The volcano Gjálp is located between the volcano Bárðarbunga and the volcano Grímsvötn in the center of the Vatnajökulll ice cap.  The last volcanic eruption there was in the year 1996. The eruption was quite big and there was a lot of  glacier ice melting that caused a lot of water gathering up inside the glaciers but in the end the water broke through from the underneath the glacier  through Grímsvötn volcano and down through Skeiðarárjökull outlet glacier and down to Skeiðarársandur floodplain area. 

Photo from Wikipedia

The last volcanic eruption before the year 1996 was in 1938. During the volcanic eruption in 1996 there was a mountain that formed inside the glaciers. At the time people took a helicopter there with the Icelandic flag and gave the mountain the name Gjálp, related to the Nordic myths. Gjálp was the daughter of the troll Geirröður and each time Þór had to cross the river Vimur Gjálp did cause a flood in the river to make it hard for Þór to cross it. Years later the mountain Gjálp was again covered with glacial ice.

The glacier outburst that came down during the eruption in 1996 was estimated to be around 50.000 cubic metres per second and was the biggest one in Iceland since the eruption in the volcano Katla in the year 1918. It did cause a lot of damage on roads and bridges on Skeiðarársandur floodplain.