The highest volcanoes in the solar system are shield volcanoes
on Mars, the highest of all being Olympus Mons. Its summit is
26 km above the level of the surrounding plain, and it is nearly
500 km across. By comparison, the Hawaiian islands on Earth
rise about 9 km above the surrounding sea bed.
Shield volcanoes grow in height gradually because repeated
eruptions take place from one vent. The shield volcanoes on
Mars are much larger than those on Earth for several reasons.
Though apparently no longer active, they probably formed early
and remained active for a much longer time than the age of any
volcanoes on Earth. On Earth, volcanic hotspots move as the
continental plates gradually shift, so volcanoes do not have time
to build very high in a single place. In addition, the lower gravity
on Mars allows the erupted material to pile up much higher,
before slumping down under its own weight.
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