Evolution of Hawaiian volcanoes
[1] Hawaiʻi's volcanoes rise an average of 4,600 meters (15,000 ft) to reach sea level from their base.
[2] As shield volcanoes, they are built by accumulated lava flows, growing a few meters or feet at a time to form a broad and gently sloping shape.
[2] Hawaiian islands undergo a systematic pattern of submarine and subaerial growth that is followed by erosion.
[1] The "assembly line" that forms the volcanoes is driven by a hotspot, a plume of magma deep within the Earth producing lava at the surface.
The age and location of the volcanoes are a record of the direction, rate of movement, and orientation of the Pacific Plate.
The pronounced 43-million-year-old break separating the Hawaiian Ridge from the Emperor Chain marks a dramatic change in direction of plate movement.
The understanding of the process was advanced by frequent observation of volcanic eruptions, study of contrasting rock types, and reconnaissance mapping.
High rainfall due to the trade wind effect impacts on the severity of erosion on many of the major volcanoes.
[3] When a volcano is created near the Hawaiian hotspot, it begins its growth in the submarine preshield stage, characterized by infrequent, typically low volume eruptions.
[4] 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.
Calderas form, fill, and reform at the volcano's summit and the rift zones remain prominent.
This volcanic phase, so named for the explosive reactions with lava that take place, begins when the volcano just breaches the surface.
[1] Once a volcano has added enough mass and height to end frequent contact with water, the subaerial substage begins.
[4] Eruption rates and frequencies peak, and about 95% of the volcano's eventual volume forms during a period of roughly 500,000 years.
During this subaerial stage, the flanks of the growing volcanoes are unstable and as a result, large landslides may occur.
This stage is arguably the most well-studied, as all eruptions that occurred in the 20th century on the island of Hawaii were produced by volcanoes in this phase.
The new lava flows increase the slope grade, as the ʻaʻā never reaches the base of the volcano.
[1] Eruption rate gradually decreases over a period of about 250,000 years, eventually stopping altogether as the volcano becomes dormant.
Eventually, the Pacific Plate carries the volcanic atoll into waters too cold for these marine organisms to maintain a coral reef by growth.
[1] Volcanic islands located beyond the warm water temperature requirements of reef-building 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.
