Types of Lava – Pahoehoe and A’a   Recently updated !


Types of Lava Pahoehoe Aa
The two main types of lava flow are pahoehoe and a’a. But, in terms of chemistry, lava is classified by its silica content.

Lava is molten or partially molten rock that erupts onto the surface. Also, once the rock cools and solidifies, it’s still called lava. When the molten rock is below the surface, it is called magma. The word “lava” likely comes from the Latin word labes, which means to fall or slide. Here is a look at the types of lava and the flows they produce.

Types of Lava by Chemical Composition

Petrologists (scientists who study rocks) and geologists classify lava according to its chemical composition, specifically regarding its abundance of silicates. Silicates are compounds that contain the element silicon chemically bonded to oxygen. Silicate lava also contains varying amounts of aluminum, calcium, magnesium, iron, sodium, potassium, and other elements. There are also nonsilicate lavas that either form from the melting of nonsilicate minerals or from the separation of these minerals from silicate-based lava.

The four types of lava, based on silica content, are felsic (silicic), intermediate (andesitic), mafic (basaltic), and ultramafic.

Felsic Lava (Silicic)

  • Silica content >63%

Felsic or silicic lava is rich in lava, making it highly viscous. Usually, this type of lava is too thick to flow. Instead, eruptions explosively release fragments. However, sometimes the magma forms lava domes or spines that extrude blocks of lava.

Intermediate Lava (Andesitic)

  • Silica content 52-63%
  • Usually low in aluminum, but enriched in iron and magnesium

Andesitic lava is thick and viscous (about the same as peanut butter), but much less viscous than silicic lava. Partly this is because it’s usually hotter than silicic lava. It forms block lava and andesite domes.

Mafic Lava (Basaltic)

  • Silica content 52% to 45%
  • Relatively high in magnesium oxide and iron oxide

Basaltic lava has a relatively low viscosity (about the same as ketchup). When you see volcanic eruptions with lava fountains or flowing rivers of lava, it is basaltic.

Ultramafic Lava

  • Silica content <45%
  • High in magnesium

Ultramafic lava is very fluid, comparable to light motor oil. This type of lava flow hasn’t happened on Earth since the Proterozoic and Phanerozoic eras (538.8 million years ago) because the Earth’s mantle has cooled too much to produce them.

Alkaline Lava

Silicate lava containing elevated levels of sodium and potassium oxides (alkali metal oxides) is called alkaline lava. The silica content ranges from felsic to ultramafic. Scientists believe alkaline lava originates from deeper in the mantle than other lava.

Non-Silicate Lava

Although less common than silicate lava, there are eruptions with a different chemical composition.

  • Sulfur lava or blue lava comes from molten sulfur deposits. The lava is yellow, but it appears electric blue at night from the hot sulfur emission spectrum.
  • Carbonatite and natrocarbonatite lava contains molten carbonate minerals. This lava sometimes forms when the carbonate lava separates from the silicate lava.
  • Iron oxide lava probably comes from the separation of iron oxide magma from an alkaline parental magma.
  • Ice lava is a molten ice mixture which erupts from the moons of gas giants.

Viscosity

The chemical composition and its temperature largely determines its characteristics, such as viscosity. A viscous fluid resists flow (like ketchup), while a low viscosity fluid readily flows (like water or oil).

Silicate lava tends to be viscous because each silicon atom can bind four oxygen atoms very strongly. The presence of high levels of aluminum in lava together with sodium or potassium oxides also leads to a thick, viscous lava. On the other hand, the metals iron, calcium, and magnesium bind to oxygen more weakly, producing a less viscous lava.

Types of Lava Flow – Pahoehoe and A’a

  • The three types of lava flow are pahoehoe, a’a, and pillow lava.
  • Most lava flow is mafic or basaltic.
  • Pahoehoe is smooth, a’a is rough, and pillow lava is rounded and only forms under water.
  • Block lava doesn’t exactly flow, but it is another common form.

When you see lava flow or walk on old lava, the two types of flows are pahoehoe and a’a. Pahoehoe (meaning “smooth lava in Hawaiian) is smooth, undulating, or ropey. A’a is rough, rubbly, and often sharp. The word “a’a” means rough, sharp lava in Hawaiian, but it’s also what you’re likely to say if you walk barefoot on this type of flow. The same volcano and magma can produce both types of lava flows. Pahoehoe lava flows close to the source of the eruption, where the higher temperature makes the lava less viscous, and down steep inclines where gravity aids flow. A’a forms further from the eruption or on flatter surfaces. Pahoehoe flow transitions into a’a flow when the lava temperature drops to around 1200 to 1170 °C. Both pahoehoe and a’a typically cool into black lava. Older flows often look more reddish or brownish from oxidation and weathering of their minerals.

The third type of lava flow is pillow lava. Pillow lava is rounded or globular and forms when lava flows underwater. Since most eruptions occur in the ocean, this is the most common type of lava flow.

Lava flows or slides from a volcanic eruption or a fissure. But, some volcanoes slowly push out lava chunks, while other explosively eject rock. Block lava is andesitic lava from stratovolcanoes that results in a surface of smooth angular blocks. Explosive eruptions of silicic or andesitic lava eject ash and rocky fragments called tephra.

Most Common Type of Lava

Pillow lava is the most common form of lava. Usually, it is mafic (basaltic) lava. However, pillow lava also forms from komatiite, basaltic andesite, andesite, rhyolite, and other minerals. The richer the lava is in silica, the larger the pillows. The ocean crust has a layer of pillow lava.

References

  • Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of Igneous and Metamorphic Petrology (2nd ed.). Cambridge, UK: Cambridge University Press. ISBN 9780521880060.
  • Takeuchi, Shingo (2011). “Preeruptive magma viscosity: An important measure of magma eruptibility”. Journal of Geophysical Research. 116 (B10): B10201. doi:10.1029/2011JB008243
  • Sehlke, A.; Whittington, A.; Robert, B.; Harris, A.; Gurioli, L.; Médard, E. (2014). “Pahoehoe to ‘a’a transition of Hawaiian lavas: an experimental study”. Bulletin of Volcanology. 76 (11): 876. doi:10.1007/s00445-014-0876-9
  • Schmincke, Hans-Ulrich (2003). Volcanism. Berlin: Springer. ISBN 9783540436508.