You'd think that asking the question "where is the highest volcano?" would be relatively simple. But not when you get into the geologists' squabble, where they split hairs about how you measure them and from what point.

For example, Llullaillaco (22,109'above sea level) in Chile/Argentina, is deemed to be the world's tallest and historically active volcano, having erupted three times in the last century. But less than 300km to the south is Ojos del Salado, which at 22,595' above sea level, shows fumaroles around the ridge that indicate activity. It has been surmised that the lack of historical record, is due to its isolated location, never having been seen and written about. Then again, sometimes things were seen, that didn't actually happen. Such as Charles Darwin reporting during a voyage on the ship Beagle, that he observed Mount Aconcagua showing activity. But Aconcagua is not a volcano, merely a plain mountain, the highest in the Americas. It owes its height not to lava, but imbricate thrust faulting.

Now, if you want to get truly picky, you could say that Ojos del Salado is the highest volcano above sea level, but its height from the base, is only about 6500'. The Mauna Lao shield volcano comes up some 8500' short of Salado's sea level height, but has a base that is ten times taller.

Of course, if you want a really big volcano, you'd have to look to the planet Mars. There you'll find Olympus Mons, the largest volcano known anywhere. The peak rises approximately 14.9 miles into the frigid Martian atmosphere. The ridged slope that drops down from the peak is nearly 340 miles wide.