Maps of the natural environment''s potential in West Germany and Norway as an aid for mineral resources management and protection

Mattig, U., 1990. Maps of the natural environment's potential in West Germany and Norway as an aid for mineral resources management and protection. Eng. Geol., 29: 321–331.

The applied geosciences are of increasing importance for finding an answer to many current problems in spatial management and environmental protection.

Based on the problem of an intelligent use of the geopotential, the paper deals with the achievement and optimization of an appropriate concept for designing geoscientific maps of the natural environment's potential, relevant to areal planning, by comparing a densely populated country like Germany and a more sparsely populated country like Norway. The study carried out is following the example of a map for reserve areas for mineral resources/near surface raw materials—sand and gravel—which enables a comparison and an assessment of the translation of information, relevant to land-use planning, in both countries.

The appropriateness of the interpretation of geoscientific information is ensured by drafting a comprehensive, clearly arranged cartographic presentation, for which uniform and comparable methods, criteria and standards will be developed. The basic concepts of areal planning have to be simple, distinct, adaptable and capable of development. They should enable the geoscientist to contribute his complex and expert knowledge to decisions of regional planning and spatial management.     

The channel between Gran Canaria and Tenerife: constructive processes and destructive events during the evolution of volcanic islands

Seismic, sidescan sonar, bathymetric multibeam and ODP (Ocean Drilling Program) data obtained in the submarine channel between the volcanic islands of Gran Canaria and Tenerife allow to identify constructive features and destructive events during the evolution of both islands. The most prominent constructive features are the submarine island flanks being the acoustic basement of the seismic images. The build-up of Tenerife started following the submarine stage of Gran Canaria because the submarine island flank of Tenerife onlaps the steeper flank of Gran Canaria. The overlying sediments in the channel between Gran Canaria and Tenerife are chaotic, consisting of slumps, debris flow deposits, syn-ignimbrite turbidites, ash layers, and other volcaniclastic rocks generated by eruptions, erosion, and flank collapse of the volcanoes. Volcanic cones on the submarine island flanks reflect ongoing submarine volcanic activity. The construction of the islands is interrupted by large destructive events, especially by flank collapses resulting in giant landslides. Several Miocene flank collapses (e.g., the formation of the Horgazales basin) were identified by combining seismic and drilling data whereas young giant landslides (e.g., the Güimar debris avalanche) are documented by sidescan, bathymetric and drilling data. Sediments are also transported through numerous submarine canyons from the islands into the volcaniclastic apron. Seismic profiles across the channel do not show a major offset of reflectors. The existence of a repeatedly postulated major NE-SW-trending fault zone between Gran Canaria and Tenerife is thus in doubt. The sporadic earthquake activity in this area may be related to the regional stress field or the submarine volcanic activity in this area. Seismic reflectors cannot be correlated through the channel between the sedimentary basins north and south of Gran Canaria because the channel acts as sediment barrier. The sedimentary basins to the north and south evolved differently following the submarine growth of Gran Canaria and Tenerife in the Miocene.     

The Commission for the Geological Map of the World (CGMW) and small-scale earth-science cartography of Europe

The origin of CGMW dates back to 1881; in its initial phase it concentrated exclusively on geoscientific cartography of Europe. Since 1911, CGMW has taken up the promotion, coordination and publication of small-scale geo-scientific maps world-wide. Of this map type, recently published examples of modern European map series are presented:

Geological Map of Europe and the Mediterranean Regions 1:1.5 Mio (in 49 sheets) Hydrogeological Map of Europe 1:1.5 Mio (in 30 sheets with explanatory notes) Tectonic Map of Europe 1:2.5 Mio (in 16 sheets)

Quaternary Map of Europe 1:2.5 Mio (in 15 sheets)

Metallogenic Map of Europe 1:2.5 Mio (in 9 sheets with an explanatory memoir, 1984)

International Map of Iron Ore Deposits of Europe 1:2.5 Mio (in 16 sheets with explanatory notes, 1970-73)

A series of different geoscientific thematic maps comprising the West European Atlantic Region will result from the Circum-Atlantic Project. Its objectives are to inventory and use the available geoscience and resource data in analyzing the geological evolution and resource potential of the Atlantic Basin. The present status of the Circum-Atlantic Project is explained.     

Oxygen isotope composition of xenoliths from the oceanic crust and volcanic edifice beneath Gran Canaria (Canary Islands): consequences for crustal contamination of ascending magmas

Xenolith samples of marine terrigenous sediments and altered Jurassic MORB from Gran Canaria (Canary Islands) represent samples of sub-island oceanic crust. These samples are postulated to define end-members for crustal contamination of basaltic and felsic ocean island magmas. The meta-igneous rocks show great heterogeneity in oxygen isotope compositions (δ¹⁸O 3.3–8.6‰), broadly correlating with their stratigraphic position in the oceanic crust. Gabbros interpreted as fragments of oceanic crust layer 3 have δ¹⁸O values of 3.3–5.1‰, which is lower than MORB (5.7–6.0‰). Layer 2 lavas and dykes show a broader range of δ¹⁸O of 4.1–8.6‰. Therefore, high-temperature metamorphism seems to have been the dominant process in layer 3, while both high- and low-temperature alteration have variably affected layer 2 rocks. Siliciclastic sediments have high δ¹⁸O values (14.1–16.4‰), indicating diagenesis and low-temperature interaction with seawater. The oxygen isotope stratigraphy of the crust beneath Gran Canaria is typical for old oceanic crust and resembles that in ophiolites. The lithologic boundary between older oceanic crust and the igneous core complex at 8–10 km depth—as postulated from geophysical data—probably coincides with a main magma stagnation level. There, the Miocene shield phase magmas interacted with preexisting oceanic crust. We suggest that the range in δ¹⁸O values (5.2–6.8‰) [Chem. Geol. 135 (1997) 233] found for shield basalts on Gran Canaria, and those in some Miocene felsic units (6.0–8.5‰), are best explained by assimilation of various amounts and combinations of oceanic and island crustal rocks and do not necessarily reflect mantle source characteristics.     

Peralkaline acid tendencies in Gran Canaria (Canary Islands)

The study of a volcanic series from the island of Gran Canaria (Canary Islands) in which alkaline and peralkaline, saturated and undersaturated rocks coexist, is reported here. Materials with high volatile content (ignimbritic trachytes) were first emitted and the series ended with the eruption of phonolitic lavas. The average peralkalinity index in these rocks is typically about 1.0 and, therefore, peralkaline rocks coexist with non-peralkaline ones. However, a maximum in peralkalinity is found in the ignimbritic rocks of the lower part of the series. In spite of the evident acid peralkaline tendencies of these rocks, it does not seem appropriate to classify them as pantellerites or comendites. Nor are they consistent with the genetic processes proposed for rocks of similar composition and oceanic environment.The crystallization of the feldspars controls the variation trends among the different magmas but the fractionation alone does not sufficiently explain the genesis of successive fluids. Various factors seem to point to the important role which a gas-transfer process causing a geochemical stratification inside the magmatic chamber may have played.The occurrence of peralkaline silicics at Gran Canaria, which is located for away from the active Mid-Atlantic ridge, is not related to transitional basalts. These rocks are a deviation from the main undersaturated alkalic trend which characterizes the volcanism of the Canary Islands, their genesis being related to the realization of favourable local volcanic conditions.     

Analysis of sedimentary processes on the Las Canteras beach (Las Palmas, Spain) for its planning and management

The sedimentary dynamics of a sandy beach located on the North coast of the Island of Gran Canaria has been studied in relation to physical processes.

The morphodynamic behaviour of the beach has been established and interpreted. Special emphasis has been placed on the characterization of erosive and accretionary processes by means of sediment transport calculations. Two sedimentary indices have been used to define these processes qualitatively and quantitatively. These indices are of importance for management.     

The Holocene volcanic history of Gran Canaria island: implications for volcanic hazards

Previous published data, combined with our results of 13 new radiocarbon ages and extensive geological fieldwork, indicate that during the past 11 ka 24 monogenetic basaltic eruptions occurred in the north sector of Gran Canaria. These eruptions can be grouped into three periods of eruptive activity: 1900–3200 ¹⁴C a BP; 5700–6000 ¹⁴C a BP; and an older period represented by only one eruption, El Draguillo, dated at 10 610 ± 190 ¹⁴C a BP. Archaeological studies have shown that the more recent eruptions affected prehistoric human settlements on the island. Field studies demonstrate that the eruptions typically built strombolian cones (30–250 m in height) and associated relatively long lava flows (100–10 350 m in length); a few eruptions also produced tephra fall deposits. The total erupted volume of these eruptions is about 0.388 km³ (46.1% as tephra fall, 41.8% as cinder cone deposits and 12.1% as lava flows). The relatively low eruption rate (～0.04 km³ ka^?1) during the past 11 ka is consistent with Gran Canaria's stage of evolution in the regional volcano‐tectonic setting of the Canary Archipelago. The results of our study were used to construct a volcanic hazards map that clearly delimits two sectors in the NE sector of Gran Canaria, where potential future eruptions would pose a substantial risk for densely populated areas. Copyright © 2009 John Wiley & Sons, Ltd.     

Preparation and application of engineering and environmental geological maps in The Netherlands

The Netherlands has no nation-wide mapping programme for either engineering geology or environmental geology, but on request the Geological Survey of The Netherlands, together with other geoscientific institutes, prepares: (1) detailed (1:5,000) engineering geological maps of urban areas, specifically meant for urban planning and management as well as for feasibility studies for civil engineering works; and (2) small-scale (1:250,000 to 1:50,000) thematic environmental geological maps, meant for regional landuse planning. The preparation and application of these maps is discussed, and some examples are given.     

A geohazard reconnaissance study based on geoscientific information for development needs of the western region of Istanbul (Turkey)

The role of geoenvironmental information is becoming increasingly important as legislative changes have forced developers and planning authorities to consider more implications and impact on the environment of large-scale development initiatives. Therefore, integration of surface and subsurface geoscientific information for development needs has prime importance and provides a means of identifying potential problems and opportunities at an early stage in any planned development. However, from the experience of recent natural disasters, it is evident that this was not case the taken into consideration in many countries. In addition to thousands of casualties, many urbanized areas, industrial districts and large-scale engineering structures suffered severe damages from the natural hazards due to many reasons including the lack of preliminary engineering geological maps and zoning maps of the settlement areas. Turkey is one of the countries which is exposed to natural hazards such as earthquakes, landslides and floods. In particular, the devastating 1999 Kocaeli earthquake, which affected the Marmara Region of Turkey, focused the attention on densely urbanized and industrialized metropolitan areas such as Istanbul. The rapid growth of Istanbul, particularly towards west with minimal geoscientific information resulted in an overwhelming pressure on the natural environment. In addition, a large earthquake, which is expected to occur in the Marmara Sea within the next 30 years, also pose a threat to the city and its surroundings. In this study, on the basis of the geological, geomorphological and geophysical reconnaissance study, an integrated geoscientific data were collected from the western region of Istanbul and evaluated for geohazards. The paper focuses on the geological and geomorphological aspects that control the occurrence of some geohazards such as earthquake-induced liquefaction, landslides and flooding. In this context, the geological map of the region was revised and Quaternary deposits were classified into 11 units, in detail. Liquefaction-prone areas were evaluated by using geomorphological criteria based on field investigation, by the examination of the available records from 88 boreholes drilled on recent deposits and by the data from resistivity profiles. The landslides within the region were classified according to their type, relative depth and activity. In addition, fluvial and marine flood-prone areas were also delimited within the region. Finally, a series of maps such as landslide inventory maps, and maps showing liquefaction- and flood-prone areas were produced with the aid of Geographic Information Systems (GIS) to assist in designing further detailed site investigations and to reduce costs by ensuring a more focused approach to strategic planning and site selection.     

Developing an Erosion Risk Map Using Soft Computing Methods (Case Study at Sifnos Island)

The erosional processes depend on various variables such as lithology, topography,drainage system, rock structure and climatic conditions. The quantitative evaluationof some of the above geological and geomorphological parameters is of primaryimportance for the purpose of developing erosion risk maps. These maps can becomeuseful tools for developing appropriate strategies on environmental protection, hazardassessment and regional planning.The island of Sifnos, in Cyclades, was chosen as a case study, where in the final map of the sub-basins of this island, erosion risk distribution is presented.     

Kinematic model for Tenerife Island (Canary Islands,Spain): Geodynamic interpretation in the Nubian plate context

Establishment of a geodetic network in Tenerife is the starting point for the use of GPS and other precise geodetic techniques in the support of the study of kinematics and their relation with island volcanic activity. This paper is focused on the characterization of volcanotectonic activity of Tenerife, to determine the geodynamic framework for volcanic surveillance.TEGETEIDE network, set up in 2005 and re-observed each year, is composed of seven GNSS–GPS stations scattered throughout the island. A horizontal deformation model is presented in order to explain the observed island displacement pattern in the geodynamic context of the Nubian plate.According to the models obtained, the most important geologic structures, such as the volcanic rifts and the caldera, determine the current deformation pattern of Tenerife. The geodynamics of the most stable areas of the island behave similarly to that observed from the permanent GNSS–GPS reference stations located in La Palma and Gran Canaria Islands.Anomalous geodynamic behaviour has been detected in two zones of Tenerife, which configure an NW–SE axis crossing the central sector of the island, related with the volcanotectonic activity of the island and its surroundings.     

中国历史文献档案中的古环境记录

我国的历史文献记录是古气候、古环境信息的重要来源,对古全球变化（PAGES）研究计划的目标时段——近2000年（人文记录和自然记录并存的时段）来说,尤其具有重要的科学价值。介绍了我国古文献中的古气候、古环境记录的概况和各项记录如水、旱、雨、雪等的数量统计,综述了这些记录被用于重建定量的气候序列、绘制历史气候复原图、编制古环境事件年表、获取高分辨率的古气候信息诸方面的主要进展。     

Carbonate tufas as archives of climate and sedimentary dynamic in volcanic settings,examples from Gran Canaria (Spain)

Three Holocene tufas from Gran Canaria volcanic island were studied with the aim of deciphering their sedimentary evolution through space and time. Las Temisas tufa (south-eastern arid part of the island) is dominantly composed of oncoids, intraclasts, phytoclasts, coated stems, minor thin stromatolites, and a high amount of siliciclastics. It was deposited in a fluvial system with variable flow velocities and palustrine conditions areas, which alternated with high energy events. Azuaje tufa (northern humid part of the island) is composed of coated stems, stromatolites, oncoids and phytoclasts, with relatively low amounts of siliciclastics, suggesting slow-flowing and palustrine conditions and a relatively low incidence of (high energy) floodings. Los Berrazales tufa (north-west of Gran Ganaria, the most humid one), is mainly composed of coated stems and crystalline crusts, formed in a laminar flow regime. Dominant clastic sedimentation in Las Temisas and high calcite growth rates in Los Berrazales led to a poor development of stromatolites in comparison with Azuaje. Las Temisas and Azuaje deposits have similar upward evolution with decreasing trend in siliciclastics and increasing trend in carbonates. However, Las Temisas has higher siliciclastic and lower phytoclastic contents suggesting a less vegetated area and more arid climate than in the other deposits. Additionally, tufas record local events common in volcanic terrains. Azuaje presents three units bounded by erosive discontinuities, which reveal significant erosion by enhanced runoff that could be caused by loss of vegetation due to wildfires related to volcanic eruptions at headwaters. Las Temisas record a possible interruption in sedimentation represented by aligned boulders due to rockfalls from the hillsides. These deposits formed from waters with similar chemistry providing to the carbonates their similar signals in δ¹³C–δ¹⁸O stable isotopes and ⁸⁷Sr/⁸⁶Sr ratios like that of the volcanic rocks. This work shows how, in volcanic areas, tufas are unique archives of the climate, vegetation and volcanic-related processes, because all imprint the sedimentary regime of tufa deposition.     

中塔合作帕米尔地球化学调查成果与展望

帕米尔高原是由喜马拉雅山脉、喀喇昆仑山脉、昆仑山脉、天山山脉、兴都库什山脉交汇于此形成的一个巨大的山结。由于其特殊的地质构造位置和复杂的构造演化历史,形成了良好的成矿地质背景条件。依据中塔两国政府“双方地学领域科技合作谅解备忘录”精神,中塔两国合作组织实施了“塔吉克帕米尔成矿带地球化学调查”工作。完成3万km²地球化学填图任务,建立了工作区地球化学数据库,编制了系列地球化学图件。应用地球化学分析方法回答了测区资源潜力评价有关的地学问题,在此基础上编制了Ag、Au、Pb-Zn等8个元素(组)找矿预测图,共圈定各类找矿预测区136个。同时,也从地球化学角度解决了许多地质问题。为开展塔吉克帕米尔地区地质科学研究,了解帕米尔地区优势资源种类、分布地域、资源潜力等评价信息,快速缩小找矿靶区,实现找矿突破,改变地区资源分布格局等工作提供了可靠而丰富的地球化学基础资料。     

Geomorphological reconstruction and morphometric modelling applied to past volcanism

Accurate and precise morphometric modelling can be extended to past eruptions after a careful palaeogeomorphological reconstruction of volcanic landforms in an oceanic island. This reconstruction was used to derive the pre-, post-eruption and present-day digital elevation models (DEM). The correct pixel size, interpolation method and quality of these DEMs are discussed. The process in a Geographical Information System framework of the geological information of cone, lava flow and tephra fall deposits together and the aforementioned DEMs allowed the determination of main morphological features of these volcanic landforms and their derivatives. The calibration and validation of morphometric modelling were performed on simulated volcanic landforms, and verified by a case study: a typical Holocene monogenetic basaltic eruption at Gran Canaria Island (Spain).     

Alkali feldspar liquid equilibrium relationships in peralkaline oversaturated systems and volcanic rocks

A critical review of both experimental data and natural rock sequences allows a comparison of liquid lines of descent in peralkaline oversaturated systems. Available experimental data have been used to draw the liquidus surfaces and the fractionation curves in various planes including the alkali feldspar join. The thermal valley becomes steeper, the fractionation curves being channeled faster towards the thermal valley with increasing alkali-silica ratio. The slope of the thermal valley varies so that the higher the peralkalinity, the more sodic the most fractionated liquids. Rock series from Easter Island, Gran Canaria, Pantellaria Island, Boina (Afar), Fantale (Ethiopia) and Menengai (Kenya) in which there is good evidence for alkali feldspar fractionation, show that such variations do occur in nature. Factors other than the peralkalinity of the series, such as additional components and changes in the confining pressure are shown to affect the orientation and location of the thermal valley.     

多模态数据的地质图关联网络构建及知识服务

地质图件及其附属资源（如描述文本）作为地质知识的重要存储媒介,蕴含着大量的地学知识及专家经验知识及隐式知识。快速精准地对多模态地质数据进行知识规范化定义、形式化表达、语义化关联和精准化推理是目前地学知识抽取及挖掘的前沿。当前,海量的地学数据中图件及附属描述信息知识分布零散,图件中的信息无法有效地与描述信息进行关联并提供知识服务。本文以多模态地质数据为数据源,建立了多源数据驱动下的地质图知识表达模型,通过地质基础知识、规则知识及决策知识来表达图件中蕴含的显隐式地质知识,从语义、空间、属性三个维度表示地质对象间的关系;并基于领域知识针对地质图件及描述信息开展了地质实体及关系的精准抽取及存储研究,最后以江西省于都县银坑幅G50E011007图幅150000矿产地质调查矢量数据为例进行了验证与分析,实验结果验证了本文所提出的知识表达模型及知识抽取方法, 能够较好地解决地质知识建模中图文关联弱、知识挖掘不充分等问题。     

The evaluation of “social risk” due to volcanic eruptions of Vesuvius

A re-evaluation of the demographic risk per number of inhabitants due to volcanic eruptions in the Vesuvius area was made on the basis of the Census data of 2001. We introduced other variables (population density and number of houses) which permit to upgrade the existing models. Using the Geographical Information Systems (GIS) and particularly ArcGIS 9.0 software we have elaborated a land use map, an urbanization map and a series of new risk maps which lead us to obtain a map of what we call “social risk” due to volcanic eruptions, derived from the combination of the data used and the overlay of the maps. We have proposed an integrated model which can be easily updated to follow the evolution of the volcanic risk in the overpopulated Vesuvius area, with the aim of supporting the planning of Civil Protection and Local Authorities, for an evacuation scenarios and the possibility of taking into account the potential infrastructural damages. This methodology can be tested in other volcanic regions.     

The Role of Partial Melting in the 15-Ma Geochemical Evolution of Gran Canaria: A Blob Model for the Canary Hotspot

The subaerial portion of Gran Canaria, Canary Islands, was builtby three cycles of volcanism: a Miocene Cycle (8•5–15Ma), a Pliocene Cycle (1•8–6•0 Ma), and a QuaternaryCycle (1•8–0 Ma). Only the Pliocene Cycle is completelyexposed on Gran Canaria; the early stages of the Miocene Cycleare submarine and the Quaternary Cycle is still in its initialstages. During the Miocene, SiO₂ saturation of the mafic volcanicsdecreased systematically from tholeiite to nephelinite. Forthe Pliocene Cycle, SiO₂ saturation increased and then decreasedwith decreasing age from nephelinite to tholeiite to nephelinite.SiO₂ saturation increased from nephelinite to basanite and alkalibasalt during the Quaternary. In each of these cycles, increasingmelt production rates, SiO₂ saturation, and concentrations ofcompatible elements, and decreasing concentrations of some incompatibleelements are consistent with increasing degrees of partial meltingin the sequence melilite nephelinite to tholeiite. The maficvolcanics from all three cycles were derived from CO₂-rich garnetlherzolite sources. Phlogopite, ilmenite, sulfide, and a phasewith high partition coefficients for the light rare earth elements(LREE), U, Th, Pb, Nb, and Zr, possibly zircon, were residualduring melting to form the Miocene nephelinites through tholeiites;phlogopite, ilmenite, and sulfide were residual in the sourceof the Pliocene–Quaternary nephelinites through alkalibasalts. Highly incompatible element ratios (e.g., Nb/U, Pb/Ce,K/U, Nb/Pb, Ba/Rb, Zr/Hf, La/Nb, Ba/Th, Rb/Nb, K/Nb, Zr/Nb,Th/Nb, Th/La, and Ba/La) exhibit extreme variations (in manycases larger than those reported for all other ocean islandbasalts), but these ratios correlate well with degree of melting.Survival of residual phases at higher degrees of melting duringthe Miocene Cycle and differences between major and trace elementconcentrations and melt production rates between the Mioceneand Pliocene tholeiites suggest that the Miocene source wasmore fertile than the Pliocene–Quaternary source(s). We propose a blob model to explain the multi-cycle evolutionof Canary volcanoes and the temporal variations in chemistryand melt production within cycles. Each cycle of volcanism representsdecompression melting of a discrete blob of plume material.Small-degree nephelinitic and basanitic melts are derived fromthe cooler margins of the blobs, whereas the larger-degree tholeiiticand alkali basaltic melts are derived from the hotter centersof the blobs. The symmetrical sequence of mafic volcanism fora cycle, from highly undersaturated to saturated to highly undersaturatedcompositions, reflects melting of the blob during its ascentbeneath an island in the sequence upper margin-corelower margin.Volcanic hiatuses between cycles and within cycles representperiods when residual blob or cooler entrained shallow mantlematerial fill the melting zone beneath an island.     

The Petrology of the Tholeiites through Melilite Nephelinites on Gran Canaria, Canary Islands: Crystal Fractionation, Accumulation, and Depths of Melting

We report major and trace element X-ray fluorescence (XRF) datafor mafic volcanics covering the 15-Ma evolution of Gran Canaria,Canary Islands. The Miocene (12–15 Ma) and Pliocene-Quaternary(0–6 Ma) mafic volcanics on Gran Canaria include picrites,tholeiites, alkali basalts, basanites, nephelinites, and melilitenephelinites. Olivineclinopyroxene are the major fractionatingor accumulating phases in the basalts. Plagioclase, Fe–Tioxide, and apatite fractionation or accumulation may play aminor role in the derivation of the most evolved mafic volcanics.The crystallization of clinopyroxene after olivine and the absenceof phenocrystic plagioclase in the Miocene tholeiites and inthe Pliocene and Quaternary alkali basalts and basanites withMgO>6 suggests that fractionation occurred at moderate pressure,probably within the upper mantle. The presence of plagioclasephenocrysts and chemical evidence for plagioclase fractionationin the Miocene basalts with MgO<6 and in the Pliocene tholeiitesis consistent with cooling and fractionation at shallow depth,probably during storage in lower-crustal reservoirs. Magma generationat pressures in excess of 3•0–3•5 GPa is suggestedby (a) the inferred presence of residual garnet and phlogopiteand (b) comparison of FeO¹ cation mole percentages and the CIPWnormative compositions of the mafic volcanics with results fromhigh-pressure melting experiments. The Gran Canaria mafic magmaswere probably formed by decompression melting in an upwellingcolumn of asthenospheric material, which encountered a mechanicalboundary layer at {small tilde}100-km depth.