![]() ![]() ![]() 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). These slabs form when a flow initially forms a flat pahoehoe surface that is later disrupted from within. The name is derived from the abundance of slabs of pahoehoe in the disrupted upper crust. Slab pahoehoe involves the emplacement of relatively low-viscosity lava under very high strain rates. Spiny pahoehoe flows were not encountered during Leg 183 and are therefore not discussed further in this chapter. These observations are shown in graphical form in Figure F1, using the plot first proposed by Peterson and Tilling (1980). 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). ![]() 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. The flow then advances over this breccia, looking much like the advance of bulldozer treads (Macdonald, 1953). The breccia rides on top of the flow and is dumped at the flow front. The torn surfaces are the spinose protrusions that are characteristic of aa clinker. Chunks of lava that are torn off of the main flow are tumbled into irregular, angular shapes. ![]() Instead, the hot plastic lava is ripped apart. 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. It is able to stretch, and the lobes advance much like a rubber balloon filling with water (Keszthelyi and Denlinger, 1996). On an active pahoehoe flow, the surface is a plastic fluid. Watching the transition from pahoehoe to aa in active lava flows allows one to see how both strain rate and viscosity control the transition. 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. (1999) show that both high crystallinity and significant motion of the lava are needed to form aa. Instead, the observations of Cashman et al. Clearly, pahoehoe flows that crystallize after they have stopped do not transform to aa. (1999) found that the transition from pahoehoe to aa took place in a Kilauea lava channel as the lava crystallinity increased past ~50%. In the Columbia River Basalt Group, classic pahoehoe flows have formed despite eruption rates on the order of 4000 m 3/s (Thordarson and Self, 1998). This is only true for viscosities and slopes typical in Hawaii. For example, Rowland and Walker (1990) found that in Hawaii all eruptions >5-10 m 3/s form aa and those <5-10 m 3/s form pahoehoe. Studies suggesting that a single parameter controls the pahoehoe to aa transition have not considered a wide enough region in parameter space. 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. The transition between aa and pahoehoe is controlled by two factors, viscosity and strain rate (Fig. 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). Most recently, another type of intermediate lava flow has been recognized. 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). 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). Spiny pahoehoe has the same centimeter-scale morphology as classical pahoehoe but has a spinose surface (e.g., Rowland and Walker, 1987). 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). Pahoehoe is characterized by having a smooth surface, and aa has a spinose autobreccia surface. Mafic lava flows have been classically divided into two categories: pahoehoe and aa (e.g., Dutton, 1884 Macdonald, 1953). ![]()
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