Evolution of Hawaiian volcanoes
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Most of the Hawaiian volcanoes (that is, the volcanoes that make up the Hawaiian Islands) go through several defined stages of evolution during their lifespans. These stages of growth are influenced by the position of the volcano in relation to the hotspot, whether the volcano's summit lies below, near, or above sea level, and by the composition of the lava being erupted.
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[edit] Submarine Preshield Stage
When a volcano is first born near the Hawaiian hotspot, it begins its growth in the submarine preshield stage. This stage is characterized by infrequent, low volume eruptions. The volcano is steep-sided, and it usually has a defined caldera and has two or more rift zones radiating from the summit. The type of lava erupted in this stage of activity is alkalic basalt.
Because this stage occurs when the volcano is near the sea floor, the form of lava flow created by eruptions is called pillow lava: lava erupted in water so deep that the water pressure prevents the lava from exploding upon contact with the cold ocean water. This stage is thought to last about 200,000 years, but lavas erupted during this stage make up only a tiny fraction of the final volume of the volcano. As the stage progresses, eruptions become more frequent.
The only example of a Hawaiian volcano in this stage is Lo‘ihi Seamount, which is thought to be transitioning from the submarine preshield stage into the submarine phase of the shield stage. All older volcanoes have had their preshield stage lavas buried by younger lavas, so everything that is known about this stage comes from research done on Loi‘hi Seamount.
[edit] Shield Stage
The shield stage of the volcano is subdivided into three phases: the submarine, explosive, and subaerial. During this stage of growth, the volcano accumulates about 95 percent of its mass and it takes on the "shield" shape that shield volcanoes are named for. It is also the stage where the volcano's eruptive frequency reaches its peak.
[edit] Submarine Phase
As eruptions become more and more frequent at the end of the preshield stage, the composition of the lava erupted from the Hawaiian volcano changes from alkalic basalt to tholeiitic basalt and the volcano enters the submarine phase of the shield stage. In this phase, the volcano continues to erupt pillow lava. Calderas form, fill, and reform at the volcano's summit and the rift zones remain prominent. The volcano builds its way up to sea level. The submarine phase ends when the volcano is only shallowly submerged.
The only example of a volcano in this stage is Lo‘ihi Seamount, which, as noted above, is transitioning into this phase from the preshield stage.
[edit] Explosive Phase
When the volcano comes close to sea level, the pressures that prevented explosive reactions between erupting lava and water no longer exist. As this point is reached, the volcano enters the explosive phase of the shield stage. In this phase, lava and seawater interact to cause explosive eruptions, rich in ash. These explosive eruptions continue intermittently for several hundred thousand years. Calderas continually develop and fill, and the rift zones remain prominent. The phase ends when the volcano has sufficient mass and height (about 4,000 feet above sea level) that constant interactions between sea water and erupting lava at vent locations no longer occur.
[edit] Subaerial Phase
During this stage of activity, the explosive eruptions become much less frequent and the nature of the eruptions become much more gentle. The lava erupted in this stage form flows of pāhoehoe or ‘a‘a. During this subaerial stage, the flanks of the growing volcanoes are unstable and as a result, large landslides may occur. At least 17 major landslides have occurred around the major Hawaiian islands. This stage is arguably the most well-studied, as all eruptions that occurred in the 20th century in Hawai‘i were produced by volcanoes in this phase.
Mauna Loa and Kīlauea volcanoes are in this phase of activity.
[edit] Postshield Stage
As the volcano reaches the end of the shield stage, the volcano goes through another series of changes as it enters the postshield stage. The type of lava erupted changes from tholeiitic basalt back to alkalic basalt and eruptions become slightly more explosive. The rate of eruption also decreases steadily. Caldera development stops, and the rift zones may become inactive. Instead of the shield development characteristic of the previous stage, activity during this stage creates cinder cones through spectacular fountaining. A steep "cap" covers the smooth, shallow-angled shield volcano.
This stage continues for some 250,000 years, as the rate of eruptions steadily decrease until they finally cease.
Mauna Kea, Hualālai, and Haleakalā volcanoes are in this stage of activity.
[edit] Erosional Stage
After the volcano becomes dormant, the forces of erosion gain control of the mountain. The volcano subsides into the oceanic crust due to its immense weight and loses elevation. Meanwhile, rain also bites at the volcano, creating spectacular valleys. Coral reefs grow along the shoreline. The volcano, altogether, becomes a skeleton of its former self.
Kohala, Lānai, and Wai‘anae volcanoes are examples of volcanoes in this stage of development.
[edit] Rejuvenated Stage
After a long period of dormancy and erosion of the surface, the volcano might become active again, entering a final stage of activity called the rejuvenated stage. During this stage, the volcano erupts small volumes of lava very infrequently. These eruptions are often spread out over several millions of years. The composition of the lavas erupted in this stage are usually alkalic.
Ko‘olau and West Maui volcanoes are examples of volcanoes in this stage of development. Note, however, that because in this stage eruptions are very infrequent (occurring thousands or even tens of thousands of years apart), erosion is still the primary factor controlling the volcano at this point.
[edit] Coral Atoll Stage
Eventually, erosion and subsidence break the volcano down to sea level. At this point, the volcano becomes an atoll, with a ring of coral and sand islands surrounding a lagoon. All the Hawaiian islands west of the Gardner Pinnacles in the Northwestern Hawaiian Islands are in this stage.
[edit] Guyot Stage
Atolls are the product of the growth of tropical marine organisms, so these islands are only found in warm tropical waters. Eventually, the Pacific Plate carries the volcano supporting an atoll into waters too cold for these marine organisms to maintain a reef by growth. Volcanic islands located beyond the warm water temperature requirements of reef building (hermatypic) organisms become seamounts as they subside and are eroded away at the surface. An island that is located where the ocean water temperatures are just sufficiently warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin Point. Islands more polar evolve towards seamounts or guyots; islands more equatorial evolve towards atolls (see Kure Atoll).
After the reef dies, the volcano subsides or erodes below sea level and becomes a coral-capped seamount. These flat-topped seamounts are called guyots. Most, if not all, the volcanoes west of Kure Atoll as well as most, if not all, the volcanoes in the Emperor Seamount chain are guyots.
[edit] Other patterns
Not all Hawaiian volcanoes go through all the stages of activity described here. For example, Ko‘olau Volcano on O‘ahu—which prehistorically was mostly destroyed by a cataclysmic landslide—never went through the postshield stage and became dormant for hundreds of thousands of years after the shield stage before coming back to life for the rejuvenated stage of activity; some volcanoes apparently never rose above sea level; and there is no evidence to suggest that West Moloka‘i volcano ever went through a rejuvenated stage, while its younger neighbors, East Moloka‘i and West Maui volcanoes, have evident rejuvenated lavas.
