On Earth, the majority of volcanic eruptions occurs in the ocean. Oceanic volcanism can be divided into interplate and intraplate volcanism regarding to their location on the ocean floor. These volcanisms are involved in constructing the topographic features of the Earth surface. The essay aims at discovering the features of the volcanism in the ocean. It begins with a brief introduction on submarine volcanoes and volcanic vents. It then focuses on the two types of oceanic volcanism including their occurrence with specific examples. The last part of the essay investigates the application of the intraplate volcanism and the problems of the mantle plume theory of this intraplate volcanism.
introduction on submarine volcanoes
Active submarine volcanoes and volcanic vents are found in shallow water revealing their existence through ejecting steam and rock debris over the ocean surface (Tilling 1985). The unlimited supply of seawater surrounding the volcanoes could result in different behavior between submarine volcanoes and those on land. When sea water pours into the shallow volcanic vents, violent eruptions with steam blasting could occur (Tilling 1985). However, most of them lie on the deep ocean floor. These submarine volcanoes are confined with great pressure exerted by enormous weight of seawater. As a result they are prevented from explosive eruptions that even a large eruption may not disturb the ocean surface (Tilling 1985). The lava erupted from submarine volcanoes will cool rapidly when making contact with the sea water and large amounts of fragmented volcanic debris will be the resulted products on the sea floor (Tilling 1985). The accumulation of the debris allows the volcanoes to build in height. After the submarine volcanoes go extinct, they become seamounts.
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Fig. 1 Submarine eruption of Myojin-sho Volcano, Izu Islands, Japan, on September 23, 1952.
(Source: Volcanoes: On-Line Edition)
What are the types of oceanic volcanism? Where are they?
Submarine volcanoes are not distributed randomly throughout the ocean surface. Most oceanic volcanisms are situated at the mid-ocean ridges that are located at the divergent plate boundaries (Alden 2013) where two plates spread away from each other. Besides, there are also submarine volcanoes at the convergent margins where two oceanic plates collide and subduct (Alden 2013). Nonetheless, no oceanic volcanism can be seen at the continental-oceanic convergent boundaries and the transform plate boundaries. These oceanic volcanisms located at plate margins can be defined as interplate oceanic volcanism. Submarine volcanoes can also be found in the middle of a plate (Brantley 1994) which is due to hotspot volcanism. This type of oceanic volcanism is defined as intraplate volcanism. The following sections investigate these two types of volcanism one by one.
Fig. 2 Locations of volcanism on Earth (Source: USGS)
Interplate Oceanic Volcanism at Divergent Plate Boundaries
Divergent plate margins are where sea floor spreading centers locate. This type of margin is indicated by the series of mid-oceanic ridges running throughout the ocean basins including the Mid-Atlantic Ridge and the East Pacific Rise (Fig. 3).
Fig. 3 Location of mid-oceanic ridges (source: USGS)
Under plate tectonics, the diverging mantle convection at divergent margins drives the two plates to move apart. Volcanism happens when magma rises up through the fissures along the ridge and cools to form new oceanic crust (Fig. 4). The lava extruded often forms unique pillow lava flows as illustrated in Fig. 5. The magma involved is basaltic as it is derived from the partial melting of the mantle (Skinner and Murck 2011).
Fig. 4 Divergent plate boundary (Source: scullyproject.wikispaces.com)
Fig. 5 Pillow lavas at mid-oceanic ridge
(Source: The Blue Planet: An introduction to Earth System Science 3rd edition)
For instance, Juan de Fuca Ridge illustrates interplate oceanic volcanism at the divergent boundary between Pacific Plate and Juan de Fuca Plate (Brantley 1994). Volcanoes and lava flows can be found in the broad valley along the ridge (Brantley 1994) (Fig. 6). Axial Seamount is the youngest submarine volcano found on the ridge as shown in Fig. 7. It is the current eruptive center of the Cobb-Eickelberg Seamount chain (Johnson and Embley 1990). Not only does this Axial Seamount sit on the Juan de Fuca ridge, it also locates on the Cobb hotspot (Johnson and Embley 1990). Therefore, the Axial Seamount may be originated from the hotspot instead of the oceanic ridge or in a combined manner.
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Fig. 6 Volcanism at the Juan de Fuca Ridge (Source: USGS)
Fig. 7 Juan de Fuca Ridge, Gorda Ridge and Axial Seamount (Open blue arrows, ridge-spreading directions; solid blue arrow, convergence direction) (Source: USGS)
Inter-plate Oceanic Volcanism - At Convergent Plate Boundaries
Convergent plate boundaries are the places where two plates move towards each other driven by the converging mantle convection. Oceanic volcanism occurs when two oceanic plates collide and one of the oceanic plate subducts beneath the other at the subduction zones. Ocean trenches are formed at the subduction zone. The wedge of the mantle undergoes partial melting and Andesitic magma is formed (Skinner and Murck 2011). The magma then intrudes through the lithosphere and extrudes to the Earth surface as lava flow. The lava flow cools down and volcanic rocks or debris are deposited. After millions years, the piling up of volcanic debris on the ocean floor gives rise to submarine volcanoes which eventually rise above ocean surface forming volcanic islands (Kious and Tilling 1996). Volcanic island arcs are resulted from a chain of volcanic islands. Furthermore, these island arcs are parallel to and curved with the trenches (Kious and Tilling 1996).
Fig. 8 Oceanic-oceanic subduction zone showing trench and island arc
The Pacific Ring of Fire comprises the convergent plate boundaries indicated with volcanic island arcs and ocean trenches such as the Japan trench and the Mariana trench (Fig. 9). For example, submarine volcanoes are in significant amount from the Japan trench extending to the Izu Bonin trench and the Mariana trench (Simkin and Siebert 1994). These volcanic island arcs are resulted from the subducting of Pacific Plate under the Eurasian Plate, Philippine Plate and Mariana Plate. This region has the greatest number of submarine volcanoes and thus with the top number of eruptions reported (Simkin and Siebert 1994). These submarine volcanoes are basaltic and much more explosive than those at hotspots (Simkin and Siebert 1994). Kikai, a submarine volcano had generated one of the most explosive eruptions on Earth during Holocene (Simkin and Siebert 1994).
Fig. 9 Ring of Fire (Source: USGS)
Another region that submarine volcanoes are observed is the Solomon Islands. Solomon Islands are located near the convergent boundary where the Indo-Australian Plate subducts under the Pacific Plate (Simkin and Siebert 1994). Within the Solomon Island, volcano Kavachi is the most active submarine volcanoes in the Sourhwestern Pacific Ocean. It is situated 30 kilometers north of the subduction zone (Simkin and Siebert 1994).
Fig. 10 Major Volcanoes of Solomon Islands (Source: USGS)
Intraplate Oceanic Volcanism - Hotspot Volcanism
Oceanic volcanism that is not happening on plate boundaries or not caused by subduction or diverging of plates can be regarded as the hotspot volcanism. According to the mantle plume theory, magma comes to the surface at the hotspots from deep upwelling hot mantle plumes (Skinner and Murck 2011). These hotspots are long-lived, in fixed positions and are not moving with the plates (Skinner and Murck 2011). The magma involved in hotspot volcanism is basaltic as it is generated at the upper mantle. The following figure shows the distribution of hotspots on Earth.
Fig. 11 Plume locations (Source: Foulger 2010)
Among these hotspots, Hawaii hotspot volcanism is the most well-known. The Hawaiian Islands are a series of volcanic mountains resulted from repeated lava flow over millions of years (Tillings et al. 1987). The islands observed above ocean surface are only small proportion of the submarine volcanic mountain chain (Tillings et al. 1987). There are more than eighty large volcanoes in the Hawaiian Ridge-Emperor Seamount Chain stretching across Pacific Ocean to the Aleutian Trench near Alaska (Tillings et al. 1987). Within the islands, Mauna Loa and Kilauea are the most active volcanoes on Earth (Tillings et al. 1987). The nearby Loihi Seamount is the youngest Hawaii submarine volcano which is still submerged in water. It is believed to be sitting on the southeast edge of the hotspot at present. Other Hawaiian volcanoes had drifted in northwest direction above the hotspot. They are eventually cut off from the magma source becoming inactive (Tillings et al. 1987). The drift of islands started 5.6 million years ago in Kauai and still continuing in Hawaii at the moment. The resulted volcanic island chain supported the mantle plume theory that the volcanisms are caused by a fixed and deep mantle plume. Although the size of the Hawaii hotspot is still unknown, scientists believe that it is approximately 200 miles across, large enough to support the volcanism at Mauana Loa, Kilauea, Loihi, Hualalai and Halekala (Tillings et al. 1987).
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Fig. 12 Volcanism at Hawaiian Islands (Source: USGS)
The Hotspots Theory - Application
If there are long lived, fixed hotspots like in Hawaii, the Hawaii volcanic island chain can be the tool for measurement of the plate movement and speed. As Loihi is the newest volcano sitting on the hotspot of Hawaii, the older volcanoes is in a further distance from the hotspot as the plate moves (Skinner and Murck 2011). It is observed that there is a bend in the Hawaiian Ridge-Emperor Seamount Chain. This was due to the Pacific Plate changed its moving direction from north to northwest 43 million years ago (Skinner and Murck 2011). As the plate movement continues, Hawaii and Loihi may be moved away from the hotspot and cutoff while new submarine volcanoes will be created (Skinner and Murck 2011).
Fig. 13 Age of islands in the Hawaiian Ridge-Emperor Seamount Chain
(Source: The Blue Planet: An introduction to Earth System Science 3rd edition)
Problems of the Mantle Plume Theory
Is the mantle plume theory the only theory for this type of volcanism? There is another theory involving the breaking up of lithosphere resulting in magma rising from upper mantle (Alden 2012). In this theory, the hotspots are named melting anomalies. Though the mantle plume theory is supported by evidence such as the linear progressing volcanic chains, still, there are problems in the theory. For instance, no plume is found deeper than the upper mantle from the studies using seismic waves (Alden 2012). A study at the Azores hotspot next to the Mid-Atlantic Ridge suggested that the Azores hotspot is not as hot as other hotspots in the world (Bonatti 1990). Bonatti believed that some hotspots may be melting anomalies rather than thermal plumes (1990). Another research conducted at the volcanoes of the Cook-Austral chain in South Pacific also stated the failure of the mantle plume theory. The result pointed out that the radiometric dates from the volcanic islands there are not equal to the age predicted by the mantle plume theory (McNutt et al. 1997). Thus, the plume alone beneath the active Macdonald seamount may not account for all the southern Austral volcanoes (McNutt et al. 1997). McNutt and his colleagues believed that the volcanism in the region is due to the stress in the lithosphere instead of the deep mantle plume (1997).
In conclusion, there are mainly two types of oceanic volcanism, interplate and intraplate volcanism. Submarine volcanoes are discovered at the oceanic divergent or convergent plate boundaries and at hotspots with respect to these two types of volcanism. Specific locations of these volcanisms are also illustrated. The application of the hotspots and the problems associated with the mantle plume theory are discussed near the end of the essay.