Comparison of Lava Flows and Lahars: Travel Distance and Physical Properties

Still looking at the LiDAR from Mount St. Helens, you can see multiple different lava flows. Do you think these lava flows travel as far as lahars? Why or why not? Explain your reasoning with evidence from the physical properties of the two kinds of flows.

Answer with Step by Step Explanation

1. • Viscosity: Lava’s viscosity is a key factor influencing its flow distance. Basaltic lava, which is low in viscosity, can flow relatively far from the eruption source. More viscous lavas, such as andesitic or rhyolitic, flow less easily and cover shorter distances.

   • Temperature: Lava flows are extremely hot, with temperatures ranging between 700°C and 1200°C (1300°F to 2200°F). This high temperature keeps the lava in a molten state for a longer period.
2. • Temperature: Lahars are typically cooler than lava flows, as they are primarily composed of water and volcanic ash rather than molten rock.

   • Flow Behavior: Lahars can travel at much higher speeds than lava flows, ranging from a few meters per second to over 50 km/h (31 mph). Their ability to travel quickly and the fact that they can follow river valleys means they can cover much greater distances.

2. • Travel Distance: Lahars can travel much farther than lava flows. Due to their fluid nature and the ability to follow river valleys and natural channels, lahars can travel tens to hundreds of kilometers from the volcanic source. For example, the lahar from Mount Pinatubo in 1991 traveled over 100 km (62 miles) from the eruption site.

Reasoning