Mars, often called the Red Planet, is defined by geological features that dwarf anything found on Earth, especially its massive, long-extinct volcanoes. The largest mountain on Mars, and indeed the entire solar system, is the shield volcano known as Olympus Mons. Its size delivers a sense of the scale of Martian geology.
Identifying the Largest Mountain
Olympus Mons is a shield volcano, characterized by its broad base and gently sloping sides, which resembles a warrior’s shield lying on the ground. It towers far above the surrounding plains of Mars to a height of nearly 22 kilometers (approximately 13.6 miles). This makes it almost three times taller than Earth’s Mount Everest measured from sea level.
The volcano’s base stretches across a diameter of roughly 600 to 700 kilometers, an area comparable to the entire state of Arizona or the country of France. Despite its height, the mountain’s flanks have an average slope of only about 5 percent. This gradual incline is a characteristic feature of a shield volcano, built up by highly fluid lava flows over eons.
At its summit, Olympus Mons features a complex structure of six nested calderas, which are collapsed craters that formed as magma chambers emptied beneath the surface. This entire caldera complex measures about 85 kilometers across, a depression large enough to contain a major metropolitan area.
The Geological Context of Formation
The primary reason Olympus Mons grew so large is the absence of plate tectonics on Mars. On Earth, moving tectonic plates constantly shift the crust over underlying magma hotspots, limiting volcanic growth. Since the Martian crust remains fixed, a single magma plume continuously erupted at the same location for billions of years, piling material into one structure.
This extended, uninterrupted period of volcanic activity allowed the mountain to accumulate a large volume of basaltic rock. The low-viscosity lava flowed easily and spread widely, creating the characteristic broad, shallow slopes of the shield volcano type. Furthermore, the lower surface gravity on Mars played a significant role, allowing the structure to grow much taller before collapsing under its own mass.
The combination of a stationary crust and reduced gravitational pull resulted in a single, stable vent that continually fed the volcano. This process allowed the structure to achieve a height and volume not physically possible under Earth’s geological conditions.
Comparing Martian and Terrestrial Giants
Comparing Olympus Mons with the largest mountains on Earth helps put its scale into perspective. Earth’s highest mountain above sea level, Mount Everest, reaches a peak of about 8.8 kilometers. Olympus Mons is roughly two-and-a-half times that height, making it a different class of mountain.
When considering Earth’s largest shield volcano by volume, Mauna Kea in Hawaii, the difference is clear. Mauna Kea rises approximately 10 kilometers from its base on the ocean floor to its summit. However, the base of Olympus Mons is nearly six times wider than Mauna Kea’s base, demonstrating a difference in overall volume and footprint.
The Martian volcano covers an area so large that its curvature causes the peak to disappear below the horizon line when viewed from its base. If a person were standing on the summit, the base would not be visible due to the planet’s curvature.
The Surrounding Volcanic Region
Olympus Mons is situated near the northwestern edge of the Tharsis Bulge, an elevated volcanic plateau near the Martian equator. This bulge is a region of volcanic activity raised up to 10 kilometers above the average Martian surface. The weight of this region is believed to have had significant effects on the planet’s geological history.
The Tharsis Bulge is also home to three other shield volcanoes, which collectively form the Tharsis Montes chain: Ascraeus Mons, Pavonis Mons, and Arsia Mons. These three mountains are aligned in a straight line extending for thousands of kilometers. Each of the Tharsis Montes individually dwarfs any volcano on Earth.
Ascraeus Mons, for example, is the tallest of the three, rising 18 kilometers above the Tharsis plateau itself. The presence of these four volcanoes underscores that the Tharsis region was the center of prolonged volcanic activity on Mars.