Magma, formed by melting of the mantle
at the edge of the hotspot rises from the base at the plate
through cracks and fissures.
Erupted lava is quenched by cold water forming pillow basalt.
As the plate carries the volcano directly over the hotspot
the chemistry of the lava changes
to less viscous lava than the previous, more-alkali rich phase.
The volcano builds with repeated cycles of construction and collapse.
Collapse results in explosive bursts of pyroclastic material, impressive at the surface.
It is interbedded with pillow basalt until lava can flow above the surface.
Dense lava flows perch on this moderately unstable pile.
Large landslides are common.
High eruption rates increase over the center of the hot spot
Over hundreds of thousands of years
countless lava thin flows result in a broad, gently sloping shield.
The volcanoes are characterized by having large
shallow magma reservoirs that erupt frequently.
Summit calderas alternately build and collapsed many times in the life of a shield volcano.
As the volcano moves from the center of the hotspot
eruptions decrease and the summit magma chamber solidifies.
After a non-eruptive time gap renewed eruption of more-viscous
alkali basalt produce a steep hummocky cap on some,
but not all, shield volcanos.
These reactions are short-lived and form clusters
of steep-sided cinder cones.
Erosion becomes the chief force shaping the land surface
Large gullies form as erosion outpaces production
and steep coastal cliffs develop ass the island begins to shrink
due to erosion and subsidence. Coral reefs grow
in the shallow water around the flanks. Hundreds of thousand years
and several hundred kilometers from the hot spots
lava that was stored deep beneath the volcano rises to erupt
a last burst of lava. This lava is higher in alkalis than all previous eruptions.
Continued erosion plus the increasing weight of the cooling sea floor
result in the creation of extensive reefs atop the atolls.
As the island moves further from the buoyant affect of the hotspot
it continues to sink, pulling the coral reefs too deep for coral to grow.