What
is a Shield Volcano?
Shield volcanoes are the more quiescent, lumbering
giants of the volcano world. Although these types of volcanoes are not
small by any means, the eruptions they produce can be pretty "ho
hum" compared to the enormous explosive potential of the Extreme
Volcanoes. The biggest single mountain in the world is a shield volcano
that was slowly built up from the floor of the Pacific Ocean over hundreds
of thousands of years - Mauna Loa.
This massive mountain rises just over
13,000 feet from the surface of the Pacific Ocean, but from its true
base on the sea bed Mauna Loa towers over 33,000 feet tall. Mauna Loa
is one of five massive shield volcanoes that make up the Big Island
of Hawaii. This towering giant had some pretty humble beginnings.
A shield volcano like Mauna Loa owes
its shape to the way the lava erupts from a vent in the earths crust that begins as a fissure, or
crack. Pockets of superheated magma well up from beneath the crust,
causing it to bulge upward. As the sea floor bulges from the movement
of the magma, cracks form in the crust, sort of like the way the top
of a cake cracks as it bakes in the oven. These fissures in the crust
become weak areas of thin crust that give way to the upward force of
the magma, eventually allowing it to break through. The overlying weight
and pressure of the ocean water affects the way the magma emerges from
fissures in the sea floor. The runny lava oozes out of the fissures
and spreads out around the crack, cooling as it contacts the seawater.
This slow and gradual accumulation of thin layers of lava build up over
long periods of time, forming a long, shield-shaped volcano.
Shield volcanoes are not the only
type of volcano that forms on the ocean floor, nor are all shield volcanoes
formed only in the sea. As shield volcanoes like the Hawaiian Islands
build up from the sea floor they are known as sea mounts - undersea
mountains. But once they reach the surface of the sea they become islands.
As the eruptions of fluid lava continue unimpeded by the weight of overlying
seawater, the runny nature of the liquid lava continues to build wide
mountains with long, gentle slopes. Basalt lava tends to build enormous,
low-angle cones because it flows across the ground easily and can form
lava tubes that enable lava to flow tens of kilometers from an erupting
vent with very little cooling. ....
Stratovolcanoes-->
