Olympus Mons formed primarily as a massive shield volcano through prolonged volcanic activity driven by a magma plume beneath the surface of Mars. This plume erupted repeatedly over billions of years in the same stationary location because Mars lacks tectonic plate movement, allowing lava to pile up in one spot and build the enormous height and size of Olympus Mons. Additionally, Mars' lower surface gravity compared to Earth allowed the lava structures to grow much taller without collapsing. The volcano's base features steep escarpments formed by gravitational slumping or interaction of lava with possible subsurface water or sediments. This formation began about 3 billion years ago and is linked to the volcanic plateau of Tharsis, where other large volcanoes also exist. Key factors in Olympus Mons formation are:

• A stationary magma hotspot beneath Mars' crust.
• No tectonic plates to move the crust away from the magma source.
• Low Martian gravity supporting taller volcanic structures.
• Repeated lava flows building a broad, gently sloping shield volcano.
• Possible influence of underlying weak sediment layers and water for certain flank features.

Thus, Olympus Mons is a giant shield volcano built by continuous lava flows over a hotspot on Mars, amplified by the planet's unique geophysical conditions.