Hank: When it comes to the rocky planets of our solar system, the Earth is the biggest one in the family.  Good for us!  So you'd think that Earth would also have the biggest rocky features like mountains and volcanoes.  But when the Mariner 9 probe arrived on Mars in 1971, it found the gigantic volcano Olympus Mons, towering far above the Martian clouds, and it turns out to be the largest volcano anywhere in the solar system.  Olympus Mons is more than 600 km wide at the base, sprawling across an area nearly the size of France.  Because there's no sea level on Mars to use as a reference, it's exact tide is a little tricky to define, but even the most conservative method measures Olympus Mons at more than 20km higher than the surrounding planes.  That's more than twice as tall as Mount Everest, and five times as high as Earth's tallest volcano, Mauna Loa in Hawaii.  Because it's so broad at the base, its slope is so gradual that if you were trying to climb to the top, you'd hardly even be able to tell that you were climbing the tallest volcano in the solar system.  But it is surrounded by steep cliffs that lead to about a 6km drop so it might not be the best place for a hike.  Since it first formed more than 3 and a half billion years ago, Olympus Mons has had so many eruptive periods, separated by long dormant periods, that at least six caldera, or volcanic craters, have merged into one 25km wide mega-crater with walls more than 2.5km high.  And Olympus Mons isn't even the only giant volcano on Mars.  Not far away, three only slightly smaller volcanoes join Olympus Mons in the area called the Tharsis Bulge, which is coincidentally my stripper name.  

But this collection of huge features raises an interesting question: Why does a planet that's smaller than Earth have so many volcanoes that are bigger than any found here?  As we've gotten to know the Red Planet, we've learned that its volcanoes are bigger because they stayed active nearly a thousand times longer than typical Earth volcanoes.  And oddly enough, that's because its surface is less active than ours.  Here on Earth, volcanoes like Mauna Loa form when one of the tectonic plates slides over a hot spot in the mantle.  The hot spot pushes liquid magma up through weak points in the crust, gradually building up a volcano.  But Earth's crust is constantly in motion.  Its plates are adrift on a slowly bubbling underground ocean of molten rock that keeps the plates moving.  So over time, the drifting of plates moves the volcano away from a hot spot after just a couple million years or less, and a new volcano pops up just downstream.  

On Mars, though, the crust cooled and grew thick too quickly to separate into tectonic plates, at least as we know them now, so the crust has probably stayed stationary relative to volcanic hotspots for billions of years.  Once the magma started escaping, the volcanoes just stayed put.  

But there's another reason Martian volcanoes grew so incredibly large.  On Earth, mountains and volcanoes stop growing when they become so massive that they actually squish the crust underneath them and pull themselves back down.  But Mars' crust is much thicker and stronger.  That, combined with the planet's weaker gravity, allows huge rocky features to grow larger without collapsing.  In fact, scientists recently learned that the youngest lava flows from Olympus Mons are actually only a couple million years old.  In geologic time, that's almost nothing.  So rather than being an extinct volcano, Olympus Mons may just be a sleeping giant slowly gathering heat below the surface and preparing for a future eruption that will make it even bigger.  So while being the bigger planet might be fun for us to brag about, when it comes to having giant volcanoes all over the place, it's probably better to let other planets have those.

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