Columnar Joints Research Articles

Geological investigations of Luna 17, Apollo 15 and Chang’E-3 landing sites at Mare Imbrium of the Moon

The Imbrium basin is one of the most prominent geologic units on the lunar nearside, and also significant exploration target for soft-landing missions. Till now, three lunar missions, USSR’s Luna 17, American Apollo 15 and Chinese Chang’E-3, have successfully soft-landed and conducted surface investigation at this basin. Employing the surface images and other data acquired by the three missions, we analyze the geologic characteristics of the three landing sites, mainly focusing on the regional geologic context, impact craters, surface boulders and regolith materials. We find that, the Lunar 17 and Chang’E-3 spacecraft landed on the relatively younger Eratosthenian mare basalt units, respectively, where massive basaltic bedrocks are exposed at the wall and rim of some small craters, and some of these exposed boulders present columnar jointing, while the Apollo 15 landed on the relatively older Imbrian lava plains, where surface boulders are seldom present. There exposed some layered basalts at the wall of the Hadley Rille, and columnar jointing structure is also observed at some boulders exposed at the rille floor and rim. We observe some possible lineaments on the surface images. We propose that these possible lineaments are caused by slightly undulant lunar surface illuminated under relatively low sun elevation angles, which is supported by our simulation experiments. The comparison of the three landing sites indicates that impacting history, volcanic activities, space weathering and other important geologic processes are regionally and locally inhomogeneous on the Moon, future lunar surface in-situ and sample-return missions will strengthen our understanding of the geologic evolutionary history of the Moon.

Feasibility of columnar jointed basalt used for high-arch dam foundation

Columnar jointed basalt, with a lot of small-spacing structural planes and poor integrity, is a kind of fractured rock mass. Through comprehensive study of columnar joints shape, roughness of fracture surface and chemical composition of basalt, it is known that columnar joints of Baihetan dam area were formed as a result of cooling and shrinkage effects of magma. The columnar jointed basalt is mainly formed through chemical reaction of chlorite, kaolinite, epidote and tremolite, and the columnar joints mainly consist of chlorite according to slice identification and chemical analysis. Test results show that the columnar jointed basalt has high uniaxial compressive strength, low friction coefficient, and high cohesion, shear strength and deformation index. Meanwhile, the columnar jointed basalt is closely locked, and joint surfaces are well closed. The permeability of the rock is quite weak, and the P-wave velocity in the rock could get up to 5 000m/s. All these show good rock properties. The columnar joints develop regularly, different from the general fractured rock masses. In summary, the columnar jointed basalt can be used directly as a foundation of dam.

Silica gossan as a prospective guide for amethyst geode deposits in the Ametista do Sul mining district, Paraná volcanic province, southern Brazil

A new frontier of amethyst geode deposits (amethyst, agate, gypsum, calcite and zeolites) driven by the prospective guide termed silica gossan may change the metallogenetic scenario previously known in the Paraná volcanic province. Silica gossans, which are related to basalt, basaltic andesite and rhyodacite, occur throughout this province. In this study, we focus on the Ametista do Sul mining district, Rio Grande do Sul, Brazil, the largest producer of amethyst geodes in the world (400t/month). In this district, a silica gossan occurs at the top of the Coogamai flow above the Mina do Museu. This basalt (47.9wt.% SiO2) flow belongs to the intermediate-Ti Paranapanema magma type (2.25wt.% TiO2 and Ti/Y=416) exhibiting columnar jointing. The gossan as prospective guide displays polygonal shape associated with the enrichment of clay minerals at the surface, the concentration of iron oxides and the presence of silicified sandstones. In addition, marshes with ponds and a change in surface color (ocher to brownish) and vegetation type were observed. A gamma spectrometry survey at the Mina do Museu silica gossan showed negative anomalies in %K, eU and eTh above the amethyst deposit with values of 55 counts per second (cps) outside the silica gossan and 43cps within it. These anomalies are caused by hydrothermal events and subsequent weathering of the silica gossan. Bulk rock analyses of the silica gossan demonstrate a loss on ignition up to 1.8wt.% whereas this value is around 0.5wt.% for rocks outside the silica gossan. SiO2, K2O, MgO, CaO and Na2O display negative correlations with loss on ignition. X-ray diffraction proved the presence of smectites and kaolinite in the soil on the silica gossan. In addition, dioctahedral and trioctahedral interlayered smectites were identified. Mg-smectite is present in the silica gossan whereas K-smectite occurs in the Coogamai flow.

Genesis of the columnar joints from welded tuff in Mount Mudeung National Geopark, Republic of Korea

Mount Mudeung National Geopark (MMNG), Gwangju, Republic of Korea (1187 masl), is known for its huge, broad occurrences of columnar jointed colonnades in the Cretaceous Mudeungsan Tuff. To understand the genesis of columnar joints infilling a volcanic vent, integrated geochemical and geophysical studies were conducted. Most colonnades in the Geopark are located in regions higher than 700 masl and show elevation-dependent variations in the mean face width of the columns. These mean widths are approximately 1.7, 1.3, 2.9, and 1.2 m for the summit (>1100 masl), upper (950–1100 masl), lower (850–950 masl), and lowest (700–850 masl) colonnades, respectively. This variation implies that columnar jointing resulted from discontinuous tuff depositions on relatively planar ground, which is associated with caldera structures and landslides from the emptied caldera rim. Geochemical analyses of major, minor, and trace elements show that the andesitic-dacitic Mudeungsan Tuff resulted from the arc magmatism associated with subduction of the Izanagi Plate and involved crustal components. Few geochemical variations of the Mudeungsan Tuff indicate that tuff originated from a single magma chamber. U-Pb age dating indicates that the Mudeungsan Tuff was deposited at ~85 Ma except for the Chotdae-bong colonnades, which were dated to ~87 Ma. However, a scaling analysis of the elevation-dependent variations of face widths and simplified numerical model calculations indicate that the variation in face widths with elevation indicates the sequential emplacement, cooling, and compaction of separate welded tuffs. Thus, the welded tuffs of MMNG can be thought to be formed by three discontinuous large explosive eruptions with periods of repose after each eruption. These tuffs include 1) the highest-elevation columnar joints of colonnades in the summit area (>1100 m), corresponding to the lower colonnades of the third tuff; 2) the intermediate-elevation columnar joints of colonnades, corresponding to the upper (950–1100 m) and lower (850–950 m) colonnades of the second tuff; and 3) the lowest columnar joints of colonnades (700–850 masl), corresponding to the upper colonnades of the first tuff.

Analysis of EDZ Development of Columnar Jointed Rock Mass in the Baihetan Diversion Tunnel

Due to the time dependency of the crack propagation, columnar jointed rock masses exhibit marked time-dependent behaviour. In this study, in situ measurements, scanning electron microscope (SEM), back-analysis method and numerical simulations are presented to study the time-dependent development of the excavation damaged zone (EDZ) around underground diversion tunnels in a columnar jointed rock mass. Through in situ measurements of crack propagation and EDZ development, their extent is seen to have increased over time, despite the fact that the advancing face has passed. Similar to creep behaviour, the time-dependent EDZ development curve also consists of three stages: a deceleration stage, a stabilization stage, and an acceleration stage. A corresponding constitutive model of columnar jointed rock mass considering time-dependent behaviour is proposed. The time-dependent degradation coefficient of the roughness coefficient and residual friction angle in the Barton–Bandis strength criterion are taken into account. An intelligent back-analysis method is adopted to obtain the unknown time-dependent degradation coefficients for the proposed constitutive model. The numerical modelling results are in good agreement with the measured EDZ. Not only that, the failure pattern simulated by this time-dependent constitutive model is consistent with that observed in the scanning electron microscope (SEM) and in situ observation, indicating that this model could accurately simulate the failure pattern and time-dependent EDZ development of columnar joints. Moreover, the effects of the support system provided and the in situ stress on the time-dependent coefficients are studied. Finally, the long-term stability analysis of diversion tunnels excavated in columnar jointed rock masses is performed.

Geologic characteristics of the Luna 17/Lunokhod 1 and Chang'E-3/Yutu landing sites, Northwest Mare Imbrium of the Moon

We compared and assessed the results of measurements and observations by the Lunokhod 1 and Yutu rovers, both of which explored the northwestern part of Mare Imbrium. Both sites are within the distinctive Eratosthenian-aged lava flow geologic unit and our comparisons showed that the geologies of these exploration sites are very similar. As in the majority of other areas of the Moon, the dominant landforms in these sites are small impact craters, having various degrees of morphologic prominence and states of preservation, and rock fragments, mostly associated with the rims and interiors of fresh craters. The shape and the degree of preservation of the observed rock fragments in these two sites are similar. In both sites sporadic rock fragments were observed whose morphologies suggest that their source rocks had columnar jointing. Localization of these specific rocks on the rims of 450–470m in diameter craters implies that the source rocks are at depths of 40–50m. Regolith in the study areas is typically a few meters thick, but locally can be much thicker. The ground penetrating radar of the Yutu rover revealed the multilayer regolith structure, which is determined by superposition of crater ejecta; with some local variations, this type of the regolith stratigraphy should be typical of the majority of lunar mare sites. The physico-mechanical properties of the regolith in these two sites appear to be rather similar: the bearing capacity ranges from 0.04 to 1.44kg/cm2, with a modal value ∼0.45kg/cm2, and the shear strength ranges from 0.02 to 0.1kg/cm2, with a modal value ∼0.05kg/cm2. Both these factors decrease by a factor of 3–4 with an increase of surface slope from ∼2 to 12°. The chemical composition of surface materials determined by the rover instruments at these two sites differ from those derived from the remote sensing data for the Eratosthenian-aged basalts on which the two sites are located. This could be partly due to low measurement accuracies, especially in the case of Lunokhod 1, but may also represent real variations in the composition of the surface materials compared to returned lunar samples. Difference in the spatial resolution of the in-situ and remote-sensing analyses should also be taken into account. Recommendations for future lunar rover missions are as follows: (1) to use ground penetrating radar and a robotic arm, and (2) to employ radial study tactics for impact crater documentation and analysis.

Geomorphic Evolution of Chambal River Origin in Madhya Pradesh using Remote Sensing and GIS

The Fluvial geomorphology of Chambal River basin is so vast in terminology; therefore remote sensing and GIS application and technique to analyzing the whole Chambal geomorphology in interval of space and time has been used. Chambal River is a perennial river and carries large quantity of water and sediment load to transportation. Chambal River is famous for the past prehistoric and as unpolluted river. Chambal origin considered from the Janapav valley (Janapav Shiv Temple) at Malwa ridge surrounded by Manpur Reserve Forest, Madhya Pradesh, India. The source of Chambal water is considered from seepage of groundwater from the Basaltic columnar joint rock (Malwa Ridge) where the water flows from the weak columnar joints of Basalts and follows gradient slop facing in Northward direction. Chambal River has Deccan trap Basalt as river bedded rock, Mesozoic age formation with dendritic drainage pattern at Malwa region. Chambal River origin lies at an elevation of 840.6 meter with Latitude 22 0 27′36.5″N and Longitude 75 0 41′14.4″E (source by: ground truth field data) . Physiographically the area to the North of Chambal is characterized by moderately dissected plateau and formed undulating topography. However, to determine the changes in flow, meandering, and geomorphic landforms development in the Chambal river origin, primarily the toposheet no 46N, 46M and Landsat imagery ETM+ data are used for geomorphic evolution maps and digitization and georeferencing has done by using ArcGIS v10 software. Identification of geomorphological landforms with their specific geomorphological processes either erosional or depositional in the fluvial system of the Chambal River will certainly help in assessing the environmental and hydrological problems in this Malwa region.

Relations between thermal history and secondary structures of ignimbrites exclusive of rheomorphism

Newly calculated model density profiles of ignimbrites also provide model thermal histories, which serve as a framework to help analyze the development of cooling joints, devitrification textures, and lithophysal cavities. Only those parts of sections of Rattlesnake Tuff in central Oregon that resided >600 °C for at least two years show devitrification. Low tensile strength means that joints form by thermoelastic contraction after cooling by as little as 25 °C. This means that columnar joints formed in as little time as a few weeks after deposition of the Aravaipa Tuff in SE Arizona. Compaction is rapid as well, so both columnar jointing and compaction are complete before the onset of devitrification in deposits <40 m thick. A section of Rattlesnake Tuff shows stratabound occurrences of devitrified spots and cavities; devitrification appears to have begun at scattered spots. Most spots are bounded by crescentic cavities in formerly ductile shards. The equation for conductive cooling of a spherical heat source shows that for rock volumes larger than ∼15 cm diameter, the rate of cooling is insufficient to prevent heating by latent heat by as much as 12 °C. Inflation therefore appears to have been caused by vapor released by devitrification and its slight adiabatic expansion. Eventual wholesale devitrification of matrix resulted in more-devitrified spots scattered in less devitrified matrix. Lithophysal cavities in the Rattlesnake Tuff and in the Peach Springs Tuff in NW Arizona are more abundant in lower density horizons, showing that cavity growth is favored in permeable zones between impermeable horizons. The Peach Springs Tuff and other deposits described in the literature have horizontal joints that formed during cooling and whose origin has yet to be deterministically modeled. The longest joints are most closely spaced in the zone of cavities and formed after columnar joints. It is inferred that after formation, each vertical column responds independently to evolving stresses; at this time, asperities together with gas pressures accompanying cavity formation result in horizontal joints. Columnar joints do not completely relieve growing gas pressures during devitrification, likely because of sealing by wholesale inflation of the rock mass and by mineral deposition. Some devitrified horizons of Rattlesnake Tuff and Peach Springs Tuff are densely fractured, resulting in a rubbly surface. These small fractures show zigzag traces, bifurcations, abrupt terminations, and pinch-and-swell walls, similar to ductile fractures described by [Eichhubl (2004)][1]. Their origin is interpreted to be the result of tensile stress due to porosity increase during later stages of devitrification. Closely adjacent sections of Rattlesnake Tuff at one site differ by 18% in thickness, reflecting buried paleotopography. Each section is devitrified. The thicker profile has a zone of lithophysal cavities near its base; the thinner profile has few cavities. The greater thickness translates to only 0.18 MPa additional lithostatic pressure at the base of the thicker profile, which serves to emphasize the subtle differences in initial conditions that can lead to formation of cavities. [1]: #ref-17

Large Lateral Deformation Characteristics of Simulated Columnar Jointed Rock Mass under Uniaxial Compression Tests

The columnar jointed rock mass is a common structure in extrusive igneous rocks. Due to the columnar joints network, large lateral deformation may result from slipping along these columnar joints and the corresponding lateral strain ratio may greatly exceed the upper limit of Poisson's ratio. Correct understanding of the large lateral deformation characteristics of columnar jointed rock mass is essential to the design of tunnels and underground caverns where the uniaxial compression condition usually occurs. Therefore, in order to perform uniaxial compression tests, simulated columnar jointed rock mass specimens with different dip angles were prepared using plaster mixtures, and the curve of variation of lateral strain ratio to dip angle was obtained. The shape of the curve resembles as inverted U-typed and the maximum of lateral strain ratio occurs at β = 30°. The mechanism of large lateral deformation is explained in light of the dip angle and failure modes, and an experimental equation is presented to predict the variation of lateral strain ratio to dip angle.

宮城県北部，石越安山岩の地質・岩石学的特徴とマグマプロセス

Ishikoshi Andesite is the Middle Miocene, amphibole-bearing, two pyroxene andesite lava flow located in the forearc area of the Northeast Japan arc, which covers the area of 8 km EW and 7 km NS and has the thickness of >180 m. In this study it is divided into peripheral lava (PL) and central lava (CL), which are distributed in the peripheral and central parts of the volcanic edifice, respectively. PL bears groundmass pigeonite or shows higher FeO*/MgO trend with fractionation and CL does not bear pigeonite and lower FeO*/MgO trend, therefore PL is tholeiitic and CL is calc-alkaline. PL shows petrological features that are compatible with crystallization differentiation. CL has several features that suggest magma mixing such as non-equilibrium plagioclase phenocrysts, but evidences for magma mixing are not robust. In comparison to similar lava flows, PL and CL seem to form a composite lava flow because clear geological boundary between PL and CL cannot be observed and FeO*/MgO ratio of CL tend to be higher in the marginal portion. We also find that two-pyroxene equilibrium temperature of CL (760-850 ℃)is lower than that of PL (970-990 ℃). CL should have been too viscous at this temperature (>1010 Pa·s in anhydrous state) to form a dike, thus it should have effused through the vent which PL formed to be composite lava flow with PL. Ishikoshi Andesite has columnar joints rather than platy joints. In comparison to other lava flows which have platy joints in Japan, it is possible that less anisotropic crystals, low eruption temperature (high viscosity) and/or large phenocryst size have prevented Ishikoshi Andesite from formation of foliation along which platy joints develop in the lava.

우리나라 주상절리에 대한 연구 전략

우리나라 주상절리에 대한 연구 전략

Experimental study on the mechanical properties of simulated columnar jointed rock masses

Columnar jointed rock mass is a kind of structural rock mass commonly encountered in igneous rocks. Due to the effects of columnar joint networks, anisotropy is the typical mechanical property of columnar jointed rock mass, i.e. deformation and strength varying with loading direction. Correct understanding of the mechanical anisotropy of columnar jointed rock mass is a key problem that should be solved for demonstration and design of large scale rock mass projects such as dams and underground cavern excavations constructed in it. Plaster simulated columnar jointed rock mass specimens at dip angles varying from 0° to 90° with respect to the axial stress were tested under uniaxial compression conditions to investigate the mechanical anisotropy and failure modes. Based on analyses of experimental results, it was found that the strength and deformation of columnar jointed rock masses had pronounced 'U-shaped' anisotropy. In the anisotropic curves, the maximum and minimum values occurred at β = 90° and β = 45°, respectively. It was also shown that the lateral strain ratio was relatively high, especially when the dip angle was close to (45° − j/2), where j was the joint friction angle. An empirical expression was adopted to predict the 'U-shaped' anisotropy of deformation and strength and the predicted anisotropic curves agreed reasonably well with experimental data. Furthermore, four types of failure modes were summarized based on experimental results and corresponding mechanisms were also discussed.

Analysis of Causes of Deformation Modulus Mismatches Degree of Columnar Jointed Basalt Blocks

Columnar jointed basalt rock block is small, but has a longitudinal wave velocity, high modulus characteristics. This article is based on the structure of surface measurements, acoustic testing, and field distortion tests results, from the liquid structure, structural plane development situation, etc., analyses the causes of this phenomenon.

Geochemical and Sr–Nd isotopic constraints on the mantle source of Neoproterozoic mafic dikes of the rifted eastern Laurentian margin, north-central Appalachians, USA

Abundant and widely distributed unmetamorphosed mafic dikes intrude Mesoproterozoic rocks of the New Jersey Highlands. The age of the dikes is imprecisely known but interpreted to fall between 615 and 576 Ma, which is consistent with the range of ages of mafic dikes from Labrador and Nova Scotia south to Pennsylvania that were emplaced along the rifted eastern Laurentian margin. New Jersey Highlands dikes are a few cm to 18 m wide and have lengths of as much as several km. They have sharp, largely discordant contacts against enclosing Mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors. Columnar joints are present locally and suggest emplacement at a shallow crustal level. Geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high TiO 2 , P 2 O 5 , Zr, Nb, Y, and La/Yb, and low MgO, Cr, and Ni. TiO 2 contents define high-Ti and low-Ti dikes that differ in high field strength elements (HFSE) and light rare earth elements (LREE) but overlap in abundances of most other elements. Dike magma evolved in an ascending mantle plume of OIB-like asthenosphere from enriched higher TiO 2 compositions to more depleted lower TiO 2 compositions. Subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation. Sr–Nd isotope values of both dike compositions overlap and are characterized by ε Nd (T) of + 1.5 to + 3.8 and initial 87 Sr/ 86 Sr ranging from 0.7032 to 0.7077. Higher Sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous, high 87 Sr/ 86 Sr lithospheric mantle having EMI or EMII-like geochemical characteristics. Dikes form tabular structures that have long segments striking an average of N44°E and short segments striking about east–west. Their regional geometries form right-stepping, rhomb-shaped patterns due to emplacement into rift-related dilational fractures likely formed through a combination of southeast-directed extension and strike-slip shear stresses. Geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event. They, along with other mafic dikes in the north-central Appalachians, were emplaced in a within-plate tectonic setting along the rifted margin of eastern Laurentia, prior to opening of the Iapetus Ocean. • Mantle sources of Neoproterozoic mafic dikes were modeled from Sr–Nd isotopes and geochemistry. • Dikes formed from partial melting of a garnet-free, enriched OIB-like source. • Geochemical similarity to other regional Neoproterozoic dikes along the eastern Laurentian margin • Dikes emplaced from single magmatic event during rifting that formed the Iapetus Ocean

Tunnel reinforcement in columnar jointed basalts: The role of rock mass anisotropy

The extent of the loosening zone in tunnels excavated through columnar jointed basalts is studied with the numerical discontinuous deformation analysis (DDA) method. The structure of the rock mass and tunnel geometry are modeled on the basis of a real field case study of deep tunneling performed in such a rock mass in south west China. Slender prismatic keyblocks formed by the intersection of broadly-spaced, gently-dipping breccia layers and closely-spaced, orthogonally oriented, steeply dipping columnar joints result in a highly anisotropic rock mass structure and give rise to sliding and toppling failure modes in the sidewalls and to an excessive height of loosening zone in the roof, the geometry of which is shown to be controlled by the orientation of the steeply inclined columnar joints. Results of displacement monitoring performed during tunnel excavation in a similar rock mass with multiple point borehole extensometers confirm the numerically obtained depth of the loosening zone both in the sidewalls and the roof. We find that the height of the loosening zone in the roof as obtained with DDA is greater than would have been predicted by Terzaghi’s empirical rock load classification for “blocky” rock masses, and show that its shape and orientation are controlled by the anisotropy of the rock mass structure. Moreover, we demonstrate that dimensioning rock bolt reinforcement using well-established empirical criteria without consideration of the anisotropic nature of the rock mass may lead to un-conservative design.

Fractures in a trachyandesitic lava at Öræfajökull, Iceland, used to infer subglacial emplacement in 1727–8 eruption

We present detailed field observations of cooling fractures in a small-volume trachyandesitic lava, informally named the Slaga lava, on the south west flank of Öræfajökull volcano, south east Iceland. Columnar joints, pseudopillow fracture systems, and curved platy jointing occur in the lava, whose exposed section is approximately 600m in length and generally 2–3m in thickness. Columnar jointing may occur at the base of flow lobes, whereas pseudopillow fracture systems occur throughout the lava in an outer, glassy, fractured carapace, and curved platy fractures occur in the centres of larger flow lobes. Pseudopillow fracture systems, composed of a single master fracture and multiple subsidiary fractures formed normal to the master fracture, are of two types: G-type pseudopillow fracture systems have very narrow striae (chisel marks) on their master fractures, indicative of rapid cooling and brittle fracture; SR-type pseudopillow fracture systems display alternating smooth and rough master fracture surface textures, evidence of alternating brittle and ductile fracture propagation mechanisms. Subsidiary fractures in both types show curved striae on their fracture surfaces, which enable the determination of fracture propagation directions. Pseudopillow fracture systems are thought only to form in the presence of water, including water caused by the melting of ice and snow. The curved platy fractures display prominent river lines and may have resulted from cooling contraction, post-emplacement degassing, flow deflation or shearing in the flow against the outer solid crust of the flow during inflation. Due to recent advances in the understanding of the formation mechanisms of pseudopillow fracture systems they, and the other fractures present in the flow, can be used to reconstruct the cooling environment. The lava is inferred to have been emplaced within subglacial drainage channels incised into or beneath a thin alpine-type glacier, with coolant infiltrating the lava radially from all sides and ponding or draining of water at or along the lava base. We link this flow to the 1727–8 eruption of Öræfajökull volcano on the basis of historical records, which describe lava effusion in this approximate location and striking chemical compositional similarity between 1727 tephra and the Slaga lava.

Study on the Failure Mechanism of Basalts with Columnar Joints in the Unloading Process on the Basis of an Experimental Cavity

Basalt with columnar joints is a kind of jointed rock body cut by both original joints and aphanitic microcracks. After unloading during excavation, such rock bodies manifest obvious mechanical phenomena such as discontinuities, anisotropy, and liability to fracture. In this work, the failure modes of basalts with columnar joints are studied based on monitoring of test tunnels on the one hand, and also through the mechanical properties of the original joints and those of microfissures of aphanitic microcracks in the unloading process analyzed by a discrete element method using a strain-softening constitutive model. Through comprehensive analysis of the numerical simulation results and the monitored behaviors of the basalts with columnar joints, their failure mechanisms are revealed, which may provide a basis for choosing suitable support plans for this kind of rock mass.

Cooling fractures in impact melt deposits on the Moon and Mercury: Implications for cooling solely by thermal radiation

We study the distribution, morphology, and geometrical properties of fractures in several young impact melt deposits on the Moon and Mercury, and the ways that these fractures may form from cooling by thermal radiation. In each impact melt complex, the topography of the underlying terrain determines the orientation of cooling fractures, such that interior fractures that formed in the relatively thick interior areas of the melt unit are wider and have a larger spacing than marginal fractures that formed in the relatively thin areas near the unit's margins. Solid debris entrained in molten deposits provides prefracture flaws that can seed cooling fractures, but too much solid debris prevents cooling fractures from growing to macroscopic sizes. The appearance of subparallel fractures is mainly caused by subsidence of the deposits during the process of cooling and solidification. Tensile stresses caused by thermal radiation are large enough to initiate cooling fractures on both the Moon and Mercury, which may represent the initial stage of columnar joints formation, but the cooling rate caused solely by thermal radiation is not large enough to form well-organized columnar joints that feature polygonal colonnades. We therefore propose that thermal conduction and convection are the major contributors in the formation of columnar joints on planetary bodies.

남한에서 주상절리의 분포와 암석학적 특성

남한에서의 주상절리 분포지역과 그들의 형태학적, 암석학적 특징을 연구하였다. 현재까지 보고된 남한의 주상절리 분포지역은 최소한 68개 지역이다. 이들은 지리적 여건과 화산활동에 따라 5개의 그룹으로 나눌 수 있다. 1) 한탄강유역에는 16개 지역에 주상절리가 발달하는데, 15개 지역은 제4기의 현무암질 용암이며, 나머지 1지역은 백악기 화산암이다. 2) 제주도의 주상절리는 18개 지역이다. 이들 모두 신생대 제4기에 분출하였으며, 대부분은 현무암질 용암이고, 산방산과 백록담 등은 조면암이다. 제주도 주상절리는 칼러네이드와 엔태블러춰가 잘 발달하며, 끌자국도 선명하게 관찰된다. 3) 울릉도와 독도에서는 국수바위를 비롯한 5개 지역에서 관찰되며, 이들은 제4기 조면암이다. 4) 포항-경주-울산의 8개 지역에서 돔형, 방사상, 수평 및 수직 주상절리가 발달하는데, 이들 모두는 제3기 화산암이다. 충청남도 평택-아산지역, 그리고 강원도 고성의 제3기 현무암에서 주상절리가 관찰된다. 5) 남해안과 서해안에는 모두 15개 지역에서 주상절리가 관찰되는데, 이들은 백악기에 분출한 화산암으로 현무암에서 유문암까지, 용암에서 용결응회암까지 다양한 암석으로 구성되어 있다. 무등산과 주왕산의 주상절리도 백악기에 분출한 용결응회암이다. 남한의 주상절리를 지역별로 분류하면, 강원도가 5개, 경기도 13개, 충남 2개, 경북 14개, 전북 1개, 전남 10개, 경남 5개, 제주 18개 지역이다. 주상절리의 형성 시기는 중생대 백악기가 18개, 신생대 제3기가 12개, 그리고 신생대 제4기가 38개 지역이다. 화산암의 마그마계열은 알칼리암계열이 36개, 비알칼리암계열이 32개 지역이다. This study has been designed to collate distribution, morphology, petrology of columnar joint in South Korea. Reported columnar joint areas in South Korea are 68, until the present time. These can be divided into five group by geography and volcanic activity. 1) The 16 columnar joint areas are distributed in Hantangang region. The 15 areas in this region are composed of basaltic lava in the Quaternary period, and the other 1 area is composed of volcanic rocks in the Cretaceous period. 2) The 18 columnar joint areas are distributed in Jeju island. Most of them are composed of basaltic lava(alkali basalt and Hawaiite), and the Sanbangsan and Baegrokdam area are composed of trachyte in the Quaternary period. Colonnade, entablature and chisel mark of the columnar joint are typically occur in basaltic lava. 3) The 5 columnar joint areas are distributed into the Ulleung island and Dokdo including Guksubawi. These are consisted of relatively well-formed trachyte columns in the Quaternary period. 4) The 8 columnar joint areas are distributed into the Pohang, Gyeongju and Ulsan region and consist of the Tertiary period volcanic rock. It's shape are dome, radial, horizontal and vertical. The 4 columnar joint areas are reported in the Pyeongtaek and Asan city of Chungcheongnamdo and Gosung of Gangwondo. All of them are the Tertiary period basalt. 5) The 15 columnar joint areas are distributed into the west and south coast region. Those are consisted of various rock type(from basalt to dacite), various occurrences(lava flow to welded tuff), and various diameters(20 cm to several meters). The columnar joint of Mudeung mountain and Juwang mountain are welded tuff in the Cretaceous period. The columnar joint is distributed over a wide area in South Korea, 5 in Gangwondo, 13 in Gyeonggido, 2 in Chungnam, 14 in Gyeongbuk, 1 in Jeonbuk, 10 in Jeonnam, 5 in Gyeongnam, and 18 in Jeju. The columnar joints in South Korea can be arranged in order of formative period, 18 in the Cretaceous period, 12 in the Tertiary period, and 38 in the Quaternary period. By magma series, 36 are belong to alkaline series and 32 are belong to sub-alkaline series.

Geochemistry and petrogenesis of lava flows around Linga, Chhindwara area in the Eastern Deccan Volcanic Province (EDVP), India

Based on systematic three-tier arrangement of vesicles, entablature and columnar joints, three distinct quartz normative tholeiitic lava flows (I, II and III) were recognized in the area around Linga, in the Eastern Deccan Volcanic Province (EDVP). Each of the flows exhibits intraflow chemical variations marked by high Mg#-low Ti, and low Mg#-high Ti contents. The MgO (4.27–7.74wt.%), Mg# (23.45–41.89) and Zr (161.5–246.3ppm) of Linga flows suggest an evolved chemistry marked by fractional crystallization and crustal contamination processes. Positive Rb and Th anomalies, negative Nb anomalies, relative enrichment of LILE–LREE with respect to Nb, Nb/Th:3.71–6.77 indicate crustal contamination of magma by continental materials through magma–crust interaction during melt migration and contributions from sub-continental lithospheric mantle (SCLM). Negative K, Sr and Ti anomalies corroborate an intracontinental, rift-controlled tectonic setting for the genesis and evolution of Linga basalts. Chondrite-normalized REE patterns reflect low HREE abundances and prominent LREE/HREE, MREE/HREE fractionation thereby pointing towards partial melting of garnet peridotite mantle source. Nb, Zr, Y variations suggest 10–15% partial melting of mantle source for the derivation of parent tholeiitic melt that suffered crystal fractionation of phenocrystal phases and subsequent liquid immiscibility. Critical evaluation of Srinitial and Ndinitial (65Ma) isotopic compositions (87Sr/86Srinitial between 0.705656 and 0.706980 and 143Nd/144Ndinitial between 0.512523 and 0.512598) suggests that these basalts were derived from an enriched mantle (∼EM I–EM II) source. The εSr (21.84–41.27) and εNd (−0.28 to 1.10) isotopic signatures defined by higher εSr and lower εNd fingerprint a plume-related source. Positive and negative values of εNd indicate an isotopically heterogeneous mantle source marked by mixing of depleted (DM) and enriched mantle (EM I–EM II) components at the source region and together with 87Sr/86Srinitial ranging from 0.705656 to 0.706980 suggest two stage contamination of parent magma which is much similar to that of Poladpur, Toranmal, Mhow, Chikaldara flows. Ba/Y versus 87Sr/86Sr and Nb/Y versus Rb/Y variations show an Ambenali–Poladpur contamination trend for the Linga basalts thereby suggesting the role of upper continental granitic crust as the contaminant of these flows through magma–crust interaction during melt migration. The lava flows of Linga are geochemically correlatable with the Poladpur flows of southwestern and Toranmal flows of northern Deccan and show genetic coherence with the basalts of Jabalpur, Seoni, Chakhla–Delakhari of eastern Deccan.