How do lava flows cool and how long does it take?

This ‘a‘ā flow erupted from fissure 8 on Kīlauea Volcano's lower East Rift Zone on June 1, 2018, shows how the interior of a lava flow remains incandescently hot even though surface cooling forms a crust of solid rubble. Based on studies of lava flow cooling rates, it will take more than 130 days for a flow this thick (about 4.5 m, or 15 ft) to cool to a temperature of about 200 degrees Celsius (290 degrees Fahrenheit). USGS photo by A. Lerner.