Analysis of crack movements observed in an alpine bedrock cliff

Crack widths and rock temperatures were monitored on an andestic bedrock cliff in the summit area of the Daisetsu Mountains, Hokkaido, northern Japan. Sequential data recorded the gradual widening of a crack to the point of critical crack extension, which resulted in catastrophic rock breakage. The data indicate that a combination of liquid water in?ltration into crack tip and subsequent freezing is the most signi?cant factor contributing to critical crack extension. The recorded sub‐critical crack movements involved a number of minor crack extensions and contractions, the timing of which correlates well with the magnitude of the reconstructed thermal stresses at the crack tip derived from thermal deformation of the plate‐shaped rock fragment. Larger crack extensions occurred when stress at the crack tip exceeded a threshold value, possibly re?ecting the control of rock fracture mechanics by which cracks are thought to propagate when the stress intensity factor at the crack tip exceeds the threshold values for stress corrosion cracking and the fracture toughness of the material. Copyright © 2004 John Wiley & Sons, Ltd.     

Relation between chemical composition and particle-size distribution of ores in the profile of nickeliferous laterite deposits of the Rio Tuba Mine, Philippines

Lateritic Ni ore mined in the Rio Tuba Mine, Philippines, derived from ultramafic rock by tropical intense weathering, is generally composed of two accumulated zones, an upper laterite zone and a lower saprolite zone. These two zones are very different in appearance, mineral assemblage, chemical composition, and in other ways. A transitional zone may be seen between the upper and lower zones, but it does not develop to any appreciable thickness. Although serpentine and goethite are still predominant in the constituent minerals, other clay minerals are increasing.

The transitional zone may be subdivided into three groups based on chemical composition. The behaviour of the chemical composition in this horizon indicates a complicated process of component transportation under the weathering process.

In the ore, chemical components are closely related in particle size of constituent minerals. In the laterite - transitional - saprolite sequence, a common variation range and tendency in chemical composition for each particle size can be seen. However, taking samples with the same particle size the chemical composition obviously differed, representing the characteristics of the components for each zone.

Concerning the correlation coefficient of the chemical components of each zone, a somewhat different inter-component relationship can be seen in these zones. These differences of correlation suggest the different complicated lateritized conditions under which they are formed.     

Quaternary vertical offset and average slip rate of the Nojima Fault on Awaji Island, Japan

Abstract Drilling was carried out to penetrate the Nojima Fault where the surface rupture occurred associated with the 1995 Hyogo-ken Nanbu earthquake. Two 500 m boreholes were successfully drilled through the fault zone at a depth of 389.4 m. The drilling data show that the relative uplift of the south-east side of the Nojima Fault (south-west segment) was approximately 230 m. The Nojima branch fault, which branches from the Nojima Fault, is inferred to extend to the Asano Fault. From the structural contour map of basal unconformity of the Kobe Group, the vertical component of displacement of the Nojima branch–Asano Fault is estimated to be 260–310 m. Because the vertical component of displacement on the Nojima Fault of the north-east segment is a total of those of the Nojima Fault of the south-west segment and of the Nojima branch–Asano Fault, it is estimated to total to 490–540 m. From this, the average vertical component of the slip rate on the Nojima Fault is estimated to be 0.4–0.45 m/10³ years for the past 1.2 million years.     

CHIME age dating of monazites from metamorphic rocks and granitic rocks of the Ryoke belt in the Iwakuni area, Southwest Japan

Abstract CHIME (chemical Th-U-total Pb isochron method) ages were determined for monazite from gneisses and granitoids of the Ryoke belt in the Iwakuni area. The CHIME monazite ages are 99.6 ± 2.4, 98.9 ± 2.1 and 98.2 ± 5.7 Ma for the Ryoke gneiss, 90.7 ± 2.2, 89.7 ± 2.0 and 89.3 ± 2.2 Ma for the Tajiri Granite, 91.0 ± 3.2, 90.6 ± 3.2 and 89.9 ± 3.2 Ma for the Namera Granite, 89.3 ±3.3 and 88.6 ± 5.6 Ma for a small stock at Shimizu, and 87.3 ± 1.6 and 86.6 ± 2.1 Ma for the post-tectonic Shimokuhara Granite. The CHIME monazite ages, interpreted as the time of the first attainment at the amphibolite facies conditions for the gneisses and as the time of emplacement for the granites, respectively, agree with the field intrusive sequence. The present dating documented that the Ryoke metamorphism in the Iwakuni area reached the amphibolite facies conditions at ∼98 Ma, was complete at -87 Ma, the time of emplacement of the post-tectonic Shimokuhara Granite.     

Convective instability and overstability in the sunspot umbra

Convective instability and overstability of a plane-parallel medium in the presence of a uniform vertical magnetic field is studied. We take into account both the effect of penetration of disturbances into sub-adiabatically stratified semi-infinite upper and lower layers, and the effect of thermal damping of disturbances introduced by the compressibility of the medium. If the degree of super-adiabaticity in temperature stratification in the middle layer is relatively large compared with that of sub-adiabaticity in the upper and lower layers, the range of physical conditions in which overstable convection occurs is very narrow. However, if it is not the case the range is rather wide. In the normal sunspot umbra, only disturbances with selected sizes in vertical dimension will be able to become overstable. The overstable convection might be a cause of some dynamical features (such as dots) in the sunspot umbra, but will not be the main contributor for transporting mechanical energy in the sunspot region.     

Gelifluction within a solifluction lobe in the Kärkevagge valley, Swedish Lapland

Soil displacement, soil temperature, depths of thaw plane and groundwater level were continuously monitored during the period from July 1999 to June 2000 within a solifluction lobe in the Kärkevagge valley, northern Sweden. The strain–probe method was used to measure soil displacement, and we found significant soil displacements in the thawing period 2000. These displacements were the result of gelifluction. The ice content profile showed that gelifluction occurred at the same time as the thaw plane reached the layers with high ice content at shallow soil depths (0–6 and 16–25 cm deep). In contrast, gelifluction did not occur when the thaw plane reached the layers with high ice content at greater depth (46–49 cm deep). These observations indicate that thawing of ice lenses in the near–surface layer triggers gelifluction.     

An estimation of the electric potential field generated by the diurnal atmospheric tide with the first negative mode excited in the lower ionosphere

The electric potential field in the ionosphere, which is generated by the atmospheric tide with the diurnal first negative mode excited in the lower ionosphere, is estimated. An equivalent mode is used instead of the exact mode in the following approximations : (i) the horizontal structure of the energy input is composed of the first two terms of the series expansion of the Hough function with the associated Legendre functions, and (ii) the Coriolis force is assumed to be constant in the equation of motion and the value appropriate to colatitude 45 being used. It is found that the equivalent mode gives a good approximation of the exact mode at midlatitudes in the neutral atmosphere. The result shows that the oscillation with its temperature variation of about 40 K in the excitation region generates the horizontal electric potential field of 10^?3 V/m in magnitude. The horizontal pattern of the field distribution is similar to that estimated by many authors. The maximum vertical electric current is about 3 × 10^?9 A/m² in the dynamo layer. It is found that the “virtual current,” introduced by Volland (1970) as a vertical current in and above the dynamo layer, in order to obtain a better agreement between theory and observations of the location of the diurnal symmetric Sq current vortex, is actually to be expected. The vertical current is very small, but this small steady current is necessary for the electric potential field of the dynamo layer to be mapped into the magnetosphere along the lines of the geomagnetic field.     

Microearthquakes and faulting in the southern Okinawa Trough

Southern Okinawa Trough represents an early stage of back-arc rifting and is characterized by normal faulting and microearthquakes. Earthquake distribution and deep structure of fault was investigated to clarify active rifting in the southern Okinawa Trough, where two parallel grabens are located. A network of ocean bottom seismometers (OBSs) that displayed the hypocenters of 105 earthquakes were observed for a period of 4 days in southern-graben (SG). Most of the microearthquakes occurred in a cluster about 7 km wide, which on a cross-section striking N45°E dips 48° to the southwest. Relocated hypocenters, which are recorded by a local seismic network, show scattered distribution around the southern-graben. There are no remarkable surface faults in the southern-graben. On the other hand, the recalculation of hypocenter locations of 1996 earthquakes swarm recorded by a local seismic network suggests that the swarm is associated with normal faulting on the southern side of northern-graben (NG). Thus, the undeveloped southern-graben is located to the south of the developed northern-graben. Southward migration of rifting, which may be caused by migration of volcanism, could thus be occurring in the southern Okinawa Trough. The extension rate computed for the southern Okinawa Trough from the fault model of the northern-graben is 4.6 cm/year, which is 59–102% of the extension rate (GPS measurements). This result indicates that the majority of extensional deformation is concentrated within the center of the northern-graben in the Okinawa Trough.     

Shigar valley gemstones,their chemical composition and origin,Skardu, Gilgit-Baltistan,Pakistan

A variety of gemstones is being mined in the Shigar valley, Skardu, Pakistan. These include beryl (goshenite and aquamarine), tourmaline (schorl), garnet (almandine–spessartine), apatite, topaz, fluorite, zoisite, clinozoisite, and axinite, mostly occurring in complex or zoned pegmatites and metamorphic rocks. These have been analyzed using electron probe micro-analyzer and X-ray diffractometer. The mineral chemistry of each gemstone is similar to its respective typical gemstone variety with homogenous chemical composition. Field and chemical characteristics suggest that beryl, tourmaline, garnet, apatite, topaz, and fluorite are occurring in zoned pegmatites which are largely formed by magmatic hydrothermal fluids in the cavities and vugs within the intermediate zone. However, zoisite, clinozoisite, and axinite may have a metamorphic and/or metasomatic origin.