What is cold lava? The volcano phenomenon that has caused havoc in Indonesia's Sumatra

Flash floods, landslides, and a destructive stream of cold lava from Mount Marapi, which is an active volcano in Sumatra, Indonesia, caused the deaths of at least 43 people and left 19 individuals missing over the past weekend. Cold lava, also known as "lahar' in Javanese, comprises a mixture of water and rock fragments that flows rapidly down the slopes of a volcano.