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Pre-Shield Stage
The pre-shield stage, also referred to as the submarine stage, begins on the ocean floor when a hotspot from the earth's mantle creates a conduit through the oceanic crust. The hotspot is thermal plume of magma from deep in the mantle. Lo'ihi is an example of a Hawaiian volcano in the pre-shield stage. This stage is characterized by mantle-driven magma that flows through a vertical conduit to a shallow summit reservoir. This magma can erupt from summit vents or be injected into rifts. The lava is alkali in nature. This lava cools quickly under water, forming a steep-sided structure.
Shield-Building Stage
The shield-building stage occurs as the volcano emerges from the ocean and rises above sea level. Around 95 percent of the volcano's volume is created during this stage. Eruptions are still mantle-driven and rifts continue to form early in this stage. For example, Hualalai volcano has three rifts. This stage represents a transition to more fluid basaltic lava. This lava erupts in fountains and flows that produce the more gently-sloping shield shape. Mauna Loa on the Big Island, an example of a volcano in the shield-building stage, is the world's largest volcano and one of the most active.
Post-Shield Stage
Following the shield-building stage, the volcano's activity will decrease. This is caused by the volcano moving away from the mantle hotspot. The magma begins to fractionate and once again becomes more alkali. This results in small-scale eruptions that form cinder and spatter cones over the vents. This stage also is characterized by erosion and sinking. The weight continues to cause the tectonic plate to sink deeper into the mantle below the volcano. As the volcano erodes and sinks, it may form an atoll or guyot. Haleakala and Mauna Kea are examples of Hawaiian volcanoes in the post-shield stage.
Rejuvenated Stage
Some Hawaiian volcanoes enter a final stage, known as the rejuvenated stage. This stage is characterized by an increase in volcanic activity. However, these eruptions are composed of strongly alkalic lava that is thick and gas-rich. This results in more explosive, Strombolian-style eruptions. These eruptions create steep-sided cones of layered ash and pyroclastic material. Diamond Head and Hanauma Bay, both located on Oahu, are examples of Hawaiian volcanoes in the rejuvenated stage. Following this stage, Hawaiian volcanoes become permanently dormant.