British researchers who studied the Puyehue-Cordon Caulle volcano in Chile in January found the obsidian lava flow was still moving even though the volcano stopped erupting in April 2012.
Obsidian lava is very thick and flows very slowly, moving more like a glacier. Rich in silica, it forms a natural glass called obsidian when it cools and solidifies.
This volcanic glass slowly inches forward as a thick, shattering crust of black rock that covers the slowly oozing lava within, the researchers said.
"We found out that the lava was still oozing after almost a year and it advances between 1 and 3 meters (between 3 feet and 10 feet) a day, Hugh Tuffen of Lancaster University said.
"It looks like a solid cliff of crumbling rock up to 40 meters (130 feet) thick, that's as thick as ten double-decker buses, but we found that hidden beneath this crust there is hot, slowly flowing lava, at up to 900 degrees C (1,600 degrees F,) which can burst out of the edges of the lava flow and help it move forwards," he said.
Obsidian from lava flows is found at many archaeological sites worldwide, as ancient people prized it highly as a material for making sharp tools such as knives.