Most people believe that one of the major hazards of volcanoes are lava flows. They are surprised to find out that almost no one has ever been killed during an eruption by lava flows - and that sadly most people are killed by other eruption phenomenon such as volcanic gases, pyroclastic flows, tsunamis and lahars.
What determines the speed of lava flows?
Lava flows are streams of molten rock that originate from a volcanic vent. The speed that this molten material moves away from the vent is dependent on:
- The composition of the molten rock. Basaltic lava flows move much faster than rhyolitic lava flows. Also, the amount of gas dissolved in the lava can change the flows ability to flow (viscosity).
- The rate of material that the vent is spewing out. During some eruptions vast amounts of lava are released and so the flows tend to be faster. Other eruptions only small amounts of lava are released, and so the flows are slower.
- The slope of the land. Obviously a lava flow will move faster over steep slopes than shallow slopes.
- If the lava is flowing in a sheet, a channel or a tube. When lava starts to flow it will move in a slow sheet. If the eruption continues, a channel might form along which the lava will flow faster. If that channel cools on its surface it can form a tube. Now the lava is completely contained and insulated. Lava can flow very fast inside the tube.
So for example, a basaltic lava flow moving over a steep slope inside a lava tube will flow around 30 km/hr (19 miles/hr). On shallower slopes the flow will move less than 10km/hr (6 mile/hr) and most probably around 1 km/hr (thats like 1 foot a second).
Can you outrun (or walk) lava flows?
The average adult human can walk at around 5 km/hr (3.1 miles/hr) and jog at around 11 km/hr (7 miles/hr). In short bursts humans can run at 32 km/hr (20 miles/hr).
So people could easily walk, jog or run away from almost all lava flows....unless they are in situation such as a steep sided very narrow valley leading to a volcanic event that is churning out basaltic lava at a huge rate. And that situation is so rare it is probably not even worth mentioning!