The 2011 and 2004 Grímsvötn eruptions 

Grímsvötn is Iceland's most active volcano, with more than 70 eruptions occurring in the last 1.000 years. It is a subglacial volcano lying under a few hundred meters of ice from the Vatnajökull ice cap. Like the Bárðarbunga volcano it is also situated near the center of the mantle plume that feeds the volcanos with magma.

Glacier floods or jökulhlaup's often occur when the Grímsvötn volcano erupts.

A geothermal area in the ice-covered caldera constantly melts the ice and adds to the Grímsvötn caldera lake. When the lake fills, the water lifts the ice and breaches from under the ice cap, leading to a glacial flood or jökulhlaup, which is the Icelandic word and international term for the phenomenon. A jökulhlaup can also be triggered by ice melting in a volcanic eruption or the reverse; jökulhaup triggers an outbreak by releasing pressure off the magma chamber below. 

Eruptions in the Grímsvötn central volcano are always explosive or hydromagmatic eruptions. They produce a lot of tephra or volcanic ash when 1100-1200°C hot magma encounters ice and water and causes explosions that tear the magma into fine particles. 

The 2004 Grímsvötn eruption was an example of a jökulhlaup triggered eruption. It began late in the evening of November 1st, 2004, and lasted for five days. A bent-over 6-10 km high eruption plume deposited ash within 3 km from the source and within the ice cap. The eruption is classified as a small-scale eruption with a volume of tephra produced estimated as 0.05 km3.  

The eruption plume in the 2011 Grímsvötn outbreak reached up to 20 km and caused problems for aviation

The 2011 Grímsvötn eruption. After just five and a half years of recharging, an intense seismic swarm started underneath the Grímsvötn volcano on May 21st, 2011. Only 70 minutes later, an eruption had broken out of the ice cap, and the eruption column rose at high speeds, reaching heights of about 20 km in less than 30 minutes. Even though the eruption only lasted one week, it was one of the large-scale volcanic events occurring in Grímsvötn once a century. Due to the size and height of the ash plume, aviation warnings were issued and 900 flights were canceled. The eruption ended on May 28th, and it is estimated to have produced about 0.7 km3 of tephra, more than twice the amount Eyjafjallajökull spewed out in 63 days.