Cross-section of the Hawaiian Chain - Northwest to Southeast
  The Northwestern Hawaiian Islands are part of a long line of volcanoes.  A hotspot under the oceanic plate provides lava to build mountains from the bottom of the sea.  As the Pacific Plate slowly moves northwest over the hotspot, a chain of volcanic mountains is created.  The hotspot that has created the Hawaiian Islands has been erupting from underneath the Pacific Plate for more than 50 million years.
Stages of growth and decline of volcanic islands and coral atolls   As undersea volcanoes grow they eventually are exposed on the surface.  They continue to grow as long as new lava is extruded.  Eventually, when an island has moved off of the hotspot, it no longer increases in size.  

Subsidence and erosion gradually wear away volcanic islands.  Eventually, all that is left of the island is a ring of coral, an atoll.  Given enough time, this too will sink under the waves, eventually sinking so low that corals cannot grow and maintain the atoll.  These underwater mountains are called guyots.
  

  For most coral to grow, water must be relatively warm and shallow, since energy from the sun is used in photosynthesis.  Deep water blocks all sunlight and only very specialized corals can survive.

 

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