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, Myrna Martin
Slowing moving lava flows
These type of 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.
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.
Walking on pahoehoe
Hawaiians always 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.
Welded Tuff Find out how welded tuff is produced during violent eruptions and by supervolcanoes.
Intrusive Rocks Learn more about intrusive rocks that cool underground forming batholiths like the Sierra Nevada Mountains in California.
Clastic Rocks Find out how bits and pieces of older rocks form shale, sandstone and conglomerate rocks.
Pyroclastic Material This page gives all kinds of information about volcanic material that forms during volcanic eruptions.
Organic Rocks Learn how animals extract calcite out of ocean water to form shells that are organic rocks.
Aa Lava Learn more about these lava flows that occur on the Hawaiian Islands and the lava balls that often form on their surface.
Rock Cycle Learn more about igneous, sedimentary and metamorphic rocks and how they form.
Kids Fun Science The links on our home page include information about volcanoes, science activities, plate tectonics, the rock cycle and much more.