Hawaiian volcanoes - Part 5: Evolution to Atolls and Seamounts

In the last two posts I have looked at the growth of Hawaiian volcanoes. With increasing age and on their way north-westwards, away from the hotspot, the volcanoes not only stop growing due to lack of new magma input, but also get constantly worn down by continuous erosion.
Hawaiian mythology phrases this as:
No sooner does Pele create an island that Nā maka o Kahaʻi begins to destroy it.

Not only the water from ocean waves wear on the volcanoes, rain also erodes the volcano, creating deeply incised valleys.

_{KOKO Crater from the Lanai Lookout. The volcano gets worn down by rainfall and forms deep valleys that run down the flanks of the volcano.
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Additionally, the lithosphere cools as it moves away from the hot spot. The cooler lithosphere is denser and will slowly begin to subside, resulting in a gradual reduction of volcano height.
As the volcanoes continue to erode they will start to sink just below the sea surface. They are reduced to rocky islets and are home to corals that have grown on top and around them forming reefs. Charles Darwin was the first to say that coral reefs are continuing to grow as a volcano subsides and creating the low, circular groups of islands known as atolls.

<sub>Animation for the formation of coral atolls. Corals are represented in tan and purple. They begin to grow around an oceanic island forming a reef. As the reef expands, the island in the interior continues to subside, turning the reef into a full barrier reef. When the island completely subsides beneath the ocean surface. a ring of growing coral with an open lagoon in its center remains. This is the final atoll.
commons.wikimedia.org

Aerial photo of the Midway Atoll. The Midway Atoll is located to the western end of the Hawaiian Ridge.
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Eventually the islands will sink beneath the ocean surface and become seamounts. At their final stages in the northern part of the Emperor Ridge, the combination of gradual sinking and erosion has formed seamounts with flat, wave-eroded tops. These seamounts are called guyots after the geographer and geologist Arnold Guyot.

Corals grow in tropical conditions to form atolls. The Emperor seamounts are getting carried further and further north by the movement of the Pacific plate. Here the waters become colder. At some point the water is too cold, to support coral growth.

The ages for the completely submerged seamounts of the Emperor Ridge are hard to determine, but they are believed to be at least 70 million years old. There might have been even older islands that have been subducted into the Kuril-Kamtschatka Trench and melted back into the Earth’s mantle.

_{3-D depiction of the Bear guyot.
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Previous posts

Hawaiian volcanoes - Part 1: Introduction
Hawaiian volcanoes - Part 2: The Hawaiian Hotspot
Hawaiian volcanoes - Part 3: A growing volcano
Hawaiian volcanoes - Part 4: The Rejuvenation Stage

Sources

Zeilinga de Boer, J. and Sanders, D.T., (2002). Volcanoes in Human History: The Far-reaching Effects of Major Eruptions. Princeton University Press
https://commons.wikimedia.org/wiki/File:KOKO_Crater_from_the_Lanai_Lookout.jpg
https://en.wikipedia.org/wiki/Evolution_of_Hawaiian_volcanoes
https://en.wikipedia.org/wiki/Guyot
https://en.wikipedia.org/wiki/Arnold_Henry_Guyot
https://en.wikipedia.org/wiki/Midway_Atoll
https://volcanoes.usgs.gov/observatories/hvo/hawaiian_volcanoes.html