Olympus Mons as seen from space.
Olympus Mons is not only the largest volcano in the Solar System but also one of the most fascinating geological features. Standing at nearly 72,000 feet (22 km; 13.6 mi)—almost 2.5 times the height of Mount Everest—Olympus Mons holds the title for the tallest known planetary mountain. Olympus Mons is considered a shield volcano, and it is located in the western hemisphere of Mars, and understanding how Olympus Mons grew so large needs a look at its structure, the unique conditions on Mars, and the geological forces that shaped it over millions of years.
As mentioned before, Olympus Mons is a shield volcano, which is a type of volcano that has broad, gently sloping sides due to the eruption of low-viscosity basaltic lava that can travel long distances. These types of volcanoes are typically much wider than they are tall, and Olympus Mons is no exception. The base of the volcano spans about 373 miles (600 km) in diameter, which is roughly equivalent to the length of the state of Arizona. The caldera at its summit, a large, circular crater formed by volcanic activity, is about 50 miles (80 km) wide and contains multiple smaller craters.
Olympus Mons is relatively “flat,” meaning that its slopes are not as steep as those of other volcanoes, such as Mount St. Helens on Earth. This is a direct result of the flow characteristics of the lava that erupted from it and the overall geological environment of Mars.
Why Is Olympus Mons So Large?
A visualization of tectonic plate movements and mountain formation on Mars vs. Earth.
The size of Olympus Mons is the result of a combination of factors, but the primary reasons are the lack of tectonic plates, Mars’ lower gravity, and a long period of volcanic activity
First off, unlike Earth, Mars lacks tectonic plates. On Earth, there is constant tectonic movement, meaning that the land mass is never in the same spot for a long period of time. However, on Mars, due to the lack of tectonic plates, the land never moves around and stays in the same place. This allows for hotspots to continually feed magma to the same spot on Mars’ crust over millions of years, leading to large mountains like Olympus Mons to form.
In addition, Mars has only about 38% of Earth's gravity, which plays a significant role in the size of Olympus Mons. The lower gravitational pull allows volcanic structures to grow taller without collapsing under their own weight. On Earth, the greater gravity limits the height of volcanoes because the material on the slopes of the volcano would become unstable and collapse as the volcano grows taller. On Mars, however, the reduced gravity means that the lava and rock that make up the volcano’s structure are less likely to slump or collapse, allowing the volcano to grow to larger sizes.
While Earth has massive volcanoes like Mauna Loa in Hawaii, which stretches over 5.6 miles (9 km) in height, Olympus Mons makes all of these volcanoes look like dwarfs! The weaker gravitational pull of Mars allows Olympus Mons to maintain its massive height without succumbing to its own weight.
The enormous size of Olympus Mons can also be attributed to the result of prolonged and sustained volcanic activity over millions and millions of years. While it may not be as active now, Mars was much more volcanically active in the past and the large amount of lava that was once fed to Olympus Mons was what allowed it to rapidly grow to such a height. The volcanic activity that created Olympus Mons is thought to have been ongoing for more than 200 million years. During this time, vast amounts of basaltic lava were steadily released onto the surface, which slowly built up the broad, gently sloping shield that defines the volcano.
Olympus Mons and had eruptions that were mostly “effusive” in nature, characterized by the slow, steady release of lava that flows out over long distances. In contrast to explosive eruptions, which can create steep-sided volcanoes, effusive eruptions result in more flatter and broader volcanoes. The lava that erupted from Olympus Mons is relatively low in viscosity, meaning it flows easily and can travel great distances before cooling and solidifying. This allowed for the volcano to spread out and build up over a large area. Effusive eruptions also mean that Olympus Mons was built up gradually, over a long period of time, by many layers of lava flows, with each eruption adding more material to the volcano.
Mars has a very thin atmosphere as well, and it lacks the kind of weather systems on Earth. In other words, there is no rain, snow, or significant wind (at least compared to Earth). While Earth’s volcanoes are constantly being shaped by erosion (wind, water, glaciers, etc.), the lack of significant erosion on Mars has allowed Olympus Mons to retain its shape for a long time. On the contrary, Mars does experience dust storms, some of which are massive, but these are not enough to significantly alter the surface features of the planet.
The Future of Olympus Mons
The size of Olympus Mons compared to that of France.
While Olympus Mons is generally considered an “extinct” volcano, meaning it is no longer actively erupting, its massive size and geological features continue to be a subject of interest for scientists. Some think that the volcano could still be “dormant” rather than extinct, which would mean that it could theoretically erupt again in the future, although this is unlikely based on current data.
The study of Olympus Mons offers cool information into not only the history of Mars but also the geological processes that shape planets in the Solar System. The volcano’s massive size and long history make it a unique feature on Mars. Although Olympus Mons may not continue to get bigger like it used to, it is unlikely that any mountain will take its title of “tallest mountain.”