Would that be because the pressure of the water and ice above balances the pressure from below. It is well known that when glacial ice retreats land that is laid bare below rises.2nd post: 'The boiler limits of Grímsvötn also show shear and collapse effects. After the water comes out, the volcanic system is losing pressure which may generate an eruption in the next few days, creating plumes of steam and ash.'
There is a live monitoring of the seismic activity. It seems to have decreased:' The water that was under the glacier that covers the volcano Grímsvötn , began to come out. A Jökulhlaup began to be created that rose more than 4 m, it is a matter of days before we begin to see columns of steam and ash in the main crater.'
It has blown before:The graphs show the average vertical motion of a seismometer per minute at three different frequency ranges (Hz). The vertical scale represents a change in output.
The Wiki has:
Grímsvötn (Icelandic pronunciation: [ˈkrimsˌvœhtn̥]; vötn = "waters", singular: vatn) is a volcano with a (partially subglacial) fissure system located in Vatnajökull National Park, Iceland. The volcano itself is completely subglacial and located under the northwestern side of the Vatnajökull ice cap. The subglacial caldera is at 64°25′N 17°20′W, at an elevation of 1,725 m (5,659 ft). Beneath the caldera is the magma chamber of the Grímsvötn volcano.
Grímsvötn is a basaltic volcano which has the highest eruption frequency of all the volcanoes in Iceland and has a southwest-northeast-trending fissure system. The massive climate-impacting Laki fissure eruption of 1783–1784 was a part of the same fissure system. Grímsvötn was erupting at the same time as Laki during 1783, but continued to erupt until 1785. Because most of the volcanic system lies underneath Vatnajökull, most of its eruptions have been subglacial and the interaction of magma and meltwater from the ice causes phreatomagmatic explosive activity.
About the climate-impacting Laki fissure eruption of 1783-1784 the Wiki mentions, as to have an idea of what climate-impacting could mean:Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions contain juvenile (magmatic) clasts. It is common for a large explosive eruption to have magmatic and phreatomagmatic components.
And the recent historyThe system erupted violently over an eight-month period between June 1783 and February 1784 from the Laki fissure and the adjoining volcano Grímsvötn, pouring out an estimated 42 billion tons or 14 km3 (3.4 cu mi) of basalt lava and clouds of poisonous hydrofluoric acid and sulfur dioxide compounds that contaminated the soil, leading to the death of over 50% of Iceland's livestock population, and the destruction of the vast majority of all crops. This led to a famine which then killed approximately 69% of the island's human population. The lava flows also destroyed 21 villages.
The Laki eruption and its aftermath caused a drop in global temperatures, as 120 million tons of sulfur dioxide was spewed into the Northern Hemisphere. This caused crop failures in Europe and may have caused droughts in North Africa and India.
In short, it does not appear to be too serious just yet, but it is something to watch.2020-21 threats of eruption
In June 2020, the Icelandic Meteorological Office (IMO) issued a warning that an eruption might take place in the coming weeks or months, following scientists reporting high levels of sulfur dioxide, which is indicative of the presence of shallow magma. IMO warned that a glacial flood as a result of melting ice could trigger an eruption. No eruption occurred.
In September 2021, an increase in water outflow from under the Vatnajökull ice cap was reported. The water contains elevated levels of dissolved hydrogen sulfide, suggesting increased volcanic activity under the ice. Jökulhlaup (glacial lake flooding) can occur before or after an eruption.
On December 4th 2021 a Jökulhlaup occurred from Grimsvötn into the Gígjukvísl river (average flow 2600 m3/s). It is believed that volcanic eruptions can occur after sudden pressure release due to reduced water level in the lake. The last time something similar happened was in 2004, 1934 and 1922.