A ‘cold lava flow’ refers to a type of volcanic mudflow or debris flow that consists of water-saturated volcanic rock debris and mud that flows down the slopes of a volcano, often following heavy rainfall. Despite the term ‘cold,’ these flows can still be very hazardous and destructive.

Here are some key points about the ‘cold lava flow’ event you described in Indonesia:

1. It occurred on the Indonesian island of Sumatra after heavy rains in the area.
2. The rains triggered flash flooding, which was already deadly, killing at least 37 people.
3. The heavy rains also destabilized volcanic debris and mud on the slopes of an active volcano on the island.
4. This loose volcanic material combined with the floodwaters to create a fast-moving slurry called a ‘cold lava flow’ or volcanic mudflow.
5. This mudflow swept down the volcano’s slopes with tremendous force, destroying over 100 homes in its path.
6. While not liquid lava, the sheer volume and momentum of the water-saturated mud and volcanic debris made it incredibly destructive to anything in its path.

So in summary, the ‘cold lava flow’ was a volcanic mudflow triggered by the heavy rains combining floodwaters with loose volcanic debris, resulting in a fast-moving, highly destructive flow that devastated parts of the island. These types of flows are common hazards at volcanic regions during extremely heavy rainfall.