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What is a Lava Flow?

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  • Written By: Mary McMahon
  • Edited By: Niki Foster
  • Last Modified Date: 13 November 2016
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A lava flow is the result of a volcanic eruption, caused by the rise of magma to the Earth's surface. Magma is called lava after it emerges from the Earth's crust. An active flow can exceed temperatures of 1,300°F (700°C) and is highly destructive. Property damage from this natural phenomenon is not uncommon, because the lava burns or pushes aside everything in its way. People and animals are usually able to escape, however, as lava moves at slow speeds.

Magma is molten rock that circulates beneath the Earth's surface. It is usually under high pressure, and sometimes, it vents or erupts through volcanoes, causing active flows. As magma travels, it collects particles of other rocks that affect its composition and subsequent potency. Because magma is less dense than rock, it slowly rises upward through the Earth, causing the formation of magma chambers and lava domes. Magma, and the resulting lava, comes in three basic types: mafic, intermediate, and felsic.

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A mafic lava flow tends to be gentler than some other forms, because its primary component is basalt, rather than more explosive silicates. This type also tends to be of lower viscosity — they are thin and very evenly spread as a result. They are most frequently found in the neighborhood of marine environments or around the subduction zones of oceanic plates, and most pillow lava is formed from this type of flow. Pillow lava is formed when lava emerges underwater, forming an instant crust that is later burst by new lava, resulting in the formation of many small bulbs that resemble pillows.

An intermediate or andesetic lava flow has a higher amount of silica and often occurs near plate boundaries. It is much more explosive, because of the higher percentage of silicates associated with them. The active flow tends to take a very liquid form, and the appearance of unique crystals called phenocrysts inside the lava are not unusual.

Felsic lava flows are highly viscous and extremely explosive. This type usually occurs when a hot spot in the continental crust erupts, carrying particulates and silicates to the surface with it. Felsic lava tends to be stickier than other types and often contains aluminum, potassium, sodium, and calcium, forming feldspar and quartz.

In addition to pillow lava, lava can also take the form of a'a, a rough, cindery type characterized by chunks of material forming on top. An a'a flow is easily visible by radar satellite because its rough surface reflects well. Walking on cooled a'a lava is challenging because of the rough and broken surface, while walking on a hot flow is generally not advised.

Lava can also appear in the form of pahoehoe, a smoothly surfaced basaltic lava. Pahoehoe often forms very strange, undulating shapes as it flows, creating small lobes that push the lava along. Rarely, lava takes the form of a sheeted flow, in which the top and sides cool and form a crust under which hot lava still moves, eventually breaking the sides and creating a very rough edge.

Deaths associated with lava flows are rarely linked directly to the lava itself. People die from the reaction of lava and water, from toxic gases that accompany flows, and from other associated events. If lava is approaching, people should evacuate rapidly and calmly, making sure to take pets with them. If there is time, evacuees should also remove important possessions, as their homes may be buried in several feet of black rock. Authorities will advise when it is safe to return.

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