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Different Types of Lava Flows

Rutuja Ghanekar Jun 9, 2019
The word 'lava' indicates both―the molten matter erupted by a volcano and the rock formed after the matter has cooled down. Lava flows are usually the transition stage.

Did You Know?

A pahoehoe lava flow can transform into an ʻAʻā flow, but an ʻAʻā flow can never change into a pahoehoe flow.
The term lava flow is self-explanatory, and indicates the flowing of lava from a volcano's vent during its eruption. Lava flows are destructive in nature and destroy everything in their path, but since they move at a considerably slow pace, they are not always life-threatening.
Lava flows travel great distances before cooling down and solidifying into igneous rocks. A lava flow is called the same even after cooling down and forming an igneous rock. The speed of a flow depends on its composition, viscosity, steepness of the surface of its travel, manner of flow, and the rate of lava produced by the volcano.
90% of the lava flows are basaltic in composition, followed by andesite (8%), and dacite/rhyolite (2%). While basaltic flows travel at a speed of 10km/hour (steep slope) and 1km/hour (gentle slope), andesitic flows travel barely a few kilometers per hour.
Rhyolite or dacite flows are extremely slow and often end up forming lava domes near the volcano's vent. Now, these lava flows are further classified into 3 types on the basis of their mineral composition and nature. Let's delve into details of the same.

ʻAʻā Lava Flow

ʻAʻā (also spelled aa, aʻa, ʻaʻa, and a-aa) is a Hawaiian term for 'stony rough lava' and 'to burn'. This type of lava flow has a rubbly surface that is formed of broken rocks of lava called clinkers.
With a molten core and high viscosity, ʻAʻā type of lava flow moves at a considerably slow rate, resulting in increased thickness of the flow; the thickness varies between 3 - 20 m. Since ʻAʻā flows have a clinkery surface and are rubbly in nature, they exhibit a dark gray color.
As the viscous lava travels down a slope, the clinkers travel with it, and this angular texture of the flow can be observed even from an orbiting satellite.
The lava fragments that cool down, if present on the leading edge of the flow, are overridden by the remaining part of the flow and are, hence, pushed down to the bottom. Hence, a typical ʻAʻā lava flow always has a layer of fragments of lava―both on the surface and at the bottom.

Pahoehoe Lava Flow

Pahoehoe means 'smooth lava' in Hawaiian terminology, referring to the undulation, smooth, shiny, and/or ropy surface that it displays. As compared to the ʻAʻā flow, pahoehoe is less viscous in nature; hence, it travels a great distance in a short period.
Pahoehoe results in thinner flows than ʻAʻā with a thickness of 1 - 3 m (average). During its travel, a pahoehoe flow is characterized by a series of outbreaks of toes (lava toes) and lobes once the crust starts to cool down.
A pahoehoe flow is well-known for the unique structures it forms while cooling down. Owing to their shiny silvery surface, pahoehoe flows are difficult to detect from an orbiting satellite.

Blocky Lava Flow

Often, andesite, dacite, and rhyolite give rise to blocky lava flows. They exhibit similar nature to ʻAʻā flows, but are more thick (greater than 20 m). They display semi-angular lava blocks with molten interior and travel like the ʻAʻā flow, overriding the lava blocks that cool down, and pushing them to the base of the flow.

Transition Between Pahoehoe and ʻAʻā Flow

This transition is observed in Hawaiian volcanoes. Here, almost all volcanoes display pahoehoe lava flows. As mentioned earlier, one of the main factors that decide the type of lava is the interdependence of the viscosity of lava and the rate of speed of its travel.
The only manner in which a pahoehoe lava can transit into an ʻAʻā lava is when the former's viscosity is subjected to change, caused due to overall cooling of the flow, loss of gas, and/or crystallization. If the viscosity of a pahoehoe lava flow is low and the rate of disturbance of lava is high (or vice versa), it favors the formation of an ʻAʻā flow.
Usually, deaths resulting from a lava flow are uncommon since they move at a pace that's slow enough to evacuate that particular area. But if the lava approaches water, or releases toxic gases, and the same comes in contact with the nearby settlement, it can prove fateful.