Each island is essentially a mountainous volcano or group of volcanoes that has risen above the surface of the ocean and expelled lava to create land mass.
(Each island; is essentially; a mountainous volcano or group of volcanoes) (volcanoes; has risen; above the surface of the ocean) (volcanoes; expelled; lava; to create land mass)
Each island volcanoes volcanoes
is essentially has risen expelled
a mountainous volcano or group of volcanoes
above the surface of the ocean
lava to create land mass