Terrestrial heat and volcanism

Summary Although active volcanic territories are not characterised by extreme high heat flow, yet terrestrial heat is one of the main, if not hte first cause of volcanic activities. About 94 per cent of the known active volcanoes are in connection with orogenic zones including ocean ridges, which mean that processes responsible for the evolution of the continents cause the overwhelming majority of volcanism.There stages of the evolution of the continental crust can be distinguished: 1.) Growth of the continental cores by differentiation, 2.) Existence of peripheral growing zones around the first core, 3.) Intergrowth of the existing continents.Volcanism, as well as seismicity are manifestations of the evolution of the crust which is caused by the development of heat sources in the mantle. The heat partly melt the upper mantle and induce slow plastic flow in connection of differentiation. The space, left empty by continental uplift is filled by the slow plastic inflow of mantle from the ocean. This slow redistribution of mantle and curstal rocks disturbs the stress field, the rocks fail along the strained zones and zones of fractures appear sometimes with volcanic activity.     

Deccan Trap volcanism

The Deccan Traps, now occupying an area of 200,000 sq. miles, must originally have been more wide-spread. Their thickness in Western India reaches 6000 ft. They have been encountered at depths of over 1500 ft. in Kathiawar and Sind (Pakistan), and have been faulted down to a depth of over 6000 ft. in the Cambay area. They are composed of numerous flows whose thickness varies from a few ft. to 200 ft. The flows are often compact in the lower portions and vesicular in the upper portions. Over most of the area (east of the Western Ghats) the rock is a tholeiitic basalt (50 to 51.5 % silica) whose pyroxene is intermediate in composition between pigeonite and diopside, and whose CIPW norm generally shows the presence of some quartz. In the Bombay Kathiawar region there are numerous eruptive Centres associated with a large range of differentiated types comprising both very basic and acid types. The study of the analyses of the various types indicates the presence of both the alkali-olivine basalt as well as the Calc-alkali lines of differentiation, which has brought up the question of the nature of the primary magma. It is noted that while the greater part of the area shows tholeiitic rock, olivine basalts and connected types appear in the more western areas, perhaps as a result of the local tectonic conditions. Recent geophysical data point to the presence of an « oceanic basalt » layer all around the earth both in oceanic and continental crust, while a less dense (presumably tholeiitic) layer overlies it (below the sial) in the continental segments. The « oceanic basalt » should therefore be taken as representing the primary magma, and tholeiite as a major type derived from it in the continental crust.     

The relationship between glaciation and volcanism: Numerical simulation and the Holocene volcanism in Kamchatka

This study is concerned with numerical simulation of the strain due to glaciation and glacial melting, when a magma zone (a layer containing inclusions of magma and magma cumulates) is present at the crust–mantle boundary. According to analytical solutions of this problem that involves viscous relaxation of an uncompensated depression at the place of the molten glacier, the depth to the zone of increased shear stresses beneath the depression is proportional to its width, while the relaxation duration is proportional to viscosity of the lithosphere and is a few thousand years. These fundamental estimates are corroborated by our numerical simulation. According to it, the magma zone at the Moho boundary shields the zone of increased shear stresses, limiting it from below. The maximum values (12–25 MPa) with glacial thickness 500–1000 m are reached at the top of this layer of low viscosity. The directions of maximum compression (s₁) as calculated for the time after the melting indicate that the magma that rises along dikes is displaced from the center of the magma lens toward its periphery. It is found that glacial unloading makes the dipping faults in the crust above the low-viscosity layer attractors for the rising magma. Glacial unloading accelerates, by factors of a few times, the magma generation in the mantle that occurs following the mechanism of adiabatic decompression, as well as facilitating the accumulation of mantle fluids in the zone of increased shear stresses at the boundary of the low viscosity layer. The magma traverses this deep fluid collector and increases the intensity and explosivity of eruptions at the beginning of an interglacial period. Our numerical simulation results are in general agreement with published data on Early Holocene volcanic eruptions that occurred after the second phase of the Late Pleistocene glaciation in Kamchatka.     

Age of shield-building volcanism of Kauai and linear migration of volcanism in the Hawaiian island chain

Tholeiitic basalts of the Napali Formation comprise the bulk of the Kauai shield volcano. Potassium-argon ages measured on 16 samples from three separate areas in this formation lie in the range 5.14 ± 0.20 to 3.81 ± 0.06 m.y. The scatter in the measured ages in each area is greater than that expected from experimental error alone, and variable loss of radiogenic argon is regarded as at least partly responsible. Nevertheless an interval of eruption in the order of 0.8 m.y. is deduced for the Napali Formation. The results from the Napali Formation taken together with K-Ar ages measured earlier on basalts of the Makaweli Formation, the youngest formation of the dome-building phase, yield a mean age of 4.43 ± 0.45 m.y. for the construction of the main subaerial shield volcano of Kauai.When this result from Kauai is combined with estimates of the average age for the shield-building volcanism in 16 other volcanoes in the Hawaiian island chain, extending over a distance of more than 2800 km, the data are found to conform to migration of the centre of volcanism from north-northwest to south-southeast at a uniform rate of 9.4 (±0.3) cm/yr over the last 28 m.y. Non-linear models of propagation of volcanism in the Hawaiian chain are quite unnecessary, especially when uncertainties in the data base are taken into account. These results are consistent with an origin of the Hawaiian volcanic chain by eruption from a magma source situated below the Pacific lithospheric plate, as proposed under hot spot or plume models. Depending upon choice of the pole for the Pacific plate, rates of rotation about the pole of 0.9° to 1.0°/m.y. are derived. By extrapolation of the Hawaiian Island chain data an age estimate of 37.8 m.y. is derived for the Hawaiian-Emperor Seamount intersection.     

Palaeomagnetism and the Deccan Trap volcanism

The results of palacomagnetic studies made on the Deccan Traps by various workers are reviewed in the light of the recent palaeomagnetic data on these rocks and the general geological information. It is suggested that: (a) the earlier altitude-polarity classification of the Deccan Traps, suggesting that the flows below the general elevation of 2000±200 feet above mean sea level are of reversed magnetic polarity while those above this horizon are normal, is not without exceptions; (b) the geomagnetic field reversed its polarity several times during the eruption of these lavas; (c) the Deccan Trap eruptions probably consisted of several phases of volcanicity over a protracted period; and (d) the phases of Deccan Trap volcanism, the phases of Himalayan upheaval, and the northward drift of the Indian landmass were rather concrescent events.     

Miocene volcanism in eastern Oregon: An example of calc-alkaline volcanism unrelated to subduction

Three upper Miocene calc-alkaline volcanic centers are spaced about 100 km apart in the northeast-trending transition zone between the Columbia Plateau and Basin and Range geologic provinces. These centers are transverse to contemporaneous andesitic vents of the western Cascades Sardine Formation. Some of the Miocene volcanic features in northeast Oregon resemble those of linear volcanic belts that are associated with subduction, but structural patterns do not support the existence of an upper Miocene subduction zone in northeast Oregon. Major east-trending fold axes and northwest-trending faults indicate that north-south compression and east-west extension were the dominant deformational events in this region. It is suggested that the origin of these calc-alkaline rocks is related to their particular tectonic setting and not to subduction.     

The relationship between calc-alkaline volcanism and within-plate continental rift volcanism: evidence from Scottish Palaeozoic lavas

Lower Carboniferous lavas from the Midland Valley and adjacent regions of Scotland are mildly alkaline and intraplate in nature. The sequence is dominated by basalt and hawaiite, although mugearite, benmoreite, trachyte and rhyolite are also present. Basic volcanic rocks display the LIL element and LREE enrichment typical of intraplate alkali basalt terrains. Low initial⁸⁷Sr/⁸⁶Sr (0.7029–0.7046), high ε_Nd (−0.4 to +5.6) and moderately radiogenic²⁰⁶Pb/²⁰⁴Pb (17.77–18.89) ratios are also comparable with alkali basalts from other continental rifts and oceanic islands.When the Carboniferous lavas are compared with subduction-related lavas of Old Red Sandstone age, erupted in and around the Midland Valley ca. 50 Ma earlier (at 410 Ma) remarkable similarities are apparent. Significant overlap occurs in Nd and Pb isotopic compositions. Sr isotopic compositions are, however, more radiogenic in the older subduction-related lavas. This, combined with high K and Rb concentrations in ORS lavas may be explained by the incorporation of a sediment component derived from the subducted slab, which by Lower Carboniferous times had been lost from the mantle source region by convection. A pronounced negative Nb anomaly in the ORS subduction-related lavas may be explained by the retention of a Nb-bearing phase in the mantle during hydrous melting of the mantle wedge above the subduction zone.Allowing for the effects of the added component from the subducted slab, there appears to be no necessity to invoke separate mantle source regions for the two suites of lavas: both may have been derived from chemically similar portions of mantle. If volcanic arc lavas are derived from the mantle wedge, the implication is that such a source lies at relatively shallow depth within the upper mantle: the same may therefore apply to the Carboniferous continental rift basalts. This evidence, combined with the fact that there is no evident hot-spot trail across the Midland Valley despite a long period of within-plate volcanism and rapid plate movements during the Carboniferous, suggests that the alkali basalt magmatism is not the product of a deep-seated mantle plume. Rather, the volcanism appears to owe more to passive rifting and to diapiric upwelling from a source region within the uppermost mantle.     

Smithsonian Institution's global volcanism network

Summary of Recent Volcanic Activity

Smithsonian Institution's global volcanism network     

Ignimbrite volcanism in North Wales

Many episodes of ignimbrite volcanism have occurred in North Wales. Ignimbrites can be recognised amongst the Pre-Cambrian rocks and from almost every stage of the Ordovician succession. The best known and most instructively exposed ignimbrite volcanies are those of Caradocian age in Snowdonia. In this deeply incised mountainous area it is possible to demonstrate the major characteristics of ignimbrite volcanism and to examine the relationship between numerous intrusive rhyolite masses and the extrusive rocks. Both acid and basic magmas were available in North Wales during Ordovician times, and at several of the volcanic centres the rocks show a differentiation sequence from pyroxeneandesite to alkali-rhyolite. The emergence and growth of volcanic islands upon which the ignimbrites were deposited is revealed in the stratigraphical record. An intimate relationship exists between magmatism and crustal unrest, and it is possible to discuss certain problems regarding the petrogenesis of the rocks, the location and character of the volcanic vents and the palaeogeography of North Wales during the Ordovician. Definition of the terms employed and criteria used in the identification of Welsh ignimbrites is given, and the field relationships and petrology of a number of areas are described in detail.     

Quaternary volcanism of the Japanese Islands

Abstract Quaternary volcanism of the Japanese Islands is examined from the perspective of experimental petrology, geographic distribution of volcanoes and spatial geochemical variations. The dehydration of amphibole and chlorite at a 110 km depth and of phlogopite at ∼180 km in the downdragged hydrous mantle layer would result in the occurrence of two volcanic chains parallel to the trench axis. Long-term subduction of the old Pacific plate and recent subduction of the young Philippine Sea plate beneath East Japan and West Japan volcanic belts respectively, would be critical for the significant difference in intensity, style and geochemistry of Quaternary volcanism between the two volcanic belts. The geochemistry of volcanic rocks in Northeast Japan and those in the Ryukyu arc is typical of 'island-arcs' having low LIL/HFS element ratios, while alkalic basalts along the Japan Sea coast side in Southwest Japan have high LIL/HFS ratios similar to intra-continental or oceanic island basalts. Across-arc variations in eruptive volume and distributional density of volcanoes and in geochemistry are documented in Northeast Japan and are well explained by the decreasing degrees of partial melting toward back-arc side, and the difference in geochemistry of fluids supplied by the downdragged hydrous layer.     

Earthquake swarms and volcanism in New Zealand

The term « swarm » is used to describe a group of related earthquakes, concentrated in space and time, without an obvious principal event. Large shallow earthquakes are often followed by aftershocks, but the pattern in which aftershocks occur differs in detail from that of a swarm. Sequences of New Zealand earthquakes that have been called swarms differ markedly from one another. The most vigorous of them, near Taupo in 1922, appears to have been an ordinary tectonic earthquake accompanied by foreshocks and aftershocks, and by surface faulting. No fault movements accompanied the 1964 swarm in the same area. Other localities that have experienced swarms include Great Barrier Island, Matamata, Kawerau, and Opunake. Swarms are considered by some writers to be characteristic of volcanic regions. Although all New Zealand swarms have occurred in areas of Quaternary volcanism, there are still no observations showing what part, if any, volcanism plays in the generation of earthquake swarms.