Luke Wooller

Volcano spreading controlled by dipping substrata

Most volcanoes grow on slopes, and some tend to fail catastrophically on the downslope side. Many volcanoes also deform by volcano spreading, which may lead to failure. We look at the effect of dipping substrata on the potential for spreading and collapse with analogue models. The dip is found to strongly control the spreading style, rate, and direction. A distinct change from purely radial spreading occurs even with small (<1°) substrata tilt. Structures on the cone and surrounding area are modified according to the underlying dip direction. Spreading becomes concentrated on downslope sectors, where movement is predominantly in the dip direction. The degree of structural realignment is a function of the slope angle. Our models are applied to previously known and new examples of volcano spreading. The effect of dipping substrata in confining spreading to sectors increases the potential for deep-seated sector collapses. This finding provides a mechanism for failure on the downslope side. The incorporation of substrata can create very large volume collapses.

Persistent summit subsidence at Volcán de Colima, México, 1982–1999: strong evidence against Mogi deflation

This paper re-examines recent ground-deformation measurements at Volcan de Colima, Mexico, to test the hypothesis that the observed movements are in response to pressure changes within a sub-volcanic magma chamber, as suggested for other volcanoes by [Mogi (1958) Earthq. Res. Inst. 36, 99-134]. Measurements of vertical ground deformation across the summit dome complex of Volcan de Colima from a precise levelling network between 1982 and 1999, together with vertical and horizontal displacements derived from dual-frequency GPS measurements in 1994 and 1997, show continuous subsidence. The deformation pattern derived from the levelling shows that subsidence increases towards the summit. The closest stations to the summit (1.1 km distant) show a mean subsidence rate of 5 mm per year compared to the reference station at 2.3 km distance, which may be subsiding itself. Vertical displacements of individual stations since 1982 show that the subsidence has been fairly continuous. The GPS stations, which are distributed more widely and include some close to the edge of the active dome, confirm summit subsidence. The largest measured value, a decrease of 280 mm, or 93 mm subsidence per year, was obtained at the edge of the dome. Horizontal displacements measured during 1994–1997 also show the largest values at the summit, but these are much smaller than the vertical displacements, with the maximum rate of 23 mm per year recorded close to the dome. It is conclusively shown that these measured movements cannot be due to deflation of a buried Mogi source, as vertical and horizontal displacements for some stations are in contrary directions to those predicted by the model, and there is no consistent pattern to the horizontal movements. We attribute the measured deformation to downslope creep, settling and compaction of the edifice, gravitational spreading, or a combination of these processes.

Shallow sub-surface structure of the central volcanic complex of Tenerife, Canary Islands: implications for the evolution and the recent reactivation of the Las Cañadas caldera

We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain. The high-density core of the CVC and the pronounced gravity low centred in the Las Canadas caldera (LCC) in greater detail than previously available. Mathematical construction of a subsurface model from the local anomaly data, employing a 3-D inversion enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture of the complex, and shedding light on its evolution.