Ooooh! That Looks Hot!

Flow moving across Hawaiian landscape
Credit: US Geological Survey / wikimedia commons

Melted rock flows across the surface of a volcanic field in Hawaii in this image shot by a USGS scientist. The existing surface is ropey (pahoehoe), but the new flow will harden in a blocky texture, called aa. 

Two different words

Lava and magma are both words that describe molten rock, hard material that has been melted by the extreme temperatures it has encountered deep below the surface of the earth.

When the molten material is below ground, the proper word to use is "magma." If the red-hot liquid just happens to find its way up to the surface and flow out onto the ground, the correct term to apply is "lava." That word is also applied to any rocks that form when a melt cools and hardens at the surface, such as happens to lava flows. Vulcanologists, or volcano experts, use different names for this hardened rock that describe the shapes captured when the liquid hardens; names like pillow, ropey and blocky.

Why no one ever sees magma
Credit: U S National Park Service / wikimedia commons

Magma, by definition, is never seen at the surface. Instead, it's found deep underground such as the cahmber filled with magma that is known to underlie Yellowstone National Park in the U. S. A.

Aa or Pahoehoe?

Hawaiian words are commonly used as well, fun words like "aa" (AH-ah) for the blocky lava, and pahoehoe (puh-HOY-hoy) for lava that solidified with a ropey texture. Whether the shape is blocky or ropey, any rock that formed from hardening lava is an extrusive igneous rock. Rocks with the same composition that formed when magma cooled in its underground chamber get a different name, intrusive igneous rocks.

Ropey texture of pahoehoe
Credit: U S Geological Survey / wikimedia commons

This lava flow demonstrates the ropey texture that vulcanologists call by its name in the Hawaiian language, "pahoehoe."

The Importance of Composition

Although movies like the Indiana Jones and Terminator series depict pools of glowing orange liquid that flows like water, magma and lava actually cover a broad range of viscosity. Molten rock's resistance to flowing depends on variables such as temperature and the percentage of gases, like water vapor, dissolved in the melt. The most important control on viscosity, however, is the abundance of certain elements.

Melted rock with a high silica content, or silicon dioxide, is relatively thick compared to a melt with less silica. The high-silica melts do not flow as readily, instead acting like a thick goo similar to oatmeal or Cream of Wheat. Rocks with a high percentage of silica are termed “acidic” rocks. Igneous rocks that form when acidic lava hardens are typically light-colored. Because an acidic lava moves slowly and is more resistant to flow, eruptions of acidic lava are often violent and explosive as giant gas bubbles expand in the melt and pop.

Molten rock with a low silica content is runnier, like thick syrup or motor oil. Low-silica igneous rocks are called “basic,” and include one of the most familiar igneous rock types, basalt. Basic rocks are tyically black or dark green or brown. Most of the Hawaiian Islands were built by volcanoes that erupt basaltic lava. Near some of the active volcanoes in Hawaiian, basaltic lava can flow as fast as a man can walk. Because the lava is runnier and contains less gas (and gases escape more easily), eruptions of basaltic lava tend to be less violent than eruptions of acidic lava.

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Explosive eruption of thick magma
Credit: U S Geological Survey / wikimedia commons

This eruption of a volcano in Alaska demonstrates the explosive power of viscous, siilica-rich melts. Note the sideways explosion at the right side of the base of the column of smoke.

Yes, it Is Hot!

As you might assume, lava and magma are both extremely hot. The precise melting temperature of rock depends on its composition, but all lava flows reach temperatures of more than 700°C (1300°F) and reach 1200°C (2200°F). So it's no wonder that a lava flow can destroy just about anything in its path![1]