The Grábrók crater was formed in a fissure eruption less than 3000 years ago. It is 173m a.s.l. Grábrók is the largest of the craters on a 600m long fissure. The Grábrók fissure is the easten most postglacial eruption site in the Ljósufjöll volcanic system.

A volcano is formed when hot magma penetrating up from the earth through cracks in the crust and the lava solidifies when it reaches a cooler medium such as air or water. Magma can penetrate up from the mantle through the bedrock because it has a larger volume and lower density than the solid rock. Before the magma reaches the surface is collected in large magma chamber where it remains until it cracks and voids in the rock allows it to continue to rise. When magma reaches the surface or seabed, we have a volcanic eruption and the magma  is called lava.

Volcanoes can be divided into two different categories. Those who have explosive process (Grábrók) and those that flow into the surrounding area. What type of volcano formed partly depends on the concentration of silicon (SiO2), which affects how magma is loose and the amount of gas. If the magma is viscous contains more than 65% silica (SiO2). Magma texture allows the gas does not disappear in an easy manner and the pressure increases down in the House. It all could end up with that you get an explosive process where the whole bedrock bursting and lava thrown high up into the air. If the magma is loose and flowing gas easily find out and may disappear. This type of magma flowing just calmly out through openings in the earth's crust as lava. Liquid magma contains less than 52% silicon.

Volcanic eruptions are often associated with that many people die. Usually it is not the lava that kills, the river flows so slowly that the time away, but it is the cloud of toxic gas that flows down the volcano side just before the outbreak. Lava flow creates instead a huge material damage. Houses, plantation crops and burn up, fields and forests covered by the hardened lava. In an explosive process, the lava down in the form of volcanic bombs. These vary in size from a few millimetres to large blocks size of houses. These can came down from the volcano side with firestorm mixed with rock and ash. The ash is white as snow but covering and smothering everything in its path.

Source: Wikipedia

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