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. Results from the Hawaiʻi Scientific Drilling Project confirm that the eruption rates of the postshield stage volcanoes started decreasing between 600 and 400 thousand years (ka) ago.
Eruptions in the postshield stage cap the volcano with a carapace of lava, containing low silica and high alkali contents, the reverse of the stage before it. Some Hawaiian volcanoes diverge from this, however. Lava is erupted as stocky, pasty ʻaʻā flows along with a lot of cinder. Caldera development stops, and the rift zones become more inactive. The new lava flows increase the slope grade, as the ʻaʻā never reaches the base of the volcano. These lavas commonly fill and overflow the caldera. Eruption rate gradually decreases over a period of ca. 250,000 years, eventually stopping altogether as the volcano becomes dormant.
Mauna Kea, Hualālai, and Haleakalā volcanoes are in this stage of activity.
Read more about this topic: Evolution Of Hawaiian Volcanoes
Famous quotes containing the word stage:
“It is a mass language only in the same sense that its baseball slang is born of baseball players. That is, it is a language which is being molded by writers to do delicate things and yet be within the grasp of superficially educated people. It is not a natural growth, much as its proletarian writers would like to think so. But compared with it at its best, English has reached the Alexandrian stage of formalism and decay.”
—Raymond Chandler (1888–1959)