Decrease In Silica Content Research Articles

EFFECT OF BASICITY ON COMPLEX METALLURGICAL PROPERTIES OF SINTER

Metallurgical properties of sinter have a significant influence on its basicity. The dependence of cold and hot strength, softening point, reducibility and other physical and chemical properties of sinter on basicity was established by studies of many domestic and foreign scientists. However, taking into account essential increase of iron content in concentrate during decrease in silica content as a result of modernization of production in JSC Stoilensky GOK, the estimation of basicity influence on complex properties of sinter produced with the silica usage is actual. The results of laboratory research at Novolipetsk Iron and Steel Works with the purpose of study of sinter basicity influence on sintering process indexes, strength and high-temperature properties, including filterability of smelting products through the coke baffle are presented. The sinters of different basicity were produced in the laboratory conditions by sintering based on concentrate of Stoilensky GOK with iron content 68,0 %. The basicity of the charge CaO/SiO2 was varied in the range of 1.2‒2.0 units in steps of 0.2 units. In the process of research we selected the optimum rate of solid fuel for each charge from the condition of maximum vertical speed of sintering and sintering strength of sinter. It was found that increasing the basicity of sintering material leads to increasing cold and hot strength, narrowing interval of plastic-viscous state. The maximum vertical sintering rate and maximum specific capacity are reached at basicity 1.6. At basicity of 1.4 the minimum strength of sinter was obtained. It was established, that the sinter reducibility sharply increases at basicity of 1.6 units. With increase in basicity the temperature softening-melting interval (TSMI) of sinter decreases, which characterizes the cohesion zone. The widest TSMI is in sinter with basicity of 1.2, the narrowest – with basicity of 2.0. With increasing basicity of sinter the characteristics of the melt flow through the coke packing increases. The mixture of 55% of sinter with 2.0 basicity and 45% of unfluxed pellets from Stoilensky GOK had the best flow characteristics of the melt products and slag.

Linking magmatic processes and magma chemistry during the post-glacial to recent explosive eruptions of Ubinas volcano (southern Peru)

Understanding the links between the magma differentiation processes, the magma plumbing system and the magma composition at arc volcanoes is of paramount importance for volcanic hazard assessment. In this work we focus on the post-glacial, Holocene, historical, and recent eruption products of Ubinas volcano (Peru), which display an overall decrease in silica content from the older, plinian (VEI 3–5), rhyolitic eruptions (69–71 wt% SiO2) to the historical and recent (2006–2009, 2013–2017), vulcanian (VEI 1–2) basaltic andesitic eruptions (55–57 wt% SiO2). Based on a comprehensive study of the major and trace elements and the Sr-Nd-Pb isotopes, we conclude that this temporal pattern reflects the evolution of the Ubinas magmas in the middle-to-upper crust by a coupled Assimilation-Fractional Crystallization (AFC) process involving a cumulate composed of plagioclase, amphibole, clinopyroxene, orthopyroxene and Fe–Ti oxides, with minor amounts of olivine and biotite at the mafic and felsic end-members, respectively. Upper crustal assimilation is limited to 5–8 vol%, but the overall radiogenic Sr-Nd-Pb signature of the Ubinas magmas requires a larger crustal component, which must therefore occur at middle to lower crustal depths. The petrology of the Ubinas magmas also points to an overall increase in P-T conditions: the large Holocene dacitic and rhyolitic eruptions record temperatures ranging from 800 to 850 °C and pressures in the range of 200–400 MPa, whereas the historical and recent (2006–2009, 2013–2017) basaltic andesitic eruptions provide higher temperatures and pressures (1000 °C, >300–400 MPa). Overall, the thermo-barometry, phase equilibrium and geochemical constraints allow us to propose the existence of a middle-to-upper crust magma column composed of a highly crystalline magma mush containing batches of liquid magma, which seems to be continually recharged from deeper levels. On the basis of the petrological nature of the historical basaltic andesitic eruptions (1667 CE, 2006–2009, 2013–2017), we postulate that during the last centuries, Ubinas experienced a recharge-dominated process, with no evidence for a rejuvenation of the silica-rich reservoir that fed the large Holocene dacitic to rhyolitic eruptions. This study highlights the importance of detailed petrological studies of Holocene sequences at explosive arc volcanoes in order to constrain the magmatic processes and conditions that control large explosive eruptions.

The eruptive chronology of the Yucamane-Calientes compound volcano: A potentially active edifice of the Central Andes (southern Peru)

We reconstruct the eruptive chronology of the Yucamane–Calientes compound volcano in southern Peru based on extensive fieldwork and a large dataset of geochronological (K-Ar, 40Ar/39Ar, U-Pb, and 14C) and geochemical (major and trace element) analyses. This compound volcano is composed of two edifices that have experienced discontinuous volcanic activity from the middle Pleistocene to the Holocene. The Calientes volcano has been constructed in four successive stages: Calientes I is composed of andesitic lava flows dated at ~500 ka. Subsequently, the Callazas ignimbrite (Calientes II stage) was emplaced ~160–190 ka, followed by the main cone-building stage (Calientes III) at ~130–100 ka. Finally, the Holocene Calientes domes were emplaced and represent the last eruptive products of this edifice. The Yucamane volcano has been constructed in three stages: Yucamane I consists of a succession of andesitic lava flows exposed at the base of the volcano that are older than 40 ka. Yucamane II (~36–30 ka) comprises a thick sequence of block-and-ash-flow deposits that represents an episode of dome growth predating the younger Yucamane cone (Yucamane III) built after 20–25 ka. During the Holocene, Yucamane vulcanian to sub-Plinian activity has emplaced tephra-fall and pyroclastic-density-current deposits. The most recent explosive eruptions occurred ca. 3000 BP and emplaced a tephra-fall and pumice-flow deposits. Most samples from Calientes volcano are andesites and dacites (60.1–67.7 wt% SiO2), while rocks from Yucamane volcano are basaltic-andesites to dacites (53.4–66.9 wt% SiO2). The rocks have a mineral assemblage of plagioclase, amphibole, biotite, orthopyroxene, clinopyroxene, olivine, and Fe-Ti oxides. The analyzed samples are categorized within a high-K, calc-alkaline series. Calientes volcano erupted mostly andesitic magmas, but its history is punctuated by rare eruptions of silica-rich magmas. In contrast, Yucamane volcano follows a different trend characterized by a gradual decrease in silica content through post-glacial time, from the large (VEI 3) sub-Plinian andesitic eruption of ~3 ka to moderate (VEI ≤ 2) vulcanian eruptions of basaltic-andesitic. On the basis of such recurrent and recent (Holocene), low-to-moderate explosive activity, Yucamane must be considered an active and potentially threatening volcano, which may affect the province of Candarave with ~8000 inhabitants.

A multilayer hollow nanocarrier for pulmonary co-drug delivery of methotrexate and doxorubicin in the form of dry powder inhalation formulation

Lung cancer is the number one cause of cancer deaths in the global population and remains difficult to treat mainly because of the absence of targeted drug delivery stages and restrictions related with delivery of drugs to deep lung tissues. The aim of the present study was to develop a noninvasive, patient-convenient formulation for the targeted delivery of chemotherapeutics to cancer cells located in the deep lung tissue. A PEGylated paramagnetic hollow-nanosphere was fabricated, fully characterized and used as a nanocarrier for pulmonary co-delivery of Doxorubicin (DOX) and Methotrexate (MTX). In first step, magnetic-silica nanoparticles were synthesized and then coated with amino/acrylate groups and derivatized with polyethylene glycol (PEG). Finally, silica was removed to provide hollow structured nanoparticles. The biocompatibility of blank carriers and the efficiency of MTX-DOX-loaded carriers were also approved by MTT assay and DAPI staining. The aerosolization performance of nanoparticles embedded microspheres were assessed by next generation impactor. The almost natural zeta potential and obvious decrease in silica content observed by EDX and removal of intense silica characteristic peak at FTIR analyses approved the successful removal of silica layer and preparation of hollow-nanospheres. The successful PEGylation of multilayer magnetic hollow-nanospheres catalyzed by boric acid was confirmed by presence of amide groups in FTIR spectra. The developed polymeric nanoparticle showed quite appropriate loading capacity for both DOX and MTX (48%) because of designing adequate negatively and positively charged sites for both drugs. The cytotoxicity assays proved the safety of blank nanoparticles and efficacy of drugs-loaded nanoparticles on lung cancer A549 cell lines. The developed dry powder inhalation formulation of nanoparticles showed appropriate aerosolization performance (fine particle fraction around 22%). It was concluded that these outcomes may open the potentials for efficient pulmonary co-delivery of MTX and DOX to the carcinogenic tissues.

Geochemistry and evolution of the Santiaguito volcanic dome complex, Guatemala

One of the longest continuous periods of dome-forming activity on record is the ongoing eruption of Santiaguito, Guatemala. Dome-building began in 1922, within the crater formed during the cataclysmic 1902 eruption of Santa María, and continues today. The earliest erupted Santiaguito lavas are chemically and mineralogically identical to the dacite pumice erupted in 1902. The erupted magma has become progressively less evolved over time, with a decrease in whole rock SiO2 from ~66 to ~62wt.% from 1922 to 2002. New geochemical data from Santiaguito are consistent with a model of magma being extracted progressively from a chemically-stratified lower crustal (~12 to ~25km) storage zone thought to have formed during the ~25ka of quiescence that preceded the 1902 eruption. The bimodal distribution of plagioclase phenocryst core compositions in early Santiaguito lavas suggests that they were strongly influenced by magma mixing in addition to fractional crystallization. However, lavas erupted since the 1940s show a unimodal distribution, suggesting a diminishing role for mixing within the Santiaguito system. Trace element data suggest that the decrease in silica content over time is best explained by a decrease in the extent of fractional crystallization of ~15% over the 80years covered by our sample set.

Osteoinductive silk–silica composite biomaterials for bone regeneration

Osteoinductive and biodegradable composite biomaterials for bone regeneration were prepared by combining silk fibroin with silica particles. The influence of these composite systems on osteogenesis was evaluated with human mesenchymal stem cells (hMSCs) subjected to osteogenic differentiation. hMSCs adhered, proliferated, and differentiated towards osteogenic lineages on silk/silica films. The addition of the silica to the silk films influenced gene expression leading to upregulation of bone sialoprotein (BSP) and collagen type 1 (Col 1) osteogenic markers. Evidence for early bone formation in the form of collagen fibers and apatite nodules was obtained on the silk/silica films. Collagen fibers were closely associated with apatite deposits and overall collagen content was higher for the silica containing samples. Also, smaller sized silica particles (24 nm–2 μm) with large surface area facilitated silica biodegradation in vitro through particle dissolution, leading to ∼5-fold decrease in silica content over 10 weeks. These results indicate the suitability of silk/silica composite system towards bone regeneration, where degradation/remodeling rates of the organic and inorganic components can be controlled.

Origin and geodynamic evolution of late Cenozoic potassium-rich volcanism in the Isparta area, southwestern Turkey

Post-collisional potassic-rich volcanism of Gölcük Volcano in the Isparta area of southwestern Turkey consists of two groups: (i) extracaldera lavas, corresponding mainly to Pliocene activity; and (ii) intracaldera lavas and pyroclastics (ignimbrite flows and ash/pumice fall deposits) formed during the Quaternary. Extracaldera volcanic rocks mainly comprise lamprophyre (minette), basaltic trachyandesite, trachyandesite, and trachyte. A close relationship exists between the silica content and phenocryst type in the extracaldera volcanics such that trachyte–trachyandesites with SiO2 < 57 wt% and basaltic trachyandesites are characterized mainly by mafic phenocryst phases (e.g. pyroxene, amphibole, biotite–phlogopite). These features suggest suppression of plagioclase crystallization under high H2O pressure conditions. Intracaldera volcanics are composed of tephriphonolitic dikes, remnants of lava flows and domes at the caldera rim, and a trachytic lava dome on the caldera floor. The Gölcük flows and pyroclastics are mainly characterized by strong incompatible element enrichment in large ion lithophile elements (LILEs; e.g. Cs, Ba, U, and Th) relative to K, Rb, and high-field strength elements (e.g. Nb, Ta, and Ti). We conclude that the Gölcük lavas were derived from a metasomatized lithospheric mantle source containing phlogopite–amphibole garnet peridotite; the latter resulted from metasomatism by a hydrous fluid phase related to subducted sediments and oceanic crust. The parental magma for the extracaldera volcanics was lamprophyric, and that for the intracaldera volcanics was basanitic. All the geological and geochemical data show that the alkaline Gölcük lavas display a gradual decrease in silica content with decreasing eruption age, indicating that, in the Isparta volcanic province, the asthenospheric melt component became more important over time. In the extracaldera volcanics, 87Sr/86Sr isotope ratios of the evolved trachyte–trachyandesites range between 0.70366 and 0.70504, whereas these ratios are lower in the less evolved basaltic trachyandesite and lamprophyres, varying in narrow ranges around 0.70365 and between 0.70374 and 0.70453, respectively. The 143Nd/144Nd values lie between 0.51264 and 0.51273 in trachyandesites, 0.51267 and 0.51273 in basaltic trachyandesites, and 0.51270 and 0.51274 in lamprophyres. In the intracaldera lavas, the 87Sr/86Sr isotope ratio is 0.70361 in the tephriphonolite and 0.70388 in the intracaldera trachytic lava dome. The 143Nd/144Nd isotope ratio is 0.51274 in the analysed tephriphonolitic flow and 0.51271 in the intracaldera trachytic lava dome; these values are higher than that of trachyte–trachyandesites of the extracaldera volcanics. The Sr–Pb isotopic signatures indicate that crustal contamination was significant for the evolved extracaldera volcanics, but was negligible for the intracaldera volcanics. The εNd values of the Gölcük volcanics range between 0 and 2.0. The low Sr isotope ratios and positive εNd values are characteristic features of a depleted mantle source. The isotopically depleted and incompatible enriched nature of the Gölcük lavas point to recent enrichment processes prior to partial melting of the mantle source. Conversely, their radiogenic lead isotope compositions (206Pb/204Pb = 19.19–19.54, 207Pb/204Pb = 15.64–15.67, 208Pb/204Pb = 39.12–39.49) indicate an enriched mantle source region.

Neutron irradiation effects on optical and structural properties of silicate glasses

The affect of thermal neutron irradiation on the optical and structural properties of the quaternary silicate glasses of varying compositions has been studied. Experimental results reveal the formation of colour centers in the visible region of the optical spectra. The optical energy gap and the width of the energy tail above the mobility gap have been measured before and after irradiation. The optical energy gap has been found to decrease with increase in neutron fluence. The decrease in the energy gap with the decrease in silica content of the glass is also observed. The change in glass network structure as revealed by the formation of non-bridging silicon–oxygen (Si–O–NBO) groups has been observed and explained in terms of radiation induced structural defects and composition of the glass.

Glass in the submarine section of the HSDP2 drill core, Hilo, Hawaii

The Hawaii Scientific Drilling Project recovered ∼3 km of basalt by coring into the flank of Mauna Kea volcano at Hilo, Hawaii. Rocks recovered from deeper than ∼1 km were deposited below sea level and contain considerable fresh glass. We report electron microprobe analyses of 531 glasses from the submarine section of the core, providing a high‐resolution record of petrogenesis over ca. 200 Kyr of shield building of a Hawaiian volcano. Nearly all the submarine glasses are tholeiitic. SiO2 contents span a significant range but are bimodally distributed, leading to the identification of low‐SiO2 and high‐SiO2 magma series that encompass most samples. The two groups are also generally distinguishable using other major and minor elements and certain isotopic and incompatible trace element ratios. On the basis of distributions of high‐ and low‐SiO2 glasses, the submarine section of the core is divided into four zones. In zone 1 (1079–∼1950 mbsl), most samples are degassed high‐SiO2 hyaloclastites and massive lavas, but there are narrow intervals of low‐SiO2 hyaloclastites. Zone 2 (∼1950–2233 mbsl), a zone of degassed pillows and hyaloclastites, displays a continuous decrease in silica content from bottom to top. In zone 3 (2233–2481 mbsl), nearly all samples are undegassed low‐SiO2 pillows. In zone 4 (2481–3098 mbsl), samples are mostly high‐SiO2 undegassed pillows and degassed hyaloclastites. This zone also contains most of the intrusive units in the core, all of which are undegassed and most of which are low‐SiO2. Phase equilibrium data suggest that parental magmas of the low‐SiO2 suite could be produced by partial melting of fertile peridotite at 30–40 kbar. Although the high‐SiO2 parents could have equilibrated with harzburgite at 15–20 kbar, they could have been produced neither simply by higher degrees of melting of the sources of the low‐SiO2 parents nor by mixing of known dacitic melts of pyroxenite/eclogite with the low‐SiO2 parents. Our hypothesis for the relationship between these magma types is that as the low‐SiO2 magmas ascended from their sources, they interacted chemically and thermally with overlying peridotites, resulting in dissolution of orthopyroxene and clinopyroxene and precipitation of olivine, thereby generating high‐SiO2 magmas. There are glasses with CaO, Al2O3, and SiO2 contents slightly elevated relative to most low‐SiO2 samples; we suggest that these differences reflect involvement of pyroxene‐rich lithologies in the petrogenesis of the CaO‐Al2O3‐enriched glasses. There is also a small group of low‐SiO2 glasses distinguished by elevated K2O and CaO contents; the sources of these samples may have been enriched in slab‐derived fluid/melts. Low‐SiO2 glasses from the top of zone 3 (2233–2280 mbsl) are more alkaline, more fractionated, and incompatible‐element‐enriched relative to other glasses from zone 3. This excursion at the top of zone 3, which is abruptly overlain by more silica‐rich tholeiitic magmas, is reminiscent of the end of Mauna Kea shield building higher in the core.

Etching and annealing studies of nuclear tracks in glasses

A series of etching and annealing experiments are designed to evaluate the characteristics of fission tracks in a number of glasses of varying silica content. A comparative study has been made by comparing the calculated value of bulk etch rate, optimum etch time and annealing rate for each glass. The present investigation suggests that both bulk etch rate ( V G) and annealing rate ( V a) increases with the decrease in silica content of glasses. A minimum value of these parameters is observed for pure silica glass.

Permian deposition in the north central Brooks Range, Alaska: Constraints for tectonic reconstructions

Two opposing tectonic models have been offered to explain the regional structural relations in the north central Brooks Range fold‐thrust belt of northern Alaska. The first suggests that rocks of the northern Endicott Mountains were thrust from south to north over the area of the present Mount Doonerak high and are therefore highly allochthonous. The second implies that the rocks of the northern Endicott Mountains were deposited in a basin that lay north of the Mount Doonerak high and later were thrust a short distance southward onto the northern flank of the high and are thus parautochthonous. To provide stratigraphic constraints for these models, this study examines Permian facies of the north central Brooks Range. Permian rocks in the north central Brooks Range comprise a thin (40 to 160 m thick), fining‐upward succession of clastic, storm‐influenced shelf deposits. When the rocks of the northern Endicott Mountains are restored south of the Mount Doonerak area, a minimum distance of 80 km, the Permian deposits grade systematically from distal facies (Siksikpuk Formation) in the southwest to proximal facies (Echooka Formation) in the northeast. Facies trends in the reconstructed Permian basin include, from southwest to northeast, (1) an increase in carbonate content and corresponding decrease in silica content, (2) a general darkening and thickening of shaley intervals, (3) an increase in proximal features of storm beds, including coarser, thicker, more abundant, and more closely spaced beds, and (4) an increase in abundance and diversity of the faunal assemblage with a corresponding decrease in age. These stratigraphic relations imply that rocks of the northern Endicott Mountains are allochthonous and structurally overlie a proximal stratigraphie succession similar to that exposed in the Mount Doonerak area and northeastern Brooks Range.

Seismic velocity structure and composition of the continental crust: A global view

Seismic techniques provide the highest‐resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high‐pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5‐km depth intervals, both in the form of histograms and as an average velocity‐depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5‐km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low‐grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

Effects of silica level on some properties of Oryza sativa straw and hull

Reduction of silica content of two rice varieties obtained by growth in hydroponic solutions (0–400 ppm added SiO2) increased leaf openness and decreased both stem bending hardness and hull weight. The number of silica bodies in the surface of leaf blades was similarly decreased. Mapping by energy dispersive X-ray analysis techniques showed high concentrations of silicon in silica bodies inside silica cells on leaf blades and uniform distribution on lemma and palea surfaces. Vascular tissues and bulliform cells were also rich in silica. Decrease in silica content to less than 10% dry weight increased the in vivo dry matter digestibility of straw in both varieties and increased the in vitro organic matter digestibility of IR36 straw.

Analcime from the L. Lias on the W. Somerset coast

SummaryAnalcime, occurring both as small, pale-brown trapezohedral crystals and pale-brown slaggy masses, is reported growing into and replacing aragonitic shell debris in a heavily faulted, 15-km long, coastal exposure of L. Jurassic marine sediments. Associated minerals include authigenic baryte, sphalerite, pyrite (some framboidal), celestine, aragonite, calcite, dolomite, quartz, and solid hydrocarbon. No other zeolites have been found. Its modes of occurrence, crystal morphology, and relationships to these minerals are described. The analcime appears to be restricted in occurrence to the W. Somerset coast. Partial chemical analysis of one sample is given. The brown colouration is suggested to be due to organic compounds. XRD analysis of five samples reveals progressive decrease in silica content towards western extremity of the exposure. No evidence of volcanic ash contamination of the sediments is found. A hydrothermal origin is discounted; formation of the analcime by purely diagenetic processes is suggested.

Distribution, age and chemical composition of tephra layers in deep-sea sediments off western Indonesia

Tephra layers occur in deep-sea sediments of the northeastern Indian Ocean, adjacent to western Indonesian are. The layers range in age from Recent to Late Miocene. Relative abundance of light and heavy mineral species in all tephra layers have been determined, and pure glass shards from representative samples have been analyzed chemically for major oxides. On the basis of the chemical data, three distinct provinces can be recognized: (1) an extensive province of rhyolitic tephra layers, ranging in age back to Late Miocene, is found adjacent to Sumatra; (2) a more restricted province of dacitic layers, adjacent to Sunda Strait and western Java; and (3) a province of andesitic layers, found adjacent to eastern Java and the Lesser Sunda Islands. Chemical composition of tephra layers in each province remains constant with time. As an example, tephra layers from the rhyolitic province are characterized by a high and restricted range of SiO 2 (75–77%) when expressed on an H 2O-free basis. Tephra layers recovered from the study area were examined for chemical evidence of secondary alteration. The analyses revealed that H 2O is the only major oxide in the glass shards which increases progressively with the age of the tephra layers regardless of the bulk composition. H 2O, however, reaches a “saturation point” of 4–5% in the layers 250–400 thousands of years old and remains constant to the oldest recovered tephra layer (7.5 m.y. old). The decrease in silica content in deep-sea tephra layers eastward along the Indonesian volcanic arc coincides with a similar eastward decrease in average silica content in Indonesian lavas. A relatively high silica content in lavas from Sumatra, with associated ignimbrites and their deep-sea ash-fall equivalents is closely linked to thick pre-Cenozoic crust. In the portion of the arc to the east of Sumatra, the crust is Cenozoic and thin. Difference in silica content of both the lavas and deep-sea tephras along the Indonesian arc is considered in regard to the hypothesis of “magma filtering” which is based on the contrasting density gradients of ascending magma and the upper crust.