Let’s start small – relatively – with the biggest volcano in the Solar System. Olympus Mons can be found in the Tharsis Montes region of Mars and rises to a peak of 25 kilometres (16 miles) high and 624 kilometres (374 miles) wide with an 80-kilometre (50-mile) wide caldera. It towers over even the tallest mountains on Earth, Everest at 8.8 kilometres (5.5 miles) and Mauna Kea (which is 10 kilometres/6.2 miles if measured from the ocean floor), while dwarfing our biggest volcanoes with around 100 times more volume than Hawaii’s Mauna Loa.
The volcanic Tharsis Montes region of Mars is actually home to several of the biggest volcanoes in the Solar System, including Ascraeus Mons and Elysium Mons, which are 14.9 kilometres (9.3 miles) and 12.6 kilometres (7.8 miles) high respectively. The reason why Mars is a great breeding ground for super-sized volcanoes is down to its geology and its gravity. On Earth, the tectonic plates are continuously moving over and under each other on top of the mantle, so that the lava is distributed over a wide area between many volcanoes instead of just one. On Mars, the crust doesn’t move in the same way, so the lava just piles up in the same spot. Because of the lower Martian gravity and higher rates of eruption, the lava flows are much longer, too.