aa

"Aa" is a specialized geological term used to describe a specific type of lava. It has one main meaning in English, borrowed from Hawaiian.

1nounrough lava

Aa is a type of hardened lava that has a very rough, jagged, and sharp surface. Imagine a rocky field covered in broken, spiky chunks of rock — that is what aa looks like after a volcano erupts and the lava cools down. Walking on it would be very painful! The word comes from the Hawaiian language, and geologists (scientists who study rocks and the Earth) use it in English too.

geology / science · Modern, used in scientific contexts

The hikers had to be very careful walking across the field of aa because the sharp rocks could cut through their boots.

After the eruption, the hillside was covered in aa, making it impossible to cross.

Geologists studied the aa flow to understand how quickly the lava had cooled.