Mafic lava flows have been classically divided into two categories: pahoehoe and aa (e.g., Dutton, 1884; Macdonald, 1953). Pahoehoe is characterized by having a smooth surface, and aa has a spinose autobreccia surface. In more recent years, some transitional types of basaltic lava have been noted, including slab pahoehoe and spiny pahoehoe (also called "toothpaste" or "sharkskin" pahoehoe). Spiny pahoehoe has the same centimeter-scale morphology as classical pahoehoe but has a spinose surface (e.g., Rowland and Walker, 1987). Slab pahoehoe has the same meter-scale morphology as an aa flow, but the autobreccia is dominated by slabs of broken pahoehoe surfaces (e.g., Macdonald, 1972). There are also many subvarieties of classic pahoehoe in Hawaii, such as S- and P-type pahoehoe (Walker, 1989), dense blue glassy pahoehoe (Hon et al., 1994), and shelly pahoehoe (Swanson, 1973). Most recently, another type of intermediate lava flow has been recognized. This flow type, dubbed "rubbly pahoehoe," is characterized by a flow-top autobreccia comprised primarily of broken pahoehoe lobes (Keszthelyi, 2000; Keszthelyi and Thordarson, 2000).

Pahoehoe and Aa

The transition between aa and pahoehoe is controlled by two factors, viscosity and strain rate (Fig. F1) (Peterson and Tilling, 1980). However, each of these factors is controlled by a vast array of parameters, including crystallinity, dissolved gas content, temperature, bubble content, slope, eruption rate, and lava composition. Studies suggesting that a single parameter controls the pahoehoe to aa transition have not considered a wide enough region in parameter space. For example, Rowland and Walker (1990) found that in Hawaii all eruptions >5-10 m3/s form aa and those <5-10 m3/s form pahoehoe. This is only true for viscosities and slopes typical in Hawaii. In the Columbia River Basalt Group, classic pahoehoe flows have formed despite eruption rates on the order of 4000 m3/s (Thordarson and Self, 1998). Cashman et al. (1999) found that the transition from pahoehoe to aa took place in a Kilauea lava channel as the lava crystallinity increased past ~50%. Clearly, pahoehoe flows that crystallize after they have stopped do not transform to aa. Instead, the observations of Cashman et al. (1999) show that both high crystallinity and significant motion of the lava are needed to form aa. Since lava viscosity is proportional to crystallinity, this further supports the Peterson and Tilling (1980) hypothesis that both high viscosity and strain rate are necessary to form aa lava.

Watching the transition from pahoehoe to aa in active lava flows allows one to see how both strain rate and viscosity control the transition. On an active pahoehoe flow, the surface is a plastic fluid. It is able to stretch, and the lobes advance much like a rubber balloon filling with water (Keszthelyi and Denlinger, 1996). If the lava becomes more viscous (i.e., due to crystallization) or if it is subjected to increased strain rate (i.e., by advancing over a steeper cliff), the lava is no longer able to stretch in a ductile manner. Instead, the hot plastic lava is ripped apart. Chunks of lava that are torn off of the main flow are tumbled into irregular, angular shapes. The torn surfaces are the spinose protrusions that are characteristic of aa clinker. The breccia rides on top of the flow and is dumped at the flow front. The flow then advances over this breccia, looking much like the advance of bulldozer treads (Macdonald, 1953).

Hawaiian Transitional Lavas

Slab pahoehoe flows form when the strain rates are high enough to form aa but the lava is too fluid to tear in a brittle manner. Spiny pahoehoe forms under very low strain rates but when the lava is too crystalline and viscous to form a smooth glassy surface (Rowland and Walker, 1987). These observations are shown in graphical form in Figure F1, using the plot first proposed by Peterson and Tilling (1980). Spiny pahoehoe flows were not encountered during Leg 183 and are therefore not discussed further in this chapter.

Slab pahoehoe involves the emplacement of relatively low-viscosity lava under very high strain rates. The name is derived from the abundance of slabs of pahoehoe in the disrupted upper crust. These slabs form when a flow initially forms a flat pahoehoe surface that is later disrupted from within. The individual slabs usually demonstrate the full range of brittle to ductile deformation; the upper chilled portion cracks while the lower hot portion deforms plastically (Macdonald, 1972). Such disruption is most often caused by high strain rates associated with surges of lava, most common in larger, sheetlike lobes. In Hawaii, slab pahoehoe lavas rarely extend for more than a kilometer before transitioning to classic aa (as the lava becomes more viscous) or pahoehoe (if the flow rate diminishes).

Rubbly Pahoehoe

Although the pahoehoe vs. aa classification scheme is applicable to the vast majority of the Hawaiian basaltic lava flows, it fails to describe many lava flows seen in Iceland, the Columbia River Basalt Group, or Leg 183 drill sites on the Kerguelen Plateau. The descriptive name "rubbly pahoehoe" has been suggested for a lava type that has a flow top composed of broken pieces of smaller pahoehoe lobes rather than spinose aa clinker (Keszthelyi, 2000; Keszthelyi and Thordarson, 2000). These breccia clasts are also distinct from pahoehoe slabs in that they have glassy chills on both sides—indicating that the lobe was broken by external forces rather than being torn apart from within. In cross section, rubbly pahoehoe flows have a four-part structure, passing from autobreccia top to coherent vesicular upper crust to dense core to lower vesicular crust. These flows often have a smooth pahoehoe base.

At this time, it is unclear where a rubbly pahoehoe flow would plot on Figure F1. Rubbly pahoehoe autobreccias lack aa clasts, implying relatively lower strain rates or viscosities. The examples of rubbly pahoehoe seen in the Columbia River Basalts have relatively high silica contents, suggesting that low viscosity is unlikely. Although this indicates that strain rates should have been quite low in the liquid portion of these flows, the breaking of pahoehoe lobes indicates high stresses.

It is clear that rubbly pahoehoe flow top autobreccias form over an extended period of time because younger clasts that engulf older cooled clasts can be found. Some lobes appear to have been broken after they were completely solidified, whereas others underwent some plastic deformation. As in aa flows, there is evidence of partially resorbed breccia clasts in the interior of the flows and there are "arms" or "lobes" of the core material pushing up into the breccia. Although these flows have many characteristics of aa flows, they have many of the internal features indicating inflation and a pahoehoe flow base.