All lava isn’t the same. There can be tremendous differences in the viscosity (thickness), temperature and chemical composition of lava. The least viscous (least syrupy) lava is known as pahoehoe, and it can flow for many kilometers away from the source of a volcanic eruption. One of the longest flows ever recorded was an eruption from Mauna Loa that was 47 km long.
In fact, when you think of an erupting volcano, with vast rivers of lava flowing out, that’s pahoehoe – it’s a Hawaiian term. It’s a basaltic lava that once hardened has a smooth, ropy surface. In fact, it can have such beautiful shapes that people call it lava sculptures. The strange shapes happen because the front of the lava flow forms a thin shell, and then blobs continually break out from the crust. These cool and then more lava breaks out from that.
Once the pahoehoe lava flows finally cool, the resulting rock is incredibly smooth; they’re smooth down to a scale of just a few millimeters. This is very different to aa lava flows, which feel like jagged glass once they harden. Pahoehoe is smooth and nice to walk across, while a’a lava will ravage your shoes and give you a nasty cut if you happen to fall on it.
We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.