I asked Gemnini why Olympus Mons could grow so tall. It said...
"Yes, the lower gravity of Mars compared to Earth is considered one of the key reasons why Olympus Mons was able to grow so incredibly tall.
Here's the breakdown of why:
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Lower Gravity: Mars has about 38% of Earth's surface gravity.
This means that the volcanic rock making up Olympus Mons weighs significantly less than it would on Earth. Consequently, the mountain could grow much taller before its own weight caused it to collapse or before the Martian crust could no longer support its immense load. Taller structures are simply more stable under lower gravity. -
Lack of Plate Tectonics: This is arguably just as important, or even more important, than lower gravity. On Earth, the tectonic plates are constantly moving over stationary "hot spots" in the mantle (plumes of magma).
As a plate moves, new volcanoes form over the hotspot, creating chains like the Hawaiian Islands. No single volcano stays over the magma source long enough to grow to the scale of Olympus Mons. On Mars, the crust appears to be largely stationary (no active plate tectonics). This means Olympus Mons likely sat over a persistent hotspot for potentially billions of years, allowing lava to continuously pile up in the same location, building one enormous structure instead of a chain of smaller ones. -
Longer Eruption Duration: Related to the lack of plate tectonics, the magma source feeding Olympus Mons was likely active over a much longer period than typical terrestrial volcanoes experience while remaining in one place, contributing to its massive volume.
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Lower Erosion Rates: While not directly related to its peak height during formation, the lack of significant water and wind erosion on Mars compared to Earth has helped preserve Olympus Mons's immense size over geological time.
In summary: Lower gravity definitely played a significant role by allowing the structure to support itself at a much greater height. However, the lack of plate tectonics, allowing the volcano to remain over its magma source for an exceptionally long time, is the other critical factor explaining the sheer scale of Olympus Mons compared to anything found on Earth."
