Could ancient tectonic activity on Mars be due to it having oceans of sulphuric acid?
First, tectonic activity have not ceased on Mars, although the planet is in a &quot;stagnant lid&quot; stage, which represents the last stage before a body becomes tectonically dead. See the answer to Is there convection in Mars&#39; mantle?
Second, tectonic activity is caused by internal heat. As a planet gradually loses its internal heat, tectonic activity slows down, until it eventually stops. There is no need for more convoluted explanations.