Geology of a complex kimberlite pipe (K2 pipe,Venetia Mine,South Africa): insights into conduit processes during explosive ultrabasic eruptions

K2 is a steep-sided kimberlite pipe with a complex internal geology. Geological mapping, logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes, lithic contents and internal structures. The pipe comprises two geologically distinct parts: K2 West is a layered sequence of juvenile- and lithic-rich breccias, which dip 20–45° inwards, and K2 East consists of a steep-sided pipe-like body filled with massive volcaniclastic kimberlite nested within the K2 pipe. The layered sequence in K2 West is present to > 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass-wasting breccias generated by rock fall and sector collapse of the pipe walls: both processes occurred in tandem during the infill of the pipe. Several breccia lobes extend across the pipe and are truncated by the steep contact with K2 East. Dense pyroclastic rocks within the layered sequence are interpreted as welded deposits. K2 East represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption. This phase may have involved fluidisation of trapped pyroclasts, with loss of fine particles and comminution of coarse clasts. We conclude that the K2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement, followed by alternating phases of accumulation and ejection.     

Swelling of a lava plug associated with a Vulcanian eruption at Sakurajima Volcano,Japan, as revealed by infrasound record: case study of the eruption on January 2, 2007

In order to clarify the time relation of the expansion of a gas pocket and failure of its overlying plug of lava during Vulcanian eruptions, infrasound records and video images of the Vulcanian eruption that occurred at Sakurajima volcano on January 2, 2007 were analyzed with respect to their origin times. Weak (≤3 Pa) and slowly increasing air pressure preceded the impulsive compression phase by 0.25–0.32 s, and a longer-period rarefaction phase of infrasound waves was recognized at all microphone stations. The velocity of the compression phase was assumed to be supersonic (ca. 400 m/s) up to 850 m above the crater bottom from other recent explosions. On the other hand, the propagation velocity of the preceding weak signal was regarded to be similar to the air sound velocity because the lack of impulsiveness is unlikely to be related to the main compression phase. Therefore, the estimated origin time of the main compression phase was delayed by 0.5–0.7 s from the preceding phase. The origin time of the preceding phase coincided with the onset of the isotropic expansion process of the pressurized gas pocket, which was obtained by the waveform inversion of the explosion earthquake. In contrast, the origin time of the main impulsive phase coincided with the time when the expansion rate reached its peak. This observation suggests that the volumetric increase of the gas pocket caused swelling of the surface of the crater bottom and its subsequent failure. When the expansion velocity exceeded a threshold level, the main impulsive compression phase radiated with a high velocity by the sudden releases of the pressurized gases. The volumetric change at the source was estimated to be 280–560 m³ from the preceding phase of the infrasound. This volume change indicates that the vertical displacement of the swelling ground was on the order of 1.0 m, assuming the radius of the lava plug was ca. 10 m.     

Sulfur,chlorine, and fluorine degassing and atmospheric loading by the 1783–1784 AD Laki (Skaftár Fires) eruption in Iceland

 The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have generated ∼250 Mt of H₂SO₄ aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements. Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and 15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years. The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of aerosols estimated by other methods. Received: 30 May 1995 / Accepted: 19 April 1996     

Seismic activity related to the 2000 eruption of the Hekla volcano, Iceland

The 2000 Hekla eruption took place from February 26 to March 8. Its seismic expressions were a swarm of numerous small earthquakes related to its onset, and low-frequency volcanic tremor that continued throughout the eruption. A swarm of small earthquakes was observed some 80 min before the onset of the eruption, and the size of the events increased with time. Low-frequency volcanic tremor, with a characteristic frequency band of 0.5–1.5 Hz and dominant spectral peak(s) at 0.7–0.9 Hz, became visible at 18:19 GMT on February 26, marking the onset of the eruption. The tremor amplitude rose quickly and was very high in the beginning of the eruption. However, it soon began to decrease after about an hour. In general, the seismic activity related to the 2000 Hekla eruption was very similar to what was observed in the previous eruption in 1991. Based on knowledge gained from seismicity and strain observations from 1991, this was the first time that a Hekla eruption was predicted.Editorial responsibility: J Stix     

Progressive assembly of a massive layer of ignimbrite with a normal-to-reverse compositional zoning: the Zaragoza ignimbrite of central Mexico

The Zaragoza ignimbrite and two enclosing rhyodacite pumice fall layers were emplaced during the 15 km³ (DRE), ∼0.1 Ma Zaragoza eruption from Los Humeros volcanic centre, 180 km east of Mexico City. The ignimbrite comprises several massive flow-units, the largest of which locally exceeds 20 m in thickness and is regionally traceable. It comprises massive lapilli-ash with vertical elutriation pipes, and has a fine-grained inverse-graded base and a pumice concentration zone at the top. It also exhibits an unusual gradational ‘double’ vertical compositional zonation that is widely traceable. A basal rhyodacitic (67.6–69 wt% SiO₂) zone grades up via a mixed zone into a central andesitic (58–62 wt% SiO₂) zone, which, in turn, grades up into an upper rhyodacitic (67.6–69 wt% SiO₂) zone. Zoning is also defined by vertical variations in lithic clast populations. We infer that pyroclastic fountaining fed initially rhyodacite pumice clasts to a sustained granular fluid-based pyroclastic density current. The composition of the pumice clasts supplied to the current then gradually changed, first to andesite and then back to rhyodacite. Inverse grading at the base of the massive layer may reflect initial waxing flow competence. The pumice concentration at the top of the massive layer is entirely rhyodacitic and was probably deposited during waning stages of the current, when the supply of andesitic pumice clasts had ceased. The return to rhyodacitic composition may have been the result of eruption-conduit modification during collapse of Los Potreros caldera, marked in the ignimbrite by a widespread influx of hydrothermally altered lithic blocks, and/or a decrease in draw-up depth from a compositionally stratified magma chamber as the eruptive mass flux waned. The massive layer of ignimbrite thins locally to less than 2 m, yet it still shows the double zonation. Correlation of the zoning suggests that the thin massive layer is stratigraphically condensed, and aggraded relatively slowly during the same time interval as did the much thicker (≤50 m) massive layer.Editorial responsibility: J McPhie     

Multi-stage volcanic events: A statistical investigation

Volcanic events comprising multiple eruptive stages are common in the historical and geological record and display activity of variable intensity ranging, in some cases, through to several centuries. To better understand the characteristics of such events globally, this study explores a database of historical events having Volcanic Explosivity Indices 4 or greater. The database was compiled for the most part from published and unpublished material provided by the Smithsonian Institution. The database was examined to find common statistical relationships, within and between the variables characterising multi-stage explosive events.     

1750-2010年强火山喷发事件的时空分布特征(英文)

Using the dataset provided by the Smithsonian Institution's Global Volcanism Program, we have extracted the large volcanic eruptions(volcanic explosivity index ≥ 4) from the period 1750–2010 and have then analyzed the main characteristics of large volcanic eruptions since 1750 according to their geographic latitudes, their elevations, and the years and months in which they occurred. The results show that most large volcanic eruptions were located around the margins of the Pacific Ocean and the islands of Sumatra and Java, especially in the equatorial regions(10°N–10°S). Large volcanic eruptions were concentrated at 1000–2000 m elevations and in the months of January and April. There were more eruptions in the summer half-year(from April to September) than in the winter half-year(from October to the next March). Large volcanic eruptions have interdecadal fluctuations, including cycles of 15–25 years and 35–50 years, which were detected by Morlet wavelet analysis, with the fluctuations being more frequent after 1870 than before. During the periods 1750–1760, 1776–1795, 1811–1830, 1871–1890, 1911–1920 and 1981–1995, there were relatively many large volcanic eruptions.     

长白山天池火山气象站期喷发类型研究

全面系统地了解一座活火山的喷发特征与历史,对于预测其未来喷发形式及灾害性具有重要意义.长白山天池火山晚更新世或全新世以来存在3期大规模的喷发活动,天文峰期喷发与千年大喷发皆为典型的大规模的布里尼式爆炸喷发,而气象站期喷发方式却在爆炸式或溢流式的喷发类型上存在着激烈的争论.气象站期喷发物在遥感影像上表现为以气象站为寄生火口,向北流动长约5.4km,形貌上似一条熔岩流,但文中研究认为是小规模的脉冲式爆炸喷发而形成的岩层形态.主要依据为:1)岩石成分为高黏度的酸性岩浆;2)野外露头与剖面的薄层状堆积特征;3)晶屑的棱角状破碎形态;4)岩石的颗粒支撑结构.研究结果表明,气象站期的堆积物是爆炸喷发产生的碎屑物沿山坡以火山灰流的形式快速流动,在高温与高流速的剪切力作用下,火山灰流形成1套薄层状的熔结凝灰岩.因此,长白山天池火山晚更新世或全新世以来3期喷发活动均为爆炸式喷发,而非以前认为的爆炸-溢流-爆炸式喷发.     

辽西阜新地区义县组的喷发旋回特征

通过581.64km的路线地质调查和对20.16km的实测剖面的研究,将辽西阜新地区义县组自下而上划分为喷发旋回Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ;每个喷发旋回的下部为火山碎屑岩组成的爆发相、中上部为熔岩组成的溢流相;熔岩类型主要是玄武岩、玄武安山岩、安山岩及部分英安岩、流纹岩、珍珠岩、黑曜岩、粗安岩;各喷发旋回内部及不同喷发旋回中,具有熔岩的酸性或碱性程度向上逐渐增强的总体变化规律。义县组在紫都台—七家子地区较老河土—十家子、后新秋—苇子沟地区的火山碎屑岩的总厚度偏大,而玄武岩的发育厚度偏小,且老河土—十家子地区缺乏喷发旋回Ⅴ等,空间分布有差异性。     

Shallow submarine volcano group in the early stage of island arc development: Geology and petrology of small islands south off Hahajima main island,the Ogasawara Islands

Small Islands south off Hahajima, the southernmost of the Ogasawara Archipelago, consist of primitive basalts (<12 wt.% MgO) to dacite erupted during the transitional stage immediately following boninite volcanism on the incipient arc to sustained typical oceanic arc. Strombolian to Hawaiian fissure eruptions occurring on independent volcanic centers for the individual islands under a shallow sea produced magnesian basalt to dacite fall-out tephras, hyaloclastite and a small volume of pillow lava, which were intruded by NE-trending dikes. These volcanic strata are correlated to the upper part (<40 Ma) of the Hahajima main island. Volcanic rock samples have slightly lower FeO^*/MgO ratios than the present volcanic front lavas, and are divided into three types with high, medium and low La/Yb ratios. Basalt to dacite of high- and medium-La/Yb types show both tholeiitic (TH) and calc-alkaline (CA) differentiation trends. Low-La/Yb type belongs only to TH basalt. The multiple magma types are coexistence on the each island. TH basalts have phenocrysts of olivine, clinopyroxene and plagioclase, while CA basalts are free from plagioclase phenocrysts.