Stasis and Change in Forty Years of Southeast Asian Agriculture

Agricultural change in Southeast Asia over the last three decades is reviewed within the context of structural change of the region's economies. An agricultural typology is outlined and the degree to which agricultural systems have remained static is examined leading to the identification of various kinds and degrees of change amongst the systems – extensification, intensification and disintensification. The paper concludes that while macro-level structural change is the basic process driving changes in agriculture, there remain important issues of long-term sustainability, poverty and distribution of benefits.     

Thick lava flows of Karisimbi Volcano, Rwanda: insights from SIR-C interferometric topography

 We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12 km and average 40–60 m (up to >140 m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of flow thickness, which we used to calculate the volume (up to 1 km³ for an individual flow, and 1.8 km³ for all the identified flows) and yield strength of several flows (23–124 kPa). Using the DEM we estimated apparent viscosity based on the spacing of large folds (1.2×10¹² to 5.5×10¹² Pa s for surface viscosity, and 7.5×10¹⁰ to 5.2×10¹¹ Pa s for interior viscosity, for a strain interval of 24 h). We use shaded-relief images of the DEM to map basic flow structures such as channels, shear zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images. Received: 6 September 1997 / Accepted: 15 May 1998     

Penetrative convection induced by the absorption of radiation with a nonlinear internal heat source

A model for convection induced by the selective absorption of radiation in a fluid layer is investigated. The concentration based internal heat source is modelled quadratically. A linear analysis and conditional nonlinear analysis are performed. Due to the presence of significantly large regions of potential subcritical instabilities, the results indicate that linear theory may only be accurate enough to predict the onset of convective motion when the model for the internal heat source is predominantly linear. This lends much credence to the use of a linearly modelled internal heat source as used by Krishnamurti [Dyn. Atmos. Oceans 27 (1997) 367], within the constraints of the parameter ranges.     

Agricultural Value of ENSO Information under Alternative Phase Definition

The El Niño-Southern Oscillation (ENSO) effect has been found to be associated with regional climate variations in many regions of the world, and, in turn, with variation in crop yields. Previous studies have found that early releases of ENSO phase information could permit agricultural producers to make adjustments in their decisions and in turn generate an increase in agricultural sector welfare. This study examines whether the value of the agricultural responses can be enhanced by releasing more detailed ENSO information. Namely we evaluate the implications for projected agricultural welfare under release and adaptation to the Stone and Auliciems five phase definition of ENSO states as opposed to the more standard three phase definition. This value is estimated using a stochastic, U.S./global agricultural model representing 22 climate years. The results indicate that the release and exploitation of the more detailed ENSO phase definition almost doubles the welfare impact. The results also indicate that there is room for up to another doubling of information value through further refinements.     

The role of carbon and oxygen in cosmic gases: some applications to the chemistry and mineralogy of enstatite chondrites

A new condensation sequence appears if the $\text{C}\text{O}$ ratio in a gas of otherwise solar composition is increased by less than a factor of two. As the ratio increases from the solar value of 0.6 to ? 1 the gas becomes extremely reduced, the condensation temperatures of silicates and oxides are depressed markedly ～ 400 K and a new suite of refractory minerals appears: AIN, CaS, MgS, SiC, TiN, graphite, Si₂N₂O and probably metastable (Fe,Ni)³C. Many of these minerals are unique to enstatite chondrites and may be analogues of the refractory silicates and oxides found in more oxidized meteorites such as Allende.The change in chemistry is related to the stability of CO, the most stable C or O compound at high T. Since the elements occur in a 1:1 ratio in CO, only the element which is in excess is free to form other compounds. But as T decreases CO reacts with H₂ to form graphite, CH₄ or other hydrocarbons thereby freeing O to form H₂O. If equilibrium is maintained oxides and silicates form at about 1000 K ($\text{C}\text{O}\text{> 1}$, P_τ = 10^?4atm) as products of reactions among the carbides, nitrides, sulfides and the gas. The possibility that equilibrium was not maintained among the C-bearing species was also investigated. If either graphite or CH₄ does not form as predicted the stability fields of the reduced minerals expands to lower temperatures. If neither graphite nor CH₄ form as predicted, CO remains stable and the nebular gas is highly reduced at all temperatures.Enstatite chondrites appear to have originated in a region of the nebula where the $\text{C}\text{O}$ ratio was somewhat higher than the solar value. Various fractionation mechanisms are considered. An interesting possibility is that graphite, which is quite refractory under a wide range of conditions, survived the collapse of the solar nebula.     

Dislocation substructure of mantle-derived olivine as revealed by selective chemical etching and transmission electron microscopy

Cleaved and mechanically polished surfaces of olivine from peridotite xenoliths from San Carlos, Arizona, were chemically etched using the techniques of Wegner and Christie (1974). Dislocation etch pits are produced on all surface orientations and they tend to be preferentially aligned along the traces of subgrain boundaries, which are approximately parallel to (100), (010), and (001). Shallow channels were also produced on (010) surfaces and represent dislocations near the surface that are etched out along their lengths. The dislocation etch channel loops are often concentric, and emanate from (100) subgrain boundaries, which suggests that dislocation sources are in the boundaries. Data on subgrain misorientation and dislocation line orientation and arguments based on subgrain boundary energy minimization are used to characterize the dislocation structures of the subgrain boundaries. (010) subgrain boundaries are of the twist type, composed of networks of [100] and [001] screw dislocations. Both (100) and (001) subgrain boundaries are tilt walls composed of arrays of edge dislocation with Burgers vectors b=[100] and [001], respectively. The inferred slip systems are {001} 〈100〉, {100} 〈001〉, and {010} 〈100〉 in order of diminishing importance. Exploratory transmission electron microscopy is in accord with these identifications. The flow stresses associated with the development of the subgrain structure are estimated from the densities of free dislocations and from the subgrain dimensions. Inferred stresses range from 35 to 75 bars using the free dislocation densities and 20 to 100 bars using the subgrain sizes.     

Earthquake swarms and local crustal spreading along major strike-slip faults in California

Earthquake swarms in California are often localized to areas within dextral offsets in the linear trend in active fault strands, suggesting a relation between earthquake swarms and local crustal spreading. Local crustal spereading is required by the geometry of dextral offsets when, as in the San Andreas system, faults have dominantly strike-slip motion with right-lateral displacement. Three clear examples of this relation occur in the Imperial Valley, Coso Hot Springs, and the Danville region, all in California. The first two of these areas are known for their Holocene volcanism and geothermal potential, which is consistent with crustal spreading and magmatic intrusion. The third example, however, shows no evidence for volcanism or geothermal activity at the surface.     

Coupled Sr-O isotope variations as an indicator of source heterogeneity for the Northern peninsular ranges batholith

Primary ¹⁸O values for tonalitic rocks from the San Jacinto Intrusive Complex range from +9.0 to +10.6; initial⁸⁷Sr/⁸⁶Sr for the same samples varies from 0.7058 to 0.7076. Rocks with low ¹⁸O tend to have higher initial⁸⁷Sr/⁸⁶Sr. The majority of samples have a limited range of ¹⁸O values (+10.0–+10.4) that is anticorrelated with colour index; this variation is compatible with crystallization of rocks with varying mineral abundances from an homogeneous (for oxygen) reservoir. More mafic rocks show a large range of ¹⁸O values, reflecting original variations in source material compositions. The combined Sr-O isotope data demonstrate that three or more distinct components were involved in the genesis of these rocks; these three components are also seen in batholithic rocks from the Mojave Block and Sierra Nevada to the north. These data are interpreted as indicating a 0–35% (oxygen atom basis) contribution to the San Jacinto rocks from an old continental lithosphere source.Division Contribution No. 4064     

Palynological evidence for early Holocene aridity in the southern Sierra Nevada, California

Sediments of Balsam Meadow have produced a 11,000-yr pollen record from the southern Sierra Nevada of California. The Balsam Meadow diagram is divided into three zones. (1) The Artemisia zone (11,000–7000 yr B.P.) is characterized by percentages of sagebrush (Artemisia) and other nonarboreal pollen higher than can be found in the modern local vegetation. Vegetation during this interval was probably similar to the modern vegetation on the east slope of the Sierra Nevada and the climate was drier than that of today. (2) Pinus pollen exceeded 80% from 7000 to 3000 yr B.P. in the Pinus zone. The climate was moister than during the Artemisia zone. (3) Fir (Abies, Cupressaceae, and oak (Quercus) percentages increased after 3000 yr B.P. in the Abies zone as the modern vegetation at the site developed and the present cool-moist climatic regime was established. Decreased fire frequency after 1200 yr B.P. is reflected in decreased abundance of macroscopic charcoal and increased concentration of Abies magnifica and Pinus murrayana needles.