Lava flows are outpourings of molten rock on the Earth's surface. They're very common in volcanic eruptions, and they come in different forms, according to AccuWeather.
Pahoehoe lava is smooth and can have a ropy, billowy appearance. It's low in viscosity, the measure of fluid thickness, and generally moves slowly.
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A'a lava is much more ragged in appearance. It's more viscous and moves faster, and it carries broken lava blocks called clinkers that cover the actual lava flow.
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Pahoehoe lava can sometimes turn into a'a lava when it encounters a steep slope. Once it does, it can never revert back.