fuse together to form an aggregate called agglutinate, or agglutinated spatter. Spatter commonly builds up as banks along the fissure sides to produce spatter ramparts.
Fissure eruptions are also common on the flanks of many large volcanoes and, therefore, they are not restricted to areas undergoing regional extension. Magma-filled fissures radiating from the summit regions of active volcanoes like Mt. Etna, Mauna Loa, and Kilauea propagate outward from the central vent system. Extrusion from these propagating fissures can produce elongate volcano morphologies, such as those that are typical of many Hawaiian shield volcanoes. Note, for example, the axial elongation of the Mauna Loa shield volcano shown in the image to the left. Mauna Loa Fissures
Spatter ramparts from Mauna Loa, 1984
"Curtain of Fire" fissure eruption, Kilauea, Hawaii 1983
When fissures cease to erupt, the remaining magma residing in the fissure will cool and crystallize into an igneous rock intrusion. The resulting rock structure is called a dike. Dikes are tabular in shape, and they cut discordantly across adjacent rock layers. In areas of ancient volcanism, dikes are often delineated as resistant walls standing above more easily eroded rock types.
Dike from Shiprock, New Mexico
Hawaiian Eruptions
The calmest of all the eruption types, they are characterized by the effusive emission of highly fluid basalt lavas with low gas contents. The relative volume of ejected pyroclastic material is less than that of all other eruption types. The hallmark of Hawaiian eruptions is steady lava fountaining and the production of thin lava flows that eventually build up into large, broad shield volcanoes. Eruptions are also common in central vents near the summit of shield volcanoes, and along fissures radiating outward from the summit area. Lava advances downslope away from their source vents in lava channels and lava tubes.
Fire Fountains
Central-vent Hawaiian eruptions are noted for their spectacular jet-like sprays of liquid lava called fire fountains. These incandescent jets ascend hundreds of meters into the air. They can occur in short spurts, or last for hours on end.
The top of fire fountains are often carried away downwind to produce an airborne curtain of glowing fragments that showers downward. The indivudual liquid-to-plastic fragments (clasts) generally cool quickly by radiating their heat into the atmosphere. Thus, they are chilled and solid by the time they hit the ground, where they accumulate as cindery fragments called scoria. However, during very high eruption rates, the fire fountains become so dense that the clasts can no longer radiate heat freely into the atmosphere. These clasts are kept hot by the heat of surrounding clasts. Under these conditions the molten clasts, spatter, may hit the ground and fuse together to form agglutinated spatter cones and spatter ramparts. If the eruption rates are high enough, spatter-fed flows (clastogenic lavas) may develop as hot spatter fragments blend together on the ground and flow away.
High Lava Fountain
Pu'u O'o volcano, Big Island of Hawaii, 1985
Lava Lakes
The fluid basalt associated with Hawaiian eruptions sometimes ponds in vents, craters, or broad depressions to produce lava lakes. In some cases, lava may erupt from a vent located within a crater, or surface lava flow may pour into a crater or broad depression. As lava lakes cool, they produce a grey-silver crust
that is usually only a few centimeters thick, as shown here in the image of the Kupaianaha lava lake. Active lava lakes contain young crust that is continually destroyed and regenerated. Convective motion of the underlying lava causes the crust to break into slabs and sink. This then exposes new lava at the surface that cools into a new crustal layer which will again break up into slabs and be recycled into the circulating lava beneath the crust.
When fissures cease to erupt, the remaining magma residing in the fissure will cool and crystallize into an igneous rock intrusion. The resulting rock structure is called a dike. Dikes are tabular in shape, and they cut discordantly across adjacent rock layers. In areas of ancient volcanism, dikes are often delineated as resistant walls standing above more easily eroded rock types.
Dike from Shiprock, New Mexico
Hawaiian Eruptions
The calmest of all the eruption types, they are characterized by the effusive emission of highly fluid basalt lavas with low gas contents. The relative volume of ejected pyroclastic material is less than that of all other eruption types. The hallmark of Hawaiian eruptions is steady lava fountaining and the production of thin lava flows that eventually build up into large, broad shield volcanoes. Eruptions are also common in central vents near the summit of shield volcanoes, and along fissures radiating outward from the summit area. Lava advances downslope away from their source vents in lava channels and lava tubes.
Fire Fountains
Central-vent Hawaiian eruptions are noted for their spectacular jet-like sprays of liquid lava called fire fountains. These incandescent jets ascend hundreds of meters into the air. They can occur in short spurts, or last for hours on end.
The top of fire fountains are often carried away downwind to produce an airborne curtain of glowing fragments that showers downward. The indivudual liquid-to-plastic fragments (clasts) generally cool quickly by radiating their heat into the atmosphere. Thus, they are chilled and solid by the time they hit the ground, where they accumulate as cindery fragments called scoria. However, during very high eruption rates, the fire fountains become so dense that the clasts can no longer radiate heat freely into the atmosphere. These clasts are kept hot by the heat of surrounding clasts. Under these conditions the molten clasts, spatter, may hit the ground and fuse together to form agglutinated spatter cones and spatter ramparts. If the eruption rates are high enough, spatter-fed flows (clastogenic lavas) may develop as hot spatter fragments blend together on the ground and flow away.High Lava Fountain
Pu'u O'o volcano, Big Island of Hawaii, 1985
Lava Lakes
The fluid basalt associated with Hawaiian eruptions sometimes ponds in vents, craters, or broad depressions to produce lava lakes. In some cases, lava may erupt from a vent located within a crater, or surface lava flow may pour into a crater or broad depression. As lava lakes cool, they produce a grey-silver crust
that is usually only a few centimeters thick, as shown here in the image of the Kupaianaha lava lake. Active lava lakes contain young crust that is continually destroyed and regenerated. Convective motion of the underlying lava causes the crust to break into slabs and sink. This then exposes new lava at the surface that cools into a new crustal layer which will again break up into slabs and be recycled into the circulating lava beneath the crust.Kupaianaha Lava Lake, Hawaii
Strombolian Eruptions
Named from the small volcano island of Stromboli, which lies between Sicily and Italy, true strombolian activity is characterized by short-lived, explosive outbursts of pasty lava ejected a few tens or hundreds of meters into the air. Unlike Hawaiian eruptions, Strombolian eruptions never develop a sustained eruption column. They eject relatively viscous basaltic lava from the throat of the volcano. Build up of the high gas pressures required to fragment this somewhat pasty lava, results in episodic explosions with booming blasts. Although Strombolian eruptions are much noisier than Hawaiian eruptions, they are no more dangerous. As shown in the image below, Strombolian explosions eject bomb- and lapilli-sized fragments that travel in parabolic ballistic paths before accumulating around the vent to construct the volcanic edifice.
Strombolian activity on Mt. Etna, October 2002
Volcanian Eruptions
In contrast to basaltic strombolian eruptions, vulcanian eruptions are most often associated with andesitic to dacitic magma. The high viscosity of these magmas makes it difficult for the vesiculating gases to escape. This leads to the build up of high gas pressure and explosive eruptions. The ejected lava fragments do not take on the aerodynamic shapes common to Strombolian eruptions. This is partly due to the higher viscosity of the erupting magma, but also because the ejecta often incorporates a high proportion of crystalline material broken away from the rock plugging the throat of the volcano. These eruptions are often associated with growing lava domes, such as that at Mt. Pelée in 1902, and with the genration of pyroclastic flows from dome collapse.
Typically, these eruptions form scoria cones composed of basaltic pryoclasts. However, mafic stratovolcanoes can also exhibit common Strombolian activity, evident for example, at Mt. Eberus in Antarctica and at Stromboli itself.
Pyroclastic particles like Pele's tears, Pele's hair, and reticulite, which are common in Hawaiian eruptions, are not present in Strombolian eruptions. Spatter-fed flows are minor. Instead, Strombolian eruptions are dominated by scoria fragments, which are highly vesiculated clasts of basalt with a cindery appearance. Tephra bombs and lapilli accumulate around the vent to produce well-bedded, and often well-sorted, scoria-fall deposits
In contrast to Hawaiian eruptions, true Strombolian eruptions produce little or no flowing lava. However, during the end stages of scoria-cone formation, it is not unusual for Strombolian activity to wane and give way to the calm extrusion of basaltic lava flows. As a general rule, a'a lava flows appear to be more common than the more
fluid pahoehoe types. As the vesiculating lava is de-gased toward the end of the eruption, it may ooze out from under the volcanic edifice to produce a lava flow, or pond in the vent to produce a lava lake. This will only occur if the underlying basalt is fluid enough to flow, which has not proved to be the case at Stromboli itself.
Pyroclastic particles like Pele's tears, Pele's hair, and reticulite, which are common in Hawaiian eruptions, are not present in Strombolian eruptions. Spatter-fed flows are minor. Instead, Strombolian eruptions are dominated by scoria fragments, which are highly vesiculated clasts of basalt with a cindery appearance. Tephra bombs and lapilli accumulate around the vent to produce well-bedded, and often well-sorted, scoria-fall deposits
In contrast to Hawaiian eruptions, true Strombolian eruptions produce little or no flowing lava. However, during the end stages of scoria-cone formation, it is not unusual for Strombolian activity to wane and give way to the calm extrusion of basaltic lava flows. As a general rule, a'a lava flows appear to be more common than the more
fluid pahoehoe types. As the vesiculating lava is de-gased toward the end of the eruption, it may ooze out from under the volcanic edifice to produce a lava flow, or pond in the vent to produce a lava lake. This will only occur if the underlying basalt is fluid enough to flow, which has not proved to be the case at Stromboli itself.Volcanian Eruptions
In contrast to basaltic strombolian eruptions, vulcanian eruptions are most often associated with andesitic to dacitic magma. The high viscosity of these magmas makes it difficult for the vesiculating gases to escape. This leads to the build up of high gas pressure and explosive eruptions. The ejected lava fragments do not take on the aerodynamic shapes common to Strombolian eruptions. This is partly due to the higher viscosity of the erupting magma, but also because the ejecta often incorporates a high proportion of crystalline material broken away from the rock plugging the throat of the volcano. These eruptions are often associated with growing lava domes, such as that at Mt. Pelée in 1902, and with the genration of pyroclastic flows from dome collapse.
Sakurajima Volcano
Vulcanian deposits contain large blocks and bombs near the vent. Bread-crust bombs (left) are particularly a
bundant. These resemble a crusty loaf of bread broken by deep cracks that often expose a frothy interior. These large pyroclastic fragments form when viscous, gas-rich magma is ejected from the vent to produce a bomb whose exterior chills quickly to a glassy or fine-grained crust while in flight. The interior of the bomb, however, continues to vesiculate on the ground, which leads to expansion of the interior and cracking of the brittle outer crust.
Although blocks and bombs are common in proximal deposits, the bulk of Vulcanian deposits is very fine grained and dominated by ash. The abundance of ash indicates a high degree of fragmentation, which can only be generated by magmas with high gas contents. In some cases, these high gas contents are derived from heated meteoric water. It is likely, therefore, that many vulcanian eruptions are at least partially hydrovolcanic. Although the ash-fall deposits generated by volcanian eruptions are highly fragmented, they are only moderately dispersed. This suggests a high degree of explosiveness (high fragmentation) associated with the development of eruptive columns that are of only moderate heights (moderate dispersal).
Plinian Eruptions
These spectacularly explosive eruptions are associated with volatile-rich dacitic to rhyolitic lava, which typically erupts from stratovolcanoes. The duration these eruptions is highly variable, from hours to days. The longest eruptions appear to be associated with the most felsic volcanoes. Although Plinian eruptions typically invlove felsic magma, they can occasionally occur in fundamentally basaltic volcanoes where the magma chambers become differentiated and zoned to create a siliceous top. An example of this was the Hekla eruption (Iceland) of 1947-48. Over the past 800 years, Hekla has had a history of generating violent initial eruptions of pumice, lasting a few hours, followed by prolonged extrusion of basaltic lava from the lower part of the chamber.
Rather than producing the discrete explosions that are typical of Vulcanian and Strombolian eruptions, Plinian eruptions generate sustained eruptive columns. Although they differ markedly from nonexplosive Hawaiian eruptions, Plinian eruptions are similar to Hawaiian fire fountaining in that both of these eruption types generate sustained eruption plumes. In both, the eruption plumes are maintained because the growing bubbles rise at about the same rate as the magma moves up through the central vent system.
Plinian eruptions generate large eruptive columns that are powered upward partly by the thrust of expanding gases, and by convective forces with exit velocities of several hundred meters per second. Some reach heights of ~45 km. These eruptive columns produce widespread dispersals of tephra which cover large areas with an even thickness of pumice and ash (see pumice-fall deposits). The region of pyroclastic fall accumulation is generally asymmetric around the volcano as the eruptive column is carried in the direction of the prevailing wind, as shown here in this NASA image of the Klyuchevskaya eruption in 1994.
The regions surrounding Plinian eruptions are not only subject to large volumes of pumice airfall (from 0.5 to 50 km3), but they are also subject to the most dangerous types of volcanic phenomena: pyroclastic flows and lahars. The occasional collapse of the eruptive column will generate hot, pyroclastic flows that advance down the volcano flanks at hurricane-force speeds. In addition, large volumes of water are often generated by the melting of snow banks and alpine glaciers during the eruption. The mixing of this water with unconsolidated tephra can generate volcanic mudflows (lahars). These features have the consistency of wet concrete, yet they can advance down slopes at the same rate as a rapidly moving stream.
bundant. These resemble a crusty loaf of bread broken by deep cracks that often expose a frothy interior. These large pyroclastic fragments form when viscous, gas-rich magma is ejected from the vent to produce a bomb whose exterior chills quickly to a glassy or fine-grained crust while in flight. The interior of the bomb, however, continues to vesiculate on the ground, which leads to expansion of the interior and cracking of the brittle outer crust.Although blocks and bombs are common in proximal deposits, the bulk of Vulcanian deposits is very fine grained and dominated by ash. The abundance of ash indicates a high degree of fragmentation, which can only be generated by magmas with high gas contents. In some cases, these high gas contents are derived from heated meteoric water. It is likely, therefore, that many vulcanian eruptions are at least partially hydrovolcanic. Although the ash-fall deposits generated by volcanian eruptions are highly fragmented, they are only moderately dispersed. This suggests a high degree of explosiveness (high fragmentation) associated with the development of eruptive columns that are of only moderate heights (moderate dispersal).
Plinian Eruptions
These spectacularly explosive eruptions are associated with volatile-rich dacitic to rhyolitic lava, which typically erupts from stratovolcanoes. The duration these eruptions is highly variable, from hours to days. The longest eruptions appear to be associated with the most felsic volcanoes. Although Plinian eruptions typically invlove felsic magma, they can occasionally occur in fundamentally basaltic volcanoes where the magma chambers become differentiated and zoned to create a siliceous top. An example of this was the Hekla eruption (Iceland) of 1947-48. Over the past 800 years, Hekla has had a history of generating violent initial eruptions of pumice, lasting a few hours, followed by prolonged extrusion of basaltic lava from the lower part of the chamber.
Rather than producing the discrete explosions that are typical of Vulcanian and Strombolian eruptions, Plinian eruptions generate sustained eruptive columns. Although they differ markedly from nonexplosive Hawaiian eruptions, Plinian eruptions are similar to Hawaiian fire fountaining in that both of these eruption types generate sustained eruption plumes. In both, the eruption plumes are maintained because the growing bubbles rise at about the same rate as the magma moves up through the central vent system.
Plinian eruptions generate large eruptive columns that are powered upward partly by the thrust of expanding gases, and by convective forces with exit velocities of several hundred meters per second. Some reach heights of ~45 km. These eruptive columns produce widespread dispersals of tephra which cover large areas with an even thickness of pumice and ash (see pumice-fall deposits). The region of pyroclastic fall accumulation is generally asymmetric around the volcano as the eruptive column is carried in the direction of the prevailing wind, as shown here in this NASA image of the Klyuchevskaya eruption in 1994.
The regions surrounding Plinian eruptions are not only subject to large volumes of pumice airfall (from 0.5 to 50 km3), but they are also subject to the most dangerous types of volcanic phenomena: pyroclastic flows and lahars. The occasional collapse of the eruptive column will generate hot, pyroclastic flows that advance down the volcano flanks at hurricane-force speeds. In addition, large volumes of water are often generated by the melting of snow banks and alpine glaciers during the eruption. The mixing of this water with unconsolidated tephra can generate volcanic mudflows (lahars). These features have the consistency of wet concrete, yet they can advance down slopes at the same rate as a rapidly moving stream.
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