Hypersthene Andesite Research Articles

Eruptive history and K-Ar geochronology of the late Cenozoic Garibaldi volcanic belt, southwestern British Columbia

Research Article| April 01, 1988 Eruptive history and K-Ar geochronology of the late Cenozoic Garibaldi volcanic belt, southwestern British Columbia NATHAN L. GREEN; NATHAN L. GREEN 1Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487 Search for other works by this author on: GSW Google Scholar RICHARD L. ARMSTRONG; RICHARD L. ARMSTRONG 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B4 Search for other works by this author on: GSW Google Scholar J. E. HARAKAL; J. E. HARAKAL 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B4 Search for other works by this author on: GSW Google Scholar J. G. SOUTHER; J. G. SOUTHER 3Geological Survey of Canada, 100 W. Pender, Vancouver, British Columbia, Canada V6B 1R8 Search for other works by this author on: GSW Google Scholar PETER B. READ PETER B. READ 4Geotex Consultants Limited, Suite 1200, 100 W. Pender, Vancouver, British Columbia, Canada V6B 1R8 Search for other works by this author on: GSW Google Scholar Author and Article Information NATHAN L. GREEN 1Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487 RICHARD L. ARMSTRONG 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B4 J. E. HARAKAL 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B4 J. G. SOUTHER 3Geological Survey of Canada, 100 W. Pender, Vancouver, British Columbia, Canada V6B 1R8 PETER B. READ 4Geotex Consultants Limited, Suite 1200, 100 W. Pender, Vancouver, British Columbia, Canada V6B 1R8 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1988) 100 (4): 563–579. https://doi.org/10.1130/0016-7606(1988)100<0563:EHAKAG>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 NATHAN L. GREEN, RICHARD L. ARMSTRONG, J. E. HARAKAL, J. G. SOUTHER, PETER B. READ; Eruptive history and K-Ar geochronology of the late Cenozoic Garibaldi volcanic belt, southwestern British Columbia. GSA Bulletin 1988;; 100 (4): 563–579. doi: https://doi.org/10.1130/0016-7606(1988)100<0563:EHAKAG>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 The Cenozoic Garibaldi belt comprises 6 volcanic fields spaced at irregular intervals along an axis extending 240 km north-northwest from the head of Howe Sound to the Bridge River area; 2 additional fields, the Franklin Glacier and Silvert lie 140 and 190 km west of the north end of the main volcanic belt. The volcanoes erupted lavas ranging in composition from augite-olivine basalt, through hypersthene andesite, hornblende andesite, and hornblende-biotite andesite, to biotite rhyodacite. Many of the volcanic complexes are characterized by geomorphic features which indicate complex interactions between volcanism and the Pleistocene ice sheets, but preglacial and postglacial phases are also present.Whole-rock samples from 18 volcanic complexes have been dated by the K-Ar method. Most of the results are internally consistent with stratigraphic relationships and with limited 14C and paleomaguetic data.They suggest that volcanic activity was episodic; most of the analyzed andesitic and dacitic lavas were erupted in the intervals 2.3 to 1.7 Ma and 1.1 Ma to present in the northern part and in the intervals 1.4 to 1.0 Ma and 0.7 Ma to present in the southern part of the belt. Basaltic volcanism occurred only during the past 0.15 m.y., except in the Salal Glacier area where hawaiite and alkali-olivine basalt, which are perhaps an "edge effect" related to subduction of the Juan de Fuca plate, were erupted as early as 0.97 Ma. The timing of Garibaldi belt volcanism provides information bearing on the distribution of pre-Wisconsin glaciers in southwestern British Columbia and constrains interpretations of late Cenozoic changes in Explorer-Juan de Fuca-North American plate configuration along the continental margin. 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.

Paricutin re-examined: a classic example of crustal assimilation in calc-alkaline magma

Following its birth on the 20th of February 1943, the Mexican volcano Paricutin discharged a total of 1.38 km3 of basaltic andesite and andesite before the eruption came to an end in 1952. Until 1947, when 75% of the volume had been erupted, the lavas varied little in chemical or isotopic composition. All were basaltic andesites with 55 to 56% SiO2, δ 18O of +6.9 to 7.0, and 87Sr/86Sr ratios close to 0.7038. Subsequent lavas were hypersthene andesites with silica contents reaching 60%, δ18O values up to +7.6, and 87Sr/86Sr of 0.7040 to 0.7043. The later lavas were enriched in Ba, Rb, Li, and K2O and depleted in MgO, Cu, Zn, Cr, Ni, Sr, and Co. The isotopic and other chemical changes, which appeared abruptly over a few months in 1947, are interpreted as the result of tapping a sharply zoned and density stratified magma chamber. Xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70%, δ 18O of +5.6 to 9.9 and 87Sr/86Sr between 0.7043 and 0.7101. In many respects they resemble samples of basement rocks collected from nearby outcrops. Three analysed samples of the latter have silica contents of 65 to 67%, δ 18O of +7.7 to 8.6, and 87Sr/86Sr between 0.7047 and 0.7056. These new data provide strong support for the original interpretations of Wilcox (1954), who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight % continental crust. Except for a few trace elements, particularly Ba, Sr, and Zr, the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late-stage lavas. Some of the discrepancies may be explained by postulating a contaminant that was older and richer in Ba, Sr, and Zr than those represented by the analysed xenoliths. Others can be attributed to chemical changes accompanying disequilibrium partial melting, contact metamorphism, and meteoric-hydrothermal alteration of the country rock. Many of the xenoliths show evidence of having been affected by such processes. The lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption. The order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities.

The Neogene Alert Bay Volcanic Belt of northern Vancouver Island, Canada: Descending-plate-edge volcanism in the arc-trench gap

The Alert Bay Volcanic Belt trends northeasterly across northern Vancouver Island, coincident with the trace of the subducted Juan de Fuca—Explorer plate edge. Volcanism began in the west, at Brooks Peninsula, about 8 Ma ago, but occurred in most centers 3.5 ± 1 Ma ago. There is a suggestion of eastward migration of activity and shift from basalt to dacite or rhyolite with time. Most of the volcanism was coincident with a time of rapid changes in the geometry of subduction, as inferred from offshore magnetic patterns, and with a hiatus in mainland, Cascade volcanic arc activity. Geometry and chronometry suggest this is a descending-plate-edge volcanic belt, where disruption of steady-state plate-consumption patterns triggered magma genesis. Chemically the rocks are quite variable, with divergent fractionation trends. One trend resembles that of Mull (Hebrides), with a plagiophyric basalt of transitional alkaline-subalkaline, mildly tholeiitic, and aluminous character which differentiated to clinopyroxene andesite, and eventually to tholeiitic rhyolite and mildly tholeiitic calc-alkaline dacite, both of K-poor magma type. The other trend is like the Cascades, with aluminous, aphyric, calc-alkaline basalt, hornblende and/or hypersthene andesite, and K-poor dacite. This divergent character is also evident in Ba, Rb, Nb, and Zr fractionation trends. Major- and trace-element discriminant diagrams generally identify the basalts as within-plate types. The 87Sr/ 86Sr isotope ratio is relatively low, averaging 0.70325, and shows no trend with rock type or differentiation series. Oxygen in the entire suite is relatively heavy, δ 18O averaging 7.1%. Even the basalts are 18O enriched. Oxygen shows no trend with degree of hydration, rock type, or series. These isotopic and chemical data are compatible with minor crustal contamination of mafic primary magmas, followed by fractional crystallization under different oxidation and hydration conditions.

Fluorine geochemistry of quaternary volcanic rocks from Southwestern British Columbia: Some petrogenetic implications

Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).

Geology and Petrology of Volcan Ceboruco, Nayarit, Mexico

Volcan Ceboruco is a moderately sized stratovolcanc located in the northwestern part of the Mexican Volcanic Belt. The bulk of the volcano consists of relatively alkali and incompatible element—poor hypersthene andesites erupted prior to the formation of two concentric calderas. These precaldera andesites were followed in the eruptive sequence by the eruption 1,000 yr ago of a white rhyodacite pumice, termed the Jala pumice. The eruption apparently caused the formation of Ceboruco9s outer caldera. A low-silica dacite dome and lava flow were then erupted and partially filled this caldera. These dacites contain xenoliths of high-Al basalt that were apparently partially molten at the time of their inclusion, suggesting that the dacite magma formed as a result of magma mixing involving Jala pumice and basaltic magmas. The dacite dome later collapsed to form the inner caldera, and Ceboruco again erupted andesites. These postcaldera andesites contain both augite and hypersthene and are enriched in alkalis and incompatible elements relative to the precaldera andesites. Finally, between 1870 and 1875, rhyodacite lava flows were erupted during Ceboruco9s only historic activity. Crystal fractionation models using both major and trace elements suggest that the 1870 dacites and the Jala pumice could have been derived from magma with postcaldera andesite composition. To determine the origin of the Ceboruco andesites, trace-element partial-melting models tested three possible sources: subducted oceanic crust in the eclogite facies and the amphibolite facies and mantle peridotite. All models were found unreliable because of the many assumptions involved. It is concluded that a complex process involving partial melting, fractional crystalization, magma mixing, and, perhaps, crustal assimilation was responsible for the generation of the andesites. Such a model cannot be tested, however, because there are at present no viable constraints that can be placed on this complex sequence of events.

大分県日出地域の大分層群

This paper clarifies that the Late Pliocene to Middle Pleistocene formations distribute in the Hiji area, north of Beppu Bay and these formations are correlated to the Takio Formation of the Oita Group in the Oita area, south of Beppu Bay, where is a standard area for the late Cenozoic stratigraphy in Kyushu.The Oita Group in the Hiji area is stratigraphically divided into the Hiji Formation, Oga Formation, Katano Volcanic Rocks and Kanagoe Volcanic Rocks in ascending order. Lower member of the Hiji Formation yields marine molluscan fossils living in coastal water associated with oceanic water and they indicate Late Pliocene to Early Pleistocene. The Hiji Formation consists of hornblende andesitic pyroclastic rock and marine elastic rocks, and the Oga Formation consists of augite hypersthene andesitic pyroclastic rocks and fresh water elastic rocks. The Katano and Kanagoe Volcanic Rocks are hornblende augite hypersthene andesite and augite hypersthene andesite lavas respectively.Change of volcanic activity in the Hiji area coincides with that of the formations corresponding to the Takio Formation in surroundings of the Hiji area. Because the basal part of the Hiji Formation suffers epithermal alteration, age of the Usa Group in the marginal area of Beppu Bay which was correlated to Miocene must be reviewed.

Correlation of Pinnacles and Neenach Volcanic Formations and Their Bearing on San Andreas Fault Problem

The Pinnacles Volcanic Formation (San Benito County, California) is an early Miocene (23.5 m.y.B.P.) sequence of calc-alkaline andesite, dacite, and rhyolite flows interbedded with pyroclastic and volcaniclastic rocks. Structurally the volcanic rocks form a west-dipping homocline which is truncated on the northeast by the Chalone Creek fault. The Chalone Creek fault is interpreted to be an early Miocene trace of the San Andreas fault. The Neenach Volcanic Formation (Los Angeles County, California) is an early Miocene (23.5 m.y.B.P.) sequence of calc-alkaline andesite, dacite, and rhyolite flows interbedded with pyroclastic and volcaniclastic rocks. Structurally the rocks form a northwest-dipping homocline which is broken by many faults. The formation is truncated on the southwest by the San Andreas fault. The Pinnacles Volcanic Formation and the Neenach Volcanic Formation lie respectively west and east of the San Andreas fault zone and are separated 315 km from each other in a right-lateral sense. Ten rock types with nearly identical field, petrographic, and chemical characteristics are present in each formation in essentially the same stratigraphic order: vitric lapilli-tuff, flow-banded rhyolite, perlite, pumice lapilli-tuff, hypocrystalline hypersthene andesite, augite-olivine andesite, andesite tuff, porphyritic dacite, agglomerate, and porphyritic rhyolite. The Pinnacles Volcanic and Neenach Volcanic Formations were contiguous across the San Andreas fault at the time of their formation and subsequently have been offset 315 km along the San Andreas fault. For several reasons the Pinnacles-Neenach correlation provides some of the most conclusive documentation of large-scale, lateral displacement on the San Andreas fault yet discovered in central California: (1) a precise distance of separation can be determined because both areas are directly adjacent to traces of the fault, thus eliminating the necessity of interpretive projection into the fault; (2) the age of the formations can be determined accurately because the rocks are volcanic and suitable for radiometric dating; (3) the correlation is based on 10 different rock types, each with distinctive petrologic features, thus reducing the likelihood of chance occurrence. Other correlations of pre-Miocene rocks along the San Andreas fault show offsets of the same magnitude as the Pinnacles-Neenach rocks suggesting that 315 km may be the maximum offset along the San Andreas fault proper.

Strontium, Rubidium, Potassium, and Calcium Variations in Quaternary Lavas, Crater Lake, Oregon, and Their Residual Glasses

Strontium and rubidium contents of 22 whole-rock specimens and 15 residual glasses from Crater Lake, Oregon, range from 650 to 725 ppm and 23 to 33 ppm, respectively, for the less differentiated hypersthene andesites and basaltic andesites; to 150 to 400 ppm and 50 to 80 ppm for the more salic whole-rock and residual glass specimens. The behavior of these elements largely reflects the separation of large amounts of phenocrystic plagioclase. Sr content closely follows that of calcium. More sodic plagioclase exhibits a greater absolute affinity and a slightly greater relative affinity for Sr than more calcic compositions. K/Rb decreases progressively from about 440 at a K 2 O content of 1.2 weight percent to 330 at 3.4 percent K 2 O as a result of separation of plagioclase with a K/Rb ratio several times that of the associated magmatic liquid. Two factors support the hypothesis that the late dacites and rhyodacites of Mount Mazama were derived by low-pressure crystal fractionation of intermediate magma(s) that are very similar in composition to the much more voluminous andesites of the volcano. They are (1) the coincidence of the whole-rock–residual liquid tie lines, with general variation trends defined by the combined whole-rock and residual liquid data; and (2) the similarity of the coherent interelement variations, with trends inferred from data on elemental distribution between coexisting phenocryst minerals and magmatic liquid. Removal of 45 to 65 percent crystalline material from andesitic magma with 25 to 30 ppm Rb is required to produce the more salic rocks containing 50 to 65 ppm Rb.

ON THE HYPERSTHENE ANDESITE FROM SHODO-SHIMA ISLAND AND AJI PENINSULA, KAGAWA PREFECTURE

Hypersthene andesite, erupted on one of the volcanic centers of the Setouchi Petrographic Province, has been studied petrologically. This rock is shown as the typical calcalkaline andesite by the microscopic observations and the chemical analyses. The phenocrysts are composed of an abundance of labradorite, hypersthene, and augite, a small amount of hornblende and ore minerals. Relic of olivine, surrounded by a thick reaction rim of hypersthene, can be found in thin sections of the SiO2-poor variety sometimes. Orthopyroxene phenocrysts (En61.3-66.3) are hypersthenic in a narrow sense, and are clearly different from those (En73.5-89.1) of the phenocryst-poor andesites (Ujike, 1972) from the same area. Parallel-growth of hypersthene (inside) and augite (outside), which has been considered as a phenomenon caused by magmatic contamination process (Ota, 1958), appears in this andesite. The data obtained by high temperature melting experiments of natural rock compositions have been compiled from literature to show that magmatic chemistry, especially normative Wo/(Wo+En+Fs), would strongly control the order of crystallization of the two pyroxenes. In short, from melt being 100×Wo (Wo+En+Fs)_??_20 by weight, ortho- and clinopyroxenes have likely been to start crystallization simultaneously; from melt having the value smaller than 15, orthopyroxene has crystallized at a higher temperature than clinopyroxene very often. This compilation suggests that the augite must have nucleated at a slightly lower temperature than the hypersthene on the surface of the latter crystal from the hypersthene andesitic magma having low Wo component, and that no contamination process might be needed to explain the parallel-growth of the pyroxenes. This kind of parallel-growth may be found frequently in calc alkaline andesite since Wo content of average andesite is small.

Contributions to the geology and palaeontology of Chiloe Island, southern Chile

During the Royal Society Expedition to southern Chile in 1958 geological observations were made at Chepu and on the San Pedro tableland in the island of Chiloe. The island is composed of schist, Tertiary sediments, fluvioglacial deposits and volcanics. The schist, of uncertain age, forming the basement at Chiloe, is an extension of the schist of the coastal range of southern Chile, and falls in the chlorite zone (greenschist facies). It includes greenschists and altered gabbroic rock as well as normal quartzofeldspathic schist, flat lying at San Pedro but strongly crumpled and folded on the west coast. Tertiary or Quaternary volcanic rocks, of limited distribution, include banded rhyolite, partly spherulitic, and altered vesicular hypersthene andesite. Upper Tertiary sandstone unconformably overlying the schist on the coast south of Chepu contains abundant fossil invertebrates (mainly Mollusca) attributed to the Lower Pliocene. The Chepu assemblage is intermediate in character and apparently in age between the Miocene (Navidad and Ranquil) faunas and the Middle Pliocene (Coquimbo) fauna of Northern and Central Chile, containing persistent Miocene elements together with Pliocene immigrant elements that do not occur together in the better known faunas of more northerly districts. Two new species ofOcenebraand a new subspecies ofAcanthina crassilabrumLamarck are described.

Sediments of Inner Continental Shelf off Northern Washington Coast: ABSTRACT

Much of the inner shelf off Washington is filled with well-sorted, positively to extremely positively skewed, and leptokurtic to extremely leptokurtic sediments having mean sizes between 2-3^phgr and 3-4^phgr. Gravel-rich sediments are common opposite and north of Cape Flattery, and at about depths 14-18 fm and 28 fm opposite Grays Harbor and at 14-28 fm opposite Quinault River. Only rarely are the sediments finer than sand size. These gravel and fine sediments are relatively poorly sorted compared with the other sediments. The pebbles constituting the gravel-rich sediments are smooth and rounded. Thin-section study of the pebbles from opposite Grays Harbor reveals the following rock types: (1) chert, (2) volcanics (many of these being altered and albitized basalt and some of them andesite--a few of these are hypersthene andesites), (3) sandstone, and (4) some granite and quartz diorite. Similar lithologic types are revealed in the gravel-rich sediments north of Cape Flattery. The heavy-mineral study shows that in much of the area the sediments have low percentages of heavy minerals, although opposite Grays Harbor at depths of 10-12 fm and Hoh River at 10-28 fm, heavy minerals are more abundant. The sediments off Cape Flattery, especially in the nearshore areas, have smaller percentages of orthopyroxene nd hornblende than do those farther south. Also, the proportions of hornblende, ilmenite, and magnetite generally are less in the present study area than in the sediments opposite Columbia River. These results suggest that many of the gravel sediments (of the offshore areas), representing the glacial drifts, and the heavy-mineral-rich sediments were deposited at sea levels lower than at present. The drift toward the north of the Columbia-derived sediment appears not to have diluted considerably the sediments supplied from other sources to the areas farther north of Grays Harbor. End_of_Article - Last_Page 461------------

ANDESITE WEATHERING

SummaryUsing the isovolumetric method of analysis, it is observed that all major chemical constituents except ferric iron and lattice water suffer loss during weathering of hypersthene‐ and olivine‐andesite to saprolite (weathered rock) in an acid leaching environment. Si, Ca, K, and Na are removed rapidly during the early stages of weathering of both rocks. Mg loss is delayed somewhat because it is found in both rocks in relatively large hypersthene phenocrysts which tend to resist weathering. Total Fe, Al, and Ti show moderate losses during weathering. During weathering of the two rocks, the loss of Si, Al, and total Fe is relatively greater from the hypersthene andesite. This is believed due to greater leaching intensity and greater acidity of leaching solution at the hypersthene andesite location. In addition, more pronounced alternate wetting and drying, which occurs at the olivine‐andesite location, exert a strong influence in stabilizing Fe and Al.

ANDESITE WEATHERING

SummaryPetrographic and quantitative mineralogical analyses of two andesites and their saprolite (weathered rock) from the Cascade Range in California reveal a mineral weathering sequence in the rocks related to crystal size and composition and to weathering environment. Both the hypersthene andesite and the olivine andesite studied have been subjected to moderate to intensive leaching by acid solutions percolating through the superjacent soil bodies. Although the two parent rocks differ in chemical and mineralogical composition, their weathering to saprolite has followed a similar progression. During early stages of weathering of both rocks, relatively large hypersthene phenocrysts are most resistant. Following in decreasing order of resistance in both cases are plagioclase phenocrysts and finegrained mafic minerals, olivine, and glassy matrix material. Quartz is relatively stable in the earliest weathering stages, but it decreases rapidly with increased weathering. Free iron oxides and clay increase with increased weathering. Amorphous clay dominates the early weathering stages, but as weathering progresses, kaolin increases relative to amorphous clay.

Petrographic and petrochemical studies of MT Egmont andesites

Thin section studies of pebbles and boulders from the Rapanui Formation near Hawera have been made in an attempt to ascertain the principal trends of petrographic variation in Mt Egmont andesites. In addition to the commonly acknowledged hornblende andesites several other varieties of andesite identified included augite andesites completely devoid of hornblende and basaltic andesites containing up to 7% modal olivine. Hypersthene andesites of the type dominating volcanic rocks in the Tongariro National Park area are conspicuously absent. Petrographic and petrochemical relationships of andesites of Mt Egmont indicate that these rocks were formed either by contamination of basaltic magma with sialic materials or by fusion of crustal rocks of dioritic composition. The latter process of origin of andesitic magma is favoured.

Clay Mineralogy of Andesite Saprolite

AbstractClay mineral associations in saprolite of two andesites from the Cascade Range of northeastern California were determined. Sesquioxidic allophane with a high CEC delta value dominates the clay fraction of the least weathered saprolite in each series (47% and 37% in hypersthene andesite and olivine andesite saprolites, respectively). With further weathering, the content of amorphous clay remains high (over 30% in all cases) but the CEC delta value of the clay drops markedly. The amorphous material in the more weathered saprolite has the properties of halloysitic allophane. Halloysite, present in all saprolites, is highest in concentration (over 30%) in the more strongly weathered members of each of the saprolite series. Formation of sesquioxidic allophane during early stages of weathering and its transformation to halloysitic allophane and halloysite during later stages of weathering are supported by X-ray diffraction, electron microscopic, DTA, elemental analysis, and CEC delta value data.

Magma type and tectonic position of the Una-Una Island, Indonesia

On April 29, 1960, 19h. 32m. 13s. G.M.T. an earthquake took place originating SE of the Una-Una volcano, Celebes. In July 1960 an expedition visited the island to record the continuous aftershocks, its volcanology, and petrology. Due to the lack of a vertical component in the seismograph the exact direction of the epicenter could not be determined with certainty, but correlation between the seismograms in Djakarta and those recorded in the island, reveals two values for the epicenter, viz. 00° 21′ S - 121° 38′ E and 00° 15′ S -121° 40′ E with SE-NW direction. The absence of pP waves on the seismograms of Djakarta, Lembang and Medan stations suggests that these are shallow earthquakes. The survey for a new topographic map of the island of Una-Una reveals nine solfatara fields, one fumarole and six secondary, probably phreatic, eruption holes. The recent temperatures of the solfatara fields are somewhat higher than those before the earthquake. The collected samples are biotite andesite vitrophyres, biotite hypersthene andesites, and microdiorites. The difference between the Una-Una rocks, the andesites from other orogenic areas and trachytes from the hinterland volcanoes in Indonesia is clearly demonstrated on a Niggli-Becke projection diagram. The Una-Una rocks show transition between trachytes and andesites. Thep value of the rocks according to Rittmann’s method places them in the medium alkaline series. The petrology and chemistry of the volcanics, geological data from Una-Una and Togean islands, and the bathymetric map of the region suggest that this volcano lies outside the orogenic belt and that it occupies the intersection of two basement fissures of NE-SW and SE-NW directions. Summit observations of the volcano show similar directions for the volcanic fissures. The close relationship between tectonic earthquakes and volcanism was also demonstrated during the activity in 1898 in which the eruption was preceded by tectonic earthquakes. This, plus the presence of several epicenters SE of Una-Una indicate that both volcanic and seismic features may be associated with weak zones c. q. faults. The extinct volcanism in the Togean ridge can be explained by thickening of the earth crust caused by strong and continuous tectonic stress which is active since Tertiary time. The same tectonic condition was also responsible for the en echelon arrangement of the islands and the convexity of the Togean arc toward the hinterland.

On the phenocrystic pigeonite in a somma lava of Usu volcano, Hokkaido

Phenocrysts of pigeonite were found in a pigeonite hypersthene andesite of a somma lava of Usu volcano in the roof mountain part of Showa-Shinzan. The phenocrystic minerals of this lava are bytownite, hypersthene, pigeonite and a little augite surrounded by pigeonite, while groundmass consists of andesine_??_labradorite, pigeonite, magnetite, brown glass and apatite. The occurrence of phenocrystic pigeonite can be divided into the following three types: (1) single crystal, (2) intergrowth with augite (thick rims around augite), (3) inversion from hypersthene. The chemical composition of those pigeonite, inferred from the optical properties, are plotted in Fig. 6. The mode of occurrence and Fig. 6 show that pigeonite phenocrysts are crystallized only in the latest intratelluric stage.

Petrology of Zao volcano

Zao volcano which belongs to Nasu volcanic zone covers an area, about 20km in E-W and about 11km in N-S, on the border of Miyagi and Yamagata Prefectures, northeastern Japan. Zao volcano, as well as Ryuzan volcano on the north and South Zao volcano on the south, are lying on a general trend extending from northwest to southeast. The central part of this volcano is built up of crater rim “Umanose”, central cone “Goshikidake” and small crater lake “Okama” showing a typical structure of double volcano expressed by the successive eruptions, explosions and subsidences. The volcanic prolucts of Zao volcano, consisting of lava flows and pyroclastics, lie upon the eroded surface of Pre-Tertiary granodiorite and the Neogene Green Tuff Formations. Most of lava flows are composed of (olivine bearing) augite hypersthene andesites of calc-alkali rock series, those of tholeiitic rock series are rare. Ten new chemical analyses are made and some petrogenetic problems are discussed.

ON LANDSLIDES AT TARA VOLCANO, NORTHERN KYÛSYU

The flood-disaster that visited Isahaya city, Nagasaki Prefecture on the 25th of July, 1957, was caused by the torrential rain limited to that particular locality. Interrelation among the character of that torrential rain, the geological formation of Tara Volcano and its physical features had an important effect upon the character of the landslide, and the outflow of debris, etc. The characteristics of landslides thet occurred in that area are summarized as follows; (1) Landslides mainly occurred in the areas of the middle and lower parts of Tara Volcano composed of hypersthene andesite and of block mud. (2) Each of the landslides forms a tadpole shape making a jet point its head, and the slided material is generally shallow. (3) The greater part of these landslides broke out by the eruption of infiltrated water out of mountainslopes. These landslides are characteristic of landslip arising from ground water. (4) Relations between the types of slopes and landslides will be summrized as follows; Some landslides broke out the slopes of shallow valley-shaped hollows. Water gushed out of the place where it is convenient for some amount of ground water to get together, which gave rise to landslip. However, the greater part of landslides broke out on a part of even slope where water getting together on the slope is supposed to be small in quantity. (5) Landslides arose from the divergence of physical properties to water between hypersthene andesite and block mud the volcano is composed of. From the genetic point of view, these landslides are classified into the following four groups. (i) Landslides breaking out in the process of eroding the flat surface on the region composed of hypersthene andesitie. (ii) Landslides breaking out on the region composed of block mud or of the andesitic lava flow intermingled with the former. (iii) Old landslides. (iv) Lateral erosion of river flow. (6) Putting together the property of slided material on the slopes, types of slopes showing scar of old landslide, deposit of earthflow along the valley and deposit transported by the flood, the writer is led to the following supposition. In that region the same sort of disastrous landslides in their character and scales occurred repeatedly both in prehistoric and historical ages. And, the above-mentioned disaster caused by the torrential rain can be said to have shown a phase of disaster similar to those repeated before.

THE REGIONAL DIFFERENCE OF THE FLOOD-TYPE SEEN AT THE FLOOD WHICH VISITED THE CITY OF ISAHAYA IN KYUSHU

It has long been well known that types of flood vary greatly according to topographical elements mountains, basins, plains and so forth. Accordingly if we subclassify the topography of those flood-visited areas, we can tell their flood-types. For example, in the case of the plain, most of the plains in Japan are alluvial, formed by sand and gravel transported by the frequent flood-waters. So the micro-relief of the plain and the state of the accumulated sand and gravel show the history of the floods. From this point of view, the writer surveyed the topography of the City of Isahaya and its vicinity by means of aerial photographs and by a field survey. And he has drawn up a topographical survey map of the City of Isahaya and its vicinity showing the classification of flood stricken areas. Furthermore, he surveyed the great flood which visited the City of Isahaya on July 25th, 1957 and has drawn up a map of the City of Isahaya and its vicinity showing the state of flood there on July 25th, 1957. He found that the depth of the stagnate water, its period of stagnation, the direction of the current, the velocity of the current, the erosion and deposit vary remarkably according to its area. In such a case, the following factors—the valley plain, the fan and the delta—have a decisive influence upon deciding the type of flood. As a result of this research, the flood-types in this district were divided into the following six types. (1) The flood-type seen in the valleys situated radially at the foot of the volcano. A lot of land-slides occurred in the upper courses of the valleys made up of the following rocks—hypersthene andesite, lava and block-mud, and situated radially around the extinct crator of Taradake Volcano, and those land-slides have furnished debris to the valley-plains. The distance of the movement of the debris is not so long. (2) The flood-type seen in the valley plains surrounding the foot of the volcano. Here the Honmyo-gawa-river and Suzuta-gawa-river which run around the foot of the volcano have tributaries coming from the northeastern part but have no tributaries from the western and southern part. The velocity of the transmission of the flood-waves in these valleys was very fast and the water reached its highest level in almost all the places of the valley plains. This phenomenon is caused by the following factors: 1) it rained heavily along the valley, 2) the flood-water from the radiate valley plains flowed into the main stream about the same time, 3) the velocity of the transmission of flood-waves was very fast. No stagnation water was seen because the valley plain has steep slopes. The banks were destroyed in many places by the flood water and here and there the streams took short cuts changing their meander courses. The surface soil drifted away with the flood-water and the layer of sand and gravel was exposed here and there, but the distance of the movement of the gravel was very short. (3) The flood-type seen in the fans at the lower part of the canyon. The Honmyo-gawa-river cuts the foot of the volcano in the western part of the City of Isahaya and forms a small canyon there. At the lower part of the canyon where the City of Ishaya lies, the flood was very disastrous and the flood water flowed away radially to the east. The deposited gravel furnished from the canyon, banks and river-bed were seen at the area where banks were destroyed. Next to the deposited gravel was seen accumulated sand, and silt was seen beyond. The velocity of the flood-water was fast and the depth of the flood-water was 3 to 4 meters deep. (4) The flood-type seen in the fan situated at the foot of the volcano. There is a fan facing the coast of Omura Bay to the west of the Taradake Volcano. The fan is composed of the volcanic products when the Taradake volcano went into the eruption in ancient times, and is quite different from those fans formed by the frequent flood water.