Complex sentence: Areas as far as from an explosive eruption could be covered by pyroclastic flows and surges; this coverage depends on the location of the vent or vents, topography, and volume of magma erupted.
Split into simpler sentences: Areas as far as from an explosive eruption could be covered by pyroclastic flows and surges. The actual areas covered by pyroclastic flows or surges depends on the location of the vent or vents, topography, and volume of magma erupted.