Atsuko Namiki

The influence of boundary heterogeneity in experimental models of mantle convection

Recent global seismological observations have revealed lateral variations in the thickness of the D 00 layer. We have performed laboratory experiments to explore how undulations of various sizes in the D 00 layer aect convection patterns. We nd that topography on the lower boundary can induce plumes and that there is a critical height above which topography controls convection patterns. Observed undulations in the D 00 layer exceed this critical height, sug- gesting that they may control mantle convection patterns.

El Cobreloa: A geyser with two distinct eruption styles

We performed field measurements at a geyser nicknamed “El Cobreloa,” located in the El Tatio Geyser Field, Northern Andes, Chile. The El Cobreloa geyser has two distinct eruption styles: minor eruptions and more energetic and long-lived major eruptions. Minor eruptions splash hot water intermittently over an approximately 4 min time period. Major eruptions begin with an eruption style similar to minor eruptions, but then transition to a voluminous liquid water-dominated eruption, and finally end with energetic steam discharge that continues for approximately 1 h. We calculated eruption intervals by visual observations, acoustic measurements, and ground temperature measurements and found that each eruption style has a regular interval: 4 h and 40 min for major eruptions and ∼14 min for minor eruptions. Eruptions of El Cobreloa and geochemical measurements suggest interaction of three water sources. The geyser reservoir, connected to the surface by a conduit, is recharged by a deep, hot aquifer. More deeply derived magmatic fluids heat the reservoir. Boiling in the reservoir releases steam and hot liquid water to the overlying conduit, causing minor eruptions, and heating the water in the conduit. Eventually the water in the conduit becomes warm enough to boil, leading to a steam-dominated eruption that empties the conduit. The conduit is then recharged by a shallow, colder aquifer, and the eruption cycle begins anew. We develop a model for minor eruptions which heat the water in the conduit. El Cobreloa provides insight into how small eruptions prepare the geyser system for large eruptions.

Intermittent and efficient outgassing by the upward propagation of film ruptures in a bubbly magma

We simulated the ascent of bubbly magma in a volcanic conduit by slow decompression experiments using syrup foam as a magma analogue. During decompression, some large voids appear in the foam. The expansion of one void deep in the foam leads to another void expansion, and the void expansion then propagates upward. The void expansion finally reaches the surface of the foam to originate outgassing. The velocity of the upward propagation of void expansions is essentially the same as the rupturing velocity of the bubble film, suggesting that the rupture of films separating each void propagates upward to create the pathway for outgassing. The calculated apparent permeability of decompressed foam can become higher than that measured for natural pumices/scoriae. The upward propagation of film ruptures thus allows for efficient outgassing. This may also appear as the mechanism for energetic gas emissions originating at a depth, such as Strombolian eruptions.

The influence of boundary heterogeneity in experimental models of mantle convection with internal heat sources

Abstract The lateral variations in the thickness of the D″ layer observed by recent global seismology are suspected to control mantle dynamics. Namiki and Kurita (1999) performed laboratory experiments to explore how undulations of various sizes in the D″ layer affect convection patterns. In addition, we found that the undulation at the lower boundary can induce plumes and that there is a critical height above which the undulation controls convection patterns. Observed undulations at the top of the D″ layer exceed this critical height, suggesting that they may control the mantle convection patterns. The previous work, however, assumed basal heating. Since internal heating is an important source which drives mantle convection, we extend our previous study to a case incorporating internal heat sources using laboratory experiments. Internal heat generation is simulated by lowering the boundary temperatures at a constant rate. We observed that the undulation, which has the comparable thickness to the critical height determined in the purely basal heating case, can fix the location of the upwelling although the magnitude of the intensity of the upwelling is weaker. This situation is similar to the Earth’s hotspots and suggests that the undulation at the top of the D″ layer is the source region of the hotspots.

Earthquake model experiments in a viscoelastic fluid: A scaling of decreasing magnitudes of earthquakes with depth

We performed shear deformation experiments using quasi-Maxwell fluids. We found that, depending on the strain rates, the same material generates earthquakes associated with the elastic rebound and deforms viscously. Around the threshold, elastic rebound releases a certain fraction of the interseismic displacement, but the other fraction remains as a result of the viscous relaxation. We applied our experimental results to a subduction zone, in which the upper part of the hanging wall behaves as an elastic layer and generates seismicity, while the deeper part behaves as a viscous fluid and subducts with the slab. Our experimental results suggest that, around the boundary of the elastic and viscous layers, seismicity can occur, but only some part of the interseismic displacements is released. The experimentally obtained threshold of the seismic activity is determined by the combination of the subduction velocity vs, the viscosity of the hanging wall η, the fault length W, and the adhesive stress σa, vsη/(Wσa)>1. This threshold suggests that if the viscosity of the hanging wall decreases with depth, the maximum size of the earthquakes also decreases with depth, and, finally, seismicity disappears. This hypothesis is consistent with the observed fact that slow earthquakes, characterized by their small magnitudes, are observed at the downdip limit of the seismogenic zone.

An experimental study of the role of subsurface plumbing on geothermal discharge

In order to better understand the diverse discharge styles and eruption intervals observed at geothermal features, we performed three series of laboratory experiments with differing plumbing geometries. A single, straight conduit that connects a hot water bath (flask) to a vent (funnel) can originate geyser-like periodic eruptions, continuous discharge like a boiling spring, and fumarole-like steam discharge, depending on the conduit length and radius. The balance between the heat loss from the conduit walls and the heat supplied from the bottom determines whether and where water can condense which in turn controls discharge style. Next, we connected the conduit to a cold water reservoir through a branch, simulating the inflow from an external water source. Colder water located at a higher place than a branching point can flow into the conduit to stop the boiling in the flask, controlling the periodicity of the eruption. When an additional branch is connected to a second cold water reservoir, the two cold reservoirs can interact. Our experiments show that branching allows new processes to occur, such as recharge of colder water and escape of steam from side channels, leading to greater variation in discharge styles and eruption intervals. This model is consistent with the fact that eruption duration is not controlled by emptying reservoirs. We show how differences in plumbing geometries can explain various discharge styles and eruption intervals observed in El Tatio, Chile and Yellowstone, USA. This article is protected by copyright. All rights reserved.

Physical characteristics of scoriae and ash from 2014–2015 eruption of Aso Volcano, Japan

The activity at Aso Volcano was mainly defined as a sequence of ash emissions and occasional ejections of scoria fragments with ash. Ash emissions sometimes started without notable explosions. The measured porosity of scoriae was as high as 0.94. The scoriae had a flattened shape with a low-porosity outer rim. To elucidate the eruptive conditions causing such ash emission and generation of scoriae, we conducted three series of measurements. First, we heated the high-porosity scoriae from Aso Volcano at 900–1150 °C and found that the heated scoriae shrunk by losing the gas in the bubbles. At the highest temperature,

Response of a bubble bearing viscoelastic fluid to rapid decompression: Implications for explosive volcanic eruptions

1150\,^{\circ } \hbox {C}