Site Of Collapse Research Articles

Eruption, transport and deposition of ignimbrite: A case study from Mexico

The Acatlan ignimbrite covers 150 km 2 around a vent situated 40 km southwest of Guadalajara, Mexico. The upper and main ignimbrite flow unit is compositionally zoned, and has white or pink rhyolitic pumice in its lower part and black andesite in its upper; these parts are believed to have been erupted continuously from the same magma chamber. This zonation has been important in correlating an extremely coarse, lithic-rich deposit, a co-ignimbrite lag-fall deposit, with this ignimbrite. The co-ignimbrite lag-fall deposit indicates that the flow unit formed by continuous collapse of an explosive eruption column, and enables the source vent to be located. The vertical compositional grading of the flow unit also provides a means of testing the various proposed mechanisms of ignimbrite emplacement (turbulent vs. high-concentration flow). The preservation of compositional zonation and other features in the deposit indicate that flow was not turbulent. Details of the grain-size distribution indicate that laminar flow was important early in the flow history when collapsing material from the eruption column was superimposed in stratigraphical sequence. Initial high gas-flow rates (associated with flow segregation and formation at the site of column collapse) rapidly decayed away from vent and later in its history the flow degassed to a semi-rigid plug overriding a sheared basal layer.

The ignimbrite source problem: Significance of a co-ignimbrite lag-fall deposit

A co-ignimbrite lag-fall deposit is one that forms at or near the site of eruptive column collapse and consists mainly of pyroclasts that are too large and too heavy to be carried away in the resulting pumice flows. An example from Mexico is identified as such because it shows the same compositional zoning from rhyolite to andesite as the associated ignimbrite. It indicates that the ignimbrite originated by continuous collapse of the eruptive column and locates the ignimbrite source vent. In this example the vent is located on flat ground, apparently unrelated to any pre-existing volcano, and the ability of the ignimbrite to travel laterally some 20 km must be due to eruptive column height rather than ground slope.

Resistance and collapse in bronchial airways.

1. The Forced Expiratory Volume (FEV) was measured in normal persons and patients with obstructive airways disease at mouth pressures from 0 mmHg to levels high enough to prevent air flow altogether by using a Starling resistor at the mouth. 2. Evidence was obtained in support of the idea that, during forced expiratory flow, airways might function with the properties of a Starling resistor. This is considered to divide the airway into upstream and downstream segments at the site of airway collapse. The technique was simple, capable of being performed on outpatients and provided a means of studying the collapsibility of airways, airways resistance and alveolar pressure. 3. The FEV in normal persons and patients with disease was determined by the properties of the upstream segment. Since this segment contains only a proportion of the total airways resistance it was not surprising to find imprecise correlations between the FEV and airways resistance measured by the body plethysmograph. 4. Preliminary observations of patients during acute exacerbations of chest disease suggest that functional changes can occur throughout the length of the airway (i.e. in both upstream and downstream segments). 5. In the longer term during the natural history of obstructive airways disease, airways resistance and the more ready collapse of airways may develop independently. 6. Impaired alveolar pressure may contribute towards the reduction in air flow during acute exacerbations of chest disease.

Experimental Investigation of Cavity Collapse Pressures

Single, hemispherical spark cavities were created in water at the pressure end of a Duralumin pressure bar. At the collapse of such a cavity, a transient stress pulse (∼20 μsec duration) is propagated along the bar which is detected by an X-cut circular disk of quartz of the same diameter as the pressure bar. The charge liberated at the surface of the crystal when the X-cut quartz disk is traversed by the stress pulse was converted into a voltage pulse which subsequently was amplified and recorded photographically from a cathode ray oscillograph. These photographic records yield only information regarding the total lifetime of the cavity and the maximum amplitude and time variation of the force developed during its collapse. However, an ultra-high speed photographic technique employed in conjunction with the piezoelectric pressure bar permits an estimate of the peak pressure developed at the site of collapse of the cavity to be made together with an observation of the time variation of the pressure within the cavity in the last stages of collapse. Cavities having maximum diameters of up to 12 in. have been studied during the course of these experiments and collapse pressures of at least 10 000 atmospheres have been observed.