What is 'cold lava' that killed several people in Indonesia?

Mount Marapi's 'cold lava' and floods killed at least 52 people in Indonesia's Sumatra. Known as 'lahar' in Indonesia, cold lava is a mixture of volcanic ash, rock debris and water that flows down a volcano's slopes during wet weather. It can flow at a speed of hundreds of kmph and spread as far away as 60 km from volcano.