'A'a flommer er vanligvis flere meter tykke,
og er innpakket i skarpkantet halvstørknet
lava, som har revet seg løs fra selve hovedflommen.

De dannes ofte ved at lavaen flyter langs en
bratt skråning, og noe av lavaflommen faller
ut over kanten. 'A'a flommen kan da forme
en kaskade av hete, glødende vulkanrester,
som ruller/sklir nedover skråningen i høy hastighet.

Se også 'lava'

.
Lava flows form more than 99 percent of the above-sea parts of
Hawaiian volcanoes. Pahoehoe (pronounced "pah-hoy-hoy") and
aa (pronounced "ah-ah") are the two main types of Hawaiian lava flows,
and these two are now used by volcanologists worldwide to describe
similar lava-flow types. Pahoehoe is lava that in solidified form is
characterized by a smooth, billowy, or ropy surface, while aa is lava that
has a rough, jagged, spiny, and generally clinkery surface. In thick aa
flows, the rubbly surface of loose clinkers and blocks hides a massive,
relatively dense interior.

The contrast between the surfaces of pahoehoe and aa flows is
immediately obvious to anyone hiking Hawaiian lava fields. Walking on
dense pahoehoe can almost be as easy as strolling on a paved sidewalk
. But walking across aa is like scrambling over a building-demolition site
or battle zone, strewn with loose, unstable debris of all shapes and sizes.
The jagged rubble of aa flows quickly destroys field boots and, should
the hiker stumble or fall (not at all uncommon), it can tear clothing
and flesh. .