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Venus has some big volcanoes, but the highest one, Maat Mons, has a height of only 8 km, about that of the Big Island. For some reason, Venus has not produced any super-Hawaiis the way that Mars has, despite its lack of noticeable plate tectonics.
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Mt. Everest is about as high as any mountain can be on Earth - or Venus. At the sea level it produces 3300 atm of pressure - almost enough to cause rock to flow. Any higher, and the rock WILL flow, settling the mountain. (Below sea level the pressure from above is balanced by the pressure from the hydrosphere.)
By the same reason, your hypothetical Super-Hawaii would never grow to 15 km. Once it reached about 9 km, it would only expand outward, not up. And with Mars' gravity being 38% of Earth, 25 km also seems to be the height limit.
Note: A mountain made of solid iron could be talller