The geomorphology of planetary calderas

Satellite-derived observations of the geomorphology of calderas on Earth, Mars and Venus can be used to learn more about shield volcanoes. Examples of terrestrial basaltic volcanoes from the Galapagos Islands, Hawaii, and the Comoro Islands show how these volcanoes contrast with examples found on Mars and Venus. Caldera structure, degree of infilling, and the location of vents on the flanks are used to interpret each volcano's recent history. The geometry of the caldera floor can be used to infer some of the characteristics of the magma storage system, and the orientation of the deep magma conduits. The formation of benches within the caldera and the effects of the caldera on the distribution of flank eruptions are considered, and it is evident that most calderas on the planets are/were dynamic features. Presently, deep calderas, with evidence of overflowing lavas and ponded lavas high in the caldera wall, show that these calderas were once shallow. Similarly, shallow calderas filled with ponded lavas are evidence that they were once deeper. It is probably a mistake, therefore, to place great significance on caldera depth with regard to the position, shape, or size of subsurface plumbing.     

Epiclastic deposits and ‘horseshoe-shaped’ calderas in Tahiti (Society Islands) and Ua Huka (Marquesas Archipelago), French Polynesia

Occurrences of debris avalanche deposits newly identified in Tahiti (Society Islands) and Ua Huka (Marquesas Archipelago) are described and interpreted here. In both islands, the breccias are located within horseshoe-shaped residual calderas. In Tahiti, the epiclastic formations, up to 500 m thick, lie on the floor of the central depression and in the valley of the northwards running Papenoo River. In Ua Huka, the breccias crop out within a depression limited by a semicircular crest in four bays along the southern coast. Their thickness is ca. 100 m. A few clasts collected in the Tahitian breccias and some rocks forming their substratum have been dated (K–Ar datings) and analysed (major and trace elements, Sr–Nd isotopes) for this study. Using these data, we show that the debris avalanche(s) occurred in Tahiti Nui at the end of the growth of the shield volcano (between 570 000 and 390 000 years ago), maybe in consequence of the emplacement of the plutonic body which occupies the central part of the caldera. In Ua Huka, the collapse took place nearly 3 Ma ago, between the construction of the shield volcano and that of the inner one. The southwards orientation of the caldera, like that of the neighbouring island Nuku Hiva, might reflect a preferential direction of weakness in the substratum of the central Marquesas.     

Timing,magnitude and geochemistry of major Southeast Asian volcanic eruptions: identifying tephrochronologic markers

We review the current knowledge about Southeast Asian volcanoes and their eruption histories, and focus on identifying tephrochronologic markers representing major explosive eruptions in order to further future palaeoclimate and volcanological studies. Forty-one volcanic edifices in Southeast Asia have been classified as large calderas by Whelley et al. (2015) and thus have, or are likely to have, produced large explosive eruptions with a Volcanic Explosivity Index (VEI) of 6–8. Unfortunately, only 20 such eruptions have known ages, spanning from 1.2 Ma to 1991 ad , and fewer have geochemical data that can be used for tephrostratigraphic correlations. Volcanic products from different geodynamic regions and different sources can generally be distinguished on major element plots (e.g. K₂O versus CaO) of matrix glass composition. However, the distinction of multiple eruptions from the same source often requires additional data such as trace element compositions of matrix glass and/or mineral compositions. Biotite, but also magnetite compositions (MgO and TiO₂ content in particular) appear to be very discriminating. Up to nine tuffs in addition to the three to four Toba tuffs can be utilised as widespread tephrochronologic markers and span a range from 1.2 to 1.6 Ma to recent. As only a few Holocene major eruptions have been well characterised and dated, many large calderas are still unstudied, and many distal tephra layers are still lacking a source, more tephrochronologic markers can certainly be defined in the future.     

遥感图像在白头山火山锥及望天鹅和南胞台山破火山口研究中的应用

长白山地区是典型的近代火山活动地区之一。笔者等 应用遥感图像对白头山火山锥的形成过程、喷发期次、喷发特点进行了研究。还对白头山火山锥、望天鹅破火山口及朝鲜境内形成时代较早的南胞台山破火山口进行了 研究,为这一地区火山活动的深入研究提供了资料。     

Formation and development of normal-fault calderas and the initiation of large explosive eruptions

The ring fractures that form most collapse calderas are steeply inward-dipping shear fractures, i.e., normal faults. At the surface of the volcano within which the caldera fault forms, the tensile and shear stresses that generate the normal-fault caldera must peak at a certain radial distance from the surface point above the center of the source magma chamber of the volcano. Numerical results indicate that normal-fault calderas may initiate as a result of doming of an area containing a shallow sill-like magma chamber, provided that the area of doming is much larger than the cross-sectional area of the chamber and that the internal excess pressure in the chamber is smaller than that responsible for doming. This model is supported by the observation that many caldera collapses are preceded by a long period of doming over an area much larger than that of the subsequently formed caldera. When the caldera fault does not slip, eruptions from calderas are normally small. Nearly all large explosive eruptions, however, are associated with slip on caldera faults. During dip slip on, and doming of, a normal-fault caldera, the vertical stress on part of the underlying chamber suddenly decreases. This may lead to explosive bubble growth in this part of the magma chamber, provided its magma is gas rich. This bubble growth can generate an excess fluid pressure that is sufficiently high to drive a large fraction of the magma out of the chamber during an explosive eruption. Received: 2 January 1997 / Accepted: 22 April 1998     

Stratigraphy and structure of Deception Island, South Shetland Islands, Antarctica

Deception Island is an active volcano with a large structural collapse caldera that has been interpreted by several investigators as having been generated by different processes. The general stratigraphy and structural study of the volcano is presented, with special emphasis on the distribution and structural disposition of the different stratigraphic units. The rocks of Deception Island have traditionally been divided into pre- and post-caldera products. A subdivision of the pre-caldera deposits, called ‘pre- and syn-caldera deposits,’ has been observed in two main units. Bulk magnetic susceptibility analyses are used to support the field stratigraphy presented. The localization and distribution of dikes is used to assign the central collapse of the volcano to rifting induced by tensional tectonics occurring in the Bransfield Strait and the presence of transcurrent faults around the island.     

Geophysical imaging of buried volcanic structures within a continental back-arc basin: The Central Volcanic Region, North Island, New Zealand

Hidden beneath the ~ 2 km thick low-velocity volcaniclastics on the western margin of the Central Volcanic Region, North Island, New Zealand, are two structures that represent the early history of volcanic activity in a continental back-arc. These ~ 20 × 20 km structures, at Tokoroa and Mangakino, form an adjacent gravity high and low, respectively. Interpretations from seismic refraction arrivals and gravity modelling indicate the − 65 mgal Mangakino residual gravity anomaly can be modelled, in part, by two low-density bodies that reach depths of ~ 6.5 km, whereas the Tokoroa gravity anomaly is due to a higher density rock coming, at most, to within ~ 650 m of the surface. The Mangakino anomaly is interpreted to be due to the remnants of magma chambers that fed large ignimbrite eruptions from about 1.2 Ma. An andesite volcano or complex volcanic structure is the preferred interpretation for the Tokoroa gravity high. The size of the putative volcanic structure is comparable to the presently active Tongariro Volcanic Complex in the centre of North Island.     

The Interpretation of Gravity Changes and Crustal Deformation in Active Volcanic Areas

Simple models, like the well-known point source of dilation (Mogis source) in an elastic, homogeneous and isotropic half-space, are widely used to interpret geodetic and gravity data in active volcanic areas. This approach appears at odds with the real geology of volcanic regions, since the crust is not a homogeneous medium and magma chambers are not spheres. In this paper, we evaluate several more realistic source models that take into account the influence of self-gravitation effects, vertical discontinuities in the Earths density and elastic parameters, and non-spherical source geometries. Our results indicate that self-gravitation effects are second order over the distance and time scales normally associated with volcano monitoring. For an elastic model appropriate to Long Valley caldera, we find only minor differences between modeling the 1982–1999 caldera unrest using a point source in elastic, homogeneous half-spaces, or in elasto-gravitational, layered half-spaces. A simple experiment of matching deformation and gravity data from an ellipsoidal source using a spherical source shows that the standard approach of fitting a center of dilation to gravity and uplift data only, excluding the horizontal displacements, may yield estimates of the source parameters that are not reliable. The spherical source successfully fits the uplift and gravity changes, overestimating the depth and density of the intrusion, but is not able to fit the radial displacements.