e year without a summer in 1816 was caused by a volcanic eruption on the island of Tambora the year before (In Indonesia)
It was caused by the Indo-Australian Plate subducting beneath the Sunda Plate. The rock form the Indo-Australian plate sank into the mantle where it melted. It then rose to the surface where it created a powerful volcanic eruption, powerful enough to cool global climates for the following yearSource(s):