Science has made a strong case for the year 536 as being one of the worst in human history, a year punctuated by volcanic eruptions, drought, famine and plague - and a year long winter.
Some explosive volcanoes can send ash high up into the sky and it can travel around the world over different countries.
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When magma rises towards the surface gas bubbles start to form. Whether or not they can escape as the magma is rising affects how explosive the eruption will be.
MESSENGER enhanced colour image showing the southern half of Mercury’s Caloris basin, hosting several red spots.
NASA/JHUAPL/CIW