What Is the Difference Between Lava & Pyroclastic Material?

Lava Materials

• Lava represents streams of molten rock on the surface of the planet. The specific composition depends upon the type of lava. One type is made of basalt lava. This lava is fluid, enabling it to travel distances of up to 25 miles, at speeds of up to 18 miles per hour. There is also andesite, dacite and rhyolite lava. These lavas are much more viscous, typically traveling less than five miles and at speeds of less than three miles per hour.

Solid Pyroclastic Materials

• Pyroclastic flows represent a mixture of rock fragments and superheated gases. The rock pieces are known as tephra. This tephra material ranges in size from ash to boulders. Rock fragments smaller than 0.8 inches are called ash. Fragments between .08 and 2.5 inches are called lapilli. Fragments larger than 2.5 inches are called blocks. All three of these tephra materials are ejected from the volcano as solids. Bombs are another type of tephra, typically larger than 2.5 inches, which are ejected in a partially-molten state. In addition to solid material ejected by volcanic eruptions, pyroclastic flows pick-up materials in their path that can be as large as boulders.

Gaseous Pyroclastic Materials

• Unlike lava, pyroclastic flows consist of large amounts of hot, expanding gases. These gases include water vapor, sulfur dioxide, hydrogen sulfide, fluorine gas and hydrogen fluoride. It is these gases that give pyroclastic flows their unique nature. They reduce inter-particle friction, enabling these flows to travel much faster and much farther than lava flows. Pyroclastic flows can travel at speeds of up to 450 miles per hour, for distances as great as 125 miles. Pyroclastic flows with high ratios of gas are called pyroclastic surges or fully dilute flows. Furthermore, pyroclastic flows are unlike lava flows in that they can change in both density and viscosity as they flow across the surface.

Material Temperatures

• Lava temperatures depend upon the type of lava. Basaltic lava is the hottest, ranging in temperature from 2,000 to 2,140 degrees Fahrenheit. Andesite, dacite and ryholite lavas are slightly cooler, ranging from 1,500 to 1,800 degrees. Pyroclastic flows can have temperatures as high as 1,800 degrees, although their range is much wider than lava flows. The pyroclastic flow from the eruption of Mount Pelee in 1912 was around 1,967 degrees, while the flow from Mount St. Helens was around 662 degrees.