PHYSICAL VOLCANOLOGY

The physical volcanologists on board the JOIDES Resolution during Leg 183 sought to determine the types of eruptive activity that formed the volcanic rocks and volcaniclastic sediments recovered in the cores. This was accomplished by describing the rocks and identifying features that are diagnostic of specific physical processes to produce an integrated picture of the style of volcanism and environmental setting of each site drilled. Additional information presented in "Lithostratigraphy," "Igneous Petrology," "Downhole Measurements," and "Alteration and Weathering" of each site chapter were particularly important in producing the final interpretation.

We define volcaniclastic sediments as containing >60% siliciclastic and volcaniclastic grains and <40% biogenic material and a higher proportion of volcaniclastic than siliciclastic grains. This class includes epiclastic sediments, the volcanic detritus produced by erosion of volcanic rocks by wind, water, and ice; pyroclastic deposits, the products of the explosive degassing of magmas; and hydroclastic sediments, the products of granulation by steam (phreatic) explosions and quenching (hyaloclastite and peperite). Note that because this definition includes epiclastic sediments, it does not imply any active volcanism at the time of deposition.

The subclassification of volcaniclastic sediments followed by shipboard scientists during Leg 183 differs from the standard ODP classification (Mazzullo et al., 1988) in that we adopt a descriptive (nongenetic) terminology similar to that employed during Leg 157 (Gran Canaria and Madeira Abyssal Plain). Unless a pyroclastic origin for sediments can be defined, clasts of volcanic provenance (volcaniclastic) are described according to the classification scheme for siliciclastic sediments, noting the dominant composition of the volcanic grains. We follow the siliciclastic textural classification of Wentworth (1922) to separate the various volcanic sediments and sedimentary rocks into volcanic gravel (grain size = >2 mm), volcanic sand (2-0.0625 mm), volcanic silt (0.0625-0.0039 mm), and volcanic clay (<0.0039 mm). For unsorted or poorly sorted volcaniclastic sediments, such as those characterized by debris flows, we apply the terms volcanic breccia (angular clasts) or volcanic conglomerate (rounded clasts) and use modifiers to describe the sediment further.

Volcanic sediment can be classified by designating a principal name and major and minor modifiers. The principal name of a granular sediment defines its sediment class (e.g., sand, silt, or clay). Relative proportions of the vitric (glass), crystal (mineral), and lithic (rock fragment) components of the sediments are expressed as major (25%-40%) and minor (10%-25%) modifiers in the name. The major modifiers are placed before the principal name and minor modifiers are placed after the principal name using "with." For example, volcanic sand composed of 45% glass, 35% feldspar crystals, and 20% lithic fragments is named a crystal vitric volcanic sand with lithic fragments. Volcanological features are recorded on both sediment and hard-rock VCD (HRVCD) forms (see the "Core Descriptions" contents list). Detailed systematic information was recorded on a separate volcaniclastic sediment description log (Table T3). Volcanic breccias/conglomerates were further characterized using a point counting grid (Moore et al., 1996), and these data were recorded on the volcaniclastic breccia/conglomerate grid information log (Table T4).

Where there was evidence for a pyroclastic origin, volcanologists followed the classification of Fisher and Schmincke (1984), as adopted by Mazzullo et al. (1988), that uses the names volcanic breccia (>64 mm), lapilli (lapillistone; 2-64 mm), and ash (tuff; <2 mm).

Sedimentary structures including graded bedding, cross-bedding, planar laminations, foreset bedding, dune forms, ripples, and any evidence of agglutination, welding, or rheomorphism are recorded on the volcaniclastic sediment description log (Table T3).

General

The physical description of lava during Leg 183 was a multistage process. Unit boundaries and lithologic descriptions were selected first. The lithologic descriptions are presented in the "Igneous Petrology" section of each chapter. A general description of the volcanological features within the flows was compiled. After this, individual units were examined in detail. Often, time did not permit detailed examination of all the units, and only a selected subset was subjected to full scrutiny. Finally, an attempt was made to synthesize the observations from the different units, combining data from sections such as "Igneous Petrology" and "Downhole Measurements." The descriptions and interpretations were checked during the sampling for shore-based studies. This examination was particularly useful for comparison between different sites.

The site chapters start with a description of each unit using nongenetic terminology. Phrases such as "chill zone" or "flow-top breccia" are generally avoided since they are interpretations. This descriptive part is followed by a unit-by-unit interpretation, with explanations of the criteria used to separate the different units. For most readers, it may be helpful to first read the interpretive section and refer to the unit descriptions in conjunction with the core photographs for the observations leading us to these interpretations.

Methodology

The first step in describing the core (selection of unit boundaries) required considerable interpretation of the rocks. As a result, the initial justification for placing these boundaries sometimes shifted or evaporated as our understanding of the site improved. Also, the unit boundaries reflect major physical changes in the core (e.g., brecciated vs. massive) that were visible in the physical properties and downhole measurements. More subtle boundaries were usually not broken out, despite their possible significance to the volcanological interpretation. Finally, when the precise location of the boundary could not be determined in the initial examination, an arbitrary decision was required for the core description to proceed. Rarely did we later make changes to the unit boundary locations when additional observations were gathered. Thus, although a strong effort was made to have unit boundaries reflect individual lava packages, the term "unit" should not be considered synonymous with "lava flow" in this volume.

General observations were made during the initial description of the rocks. We first noted intervals of breccia and coherent lava and gross changes in vesicularity. We then characterized clasts in the breccias using the same techniques as sedimentologists (clast size, shape, sorting, and lithology). We also noted the volume fraction and type of matrix (commonly none or only secondary alteration minerals) and the abundance of finer lava fragments. Of particular interest was the presence (or absence) of features diagnostic of brecciation while the lava was hot. These include clasts engulfing fragments of other (earlier) clasts, welding, glassy margins surrounding the clasts, extensions of the interior of the flow into the breccia, entrained clasts within the interior of the flow, and the presence of a basal breccia. Changes in the abundance and morphology of vesicles were used to identify clasts of different lithologies within the breccia.

The second more systematic description of units concentrated on documenting vesicle features by recording (1) volume percentage, (2) size range (maximum, minimum, and average diameters), (3) number density, (4) shape (sphericity and angularity), and (5) grading (fining up or coarsening up) of the vesicles at intervals appropriate for the variability shown in the core (typically every 1-30 cm). This information is recorded on a separate vesicle description log (Table T5) (see the "Supplementary Materials" contents list). The volume fraction of vesicles was estimated using visual percentage estimate charts that provided examples at 1%, 2%, 3%, 5%, 7%, 10%, 15%, 20%, 25%, 30%, 40%, and 50%. The reported values are only accurate to ±1 of these steps. Thus, a reported value of 10% indicates a 2- confidence that the actual value is between 7% and 15%. In a few locations, these visual estimates were supplemented by measurements using a binocular microscope. Lines covering ~50 cm of the core were examined, and the fraction of the length of these lines that intersected vesicles provided a more quantitative vesicle abundance. The measurement of maximum and minimum vesicle sizes was straightforward, though vesicles <0.1 mm were usually not visible. The average vesicle size is a measure of what appeared to be a typical vesicle. It is subjective, but provides a measure for whether the vesicle size distribution is skewed toward the small or large end of the size range. Number density was determined by counting the number of vesicles in a given area. The size of the measurement area changed depending on the density of vesicles but, in general, was large enough to contain 20 to 100 vesicles. Because vesicle number density varied by ~5 orders of magnitude in the rocks recovered during Leg 183, the measurement areas ranged from 0.25 to 500 cm². Angularity and rounding were estimated using the charts produced for describing sedimentary rocks. During these measurements, notes were also taken on the presence of mesostasis blebs, orientation of elongated vesicles, changes in groundmass texture, and other features.

For the breccias, the more intensive measurements included determination of the percentages of clasts in size categories of >1, 0.1-1, and <1 cm, as well as void space. The margins of clasts were also subjected to more careful examination to characterize the brecciation process.

Finally, during the synthesis of the various data sets, we estimated the thickness of the various units and flows. It is very important to note that ODP curation procedures assume that the top of recovered material is from the top of the cored interval. This can lead to large (up to 9 m) differences between the curated depth and the actual depth. During Leg 183 there were several occasions in which the pattern of rock recovery and selection of unit boundaries combined to maximize these problems. Unit thicknesses derived from curated depths alone must be considered accurate to only ±5 m. Downhole measurements were required to produce more accurate estimates of flow thicknesses.

Interpretation

The interpretation of the lavas involved three steps. The observed features were tied to physical processes, the emplacement style of individual flows was inferred, and the environmental setting (e.g., subaerial or submarine) for the whole sequence was discussed. Interpretations relied most heavily on observations from active volcanism in Hawaii and the ~15-Ma flood basalts of Columbia River Basalt Group, in the United States.

Traditionally, mafic lava flows have been divided into pahoehoe and aa flows (Macdonald, 1953). This division is important because aa and pahoehoe flows are emplaced in fundamentally different styles. Aa flows disrupt and mix their upper crusts, which move like the treads on a bulldozer. Pahoehoe flows advance by inflating a lobe with a continuous crust, much like filling a rubber balloon with water. However, there is also a wide range of intermediate flow types. Most transitional flow types have "pahoehoe" in their names (e.g., slab pahoehoe, remobilized pahoehoe, toothpaste pahoehoe, and sharkskin pahoehoe), but many are more similar to aa than to pahoehoe. The most common transitional flow type found during Leg 183 has a flow-top breccia composed of broken pieces of pahoehoe. This type of flow has not been seen in Hawaii and has no widely accepted name.

Determining lava type is relatively straightforward if centimeter-scale morphologic features of the flow tops and bottoms are well preserved and recovered. Aa flows are characterized by angular, spinose clinker at both the flow tops and bottoms, and pahoehoe flows are characterized by smooth tops and bottoms. Transitional flows show some of the characteristics of both aa and pahoehoe lava flows. However, these types of features are the first to be destroyed by erosion and weathering and are also difficult to recover in core.

It is commonly possible to use the distribution and morphology of vesicles in the interior of the flow to determine the flow type (Fig. F9). Most lava flows have a thick vesicular upper crust, a dense core, and a thinner vesicular lower crust (Aubele et al., 1988). To avoid confusion with the drilled core, we use the term "massive interior" instead of "core" throughout this volume. Aa flows tend to have fewer, but more distorted vesicles than pahoehoe flows (Macdonald, 1953). Aa flows also commonly exhibit partially resorbed pieces of entrained crust within the dense interior of the flow. These typically appear as fist-sized clots of small, highly distorted vesicles within the nonvesicular part of the flow. Aa flows also commonly have 10- to 200-cm-wide "arms" or "fingers" of dense material pushed up into the vesicular, brecciated crust. The margins of these arms have been observed to disaggregate into sand-sized and finer fragments as they cool (L. Keszthelyi, unpubl. data).

For pahoehoe flows, we use the terminology from Self et al. (1997). Key vesicle features are horizontal vesicular zones in the upper crust, horizontal vesicle sheets defining the boundary between the upper crust and the massive interior of the flow, and pipe vesicles at the base. Pipe vesicles are typically 0.3-0.7 cm in diameter, 3-15 cm in height, and reliable indicators of a flow margin. Pipe vesicles may grade into vesicle cylinders, which in turn lead to 0.5- to 5-cm-thick horizontal vesicle sheets at the base of the upper vesicular crust. The vesicles within cylinders and sheets are usually 1-5 mm in size and are very irregular in shape. The vesicle cylinders and vesicle sheets are usually found in segregated late-stage differentiates that can have micropegmatite textures (e.g., Goff, 1996). These features are synonymous with segregation veins. However, they are very different from the horizontal vesicular zones that can be found in the upper crust of pahoehoe flows. These zones can vary in thickness from 0.5 to 500 cm and typically contain vesicles 0.5-5 cm in size. Commonly, they are marked by relatively sharp upper boundaries and fade gradually into a zone of relatively low vesicularity. Bubble sizes usually fine downward across this transition. The vesicles in the uppermost 1-10 cm and lowermost 10-50 cm of a pahoehoe flow are usually relatively small (0.1-3.0 mm in diameter) and highly spherical.

The interpretation of these vesicle features is explained at length in Self et al. (1997, 1998), but we provide a short synopsis here. The vesicles in the upper and lower crust represent bubbles formed during eruption. When the injection of fresh lava into the flow stops, the bubbles rapidly rise to the base of the upper crust, leaving a dense interior. Horizontal vesicular zones represent episodes of reactivation of the flow, whereas horizontal vesicle sheets and vesicle cylinders can only form in a stagnant lava.

This model allows the duration of active flow in an inflated pahoehoe flow to be estimated. The thickness of the upper vesicular crust is interpreted as a direct measurement of the amount of lava that solidified while fresh lava was being injected into the flow. A cooling model allows the translation of this thickness into time. We estimate the duration of active flow using the Hon et al. (1994) empirical cooling model as adopted by Thordarson (1995). This technique and its limitations and errors are described in more detail in Self et al. (1997, 1998).

Given the abundance of flows with brecciated flow tops recovered during Leg 183, the processes that disrupt lava require some additional explanation. Aa clinker forms primarily by the rupture of molten lava. This can only happen when and where strain rates are high and/or the viscosity of the fluid lava is high (Peterson and Tilling, 1980). This tearing of the lava is a result of the non-Newtonian behavior of lava under these conditions. Large "arms" of lava from the dense interior of the flow commonly extend into the breccia, especially along well-defined shear zones. Entrainment of the brecciated top into the interior of the flow is a powerful heat loss mechanism for aa flows (Crisp and Baloga, 1994), driving increased rapid crystallization of the flow interior (Crisp et al., 1994) and leading to effective degassing.

Transitional lavas in Hawaii fall into two broad categories. Lavas such as toothpaste or sharkskin pahoehoe involve emplacement of highly viscous lavas at very low strain rates (Rowland and Walker, 1987). These flows do not form brecciated tops. The other category of transitional lava is slab pahoehoe, which involves emplacement of relatively low viscosity lava under very high strain rates (Peterson and Tilling, 1980). Such high strain rates are usually not sustained and are, instead, associated with surges of lava. Also, high strain rates are associated with higher volumetric flux and larger, sheet-like, lobes. The crust on the flow at the time of the surge (typically flat slabs from the top of larger sheetlike pahoehoe lobes) is disrupted and rafted along. Jumbled piles of crustal slabs accrete wherever there are obstacles or constriction in the flow. The individual clasts usually demonstrate the full range of brittle to ductile deformation as the upper chilled portion cracks and the lower hotter portion deforms plastically to the disruption. Slab pahoehoe lavas rarely extend for >1 km before transitioning to classic aa or pahoehoe.

Another style of brecciation was observed during the Mauna Ulu eruption of Kilauea Volcano, Hawaii. Within <12 hr, pahoehoe sheet flows became disrupted and brecciated by the intrusion of new lava into the interior of the flow and remobilized the entire upper surface. In Hawaii, this kind of extreme inflation was observed only in the near-vent facies, where sudden pulses in eruption rate could be immediately transferred into a flow without a longer transport system to buffer the surges (Peterson and Tilling, 1980). However, surges disrupted the pahoehoe surfaces of flows from Laki (Iceland) >50 km from the vent (Th. Thordarson, pers. comm., 1995).

Lava flows that entered water (or were erupted subaqueously) may form pillow basalts, hyaloclastites, and peperites. The latter two are volcaniclastic sediments that form by quench fragmentation. Pillow basalts can usually be distinguished from pahoehoe flows by the thicker glass rinds, lower vesicularity, and the presence of marine sediments between the pillows. However, larger subaqueous flows may form massive sheets. The morphology of submarine flows is apparently controlled by local flow rate that is primarily a function of slope and eruption rate (Gregg and Fink, 1995).