'A'a flows are characterized most obviously by very rough top surfaces, dense interiors, and sometimes rough bottom surfaces. The loose fragments that make up the surface of flows are usually spinose (=clinkers) or more rarely smooth (=blocks). True block lavas are essentially absent from Hawaiian volcanoes (but common on some strato-volcanoes). Clinkers are formed as pasty lava is pulled apart by shearing and twisting during flow. The clinker layer is usually 1-2 m thick, but can be as thin as 10 cm. In general, the spininess of the clinkers is inversely proportional to both the thickness of the clinker layer and of the flow itself.
The dense interior is what actually flows as an 'a'a flow is emplaced, and it carries the clinkers along with it. Clinkers that fall off the front are buried by the advancing flow, generating a bottom clinker layer.
On the left are diagrams of a moving distal-type flow, showing the dense interior (that acts almost like a solid even though it is indeed flowing) with a blocky clinker carapace. The bottom clinker layer forms mostly from material that falls off the front of the flow and is run over (adapted from Macdonald 1972).
'A'a flows in Hawai'i range in thickness from 1-10 meters, and each consists of a few large flow units. The longest post-contact 'a'a flow in Hawai'i is the 1859 Mauna Loa flow (see below), at 51 km in length on land (plus a little more offshore).
Right: Map of the 1859 Mauna Loa "paired" lava flow. The 'a'a flow (orange) was active for 16 days, advanced at an average flow-front velocity of 133 meters/hour, and erupted at a volumetric flow rate of 208 cubic meters/sec. The pahoehoe part (blue) followed immediately after, was active for 285 days, advanced at an average flow-front velocity of 7 m/hour, and was erupted at a volumetric flow rate of 5 cubic meters/sec (from Rowland & Walker 1990).
High discharge-rate eruptions (usually accompanied by vigorous fountaining) lead to high volumetric flow rates, and these form 'a'a flows, which are emplaced at high flow-front velocities. The fastest recorded flow in Hawai'i was the 1950 Ho'okena 'a'a flow of Mauna Loa which advanced down a 5º slope through thick forest at approximately 10 km/hour.
Below: A plot of volumetric flow rate vs. average flow-front velocity. Note the distinct separation of 'a'a and pahoehoe at a volumetric flow rate of about 6 cubis meters per second and a flow-front velocity of about 10 m/hour (adapted from Rowland & Walker 1990).
Hawaiian 'a'a flows can be classified into two main types, proximal-type 'a'a and distal-type 'a'a (Rowland & Walker 1987).
Each can be found at any distance from the vent although the names imply otherwise. Proximal-type 'a'a flows tend to be 1-3 m thick, fast-moving, have thin layers of spiny clinker, little fine material mixed in with the clinker, and their interiors are often vesicular. When moving, the pasty interior of proximal-type 'a'a flows can be observed deforming and flowing, and can be penetrated by a thermocouple or viscometer.
To the right are examples of proximal-type 'a'a flows. In A, the incandescent core could be seen deforming as the flow advanced at a few meters/minute, and if you had been properly protected from the intense radiant heat you could have scooped out a pasty blob with a hammer. In B, note that the top clinker layer is only 10-20 cm thick and that the interior is relatively vesicular.
Distal-type 'a'a flows are often up to 10 m thick, slow moving, have thick top layers of dense smooth-surfaced blocks intermixed with much fine comminuted sand and dust, and their interiors are poorly-vesicular. When moving, the "flow" of the distal-type 'a'a interior is imperceptible, and it isn't possible to penetrate it even though it may be incandescent. There is a complete gradation between these two 'a'a types, and their differences are due to the fact that flows lose so much heat as they flow. This loss of heat increases the viscosity and yield strength of the lava, greatly changing the flow properties.
Distal-type 'a'a flows. In A, which shows the top half of a ~6 m-thick flow margin during the 1984 Mauna Loa eruption, the incandescent lava could not be perceived to flow and the main activity consisted of little particles of incandescent sand-sized particles sliding down the flow edge. Occasionally a large dense block would also tumble down off the top of the flow. Additionally, we could not penetrate the incandescent lava with a stick. In B, a pre-contact Kilauea flow, note the blockiness of the top layer and the massive, non-vesicular quality of the interior. Hammer (arrow) for scale.