The largest explosive volcanic eruptions that have occurred on Earth generated giant ash clouds from rising plumes that spread in the stratosphere around a height of neutral buoyancy, with estimated supply rates that are in the range 10¹¹ to 10¹³ m³/s. These giant ash clouds are controlled by a balance between gravity and Coriolis forces, forming spinning bodies of nearly fixed proportions after a few hours and are initially insensitive to stratospheric winds. In contrast, volcanic plumes from eruptions with small to intermediate magnitude spread as inertial intrusions under the influence of stratospheric winds, with the Earth's rotation being unimportant. In the largest eruptions the giant spinning ash clouds typically develop diameters greater than 600 km and up to 6000 km in the most powerful super-eruptions, thus explaining why areas of continental size can be covered with volcanic ash. The radial expansion and spinning velocities are calculated at tens of metres per second and increase with eruption intensity. Higher spreading velocities carry larger ash grain sizes to a given radius, so that grain size at a given distance from the source increases with eruption intensity, consistent with geological observations.