While the the surface of Mercury is very hot,
there are currently no active volcanoes on
Mercury because it no longer has a molten rock
core. Volcanoes are formed when molten rock
from a planet's core violently explodes to the
surface - without a magma to flow, there are no
volcanoes. This wasn't always the case, though.
Early surveys of Mercury (1974) noticed that its
surface is very flat, similar to lava fields on
Earth, but only found 11 volcanic domes, which
wouldn't be enough to cover the planet.
The most recent NASA probe, MESSENGER, passed
Mercury in October 2011 and found evidence of
fissure vents. This showed that lava on Mercury
flowed from rifts in the ground, like water from
an overflowing bathtub, and not in explosions like
many volcanoes on Earth. These vents erupted 1 to
4 billion years ago. So, the surface of Mercury
was shaped by volcanoes for nearly 3 billion years
after its formation (~4.5 billion years ago), but
the molten core responsible has since cooled into
solid rock - no more volcanoes for Mercury.
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