its area is
as large as the entire Hawaiian archipelago.

Gigantic volcanoes of
Mars

Mars has also been the target
for numerous robotic Earth emissaries. They have revealed that Mars
today is almost volcanically extinct - but that hasnt always
been the case. The Mariner 9 spacecraft in 1971 and the two Viking
spacecraft in 1976 first returned images that tell a story of more
dynamic times. Like Venus, Mars boasts an impressive assortment
of volcanic features.

Indeed, the largest volcano
in the Solar System is found on Mars - Olympus Mons. Situated in
the Tharsis province, a huge bulge in the planets crust, Olympus
Mons towers some 27 km high, three times higher than Mauna Loa,
Earths largest volcano. At least twelve other volcanoes are
found within the Tharsis province including Arsia Mons, Pavonis
Mons and Ascraeus Mons - three giants in their ownfor seas). We
know differently today. In truth they are some of the Solar Systems
largest expanses of basalt, the most common volcanic rock. The lunar
basalts were erupted thousands of millions of years ago, at temperatures
as great as 1400 C. The very high temperatures meant the lavas were
as fluid as engine oil, allowing the flows to cover vast distances.

Mercury also shows evidence
for past volcanism, but youll need to look a lot harder to
find it. Masked by craters that pockmark its surface, Mercury hides
lava plains similar to the lunar maria. The craters themselves actually
give some indirect evidence for past volcanic activity on Mercury.
The planet lacks craters less than 50 km in diameter, and the simplest
interpretation is that lavas buried them thousands of millions of
years ago, when Mercury was a much fierier