Aa Lava Flows Research Articles

Airfield pavement construction using basalt aggregate

75,000 m3 oc concrete were required for the Fast Exit Taxiway and Apron works for development of Wideawake airfield on Ascension Island. Locally available construction materials were used wherever possible because of the remote location. The pavement contractor obtained some 250,000 tonnes of concrete aggregate by quarrying basalt from aa lava flows associated with highly vesicular basalt scoria ejected during Strombolian-type eruptions. The methods used for ground investigation, material assessment and development include the innovative use of geostatistical techniques for resource evaluation.

Influence of crystallization and entrainment of cooler material on the emplacement of basaltic aa lava flows

A theoretical model is used to describe and investigate the effects of simultaneous crystallization, radiation loss, and entrainment of cooler material on the temperature of a well‐mixed core of an active aa lava flow. Entrainment of crust, levee debris, and base material into the interior of active flows has been observed, but the degree of assimilation and the thermal consequences are difficult to quantify. The rate of entrainment can be constrained by supplementing the theoretical model with information on the crystallization along the path of the flow and estimation of the radiative loss from the flow interior. Application of the model is demonstrated with the 1984 Mauna Loa flow, which was erupted about 30°C undercooled. Without any entrainment of cooler material, the high crystallization rates would have driven temperatures in the core well above temperatures measured by thermocouple and estimated from glass geothermometry. One plausible scenario for this flow, which agrees with available temperature and crystallinity measurements, has a high initial rate of entrainment during the first 8 hours of travel (a mass ratio of entrained material to fluid core of about 15% if the average temperature of the entrained material was 600°C), which counterbalances the latent heat from approximately 40% crystallization. In this scenario, the model suggests an additional 5% crystallization and a 5% entrainment mass ratio over the subsequent 16‐hour period. Measurements of crystallization, radiative losses, and entrainment factors are necessary for understanding the detailed thermal histories of active lava flows.

Directional variations in thermal emission from geologic surfaces

We investigated the directional emission properties of geologic surfaces using a ground‐based, hand‐held infrared radiometer and thermistor probe. Field sites involved surfaces ranging from smooth playa and sand surfaces to a very rough aa lava flow in the Lunar Crater Volcanic Field, NV, as part of the Geological Remote Sensing Field Experiment. Large directional variations in thermal emission were found; they result from the presence of surface roughness–at large scales producing spatial variations in kinetic temperature and at small scales producing emissivity variations. These variations are important in remotely determining surface structure and understanding surface energy balance and emission spectra.

The growth of AA lava flow fields on Mount Etna, Sicily

The historical lavas on Mount Etna provide a large body of data for examining the growth of aa flow fields. A flow field, the final product of an eruption, is composed of one or more flows. Flow generation depends on the ratio SR/SD of crustal retaining strength (SR) to stress driving the lava outward (SD), rate of flow thickening h, and the time for which h is sustained. At low SR/SD, lava advances by simple spreading, and new flows are formed by subdividing about topographic highs (bifurcation). At low h, growing crusts can contain thickening at all values of SR/SD. Overflow of chilled margins can occur at higher sustained h. Increasing SR/SD favors flow propagation by breaching of crusts. Flow growth is examined by extending the Bingham model of Hulme (1974). Cooling and discharge rate along a flow appear to be the main factors controlling the generation of new flows, with the result that long durations and low average effusion rates favor flow fields with large width/length ratios. Bifurcation generates flows randomly. Breaching and overflow generate flows systematically because of their dependence on cooling. With time, therefore, flow field growth evolves from predominant lengthening to widening. Such behavior is evident for Etnean flow fields, which can also be grouped into two populations on the basis of their width/length ratios. These populations may be related to two distinct mechanisms of magma ascent.

Radar observations of basaltic lava flows, Craters of the Moon, Idaho

Radar images of Craters of the Moon, Idaho were used to study the backscatter characteristics of basaltic lava flows of predominantly pahoehoe textures and to determine the ability to detct fissure vents. Four images were obtained: X-band HH, X-band HV, L-band HH, and L-band HV. Hummocky pahoehoe flows were found to have strong backscatter in all four of these images. Aa lava flows showed the greatest variation in backscatter intensities, due to an increase in multiple scattering at the L-band scale. Eruptive fissures are detectable in the radar images by virtue of associated parallel spatter ramparts which have diagnostic, strong backscatter in the X-band images that are in contrast to the weak backscatter of the surrounding shelly pahoehoe lava. The importance of look direction in the use of radar images to characterize terrains is emphasized.

Sedimentologic Succession of Uplifted Coral Community, Urvina Bay, Isabela Island, Galapagos Archipelago, Ecuador: ABSTRACT

In March 1954, along the west-central coast of Isabela Island, an upward movement of magma suddenly raised Urvina Bay over 6 m and exposed several square kilometers of carbonate deposits covering a young aa lava flow (around 1000 years old). Results from 6 transect lines across the uplift, 30 cores, and 10 trenches describe the sedimentologic and ecologic transition from barren basalt to diverse carbonate sediments with small coral reefs. Along horizontal transects spanning from 0 to 7 m paleowater depth, there is a seaward progression from beaches, mangroves, and basalt to thick deposits (> 1.6 m) of carbonate sands and small coral reefs. Variation in water depth, degree of wave exposure, and irregularity of the aa lava topography provided many microhabitats where coral, calcareous algae, and mollusks settled and grew. Eight hermatypic coral species are found throughout the shelf, and three species (i.e., Pavona clavus, Pocillopora damicornis, and Porites lobata) produced five small, isolated, monospecific, coral-reef frameworks. The vertical section seen in cores and trenches shows that calcium carbonate increased upward, whereas volcanic sediments decreased; however, episodic layers occur with high concentrations of basaltic sands. In vertical samples from the central portion of the shelf, the coral population changedmore » from small, isolated colonies of Psammocora (Plesioseris) superficalis near the basalt basement to large reef-forming colonies of Pocillopora damicornis farther upsection. Reefs of the Galapagos Islands are small and less diverse than most Pacific reefs. Nonetheless, understanding their temporal successional development should throw light on the origin and history of larger oceanic reefs in the Pacific.« less

The October 1983 eruption of Miyakejima volcano

Miyakejima volcano erupted at 1515 hours on October 3, 1985. During the first 2 hours, a 4.5-km-long flank fissure produced the curtain of fire at higher vents from 500 m to 100 m in altitude. Porous scoria drifted eastward to form a thin blanket across the island. Most of the erupted material from these vents formed aa lava flows which devastated the forest. One flow reached the sea shore and another engulfed the largest community of the island within several hours after the start of the eruption. About 430 houses were burned and buried but the 1500 residents escaped without casualties. An effort was made to control the advancing lava flow by spraying it with 4700 tons of sea water. However, by the time the watering operation started, the lava flow apparently had stopped moving so that the real effect of watering could not be confirmed. The fissure extended northward and southward at a rate of 40 m per minute during the first 30 minutes. The northward extension stopped at 505 m altitude but the southward extension continued at a rate of 26 m per minute to the 40 m deep sea floor off the southern coast of the island. Near the southern end of the fissure, violent phreatomagmatic explosions produced large craters and a pyroclastic ring on the coast. An 8000-m-high eruption column produced heavy scoria fall in the southeastern part of the island. The scoria was much denser than that erupted by the northern vents and caused great damage to farm lands and houses and broke car windows. The largest earthquake of M = 6.2 occurred at 2233 hours which shook off loose debris and gave rise to a revival of fountaining eruptions in the southernmost vents. The eruption ended before 0600 hours of the next day, October 4. The products are tholeiitic basalts (SiO 2 52–54%) containing a few percent of plagioclase phenocrysts. Much less abundant augite and titanomagnetite and very rare olivine and hypersthene phenocrysts are also found. These basalts fall on the general differentiation trend of Miyakejima volcano and are in a moderately advanced stage of differentiation. Highly vesiculated glassy xenoliths (SiO 2 56–68%) are found included in the juvenile ejecta. Chemistry of the xenoliths indicates that most of them are derived from mechanical mixtures (probably volcaniclastic sediments) of ancient ejecta of Miyakejima of various compositions.

ATTENUATION MEASUREMENTS IN THE FIELD

The amplitudes of the P head wave were measured on an aa lava flow, on unconsolidated cinders, and on compact limestone. The data are satisfied by the equation [Formula: see text], where A is the amplitude, d the distance, [Formula: see text] the attenuation coefficient for P waves, ν the frequency, and α the P wave velocity. By assuming a complex shear modulus μ* but a real λ one finds [Formula: see text], where β is the shear wave velocity. This formula is in reasonable agreement with published data.