Over its 85 million year history, the Hawaii hotspot has created at least 129 volcanoes, more than 123 of which are
extinct volcanoes,
seamounts, and
atolls, four of which are
active volcanoes, and two of which are
dormant volcanoes. They can be organized into three general categories: the
Hawaiian archipelago, which comprises most of the
U.S. state of Hawaii and is the location of all modern volcanic activity; the
Northwestern Hawaiian Islands, which consist of coral atolls,
extinct islands, and
atoll islands; and the
Emperor Seamounts, all of which have since eroded and
subsided to the sea and become seamounts and
guyots (flat-topped seamounts). The tallest mountain in the Hawaii chain, Mauna Kea, rises above
mean sea level. Measured from its base on the seafloor, it is the world's tallest mountain, at ;
Mount Everest rises above sea level. Hawaii is surrounded by a myriad of seamounts; however, they were found to be unconnected to the hotspot and its volcanism. Kīlauea erupted continuously from 1983 to 2018 through
Puʻu ʻŌʻō, a minor volcanic cone, which has become an attraction for volcanologists and tourists alike.
Landslides The Hawaiian islands are carpeted by a large number of landslides sourced from volcanic collapse. Bathymetric mapping has revealed at least 70 large landslides on the island flanks over in length, and the longest are long and over in volume. These debris flows can be sorted into two broad categories:
slumps, mass movement over slopes which slowly flatten their originators, and more catastrophic
debris avalanches, associated with flank and
sector collapse, which fragment volcanic slopes and scatter volcanic
debris past their slopes. These slides have caused massive
tsunamis and earthquakes, fractured volcanic massifs, and scattered debris hundreds of miles away from their source. Active slumping is currently taking place on the south flank of the
Big Island, where the
Hilina Slump comprises a mobile portion of the island's mass south of Kīlauea. Slumps tend to be deeply rooted in their originators, moving rock up to deep inside the volcano. Forced forward by the mass of newly ejected volcanic material, slumps may creep forward slowly, or surge forward in spasms that have caused the largest of Hawaii's historical earthquakes, in 1868 and 1975. Debris avalanches, meanwhile, are thinner and longer, and are defined by volcanic amphitheaters at their head and hummocky terrain at their base. Rapidly moving avalanches carried blocks tens of kilometers away, disturbing the local
water column and causing a tsunami. Evidence of these events exists in the form of marine deposits high on the slopes of many Hawaiian volcanoes, GPS measurements on the eastern flank of Hawaii Island over a 5 year epoch show the pattern of collapse with velocities of up to relative to the Pacific Plate
Evolution and construction |alt=Animation showing an intact volcano that gradually shrinks in size with some of the lava around its perimeter replaced by coral Hawaiian volcanoes follow a well-established life cycle of growth and erosion. After a new volcano forms, its lava output gradually increases. Height and activity both peak when the volcano is around 500,000 years old and then rapidly decline. Eventually it goes dormant, and eventually extinct. Weathering and erosion gradually reduce the height of the volcano until it again becomes a seamount. This life cycle consists of several stages. The first stage is the
submarine preshield stage, currently represented solely by
Kama'ehuakanaloa. During this stage, the volcano builds height through increasingly frequent eruptions. The sea's pressure prevents explosive eruptions. The cold water quickly solidifies the lava, producing the
pillow lava that is typical of underwater volcanic activity. As the seamount slowly grows, it goes through the
shield stages. It forms many mature features, such as a
caldera, while submerged. The summit eventually breaches the surface, and the lava and ocean water "battle" for control as the volcano enters the
explosive subphase. This stage of development is exemplified by explosive steam vents. This stage produces mostly
volcanic ash, a result of the waves dampening the lava. Mechanical collapse, indicated by large submarine landslides adjacent to landslide scars on the islands, is an ongoing process that shapes the early phases of volcano construction for each of the islands. After the subaerial phase the volcano enters a series of
postshield stages involving mechanical collapse creating subsidence and erosion, becoming an atoll and eventually a seamount. Once the Pacific Plate moves it out of the
tropics, the reef mostly dies away, and the extinct volcano becomes one of an estimated 10,000 barren seamounts worldwide. Every Emperor seamount is a dead volcano.
Coral reef development on Hawaiian Hotspot islands Reef growth and morphology often show the progression from underwater volcano to subaerial shield to seamount. The process of reef building around the margins of a volcanic island once it is formed, relates to both local island subsidence and global sea level increase. Other local factors such as water temperature and topography are important in reef formation. As island
subsidence progresses, fringing reefs develop into
barrier reefs and once the volcano becomes a seamount, barrier reefs form
atolls.
Midway Atoll is a good example of the final stage of the evolution of a
hotspot volcanic island. ==See also==