Submarine Preshield Stage
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. 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 alkali basalt. Due to stretching forces, the development of two or more rift zones is common. The lava accumulates in a shallow magma storage reservoir.
Because the eruptions occur with the volcano underwater, the form of lava typically erupted is pillow lava. Pillow lava is rounded balls of lava that was given very little time to cool due to immediate exposure to water. Water pressure prevents the lava from exploding upon contact with the cold ocean water, forcing it to simmer and solidify quickly. 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 time progresses, eruptions become stronger and more frequent.
The only example of a Hawaiian volcano in this stage is Lōʻihi 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 Lōʻihi Seamount.
Read more about this topic: Evolution Of Hawaiian Volcanoes
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