The major eruptive product of Hawaiian volcanoes is lava. Lava flows can form during fountaining eruptions or they can well out of the ground with little or no pyroclastic activity. There are two major types of basaltic lava flow, 'a'a and pahoehoe. These are Hawaiian words that have no meaning other than the type of lava, and they have been adopted by geologists working in other basaltic areas besides Hawai'i. They are different in almost every respect possible except for their chemistry.
Pahoehoe flow (left) flowing over older 'a'a, and in front of an advancing 'a'a flow (background). Note the continuous skin of the pahoehoe being crumpled at the flow front, and compare it to the broken clinker of the active 'a'a. Note also that the pahoehoe flow front is ~30 cm thick whereas the 'a'a flow front behind is almost 2 m high.
Table comparing 'a'a and pahoehoe lava flows.
All their myriad differences can be attributed to different eruption and consequent emplacement mechanisms.
'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.
The first and most obvious difference is that pahoehoe flows are smooth down to a scale of a few mm. Instead of consisting of only 1-2 large flow units, a pahoehoe flow consists of thousands on thousands of small flow units called toes. Each toe is usually <30 cm thick, 1-2 m long, and 30-50 cm wide.
Close-up photo of an active pahoehoe toe. This toe is about 30 cm wide at its widest. Note how it has erupted out of a crack in a previous toe and is flowing over yet another previous toe (with the ropy texture). Note also that with the sun shining on it, one side of the active toe doesn't look all that different from the surfaces of the older inactive toes; late afternoon and early morning (and night) are the best times for observing lava flows.
Pahoehoe flows are associated with low-effusion rate eruptions and are emplaced at low volumetric flow rates (2-5 cubic meters per second) and slow flow front velocities (1-10 m/hour) [See the A'a page for a velocity comparison chart]. Pahoehoe flows can be just as long as 'a'a flows. The longest post-contact flow was also erupted from Mauna Loa in 1859 (forming the second half of the "paired flow"; Rowland & Walker 1990), and is 47 km long. This strongly contradicts the notion that flow length is directly determined by effusion rate.
The low velocity of pahoehoe flows means that the skin that forms by air-cooling is not disrupted during flow and can maintain its smooth, unbroken, well-insulating surface. Thus the temperature and viscosity of lava do not change very much even tens of kilometers from the vent. The advancing front of a pahoehoe flow consists of hundreds or thousands of active toes. Each stops flowing after a few minutes and becomes inflated (with lava) as the eruption continues. Eventually the cooled skin fractures, often at the seam between two toes, and a new toe forms.
Cross-section of a pahoehoe flow exposed in a sea cliff. Some of the individual flow units have been outlined in white but you can see many others. Two particularly large ones were probably flowing as small lava tubes; the one labeled 'd' drained out at the end of the eruption and the one labeled 'f' solidified full. The dashed pink lines mark the top and bottom of the pahoehoe flow; above and below are 'a'a flows.