The third phase of the eruption in Eyjafjallajökull lasted from 5th to 17th of May and was characterized by increased eruption activity and ash production.  New magma intrusions occurred during the phase that mixed with older silica-rich magma, changing the composition of the magma in the eruption, and the eruption went from being a Strombolian eruption to a Vulcanian eruption. Vulcanian eruptions are quite powerful explosive eruptions and the magma in such eruptions is usually intermediate. The upper part of the magma in the eruption channel can cool and form a plug, but below it the pressure builds up until the plug gives way. That results in an explosion that ejects tephra and gas away from the crater. The in-between time of explosions can be a few seconds and up to several hours. The eruption plume in this phase went up to a height of 9 km, but was usually at a height of 6-7 km, and high production of fine ash colored the plume gray. It is estimated that the tephra discharge rate in this phase was around 150-200 tons per second, with a peak discharge of about 400 tons per second. The power of the eruption was somewhat unstable during this phase, but the activity gradually began to decrease after 11th of May. After 18th of May, the activity began to decrease significantly and marks the beginning of the fourth and final phase of the eruption.