There is no precise answer, since the molten rock that volcanoes expel during an eruption does not always advance at the same rate. The average is around 40 km / h, but it depends on the type of material and its composition, the volume of magma released and the orography of the terrain.
The face of a lava flow that contains little silica and has a basic pH – the most fluid ones – moves at about 10 km / h on a not very inclined terrain. However, when it circulates through the channels or furrows formed by the solidified lava on the side of the volcano, it reaches 60 km / h.
If the magma is acidic and rich in silica, it will be denser and more viscous, so it only travels a few meters every hour, even on a slope. The winners of the race are the pyroclastic materials expelled during an explosive-type eruption.
This occurs when the material solidifies in a dome shape over the crater, causing the pressure of gases and water vapor to release the magma at full blast, up to more than 200 km / h. The magmatic flow slows down as it cools and settles on the volcano's skirt.
