What is a lahar and when does it typically occur?

• A. A volcanic lava flow
• B. A volcanic mudflow of water-saturated debris
• C. A basaltic eruption
• D. A pyroclastic flow

Answer: (B)

A lahar is a volcanic mudflow: a mixture of water and volcanic debris that moves down the sides of a volcano and through river valleys like a fast, dangerous flood. It typically happens during or soon after an eruption when heat melts snow and ice on the summit, when crater lakes drain, or when heavy rainfall saturates loose ash and pumice on the slopes. The presence of water makes the loose material move rapidly, carrying ash, rocks, and vegetation for long distances and causing widespread damage. This is different from a lava flow, which is molten rock, and from a pyroclastic flow, which is a hot, fast cloud of gas and volcanic ash. The key idea is a water-saturated mudflow triggered by eruption-related sources of water.