Olympus Mons

Olympus Mons is a massive shield volcano located on the planet Mars.

It holds the title of the tallest volcano on Mars and the tallest planetary mountain in our solar system. It is approximately tied with the mountain Rheasilvia on the asteroid Vesta for the tallest discovered mountain in the solar system. This enormous structure is part of a larger volcanic region known as the Tharsis Montes. Because of its immense scale, Olympus Mons is a primary subject for scientists studying Martian geology and volcanic history.

The volcano is classified as a shield volcano, which means it has a very gently sloping profile. This shape is similar to the volcanoes found in the Hawaiian Islands on Earth. The average slope of its flanks is only 5%. Because the slopes are so shallow, the volcano's shape is sometimes compared to a circus tent held up by a single, off-center pole. The flanks are steepest near the middle and become even shallower toward the base, creating a concave upward profile. This gentle slope makes the mountain so large that an observer on the surface could not see its entire profile at once. The curvature of Mars would hide the edges from view, and someone standing at the summit might not even realize they were on a massive mountain.

Several specific factors allowed Olympus Mons to reach such extraordinary heights. First, Mars lacks mobile tectonic plates, which are the moving sections of the crust found on Earth. On Earth, plates move over stationary hotspots to create chains of smaller volcanoes. On Mars, the crust remains fixed over a hotspot, allowing lava to pile up in one single location for a long time. Second, Mars has weaker gravity than Earth, which helps support such a massive structure. Finally, there are less intense weather systems on Mars to cause erosion. These combined factors allowed the volcano to grow to a height of nearly 25 kilometers, which is about 2.5 times the elevation of Mount Everest above sea level.

The summit of Olympus Mons features a complex structure of six nested calderas. A caldera is a large, irregular depression formed by the collapse of a volcano's roof. This collapse happens when a subsurface magma chamber is depleted or emptied during an eruption. Each of these six calderas likely represents a separate pulse of volcanic activity. Scientists estimate that the largest caldera segment may have once been a single, massive lava lake. Based on geometric models, the magma chamber associated with this largest caldera is estimated to lie at a depth of about 6 kilometers below the floor.

Geologically, Olympus Mons was built by many thousands of highly fluid, basaltic lava flows. These flows poured from volcanic vents over a very long period of time. The composition of the volcano is roughly 44% silicates and 17.5% iron oxides, which contribute to the red color of Mars. It also contains 7% aluminum, 6% magnesium, 6% calcium, and a high proportion of 7% sulfur dioxide. These materials indicate the surface is largely composed of basalts and other mafic rocks. These rocks erupt as low-viscosity lava, which flows easily and creates the low gradients seen on the volcano's surface.

Astronomers have been aware of this feature since the late 19th century. At that time, it was known as the albedo feature Nix Olympica, which is Latin for "Olympic Snow." While they suspected it was a mountain, it was not confirmed as such until space probes arrived. The volcano formed during the Martian Hesperian Period and continued erupting well into the Amazonian Period. It last erupted approximately 25 million years ago. However, some lava flows on the northwestern flank have been dated as recently as 2 million years old. This suggests the volcano may still be volcanically active in a very quiet and episodic way.

The structure of the volcano is also asymmetrical. The northwestern flank is longer and shallower, showing signs of extension like large slumps and normal faults. This area includes a massive escarpment, or cliff, that can be up to 6 kilometers tall. This cliff may have been created by enormous landslides from the volcano's flank. In contrast, the southeastern side is steeper and shows signs of compression, such as step-like terraces. These differences may be caused by how the volcano spreads laterally at its base as it grows.

Finally, the environment at the top of Olympus Mons is extreme. The atmospheric pressure at the summit is only about 72 pascals. This is only about 12% of the average Martian surface pressure. For comparison, the pressure at the top of Mount Everest on Earth is about 32,000 pascals. Even in this thin air, high-altitude clouds and Martian dust can still be found drifting over the summit.