Volcanic eruptions are common on Earth, but the presence of volcanoes is not unique to Earth. All of the inner rocky planets, as well as a multitude of moons, have evidence of past or current volcanic activity. On Earth, the tallest volcano (and tallest mountain measured from base to summit) is Mauna Kea at 33,500 feet (10,210 meters) tall. It may seem big, but even the tallest mountains on Earth are tiny compared to a particular volcano on Mars. Mars is home to the tallest volcano in the solar system, a behemoth called Olympus Mons. Rising to 22,000 meters (72,000 feet), it is two and a half times the size of Mount Everest. Olympus Mons is 674 miles (324 kilometers) in diameter, making it about the same diameter as the state of Arizona. By area, Olympus Mons covers 120,000 square miles (300,000 square kilometers), roughly the same area as Italy. Interestingly, despite its size, Olympus Mons is a relatively flat volcano. The slope of Olympus Mons is on average only five degrees. In fact, if you were to stand on top of Olympus Mons, its lack of a steep slope and its size would make it impossible to see a slope. Rather, it looks like you’re standing on a flat surface instead of the tallest mountain in the solar system. All these factors make Olympus Mons the largest known volcano in the solar system.
How did Olympus Mons get so big?
Volcanoes on Mars are generally larger than those found anywhere else in the solar system. Two main factors contribute to this: Mars’ low surface gravity and the absence of plate tectonics. Volcanoes on Mars form in much the same way as on Earth. Molten rock from the mantle is pushed up to form a mountain, with an extensive system of tubes connecting the mountain to the magma inside the planet. However, there is one major difference between Earth and Mars. Earth’s crust is broken up into separate plates, which in turn drift to the surface as partially molten rock sinks beneath the crust. This process is called plate tectonics. Beneath the crust, in the mantle, there are stationary hot spots that lead to the formation of volcanoes. These hotspots actively spew magma from the mantle to the surface, forming a volcano in the process. Since the crust is constantly in motion, a hotspot will often form multiple volcanoes as the crust moves away. You can think of it as a sort of conveyor belt, in which magma rises to the surface, forms a volcano, and then the crust pushes that volcano away and allows another volcano to form. That’s what happens on Earth, but Mars is another story. The Martian crust is not divided into separate plates, and Mars therefore does not experience any plate tectonics. Therefore, the hotspot and the crust above it are stationary. A single hot spot on Mars will simply continue to add material to the same point in the crust, resulting in the formation of a single, gigantic volcano. Additionally, Mars’ low surface gravity allows volcanoes to grow to a much larger size in a shorter period of time than on Earth. Olympus Mons is a type of volcano called a shield volcano. Rather than forming from massive eruptions, shield volcanoes form when lava flows from the top of the volcano to its base, eventually solidifying and causing the volcano to grow larger.
Is Olympus Mons active?
If Olympus Mons were to ever erupt, it would be unlike anything we have experienced on Earth. For now, scientists do not know if Olympus Mons is active or not. Data from nearby lava flows suggest that Olympus Mons experienced small eruptions as recently as two million years ago, which is unusually short in geological terms. This suggests that Olympus Mons may still be partially active, but it is unclear if it will ever experience a full-scale eruption in the future.