Lava flows can move at very different speeds and can take on a variety of morphologies. Speeds up to 55 km/hr (34 mph) have been measured for some lava flows that are confined to channels. In general, the fastest flow front velocities are about 8 km/hr, with most much less than 2 km/hr. Many factors influence the speed of a lava flow. The most important are the viscosity and the density of the magma, the lava flows thickness, and the slope of underlying ground.

Photos courtesy of USGS

The two main forces acting on a lava flow are 1) gravity, which drives the flow downslope, 2) drag at the base of the flow and 3) resistance caused by viscosity of the flow, both of which tend to slow the progress of the flow. In order for a flow to move, the gravitational force along the direction of slope must exceed the drag at the base and the internal resistance.

The speed of a lava flow is can be estimated from the following equation:

The thickness of a lava flow can be estimated from the following equation: