Pumice-rich Layers Research Articles

Precise dating of large flank collapses by single-grain 40Ar/39Ar on pyroclastic deposits from the example of Flores Island (Azores)

Large-scale flank collapses are one of the main hazards associated with the evolution of volcanic islands. Precisely dating such events is critical to evaluate the frequency of destabilization episodes and further assess the triggering mechanism(s) associated with internal and/or external factors, such as volcano dynamics, regional tectonics, and global paleoclimatic changes. Here, we constrain the age of a pumice-rich pyroclastic deposit exposed on the eastern flank of Flores Island (Azores), which we interpret as a co-blast deposit generated by a major flank collapse that destroyed the whole western flank of the former volcanic edifice. Twelve single-grain 40Ar/39Ar analyses, performed on 250–500 µm anorthoclase feldspars (mean K/Ca close to 5) with our high-sensitivity multi-collector NGX mass spectrometer, provide a robust weighted mean age of 1.32 ± 0.01 Ma for this eruption. This new age is consistent with previous K/Ar data bracketing the flank collapse between 1.30 ± 0.04 and 1.18 ± 0.09 Ma, and indicates that this event occurred at the end of the main construction phase of the volcano. The explosion produced pumice-rich layers preceded by a lahar as attested by a polygenetic mudflow deposit underlying the dated deposit. From the geochemistry of lavas erupted just before and after the collapse, we speculate upon the possible role of magmatic processes on flank destabilization. We propose a first hypothesis where differentiation in a shallow magma reservoir could have favored edifice inflation, ground shaking, and flank failure, triggering a decompression-induced violent eruption. Overall, our study shows that high-sensitivity mass spectrometers have now reached analytical performances allowing to measure precisely and accurately ages on relatively small and moderately K-rich single feldspars, which is of the utmost importance for dating heterogeneous blasts and tephra deposits that may have been induced by large-scale flank collapses during the late Quaternary.

A first reconstruction of the evolution of Cubilche Volcanic Complex, Imbabura Province, Ecuador

Cubilche Volcanic Complex (Imbabura Province, Ecuador) includes three morphologically well-preserved eruptive centers: in increasing size, Cunrru Dome Complex (3300 m above sea level, m asl), Panga Ladera (3420 m asl) and Cubilche (3836 m asl). As this complex is poorly known, we conducted a volcanological study of its evolution by combining geomorphological, lithostratigraphic, radiocarbon (14C), and petrological analyses, along with eruption size determinations. Cunrru Dome Complex shows an eastward-opening scarp formed during a dome collapse event that emplaced a small-volume debris avalanche deposit and a pumice-rich layer from an accompanying directed explosion. The higher Panga Ladera edifice is breached towards the north, and the emplacement of the resulting debris avalanche deposit was also accompanied by a blast-forming eruption of ash and pumice. The post-collapse vent subsequently produced dilute pyroclastic density currents (PDCs) that built a low cone. The Cubilche eruptive center comprises old and young edifices. The Old Cubilche edifice emitted lavas from a summit crater and flank vents before being partly destroyed during a flank failure event associated with an explosive eruption that produced a widespread pyroclastic density current deposit dated at ca 44 ka and an associated pyroclastic fall deposit. The Young Cubilche cone grew inside the Old Cubilche avalanche crater during ~14–19 ka over five eruptive episodes that included early effusive eruptions, a dome growth period that terminated with a PDC that emplaced scoria flow deposits at ca 29.6 ka, an explosive eruption at ca 26.3 ka, and final Strombolian activity emitting more mafic products. As the adjacent Imbabura Volcanic Complex erupted in early Holocene times and the two complexes seem to share common reservoirs and plumbing systems, we cannot fully discard a possible reactivation at Cubilche. Other more short-term hazards are debris flows and landslides potentially triggered by heavy rain fall and/or seismic events.

Fluidized landslides triggered by the liquefaction of subsurface volcanic deposits during the 2018 Iburi\u2013Tobu earthquake, Hokkaido

The 6.6 Mw Iburi–Tobu earthquake struck southern Hokkaido, Japan on 6 September 2018. The earthquake triggered widespread slope collapses in the hills near the epicenter, resulting in destructive landslides that killed 36 people. Volcanic deposits covering the region slid downhill in a flow-like manner suggestive of fluidized landslides. Here, we report a distinctive example of liquefaction in the field, which could be a prerequisite for the generation of fluidized landslides triggered by large earthquakes. In the scarp of a typical landslide, an altered halloysite-bearing volcanic layer is observed at a level almost coincident with the sliding surface. The layer is intensely undulating and can be divided into an upper clay-rich layer and a lower pumice-rich layer, suggesting that the altered layer had liquefied as a result of the strong coseismic ground motion. The layer had been soaked by heavy rainfall just one day before the earthquake and could have liquefied, producing a weak and slippery plane, resulting in the catastrophic landslides in this area.

Sedimentation and welding processes of dilute pyroclastic density currents and fallout during a large-scale silicic eruption, Kikai caldera, Japan

Sedimentation and welding processes of the high temperature dilute pyroclastic density currents and fallout erupted at 7.3 ka from the Kikai caldera are discussed based on the stratigraphy, texture, lithofacies characteristics, and components of the resulting deposits. The welded eruptive deposits, Unit B, were produced during the column collapse phase, following a large plinian eruption and preceding an ignimbrite eruption, and can be divided into two subunits, Units B l and B u. Unit B l is primarily deposited in topographic depressions on proximal islands, and consists of multiple thin (< 1 m) flow units with stratified and cross-stratified facies with various degrees of welding. Each thin unit appears as a single aggradational unit, composed of a lower lithic-rich layer or pod and an upper welded pumice-rich layer. Lithic-rich parts are fines-depleted and are composed of altered country rock, fresh andesite lava, obsidian clasts with chilled margins, and boulders. The overlying Unit B u shows densely welded stratified facies, composed of alternating lithic-rich and pumice-rich layers. The layers mantle lower units and are sometimes viscously deformed by ballistics. The sedimentary characteristics of Unit B l such as welded stratified or cross-stratified facies indicate that high temperature dilute pyroclastic density currents were repeatedly generated from limited magma-water interactions. It is thought that dense brittle particles were segregated in a turbulent current and were immediately buried by deposition of hot, lighter pumice-rich particles, and that this process repeated many times. It is also suggested that the depositional temperature of eruptive materials was high and the eruptive style changed from a normal plinian eruption, through surge-generating explosions (Unit B l), into an agglutinate-dominated fallout eruption (Unit B u). On the basis of field data, welded pyroclastic surge deposits could be produced only under specific conditions, such as (1) rapid accumulation of pyroclastic particles sufficiently hot to weld instantaneously upon deposition, and (2) elastic particles' interactions with substrate deformation. These physical conditions may be achieved within high temperature and highly energetic pyroclastic density currents produced by large-scale explosive eruptions.

On the occurrence of a pumice-rich layer in Holocene deposits of western Peloponnesus, Ionian Sea, Greece. A geomorphological and geochemical approach

AbstractA single, pumice-rich sandy horizon located in Holocene deposits of western Peloponnesus, Ionian Sea, Greece has been newly detected in a littoral belt 250 m wide and more than 3km long. Pumice fragments are hosted in siliceous-cherty sand that overlies coarser clastic sediments, and occur in varying sizes. The geomorphology of the area and the development of two dune systems played an important role in the entrapment of the pumice fragments. These were transported there by the wind and marine currents, rather than by a tsunami event. The chemistry of the pumice fragments is constistent throughout the deposit. Major and trace element analysis of the pumice suggests an origin in the south Aegean Volcanic Arc, rather than in southern Italy and surroundings. The age of this deposition is thought to be younger than 4,000 years before present.

Spatiotemporal evolution of a marine caldera-forming eruption, generating a low-aspect ratio pyroclastic flow, 7.3 ka, Kikai caldera, Japan: Implication from near-vent eruptive deposits

The VEI 7, 7.3 ka caldera-forming eruption of the Kikai caldera occurred in a shallow sea, and caused devastating damage to the prehistoric human settlements of southern Kyushu, Japan. This Holocene activity at this volcano records eruptions with compositional range of 56 to 74 wt.% in SiO 2 spanning with the climactic eruption. In this study, stratigraphy, component, and lithology of the pyroclastic deposits were analyzed at near-vent islands (Satsuma Iwo-jima and Take-shima) in order to reconstruct this eruption. Stratigraphic sections are characterized by plinian pumice-fall deposits (Unit A), intraplinian flow deposits (Unit B), climactic pyroclastic flow deposits (Units C1–C3), and co-ignimbrite ash-fall deposits (Unit D). In total the estimated magma volume in the system was 70–80 km 3 (DRE) and the eruption therefore represents the evacuation of a major silicic magmatic system. The plinian stage (Phase 1) is subdivided into an initial small phase and a second large one. The column height in the second phase was estimated to be 40–43 km. The total tephra volume of this stage was estimated to be 40 km 3. The magma discharge rate has been calculated from the column height data to be 2 × 10 8 kg/s. The eruption duration is also estimated to be a minimum of approximately 28 h. Collapse of the column (Phase 2) produced Unit B, which consists of multiple thin lithic-rich or pumice-rich layers or pods, including welded pumice-fall layers. The deposits are characterized by stratified or cross-stratified facies and display various degree of welding. These sedimentary characteristics indicate that, during the plinian column collapse, high temperature turbulent density currents were generated where dense pyroclasts were well segregated, resulting in the lithic-rich layers or pods. Phase 3 is characterized by Units C1–C3. Unit C1 shows non-welded stratified facies, which consist of lithic and crystals, including quenched juvenile materials as a minor constituent. Unit C2 displays welded stratified facies, which consist of lithic-rich layers and pumice-rich layers. These two subunits occur only in topographic lows in Satsuma Iwo-jima. Unit C3 is thickest and poorly-sorted non-welded massive deposit, which includes fragments of welded tuff from underlying units in proximal regions. These facts indicate that multiple pyroclastic density currents produced Units C1 and C2 in the near-vent area, and were followed by the main sustained current producing Unit C3, a low-aspect ratio ignimbrite, distributed over a wide area of southern Kyushu across the sea. Varying extents of magma–water interactions started during Phase 2, continuing during the early stages of Phase 3, and diminished during the climactic C3 ignimbrite stage. In addition, collapse of the caldera may have started before Unit C deposition, based on the evidence of a fault overlain by Unit C on the caldera rim. The collapse may have initiated water access to the magma. The source appears to have been biased toward the western side of the caldera. The Holocene evolution of the Kikai volcano records the existence of a large silicic magma system at depths of about 7 km that coexisted with or was regularly recharged with mafic magma.

Industrial mineral occurrences associated with Cenozoic volcanic rocks of Sardinia (Italy): Geological, mineralogical, geochemical features and genetic implications

The most important Industrial Mineral (IM) deposits associated with the Cenozoic volcanics of Sardinia include kaolin, bentonite, and zeolites. Though of less significance, perlite and potassium feldspar, are also important in local and national markets. Bentonite, and to a lesser extent kaolin and perlite, form economically viable deposits and zeolites appear as very important industrial mineral commodities for future economic development. The market situation for potassium feldspar is still under evaluation for use in ceramics. All these deposit types occur as alteration products of Cenozoic volcanic rocks, which were emplaced during three main volcanic cycles: in the Eocene (EC, 55 to 51 Ma), Oligocene–Miocene (OMC, 32 to 13 Ma), and Pliocene–Pleistocene (PPC, 5 to 0.1 Ma). Kaolin, bentonite, zeolites, and potassium feldspar occur as alteration products of the OMC volcanites; perlite occurs as an alteration product of the PPC volcanites. All deposits have generally derived from the alteration of the most acidic members of the Cenozoic volcanic rocks (rhyolites to rhyodacites), but differences among the associated ore-forming phenomena are recognized. Hydrothermal sensu stricto, deuteric alteration and weathering are the most important processes involved in ore formation. Weathering, although of minor importance, can also contribute to enhance the alteration of the deposits formed by hydrothermal and deuteric processes. The extent and type of occurrences are mainly controlled by the local geological and structural features, and the environmental geochemical conditions. Kaolin occurs in three main districts: at Romana and Tresnuraghes it is hosted within ignimbritic sequences, while at Serrenti– Furtei it is found in pyroclastic flows and ash-flow tuffs. Kaolin formed by hydrothermal processes occurs in high-sulfidation ( Romana and Serrenti– Furtei) and low-sulfidation ( Tresnuraghes) type epithermal systems. The alteration is proximal to the feeder conduits and structural setting controls kaolin formation. Bentonite is widespread throughout Sardinia. Deposits occur as stratiform mineral bodies (in situ-altered) within poor welded pumice-rich and ash-rich layers and also as distal fall-out deposits, as lenticular bodies interlayered within the Miocene sedimentary sequences. The former originated by deuteric alteration and formation is mainly controlled by lithology, whereas the second type originated by weathering, which altered the volcanic ash-lenses deposited in lacustrine and/or fluvial–deltaic environments. Zeolites, mainly clinoptilolite and mordenite, are widespread in northern and central Sardinia, and deposits are distal from the vents. Zeolites form in situ-altered and epiclastic mineralization. The former shows higher zeolite contents than the latter, and zeolitization phenomena pervasively affect the ash-rich and pumice-rich layers. Principally deuteric processes gave rise to phreato-magmatic deposit-types and deposits formed in open hydrologic systems. Epiclastic mineralization originated from weathering of the volcanic ash-products. The perlite of Monte Arci (central-southern Sardinia) is associated with the PPC, which consists of a subalkaline sequence (from subalkaline basalts to rhyolitic lavas). Obsidian rhyolitic lavas, affected by weathering, host the mineralization. The weathering process involves complex interactions between non-hydrated volcanic glasses (obsidian) and meteoric water, which was heated circulating below the paleo-surface and formed hydrated glasses (perlite). Potassium feldspar mineralization is associated with low-sulfidation type epithermal systems, and was originated by hydrothermal phenomena sensu stricto. The K-feldspar-rich bodies occur preferentially within the most acidic members (rhyolitic–rhyodacitic pyroclastites) of the volcanic sequence.

Pumice layers in marine terraces: implications for tectonic uplift rates on the east and northeast coasts of Taiwan over the last hundreds of years

Two very rare pumice-rich layers were recently found at two sites in Taiwan, more precisely in the lowermost marine terrace of the Sanhsientai area on the southeast coast, and in that of the Yenliao area, on the northeast coast. The pumice-rich layer in the former is about 15 cm in thickness and is composed of 60–70% gray to dark gray, subangular to angular pumice fragments with barnacle shell fragments. The layer in the latter, on the other hand, is about 10 cm in thickness and is composed of 40–50% light gray to gray, subrounded to subangular pumice fragments with a few carbonized wood fragments. Despite their different occurrences, both pumice fragments very closely resemble each other in terms of major and trace as well as isotopic compositions and, therefore, may be considered to have come from the same source, the Luzon or Philippine arcs. 14C-dating gives the layers an equal age ranging from around 420 to 402 cal. yr BP. Based on the age and altitude of the marine terraces in the Sanhsientai and Yenliao areas, however, the uplift rates are 10.9 and 5.4 mm/yr, respectively. The results strongly indicate that, firstly, the uplift rates may have accelerated in the last thousands of years during the Holocene in the Sanhsientai–Chengkung area and that, secondly, the northern coastal area of Taiwan likely suffered from a spasmodic uplift in the very latest Holocene.

Tephra layers as correlation tools of Neogene coal-bearing strata from the Kenai lowland, Alaska

Research Article| March 01, 1995 Tephra layers as correlation tools of Neogene coal-bearing strata from the Kenai lowland, Alaska Linda M. Reinink-Smith Linda M. Reinink-Smith 1Mineral Industry Research Laboratory and Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, Alaska 99775 Search for other works by this author on: GSW Google Scholar Author and Article Information Linda M. Reinink-Smith 1Mineral Industry Research Laboratory and Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, Alaska 99775 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1995) 107 (3): 340–353. https://doi.org/10.1130/0016-7606(1995)107<0340:TLACTO>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 Linda M. Reinink-Smith; Tephra layers as correlation tools of Neogene coal-bearing strata from the Kenai lowland, Alaska. GSA Bulletin 1995;; 107 (3): 340–353. doi: https://doi.org/10.1130/0016-7606(1995)107<0340:TLACTO>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 Thirty-two tephra layers, exposed in coal beds of the Miocene and Pliocene Beluga and Sterling Formations along the shores of the Kenai lowland on the northwestern Kenai Peninsula, Alaska, were studied in detail to improve the geochronology and regional correlation of the Sterling Formation and test prior correlations that were based on palynology and physical tracing of beds over short distances. Published radiogenic isotope data suggest an age span of ∼4 m.y. for the Sterling Formation at this location but give discordant ages for individual samples depending on dating techniques.A crystal-rich tephra layer near the middle of the section was traced across the Kenai lowland as one or two ash falls based on stratigraphic position, inertinite contents of adjacent coal, geochemical and mineralogical analyses, and individual characteristics. A pumice-rich layer deposited near the top of the Sterling Formation is preserved at two localities on the northwestern and southeastern sides of the Kenai lowland. Geochemical similarities, similar glass morphologies, and an absence of opaque phases characterize this layer as a single ash fall and allow correlation. On a regional scale, these correlations concur with previous correlations and show that a shallow anticline with a northwest-southeast–trending axis extends across the Kenai lowland. 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.