'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.

'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, at 51 km in length on land (plus a little more offshore). 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.

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.

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.


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