Oor die koninkryk van God

As Johannes die Doper spreek van die koninkryk van God wat naby gekom het (Matt. 3 : 12) en Jesus dat die koninkryk inderdaad gekom het (Matt. 12 : 28; Luk. 11 : 20; 17 : 21), spreek hulle oor ’n saak wat eers ver was en nou naby is, en wat eers nie was nie of elders was en nou gekom het

Effets de la charge des édifices volcaniques sur la propagation de structures régionales compressives : exemples naturels et modèles expérimentaux

Nous présentons ici des exemples naturels d'édifices volcaniques coniques reposant sur un substratum fragile, soumis à une compression régionale ainsi que des résultats expérimentaux. Nous montrons que la charge de l'édifice induit une perturbation de la déformation régionale se traduisant par une déflexion et une horizontalisation des structures compressives régionales. Le contrôle tectonique est de nature topographique. Nous discutons ensuite certaines conséquences, en particulier concernant l'étalement gravitaire des volcans. We present natural examples and experimental models of volcanic cones located above brittle substratum undergoing regional compressive deformation. The volcanic loading induces a strain partitioning involving deflection and flattening of regional compressive structures. The main control is the topographic load. Anticlinal thrust ridges, observed around many volcanoes, have generally been interpreted as being due to gravitational spreading; however, this study shows that this is not necessarily the case, as they can also be a symptom of regional compression

A glacial control on the eruption rate of Mt Erebus, Antarctica

Mt Erebus is the most active Antarctic volcano, on the flanks of the world’s largest ice sheet. Despite this, the interactions between its eruptions and the ice cover have not been studied in detail. Focusing on the most recent deglaciation, we build a glacial retreat model and compare this to recent lava geochemistry measurements to investigate the processes involved. This analysis exposes a previously unknown link between Antarctic glaciation and eruptions, of vital importance to the understanding of volcanism in this context. We find that deglaciation led to rapid emptying of the shallow magma plumbing system and a resulting peak in eruption rates synchronous with ice retreat. We also find that the present day lavas do not represent steady state conditions, but originate from a source with up to 30% more partial melting than older >4 ka eruptions. This finding that deglaciation affects volcanism both on short and longer timescales may prompt a re-evaluation of eruptions in glaciated and previously glaciated terrains both in Antarctica and beyond

Field evidence for summit subsidence, flank instability and basal spreading at Mt Cameroon volcano, West Africa

Mt Cameroon is a steep lava-dominated volcano located on the coast of the Gulf of Guinea. This 1400 km3 edifice is one of two active centres in the Cameroon Volcanic Line. Despite recent lava eruptions along its rift zones in 1999 and 2000, little geological or monitoring data are available to understand the structure of this large volcanic system. Here we report results from a field campaign dedicated to mapping geological structures in the summit area and at the SE base of Mount Cameroon. Eruptive fissures and open fractures’ orientation, vents’ location and alignment above 3500 m a.s.l were systematically surveyed. In addition to the tectonically-controlled N40°E orientation of eruptive fissures along the rift zones, other dominant orientations were identified such as N60°E (summit vents alignment), N20°E and N90° (extension related structures). These were attributed to local instability around the summit, stress field re-orientation around the head of a deep valley cutting through the NW flank and radial pattern around the summit. Inward-dipping structures were also observed to border the relatively flat upper part of the rift zones. Geological profiles were also measured along rivers cutting through a topographic bulge at the SE base of Mt Cameroon. This topographic step was seen to be associated with deformed Miocene sediments from the Douala basin overlain by volcanic products.Weak sediments of this area are deformed by N50- 60°E trending asymmetrical folds verging toward the SE and by N10-30°E trending symmetrical folds and thrusts. Initial NE-SW trending structures formed following the sliding of sediments on the flank of a NE-SW elongated uplift dome. Later, the same area has been deformed by NNE-SSW trending compressive structures linked to the spreading of Mt Cameroon southern flank toward the SE. Combined with the interpretation of a 30 m Digital Elevation Models and multispectral satellite data, the field observations suggest that Mt Cameroon is affected by major instabilities. Both slow spreading movements and catastrophic collapses of the steep flanks are interpreted to result from complex interactions between the growing edifice, repeated dyke intrusions, the weak sedimentary substratum and tectonic structures

Intertemporal portfolio allocation and hedging demand: an application to South Africa

This paper analyses the intertemporal hedging demand for stocks and bonds in South Africa, the United Kingdom and the United States. The analysis is done using an approximate solution method for the optimal consumption and wealth portfolio problem of an infinitely long-lived investor. Investors are assumed to have Epstein-Zin-Weil-type preferences and face asset returns described by a first-order vector autoregression in returns and state variables. The results show that the mean intertemporal hedging demands for stocks are considerably smaller in SA than in the UK or the US, whilst the mean intertemporal hedging demand for bonds are not significantly different from zero in any of the countries considered. Furthermore, it is found that stocks in the US and the UK do not present a useful hedging opportunity for an investor in SA, nor do SA stocks present a useful hedging opportunity for investors from the UK or the US

GLAcier Feature Tracking testkit (GLAFT): a statistically and physically based framework for evaluating glacier velocity products derived from optical satellite image feature tracking

Glacier velocity measurements are essential to understand ice flow mechanics, monitor natural hazards, and make accurate projections of future sea-level rise. Despite these important applications, the method most commonly used to derive glacier velocity maps, feature tracking, relies on empirical parameter choices that rarely account for glacier physics or uncertainty. Here we test two statistics- and physics-based metrics to evaluate velocity maps derived from optical satellite images of Kaskawulsh Glacier, Yukon, Canada, using a range of existing feature-tracking workflows. Based on inter-comparisons with ground truth data, velocity maps with metrics falling within our recommended ranges contain fewer erroneous measurements and more spatially correlated noise than velocity maps with metrics that deviate from those ranges. Thus, these metric ranges are suitable for refining feature-tracking workflows and evaluating the resulting velocity products. We have released an open-source software package for computing and visualizing these metrics, the GLAcier Feature Tracking testkit (GLAFT).</p

Crevasse density, orientation and temporal variability at Narsap Sermia, Greenland

Mass loss from iceberg calving at marine-terminating glaciers is one of the largest and most poorly constrained contributors to sea-level rise. However, our understanding of the processes controlling ice fracturing and crevasse evolution is incomplete. Here, we use Gabor filter banks to automatically map crevasse density and orientation through time on a ~150 km2 terminus region of Narsap Sermia, an outlet glacier of the southwest Greenland ice sheet. We find that Narsap Sermia is dominated by transverse (flow-perpendicular) crevasses near the ice front and longitudinal (flow-aligned) crevasses across its central region. Measured crevasse orientation varies on sub-annual timescales by more than 45$^\circ$ in response to seasonal velocity changes, and also on multi-annual timescales in response to broader dynamic changes and glacier retreat. Our results show a gradual up-glacier propagation of the zone of flow-transverse crevassing coincident with frontal retreat and acceleration occurring in 2020/21, in addition to sub-annual crevasse changes primarily in transition zones between longitudinal to transverse crevasse orientation. This provides new insight into the dynamics of crevassing at large marine-terminating glaciers and a potential approach for the rapid identification of glacier dynamic change from a single pair of satellite images

Granular fingering as a mechanism for ridge formation in debris avalanche deposits: Laboratory experiments and implications for Tutupaca volcano, Peru

The origin of subparallel, regularly-spaced longitudinal ridges often observed at the surface of volcanic and other rock avalanche deposits remains unclear. We addressed this issue through analogue laboratory experiments on flows of bi-disperse granular mixtures, because this type of flow is known to exhibit granular fingering that causes elongated structures resembling the ridges observed in nature. We considered four different mixtures of fine (300–400 µm) glass beads and coarse (600–710 µm to 900–1000 µm) angular crushed fruit stones, with particle size ratios of 1.9–2.7 and mass fractions of the coarse component of 5–50 wt%. The coarse particles segregated at the flow surface and accumulated at the front where flow instabilities with a well-defined wavelength grew. These formed granular fingers made of coarse-rich static margins delimiting fines-rich central channels. Coalescence of adjacent finger margins created regular spaced longitudinal ridges, which became topographic highs as finger channels drained at final emplacement stages. Three distinct deposit morphologies were observed: 1) Joined fingers with ridges were formed at low (= 1.9) size ratio and moderate (10–20 wt%) coarse fraction whereas 2) separate fingers or 3) poorly developed fingers, forming series of frontal lobes, were created at larger size ratios and/or higher coarse contents. Similar ridges and lobes are observed at the debris avalanche deposits of Tutupaca volcano, Peru, suggesting that the processes operating in the experiments can also occur in nature. This implies that volcanic (and non-volcanic) debris avalanches can behave as granular flows, which has important implications for interpretation of deposits and for modeling. Such behaviour may be acquired as the collapsing material disaggregates and forms a granular mixture composed by a right grain size distribution in which particle segregation can occur. Limited fragmentation and block sliding, or grain size distributions inappropriate for promoting granular fingering can explain why ridges are absent in many deposits