Aa lava flows form when basalt lava flows down the side of a volcano and is cooler than pahoehoe lava flows. The lava flow has a rough or rubbly surface with sharp edges. The Hawaiians used this term to describe these type of lava flows on their islands for a very long time. The rocks in the lava flow where unstable and had sharp edges that could easily cut a foot when walked on with bare feet.
Giant lava balls on top of a lava flow
Slowing moving lava flows
Aa flows have a massive dense core of basalt that moves slowly away from the volcano vent. The surface cools and cracks forming clinkers which are broken lava blocks.
The clinkers are carried along on the surface of the flow. They often tumble down the front of the flow forming a broken surface on the top and bottom of the aa lava flow.
Slow moving aa lava flow on Kilauea 2018 USGS
Lava balls are like giant snowballs
Accretionary lava balls sometimes form on the surface of the flows like giant snowballs. Lava balls often are 3 meters (10 feet) in diameter. They sit on top of the lava flow after it cools. The lava balls in the picture above are located at Lava Butte just outside of Bend, Oregon.
Lava flows in Hawaii
Walking on aa lava flows is difficult and dangerous because of the sharp spiny surface. The rocks are unstable and you can easily fall when walking on them causing you to hit the sharp glassy surfaces of the clinkers. Aa lava flows move slower than pahoehoe lava flows allowing thin crusts to form that break apart as the flow continues to move down the volcano's slope.
Walking on pahoehoe
Hawaiians prefer to walk on pahoehoe, another typical type of lava flow found on the Hawaiian Islands, that is smooth and relatively safe to walk on. It is hotter when it is flowing and forms a rather smooth ropy surface.
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