Thick Lava Flows Research Articles

Tectonics and volcanism of Sierra Chichinautzin: extension at the front of the Central Trans-Mexican Volcanic belt

Because of its recent activity and position at the southern magmatic front of the Trans-Mexican Volcanic Belt (TMVB), the Sierra Chichinautzin volcanic field (SCN) is a key area for the understanding of this controversial volcanic province. Volcanic activity has built more than 220 monogenetic volcanoes (shields, scoria cones, thick lava flows, and hydromagmatic structures) during the last 40,000 years, for a total volume of about 470 km 3. The SCN basalts are geochemically similar to OIBs, while the intermediate and felsic volcanic rocks show a calc-alkaline trend and abundant evidence for magma mixing. The structural analysis of this volcanic field and surrounding areas has been based on field data, satellite images, and a method for detecting volcanic center alignments. The tectonic data, together with geophysical evidence, confirm active general N–S extensional conditions with a strike–slip component for the SCN area, the same structural setting that prevails in the rest of the Central TMVB. Extensional tectonics, a negative regional Bouger gravity anomaly, a low-velocity mantle, high heat flow, and shallow seismicity suggest a rift-type setting involving the upwelling of anomalous mantle beneath the Central TMVB. The combined petrological, structural and geophysical arguments support that the SCN volcanism is rift-related, and rule out processes involving the subduction of the Cocos plate, which casts further doubts on the standard subduction model for the TMVB volcanism.

Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan

During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km 3 in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km 3, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano.

Thick lava flows of Karisimbi Volcano, Rwanda: insights from SIR-C interferometric topography

We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12 km and average 40–60 m (up to >140 m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of flow thickness, which we used to calculate the volume (up to 1 km3 for an individual flow, and 1.8 km3 for all the identified flows) and yield strength of several flows (23–124 kPa). Using the DEM we estimated apparent viscosity based on the spacing of large folds (1.2×1012 to 5.5×1012 Pa s for surface viscosity, and 7.5×1010 to 5.2×1011 Pa s for interior viscosity, for a strain interval of 24 h). We use shaded-relief images of the DEM to map basic flow structures such as channels, shear zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images.

Role of plagioclase crystal chains in the differentiation of partly crystallized basaltic magma

Melting experiments on samples of basaltic rock from a thick lava flow reveal that when this lava originally began to crystallize, feldspar crystals linked together to form a continuous three-dimensional network of chains when the lava was no more than 25% crystallized. Formation of this network has profound implications for the behaviour and differentiation of basaltic magma. Much of the compositional variation of igneous rocks results from the separation of liquid from crystals, a process that is dramatically affected according to whether crystals occur separately or are linked together in networks.

Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico

Research Article| October 01, 1997 Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico Fred W. McDowell; Fred W. McDowell 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Jaime Roldán-Quintana; Jaime Roldán-Quintana 2Instituto de Geología, Universidad Nacional Autónoma de México, Apartado Postal 1039, Hermosillo, Sonora 83000, México Search for other works by this author on: GSW Google Scholar Ricardo Amaya-Martínez Ricardo Amaya-Martínez 3Departamento de Geología, Universidad de Sonora, Hermosillo, Sonora 83000, México Search for other works by this author on: GSW Google Scholar GSA Bulletin (1997) 109 (10): 1349–1360. https://doi.org/10.1130/0016-7606(1997)109<1349:IOSAVD>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Fred W. McDowell, Jaime Roldán-Quintana, Ricardo Amaya-Martínez; Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico. GSA Bulletin 1997;; 109 (10): 1349–1360. doi: https://doi.org/10.1130/0016-7606(1997)109<1349:IOSAVD>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Clastic sedimentary deposits and associated volcanic rocks record the progress of Tertiary extension in the Mexican state of Sonora. These deposits accumulated within basins located throughout the eastern two-thirds of the state. The rocks are slightly indurated conglomerates and sandstones; clast types reflect the local highland exposures. Volcanic units that bound and are interlayered with the sedimentary rocks provide stratigraphic and time markers of basin evolution and, by inference, of marginal fault development. This record has been examined in an east-west belt across south-central Sonora.The volcanic rocks occur in three distinct associations. At the base and interbedded within the lower portions of the sedimentary sections are lava flows of dominantly basaltic andesite composition. In most localities the overlying clastic sedimentary strata are conformable with these lava flows, and we conclude that the magmatism was triggered by early faulting along the basin margins. A second association present near the base of the sections comprises massive lava domes and flows of intermediate composition that contain distinctive phenocrysts of dark brown amphibole. The domes apparently formed when viscous lava welled up along developing marginal basin faults. In one case a transition was observed over a short distance from a structureless dome to a thick lava flow that is conformably interbedded with, and sheds clasts laterally into, the sedimentary section. A third volcanic association comprises layers of rhyolitic lava flows and ignimbrites that overlie the coarsest and thickest lower portions of the sedimentary sections.K-Ar ages for volcanic rocks in the two oldest of these settings indicate that each Tertiary basin had a distinct history with no apparent regional geographic pattern. In the Rio Yaqui basin, where exhumation by the major river of Sonora has exposed a full section of the volcanic and clastic sequences, extension was possibly as old as 27 Ma. Elsewhere, deposition of sediments was underway in most basins by 24 Ma, and in all basins by 20 Ma. K-Ar ages of the younger rhyolitic volcanic rocks fall within a narrow range from 12.8 to 10.5 Ma. These ages provide a younger time limit to deposition of the coarser and thicker portions of the clastic sedimentary sections.The mafic lava flows, dominantly basaltic andesite, are similar in major-element composition to mafic lava flows that cap felsic sections within the Sierra Madre Occidental volcanic field of western Mexico and the mid-Tertiary Datil-Mogollon volcanic field in southwestern New Mexico. They are higher in silica and lower in total alkalies than younger capping mafic lava flows within the Gulf of California extensional province of western Sonora and Neogene basaltic lava flows in the Datil-Mogollon field. They are similarly distinct from basaltic dikes and lava flows that are clearly associated with Basin and Range faulting in Trans-Pecos Texas and in northern Durango state.A convergent-margin magmatic arc was active in the present-day Gulf of California region between 24 and 11.5 Ma. The earliest extension in south-central Sonora was therefore occurring in a back-arc setting. Continued sedimentation and rotation of volcanic strata indicate that extension continued throughout the region after 10 Ma with little coeval volcanism. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Morphology of the 1984 open-channel lava flow at Krafla volcano, northern Iceland

An open-channel lava flow of olivine tholeiite basalt, 9 km long and 1–2 km wide, formed in a volcanic eruption that took place in the Krafla volcano, Iceland, on the 4–18 September 1984. The eruption started with emplacement of a pahoehoe sheet which was fed by a 8.5-km-long fissure. After two days of eruption, lava effusion from the fissure ceased but one crater at the northern end of the fissure continued to release lava for another twelve days. That crater supplied an open-channel flow that moved toward the north along the rift valley. The lava was emplaced on a slope of 1°. The final lava flow is composed of five flow facies: (1) the initial pahoehoe sheet; (2) proximal slab pahoehoe and aa; (3) shelly-type overflows from the channel; (4) distal rubbly aa lava; and (5) secondary outbreaks of toothpaste lava and cauliflower aa. The main lava channel within the flow is 6.4 km long. The mean width of this channel is 189 m (103 m S.D.). An initial lava channel that forms in a Bingham plastic substance is fairly constant in width. This channel, however, varies in width especially in the proximal part indicating channel erosion. Large drifted blocks of channel walls are found throughout the flow front area and on the top of overflow levees. This suggests that the channel erosion was mainly mechanical. The lava flow has a mean height of 6 m above its surroundings, measured at the flow margins. However, a study of the pre-flow topography indicates that the lava filled a considerable topographic depression. Combined surface and pre-flow profiles give an average lava-flow thickness of 11 m; the thickness of the initial sheet-flow is estimated as 2 m. The volume of the lava flow calculated from these figures is 0.11 km 3. The mean effusion rate was 91 m 3/s. When lava flow models are used to deduce the rheological properties of this type of lava flow, the following points must be considered: (1) when a lava flow is emplaced along tectonic lineaments, its depth and volume may be significantly larger than what the surface exposure suggests; (2) lava channels may become severely eroded during channel flow even if a lava flow was formed in a relatively short time; (3) the levee dimensions, and hence lava flow dimensions, may be significantly altered by extensive overflows.

Quaternary Basin Evolution and Basaltic Volcanism of Crater Flat, Nevada, From Detailed Ground Magnetic Surveys of the Little Cones

Ground magnetic surveys of the Quaternary (0.77‐0.98 Ma) Little Cones and surrounding areas of southern Crater Flat, Nevada, reveal detailed images of buried basaltic lava flows not evident from surface exposures. Magnetic maps derived from these surveys show an apron of buried lava extending radially up to 300 m from the center of the two cones. Modeling of the anomalies indicates burial of the lava apron by up to 15 m of alluvium on top of approximately 10 m thick lava flows. From these results, we estimate a volume >0.01 km3 of volcanic material for the Little Cones, comparable to the more completely exposed and similarly aged Red and Black Cones of central Crater Flat. Partial burial of the Little Cones by alluvium, in contrast to the exposed flows associated with Red and Black Cone, suggests differential subsidence of the southern part of the basin in the Quaternary. This pattern of subsidence is consistent with recent structural studies that show increased slip toward the south on major normal faults within and adjacent to the eastern and western margins of the basin. The depth of burial indicates sediment accumulation rates of approximately 0.03 mm/yr, similar to rates since the Miocene (Tortonian) based on the thickness of alluvium above 10.0 to 11.5 Ma basalts in the USW VH‐2 drill hole. These comparatively rapid sedimentation rates suggest that other Quaternary and Pliocene volcanic centers may be buried in southern Crater Flat without surface expression.

Four lunar mare meteorites: Crystallization trends of pyroxenes and spinels

Abstract— We studied crystallization trends of pyroxene and spinel in four Antarctic meteorites known to be derived from mare regions of the Moon: Y‐793169 and A‐881757 (YA meteorites) are unbrecciated igneous basalts, EET 87521 is a fragmental breccia, and Y‐793274 is a regolith breccia. All have relatively low bulkrock TiO2 content, and the YA meteorites are uncommonly ancient. Our electron probe microanalysis (EPMA) data indicate that the YA meteorites and the dominant mare components of Y‐793274 and EET 87521 conform to a general trend for Ti‐poor (low‐Ti and very low‐Ti) mare basalts. Their pyroxenes show a strong correlation between Fe/(Fe + Mg) (Fe#) and Ti/(Ti + Cr) (Ti#), both ratios typically increasing from core to rim. These trends presumably reflect local crystallization differentiation of interstitial melt. Previous studies (M. J. Drake and coworkers) have suggested that the detailed configurations of such Fe# vs. Ti# trends may reflect the bulk TiO2 contents of the parent magmas (basalts). As a more systematic approach to this problem, we plot bulk‐rock TiO2 as a function of the Fe# = 0.50 intercept of each rock's pyroxene Fe# vs. Ti# trend. We call this intercept the Fe#‐normalized Ti#. Based on our data for EET 87521, the YA meteorites, and Apollo 12 basalts 12031 and 12064, plus literature data for several other Ti‐poor mare basalts, we find a strong correlation between Fe#‐normalized Ti# and the bulk TiO2 content of the parent basalt. This correlation confirms that fragmental breccia EET 87521 is nearly pure very low‐Ti (VLT) basalt and that the YA meteorites, for which bulk‐rock TiO2 results scatter due to unusually coarse grain size (A‐881757) or scarcity of available sample (Y‐793169), are pieces of an uncommonly Ti‐poor, but not quite VLT, variety of low‐Ti mare basalt. Extrapolating from this correlation, the dominant mare component of regolith breccia Y‐793274 is probably of VLT affinity. Besides the normal mare pyroxene trend of strong correlation between Fe# and Ti#, Y‐793274 includes two additional pyroxene compositional trends, both showing a wide range of Ti# despite relatively constant (and low, by mare standards) Fe#. The most magnesian of these trends consists of a single clast with a mode of orthopyroxene + MgO‐rich ilmenite. These two trends are of uncertain origin. Possibly one or both represents the highland component of this regolith breccia, although, unlike most highland pyroxenes, these appear relatively unaltered by impact brecciation and metamorphism. Compositions of spinels in the coarse‐grained A‐881757 show an extraordinary distribution: chromite and ulvöspinel components vary among grains but are nearly constant within grains. Despite its old age and unusually coarse grain sizes, mineralogical evidence (i.e., heterogeneity within both pyroxene and spinel; typical pyroxene exsolution scale very coarse by mare standards but exceeded by the pyroxenes of EET 87521 and Y‐793274) indicates that A‐881757 was cooled only slightly more slowly than typical mare basalts and may have formed near the center of an uncommonly thick lava flow. Both of the VLT basaltic lunar meteorite breccias, EET 87521 and Y‐793274, are composed dominantly of pyroxenes with exsolution coarser than normal for mare basalts. Possibly VLT basalt flows tend to be systematically thicker, and thus more slowly cooled, than more Ti‐rich flows.

Contrasting volcanism in the Michoacán-Guanajuato Volcanic Field, central Mexico: Shield volcanoes vs cinder cones

El Campo Volcánico de Michoacán-Guanajuato (40,000 km2), de la Faja Volcánica Trans-Mexicana (FVTM), contiene volcanes de tamaño pequeño y mediano, y carece de grandes volcanes compuestos. Los volcanes e tamaño pequeño incluyen a 900 conos cineríticos, y 100 volcanes de otros tipos tales como conos, domos y gruesos derrames de lava no asociados con conos y mares. En contraste, los volcanes de tamaño medio incluyen mas de 300 volcanes y algunos domos de lava. Ambos grupos de volcanes coexisten en tiempo y espacio. Las lavas asociadas a conos cineríticos poseen un alto rango composicional de 47 a 67% en contenido de SiO2, con abundantes basalltos de olivino calcialcalinos y andesitas basálticas. Existen también unas cuantas rocas alcalinas. Las lavas de volcanes escudo son todas andesitas calcialcalinas que muestran un rango limitado de SiO2 (comúnmente 55%-61%), con ocurrencia común de fenocristales de ortopiroxenos. Se han encontrado composiciones similares para flujos de lava que no están asociados a conos. Estas lavas y las de los escudos, representan erupciones efusivas, menos explosivas. Los volcanes escudo tienen derrames de lava mas largos y volúmenes mayores que los derrames que no están asociados con conos, indicando con esto una tasa efusiva y un aporte de magma mayores que estos últimos. Debido a que sus lavas están más fraccionadas que las lavas calcialcalinas de conos cineríticos, pero que se grafican en el mismo tren composicional que éstas, posiblemente aquellas sean producto de cristalización fraccionada de basaltos primitivos calcialcalinos, los cuales se encuentran en algunos conos cineríticos.

Le gisement magdalénien de Neschers et sa situation par rapport à la coulée basaltique du Tartaret (Puy-de-Dôme) : état de la question

ABSTRACT A magdalenian settlement was excavated at Neschers (Puy-de-Dôme, France) by the Abbé Croizet, around the middle of the nineteenth century. The remains layed at the bottom of a thick lava flow from the Tartaret volcano; however, there is a doubt whether the remains were beneath or besides the lava (i.e., whether the eruption took place before or after the human occupation). The archaeological materials were very interesting and lead one to try to study the site again today. The last chronological data indicate that the lava and the settlement are nearly contemporaneous to each other, around 12500 BP, i.e. between 14 and 15 ka before us.

A Detailed Magnetic and Opaque Petrologic Study of A Thick Tholeiite From the Rajmahal Volcanics, India

Summary A 22 m thick Rajmahal tholeiite basalt lava flow shows wide variations in magnetic properties which have been explained by opaque petrologic studies. Microscopic ore evidence clearly demonstrates that the upper one-third of the lava is dominated by Fe-rich titanomagnetites from high temperature (deuteric) oxidation and that the lower two-thirds are dominated by unoxidized Ti-rich titanomagnetite except for a thin low temperature oxidation zone. the natural remanent magnetization (NRM) intensity appears to be related to the composition of magnetic minerals, giving rise to high values for Fe-rich titanomagnetites and lower values for their Ti-rich counterparts. Stable magnetization is confined to two narrow zones in the flow where the titanomagnetite grains are intensively subdivided by lamellar ilmenites or by wide cracks. the initial susceptibility and the concentration of magnetic oxide grains both show little variation, suggesting the dependence of the former on the latter. Although the relation of hysteresis properties and opaque mineralogy is usually complex, the ratio of remanent magnetization to the saturation remanent magnetization, JRS/JS, is chiefly dependent on the effective grain size and the coercive force Hc, tends to be higher for anisotropic grain fabrics resulting from lamellar ilmenite formation in titanomagnetite. the Q-values of the specimens suggest that the remanent magnetization is significant only in the high temperature oxidation zone of the lava.

Extended cooling and viscous flow of large, hot rhyolite lavas: implications of numerical modeling results

Abstract A common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities. Although a high-viscosity lava will indeed flow quite slowly, it may advance a great distance if the lava efficiently retains heat and thus mobility, and if the available magma supply is large. To evaluate this possibility, a finite-difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas. Cooling of such 100–300 m-thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat. Thermal history alone suggests that large, thick lava flows could remain active for several decades. To constrain the duration of flow advance of these units, their inferred apparent viscosities are compared with those of active lava flows. Mean flow velocity of some large rhyolites in southwestern Idaho, USA, was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization, indicating perhaps a 10- to 18-year flow duration, based on studies of Obsidian Dome, California. Mean flow velocities would have been roughly 0.59 to 2.5 km/yr, and calculated average apparent viscosities of the flows ranged from 1.8 to 3.6 orders of magnitude greater than the viscosity of the lava itself. This relationship also holds for active andesite and basaltic andesite block lava flows, which have apparent viscosities 2.7 to 4 orders of magnitude greater than lava viscosities. This implies that large rhyolites may indeed have been emplaced in the same manner as less-silicic block lavas, and that long distance viscous flow is not unrealistic for rhyolite lavas. A good example of a large-volume lava is the 15-km 3 Rhyolite of the Badlands (SW Idaho), where vent relations show that lava effused from a fissure and advanced up to 9 km. More extensive rhyolites could also be emplaced solely by effusion. Extended flow of these lavas may lead to the remelting and removal of basal crumble breccias, and to the formation of welded shard textures in their margins, both processes tending to obscure the lava flow origin of the units. The great dimensions of lava-like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites.

Magnetic property zonation in a thick lava flow

In this study, grain size and composition‐dependent magnetic properties of titanomagnetite minerals are used as indicators of intraflow structures and magmatic evolution in an extensive and thick (30–60 m) basaltic lava flow. Similar zonation occurs in this flow at three localities separated by tens of kilometers. The magnetic properties subdivide the flow to three zones. The upper layer, representing the top 1/3 of the lava (≤ 20 m), has higher magnetic stability due to smaller and more deuterically oxidized titanomagnetite grains, approaching pure magnetite. The central layer in the underlying 2/3 of the flow (≤ 35 m) has larger, magnetically less stable, and less oxidized grains with relatively uniform magnetic properties. The basal layer, the bottom 1/10 of the flow (≤ 5 m), has near primary, least oxidized titanomagnetites (Ulv68Mag32). The upper intraflow boundary of the magnetic properties appears to coincide with the transition from entablature (above) to colonnade (below), distinguishing between regions of faster and slower cooling. Microprobe data indicate that the intraflow oxidation state (Fe3+/Fe2+) of the initially precipitated primary titanomagnetites increases with falling equilibrium temperature from the flow margins to a maximum near the center, the position of lowest equilibrium temperature. In contrast, Curie temperature measurements indicate that titanomagnetite oxidation increases with height in the flow. Modification of the initially symmetric equilibrium titanomagnetite compositions was caused by subsolidus high‐temperature oxidation possibly due to hydrogen loss produced by dissociation of magmatic water, as well as unknown contributions of circulating air and percolating water from above. The titanomagnetites of the basal layer of the flow remain essentially unaltered.

Aeolian morpho_logy preserved by lava cover, the Precambrian Mussartut Member, Eriksfjord Formation, South Greenland

Investigations of sedimentary deposits in elastic interval 5 of the Mussartilt Member have revealed the occurrence of aeolian sandstones. The aeolian deposits rest on pebbly sandstones and conglomerates of fluvial origin, and the are sharply overlain by a c. 70 m thick lava flow. The aeolian sandstones comprise up to 10 m thick and 200 m wide dome-shaped bodies that are initiated by thin and patchily preserved aeolian sand sheet deposits. The bulk of the aeolian sandstone bodies constitute low-medium-angle, dipping lee-side deposits of dome-shaped dunes. The dome-shaped dunes migrated towards a zone of distal alluvial fans perhaps during the influence of prevailing NE trade winds. Superimposed smaller-scale dunes formed during periodic strong winds from the E. Erosion of the dunes at the base of the lava flow was insignificant and most of the original dome-shape form of the dunes seems to have been preserved. The dome-shaped dunes may represent part of a migrating erg system, but continued dune migration was stopped by extrusion of the lava flow.

The Geoscience Laser Altimetry/Ranging System

The Geoscience Laser Altimetry/Ranging System (GLARS) is a planned highly precise laser distance-measuring system to be used for geoscience measurements requiring extremely accurate geodetic observations from a space platform. The system combines the attributes of a pointable laser ranging system making observations to retroreflectors placed on the ground with those of a nadir-looking laser altimeter making height observations to ground, ice sheet, and oceanic surfaces. In the ranging mode, centimeter-level precise baseline and station coordinate determinations will be made on grids consisting of 100 to 200 targets separated by distances from a few tens of kilometers to about 1000 km. These measurements will be used for studies of seismic zone crustal deformations and tectonic plate motions. Ranging measurements will also be made to a coarser, but globally distributed, array of retroreflectors for both precise geodetic and orbit determination applications. In the altimetric mode, relative height determinations will be obtained with approximately decimeter vertical precision and 70-100-m horizontal resolution. Altimetric profiles consisting of nearly contiguous spots will be available when the system is operated at 40 pulses per second. The height data will be used to study surface topography and roughness, ice sheet and lava flow thickness, and ocean dynamics. Waveform digitization will provide a measure of the vertical extent of topography within each footprint.

Temporal variation in chemical composition of phenocrysts and magmatic temperature at Daisen volcano, southwest Japan

Abstract Daisen volcano, located in the San'in district, southwest Japan, started its activity in the middle Pleistocene and continued until at least ca. 20,000 yr B.P. The volcano is composed entirely of dacitic pyroclastic materials, lava domes and subordinate thick lava flows. Its activity is divided into two groups, Older (1.0-0.4 Ma) and Younger (0.4 Ma to ca. 17.000 yr B.P.). Chemical compositions of phenocrysts in the members of the Upper Tephra Group (the last 150,000 years) in the Younger Group were examined in detail by electron microprobe analysis. The compositions of phenocryst minerals change systematically and cyclically with the order of eruptions. Phenocrysts with less differentiated compositions were found in the products of eruptions 60,000 and 20,000 years ago. The variation patterns of inferred magma temperature (estimated by the Fe-Ti oxide geothermometer) with time are well correlated with those of the chemical compositions of phenocrysts. Orthopyroxene phenocrysts generally show both reversed and normal zoning in single rock specimens and the compositional range of rims is much smaller than that of the core, indicating that the process of re-equilibration of two compositionally distinct orthopyroxenes took place. These facts could be explained by injection of less differentiated, higher-temperature magmas from a deeper level into the shallower more differentiated magma reservoir. A relatively active (frequent and/or voluminous) injection episode seems to have taken place twice during the last 150,000 years; 60,000 and 20,000 years ago.

Large-scale landslide landforms in Tohoku District, Northern Japan

The large-scale landslide landforms, wish source areas wider than 1 km were mapped by air-photograph interpretation in the Tohoku district between latitude 38°N and latitude 42°N. On the basis of the mapping, characteristics of the distribution of those large-scale landslide landforms are discussed from geomorphological and geological points of view.Six regions are recognized as those of concentration in the central part of the Ou Mountains, the Shirakami Mountains, the Hinotodake-Kamura Mountains and the northern part of the Asahi Mountains. Those regions mostly coincide with the areas of the maximum uplift during the Quaternary time in the Tohoku district. Those are also underlain by the zones of the Lower and Middle Miocene sedimentary rocks and submarine volcanic rocks, commonly called “Green Tuff”, and the Quaternary volcanic rocks. The topography in those zones are characterized by cap rock structures.The large-scale landslide landforms of the Quaternary volcanic areas are dominantly found in the old and/or dormant volcanoes, and are roughly divided into three types from the main contributing factors of landsliding, the volume in source area, morphology and so on.(1) Type 1 is a catastrophic landslide or collapse of a volcanic cone associated with volcanic activity. Its tophographic and structural feature is composed of the so-called the collapse caldera of the source area and hummocky relief of the deposition area. The latter is a typical topographic feature of volcanic dry avala nche deposits composed of megablocks and interstitial fine material. The volume of the source area is the largest of the three types. Examples can be seen in Iwate volcano, Iwaki volcano, Gassan volcano, Cheikai volcano, Zao volcano and Shirataka volcano.(2) Type 2 shows a typical landslide landform associated with a gentle arc-shaped main scarp and a moving mass. Main scarp of this type is found on the slope of a thick lava flow near the top of the volcano.The activity of the volcanoes, where the landslides of this type occurred, ceased in the late Pleistocene and seems to be almost dormant at present.The contributing factor to the landsliding may be a large quantity of water supply related to the late Quaternary climatic change.Water are prepared in the form of the perennial snow patches and are supplied by the rapid melting of the perennial snow patches with the rising air temperature. The formative age of the perennial snow patches ranges from the late Pleistocene to the eariest Holocene (period about 18, 000y. B. P. to 8, 000y. B. P.).Because the sea level of this period is lower than that of the present, mountains were vigorously dissected. Then the stability of the mountain slopes rapidly deteriorated and a number of large-scale landslides occured.Those landslides are found in Hachimantai volcano, Yakeishi volcano, Kurikoma volcano, Funakata volcano, Gassan volcano and Zao volcano.(3) Type 3 is recognized along the dissected vally wall in the pyroclastic deposits and pumice fall deposits. Type of movement is mostly rotational. The volume in source area is smaller than that of the type 2. However, the same contributing factor may be important for the landslides of type 3 as the type 2.

Contemporaneous faulting, and the eruption and preservation of the Lough Guitane Volcanic Complex, Co. Kerry

Interbedded with the clastic fluvial sediments of the Munster Basin, the Lough Guitane Volcanic Complex represents a rare volcanic episode in the Upper Devonian of SW Ireland. From three distinctly separate volcanic centres, an exclusively rhyolitic suite of thick lava flows and tuffs were erupted. The largest of the centres possessed a system of contemporaneous faults producing a subcircular depressed area, itself enclosed within a graben 1.5 km across. These faults lead to rapid thickness variations in many volcanic units, and a pooled lava flow reaches a thickness of 300 m. The site of a major elongate vent lies along one such fault, and rapid erosion of its poorly lithified walls generated coarse sandstone boulder agglomerates and is related to the absence throughout the complex of normal air-fall tuffs unmixed with terrigenous sand. The containment of the bulk of the volcanics by contemporaneous faults, and subsequent rapid burial by terrigenous sediments has led to preservation of the volcanic succession virtually intact.

The early devonian tangerang formation of the marulan‐ windellama region, nsw — definition and palaeo‐environmental significance

The Early Devonian Tangerang Formation consists of a sequence of shallow marine conglomerate, limestone and shale interbedded with shallow marine and terrestrial volcaniclastic arenite, tuff and dacitic lava flows. Three distinctive members have been recognized within the formation, namely the Windellama Limestone Member, the Carne Dacite Member and the Aloes Tuff Member. Limestone is confined to the lower part of the formation in the south where it is overlain by an entirely shallow marine sequence of volcaniclastic arenite. The proportion of volcanic material increases northwards and includes fiamme and accretionary lapilli which are considered to represent ashflow and ashfall tuffs that accumulated, together with thick lava flows, on the flanks of subaerial dacitic volcanoes. These volcanoes were situated at the northern end of the Aloes Tuff Member and towards the top of the formation in the Marulan South area. In the south the Tangerang Formation unconformably overlies Ordovician strata and the presence of Lochkovian faunas within the lower part of the formation suggests a lateral interdigitation with the upper part of the Bungonia Limestone. The dacitic lavas and associated volcaniclastic rocks in the upper part of the formation are petrographically identical to rocks in the southern part of the Bindook Volcanic Complex. Thus the Tangerang Formation, which accumulated on an uplifted horst of Late Ordovician flysch on the eastern margin of the Wollondilly Tract, is considered to represent the basal unit of the Bindook Volcanic Complex in the Marulan‐Windellama area. The progressive filling of the Wollondilly Tract to the north of Marulan with volcanic ejecta is reflected in the change in palaeocurrent direction from a dominant northward longshore system to an E‐W onshore‐offshore system during the deposition of the Tangerang Formation.

Distribution, form, and structural control of the Alexandra Volcanic Group, North Island, New Zealand

Abstract Karioi, Pirongia, Kakepuku, and Te Kawa which show a northwesterly (300°) alignment. Volcanism was predominantly basaltic and produced approximately 55 km3 of lava, volcanic breccia, and scoria from at least 40 centres. A new formation is proposed within the Alexandra Volcanic Group, the Okete Volcanic Formation, which shows differences in form, petrography, petrochemistry, and eruptive mechanisms from the rest of the Alexandra Volcanics. The Okete Volcanics form numerous small partially eroded scoria cones and thin lava flows with occasional tuff rings, and are alkalic in nature, consisting of finegrained basanites, basalts, and rare hawaiites. The Pirongia, Karioi, Kakepuku, and Te Kawa Volcanics form large low-angle composite cones constructed of thick lava flows and volcanic breccias, with minor dikes, scoria, tuff and lahars. They are composed of tholeiitic or subalkaline coarse-grained basalts, basaltic andesites, with rare andesites. Volcanism in the Alexandra field is Plio-Pleistocene in age, spanning the interval between 3.79 m.y. and 1.80 m.y., although some volcanic centres may be considerably younger. Geomorphological evidence suggests the volcanoes of Karioi, Pirongia, Kakepuku, and Te Kawa become progressively younger to the southeast. The Okete Volcanics intercalate stratigraphically with lavas from Pirongia and Karioi, and show no distinct temporal relationships. The regional northwesterly alignment may be caused by rising magma which encountered a major northwesterly fracture at deeper crustal levels and was channelled in part along the system of north and northeast-striking faults at upper crustal levels.