The deposits of Plinian and subplinian eruptions provide critical insights into past volcanic events and inform numerical models that aim to mitigate against future hazards. However, pyroclastic deposits are often considered from either a fallout or pyroclastic density current (PDC) perspective, with little attention given to facies exhibiting characteristics of both processes. Such hybrid units may be created where fallout and PDCs act simultaneously, where a transitional phase between the two occurs, and/or due to reworking. This study presents analysis of a novel hybrid pyroclastic lithofacies found on Tenerife (Canary Islands) and Pantelleria (Italy). The coarse pumice block facies has an openwork texture and correlates with distal Plinian units, but it is cross-stratified and relatively poorly sorted with an erosional base. The facies is proposed to record the simultaneous interaction of very proximal fallout and turbulent PDCs, and it reveals a fuller spectrum of hybrid deposition than previously reported. This work highlights the importance of recognizing hybrid deposition both in the rock record and in hazard modeling.Analysis of pyroclastic stratigraphy can reveal the behavior and magnitude of explosive eruptions (e.g., Fisher and Schmincke, 1984), providing input parameters for numerical models (e.g., Pyle, 1989; Bursik and Woods, 1996; Doyle et al., 2010) and informing hazard analysis (e.g., Bonadonna et al., 2005). Pyroclastic deposits may be interpreted to record either plume (fallout) or pyroclastic density current (lateral) activity. However, during an eruption, multiple processes related to the eruption column, fountaining, and pyroclastic density currents (PDCs) may impact the same location at the same time. Deposits that capture simultaneous processes are common at tuff cones and maars (e.g., Cole et al., 2001; Zanon et al., 2009, and references therein), but there are relatively few studies of such hybrid deposits formed during Plinian eruptions (e.g., Valentine and Giannetti, 1995). In this study, we (1) disentangled hybrid lithofacies using previously reported examples, (2) defined a new proximal hybrid lithofacies based on evidence from the 273 ka Poris Formation of Tenerife (Canary Islands) and the 46 ka Green Tuff of Pantelleria (Italy), and (3) considered its significance for interpretation and modeling of volcanic hazards.Deposits classified here as hybrid exhibit characteristics of both Plinian fallout (typically clast-supported and landscape-mantling deposits; e.g., Walker, 1971) and ignimbrite deposited by PDCs (typically ash- and pumice-rich deposits that are poorly sorted; e.g., Fisher and Schmincke, 1984). Hybrid facies can vary in appearance; ignimbrite stratigraphy varies dependent on a range of factors (such as PDC concentration, on a spectrum of fully dilute to fully concentrated; e.g., Branney and Kokelaar, 2002; Sulpizio et al., 2014), and Plinian fallout units are variable due to factors including plume height and proximity to the vent (e.g., Cioni et al., 2015).Valentine and Giannetti (1995) described a hybrid lithofacies generated by primary volcanic fallout and PDC processes operating simultaneously within the White Trachytic Tuff at Roccamonfina, Italy (subunit E1). Associated ignimbrite is predominantly fine-grained ash, with minor pumice lapilli. By contrast, the hybrid lithofacies is clast-supported and consists of angular pumices ranging from coarse lapilli to small blocks. Pumice layers grade from ignimbrite, thicken and thin, and pinch out laterally. The hybrid lithofacies is interpreted to record Plinian fallout into dilute (ash-rich) PDCs that waxed and waned; the pumice-fall was either incorporated into the currents or fell through them and dominated the deposition.Alternating PDC and fallout units in volcanic successions may be interpreted as hybrid facies. A spectrum of lithofacies architecture can occur at the base of ignimbrite successions, marking the change from Plinian fallout to PDC deposition (for a review, see Valentine et al., 2019). Modeling has been used to propose that a “transitional regime” can occur between the two end members where the collapsing eruption column is oscillatory and highly unsteady (e.g., Neri and Dobran, 1994; Di Muro et al., 2004, and references therein). Di Muro et al. (2008) described a hybrid lithofacies recording this transitional regime in the 800 yr B.P. Quilotoa succession in Ecuador. The A2 submember of unit U1 is composed of alternating clast-supported pumice lapilli layers and beds of stratified ash, pumice, and lithic lapilli. Proximally, the facies is cross-stratified. Distally, regressive and progressive bed forms occur, and the facies grades laterally into a pumice lapilli bed.Alternating facies in the 1912 CE Novarupta proximal succession (Alaska, USA), were interpreted by Houghton et al. (2004) to reflect coeval regimes rather than plume oscillation. Unit Fall 2/PDC 2, comprising up to seven PDC beds with thin intervening lapilli falls, was proposed to record fallout deposited during intervals between discrete PDC units from a plume that maintained buoyant and nonbuoyant states simultaneously.Deposits that exhibit characteristics of both fallout and PDC processes may be created by reworking. Fallout units can be reworked by ambient wind or water during a hiatus in the eruption (e.g., Yellowstone, USA; Myers et al., 2016). The syneruptive involvement of strong wind currents may create a lack of clear distinction between the deposits of Plinian fallout and a fully dilute PDC (Wilson and Houghton, 2000). Wilson and Hildreth (1998) described a hybrid fall deposit in the Bishop Tuff, California, distinguished by variable cross-bedding and the presence of subrounded pumice lapilli, which was interpreted to record redeposition of Plinian fallout by wind vortices driven by air currents into coeval PDCs.Investigations of proximal pyroclastic stratigraphy are rare, in large part because of non-preservation due to caldera collapse or erosion during eruption waxing. However, where preserved, proximal exposures can give important insights into complex depositional processes (e.g., Druitt and Sparks, 1982; Houghton et al., 2004). We report a proximal hybrid lithofacies (referred to throughout as xspB) found at Las Cañadas caldera (Tenerife), and at Pantelleria.Proximal deposits of the 273 ka Poris eruption are exposed at Las Cañadas less than 4 km from the likely vent location, in the 1.9-km-wide Diego Hernandez wall (Smith and Kokelaar, 2013; Fig. 1A). Distal Poris exposures occur 15–20 km away in the coastal Bandas del Sur (e.g., Brown and Branney, 2004).The proximal Poris Formation includes a parallel-stratified to cross-stratified pumice-block facies (xspB; Fig. 1B). Typically <2 m thick, xspB consists of pumice-rich beds 50–800 mm thick bounded by ash-rich beds <100 mm thick (Smith and Kokelaar, 2013). Pumice beds are poorly sorted (σΦ = 1.7; see Figure 1C for grain-size distribution) and typically contain 70%–80% pumice lapilli and blocks (5–300 mm) with rare lithic lapilli. At one location, a pumice bed is fully clast-supported (Fig. 2A). Pumices ~20 mm in diameter are subrounded, while large lapilli and blocks (20–300 mm) are subangular to angular. Pumice blocks show no evidence of ballistic impact (such as sag structures or jigsaw-fit breakage). Pumice beds display planar and low-angle cross-stratification (Fig. 1B) and occasional internal cross-stratification of pumice clasts (Fig. 2B). Three-dimensional cuts show that xspB beds thin and thicken both laterally and longitudinally. The xspB facies is in gradational to erosive contact with stratified lithic-rich lapilli-tuff below, and it is overlain by stratified to massive lapilli-tuff with a locally erosive contact (Fig. 1B). It is poorer in ash and lithic content (by 15% and 14%, respectively, at Figure 1 locality) and better sorted than the massive lapilli-tuff. The xspB facies is distinct from bedded pumice lapilli at the base of the succession by the pumice blocks (Fig, 2A), poorer sorting (σΦ = 1.7 vs. σΦ = 1.3; Fig. 1C), cross-stratification, and variable ash content (Fig. 1B).In distal Poris Formation exposures, two discrete clast-supported pumice lapilli facies record Plinian fallout (members 1 and 5 of Brown and Branney [2013]). The proximal xspB facies stratigraphically correlates with the upper distal fallout (Smith, 2012).The 46 ka Green Tuff Formation is well exposed across Pantelleria, from the Cinque Denti caldera walls (<3 km from the vent) to coastal sections (<7 km from the vent; Williams, 2010; Williams et al., 2014). In the Cinque Denti wall at Bagno dell'Acqua (Fig. 3A), the proximal Green Tuff Formation contains discontinuous horizons of a clast-supported, poorly sorted (σΦ = 1.6; Figs. 3B and 3C), cross-stratified, pumice-block facies (xspB). The facies consists of angular pumice lapilli and blocks (<275 mm) and subordinate polylithic lapilli and blocks (<77 mm; Fig. 2C) that are not systematically smaller than pumice clasts. Local lithic- and pumice-rich lenses (Fig. 2D) occur within the unit. Cross-stratification in xspB is relatively high angle (~20° to 30°), not unidirectional and transverse to the inferred current direction. Locally, lithic-rich scours <300 mm thick and <500 mm wide, with >40% lithics, occur at the base of xspB, with basal contacts cutting into the units below (Fig. 3B).The xspB facies grades vertically from a massive pumice lapilli facies with a locally erosive contact; xspB is distinct from the underlying unit in that it contains larger pumice and lithic blocks and exhibits poorer sorting (Fig. 3C), has a wider range of lithic clast compositions, and is cross-stratified. It correlates compositionally (Zr ppm) and stratigraphically with a pumice (or ash) fall layer in coastal sections (Williams et al., 2014). The overlying facies is welded ignimbrite.The xspB facies differs from proximal lithicrich breccias (e.g., Druitt and Sparks, 1982). It is dominated by pumice (with the exception of minor lithic-rich lenses at Pantelleria), does not contain grading or elutriation pipes, and does not grade laterally into ignimbrite. The xspB facies has similarities to fines-poor ignimbrite (e.g., Walker et al., 1980); it is better sorted and coarser than massive lapilli-tuff, and it is less well-sorted than associated Plinian deposits. However, xspB is not massive, does not occur just locally (at Tenerife, it is continuous across the caldera wall), and correlates laterally with Plinian fallout. The cross-stratification makes xspB distinct from reported proximal fallout, such as the coarse, poorly sorted “Bed S” of the 1912 Novarupta eruption, which records complex fallout from a “collar” of low-fountaining ejecta (Fierstein et al., 1997). However, like Bed S, xspB contains distinctly coarse and poorly sorted pumice blocks.The xspB facies exhibits characteristics of both fallout and PDC deposits. The subangular pumice blocks, areal continuity, (variable) open-work texture, and correlation with Plinian units are suggestive of fallout (e.g., Walker, 1971). The cross-stratification, relatively poor sorting, erosional base, and lack of aerodynamic equivalence between adjacent clasts suggest PDC deposition (e.g., Branney and Kokelaar, 2002). The xspB facies differs from previously reported hybrid facies. It has a different grain size compared to associated Plinian fallout (Figs. 1B and 3B), and at Pantelleria, it displays higher-angle cross-stratification than the hybrid facies created by fallout into dilute PDCs described by Valentine and Giannetti (1995). It does not always directly overlie Plinian fallout facies (Tenerife), nor does it contain interbedded strata (cf. Di Muro et al., 2008). However, the dominance of pumice blocks is akin to the coarse proximal fallout layers in the alternating Fall 2/PDC 2 sequence at Novarupta (Houghton et al., 2004). The xspB facies is interpreted to record primary volcanic deposition; the proximal location makes extensive aqueous reworking unlikely, as there is no catchment or upslope source of water. The componentry of xspB differs to underlying and coeval pumicefall deposits, making clear-air reworking of those facies unlikely (cf. Wilson and Hildreth, 1998).At Tenerife and Pantelleria, the increase in grain size in xspB relative to underlying facies records an influx of coarser material at the vent. This may be due to vent widening and shallower fragmentation (evidenced by coarse lithics within the lithofacies at Pantelleria and underlying lithic-rich stratified tuff at Tenerife; Smith and Kokelaar, 2013). As coarse material entered the column, large blocks would have been deposited from a low-fountaining collar of fallout ejecta (as invoked by Fierstein et al. [1997]), and smaller material would have been transported in PDCs formed by contemporaneous fountaining.In the Poris eruption, PDC activity had begun prior to deposition of xspB (recorded in underlying tuff deposits; Brown and Branney, 2004; Smith and Kokelaar, 2013), but it was unsteady and marked by waxing and waning that led to changes in runout distance. During deposition of the xspB facies (~4 km from likely vent), a hiatus in distal PDC activity allowed contemporaneous Plinian fallout to be recorded at the coast (Dowey et al., 2020). On Pantelleria, xspB marks the onset of PDC activity, indicating that the vent widening episode may have instigated column collapse. The proximal currents did not travel far (<1 km); xspB is not longitudinally extensive and is absent at distal locations.The xspB facies reported here contains predominantly coarse material with variable fines, and it exhibits cross-stratification. Cross-stratification indicates traction-dominated deposition and migration of bed forms at the flow-boundary zone (sensu Branney and Kokelaar, 2002). This has typically been associated with fully dilute PDCs (also known as surges), but it is also possible in dense granular currents (e.g., Smith et al., 2020). The range of grain sizes evident in xspB and the evidence of abrasion of the smaller pumices indicate that the currents involved were not fully dilute or ash-rich (cf. Valentine and Giannetti, 1995). Minor fines-rich zones may have been generated by changes in supply to the flow-boundary zone or by variable influence of fallout material.We propose that the hybrid xspB facies reported here formed during a short-lived phase where very proximal fallout interacted with turbulent density currents, in a setting similar to the “impact zone” envisaged by Valentine (2020).This study provides a novel example of simultaneous primary volcanic deposition in the complex proximal domain, representing a previously unreported part of the spectrum of hybrid deposition.Numerical modeling exploring proximal ignimbrite-forming processes has shown that an influx of coarse material into a collapsing column can translate into formation of dense flows in a proximal “impact zone,” which are overridden by dilute currents of expelled fines (Valentine, 2020, and references therein). This modeling could explain the fines-poor nature of the xspB facies reported here. Greater recognition of hybrid processes in the rock record can inform future modeling, allowing us to more confidently understand how fallout and flow may interact and impact hazard assessments around a volcano.It is important to recognize the “gray areas” in field volcanology. It is widely appreciated that complexities such as bypass and erosion are inherent aspects of PDC activity that can be cryptic in the rock record (e.g., Brown and Branney, 2004). Hybrid processes may be similarly cryptic. Hybrid facies created by reworking during hiatuses can only be preserved where not eroded by a subsequent PDC. Those recording Plinian fallout into a PDC are likely only recorded where currents wane sufficiently to allow fallout to dominate the deposit or where Plinian material is coarse/dominant enough to be recognized (this study).We suggest that hybrid processes should be seen as inherent in Plinian eruptions and given greater consideration. Different hybrid processes are likely to occur both at different locations around the volcano and during different stages of an eruption. A snapshot of this complexity is illustrated in Figure 4. It follows that hybrid pyroclastic units may be more common than is reported. Where recorded, they can be difficult to distinguish from fallout or PDC deposits. An interpretation of ignimbrite may lead to the involvement of fallout being underestimated, while identification as fallout could lead to the existence of PDCs at a study location being overlooked. Whatever the location on the volcano, correct hazard identification is the ideal, but acknowledgment of the potential complexity and uncertainty highlighted by studies such as this one is perhaps just as important to hazard modeling and assessment.We acknowledge Natural Environment Research Council studentships NE/G523855/1 and NER/S/A/2006/14156. We thank P. Rowley, D. Brown, G. Valentine, and C. Wilson for their reviews.

中文翻译：

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火山碎屑喷发过程中的同时下降和流动：一种新的近端混合相

Plinian 和 subplinian 喷发的沉积物为过去的火山事件提供了重要的见解，并为旨在减轻未来危害的数值模型提供了信息。然而，通常从沉降或火山碎屑密度流（PDC）的角度考虑火山碎屑沉积物，很少关注表现出两种过程特征的相。这样的混合单元可以在辐射和 PDC 同时作用、两者之间发生过渡阶段和/或由于返工的情况下创建。本研究分析了在特内里费岛（加那利群岛）和潘泰莱里亚（意大利）发现的一种新型混合火山碎屑岩相。粗浮石块相具有透雕结构，与远端普林单元相关，但呈交叉分层，分选相对较差，具有侵蚀基底。这些相被提议记录非常近的沉降物和湍流 PDCs 的同时相互作用，并且它揭示了比以前报道的更全面的混合沉积谱。这项工作强调了在岩石记录和灾害建模中识别混合沉积的重要性。火山碎屑地层学分析可以揭示爆炸性喷发的行为和幅度（例如，Fisher 和 Schmincke，1984 年），为数值模型提供输入参数（例如, Pyle, 1989; Bursik and Woods, 1996; Doyle et al., 2010) 并为危害分析提供信息（例如，Bonadonna et al., 2005）。火山碎屑沉积物可以解释为记录羽流（沉降）或火山碎屑密度流（横向）活动。然而，在一次喷发期间，与喷发柱、喷泉、和火山碎屑密度流 (PDC) 可能同时影响同一位置。捕获同时过程的沉积物在凝灰岩锥和马尔斯很常见（例如，Cole 等人，2001 年；Zanon 等人，2009 年，以及其中的参考文献），但对普林尼喷发期间形成的这种混合沉积物的研究相对较少（例如，瓦伦丁和詹内蒂，1995 年）。在这项研究中，我们 (1) 使用先前报道的示例解开混合岩相，(2) 根据来自特内里费岛（加那利群岛）的 273 ka Poris 组和 Pantelleria 的 46 ka 绿色凝灰岩（意大利）的证据定义了一个新的近端混合岩相), (3) 考虑了它对解释和模拟火山灾害的重要性。此处归类为混合的沉积物具有普林沉降物（通常是碎屑支撑和景观覆盖沉积物；例如，Walker，1971 年）和 PDC 沉积的火成岩（通常是分选不良的富含灰分和浮石的沉积物；例如，Fisher和施明克，1984 年）。混合相的外观可能不同；火成岩地层的变化取决于一系列因素（例如 PDC 浓度，在完全稀释到完全浓缩的范围内；例如，Branney 和 Kokelaar，2002 年；Sulpizio 等人，2014 年），而普林尼沉降物单位因因素而异包括羽流高度和与喷口的接近度（例如，Cioni 等人，2015 年）。Valentine 和 Giannetti（1995 年）描述了由初级火山沉降物和 PDC 过程在罗卡蒙菲纳的白色粗面凝灰岩内同时运行产生的混合岩相，意大利（亚单位 E1）。伴生的火成岩主要是细粒灰，带有少量浮石青金石。相比之下，混合岩相由碎屑支撑，由从粗青金石到小块的角状浮石组成。浮石层从火成岩分级，变厚变薄，并横向尖灭。混合岩相被解释为将普林尼沉降物记录为稀薄的（富含灰分的）PDCs，这些PDCs有盈有亏；浮石落入洋流或穿过洋流落下并主导沉积。火山演替中的PDC和落尘单元交替可解释为混合相。一系列岩相结构可以出现在火成岩演替的底部，标志着从普林尼沉降物到 PDC 沉积的变化（参见 Valentine 等人，2019 年的综述）。模型已被用于提出“过渡状态”可以发生在两个端部成员之间，其中坍塌的喷发柱是振荡的且高度不稳定的（例如，Neri 和 Dobran，1994；Di Muro 等，2004，以及其中的参考资料） . 迪穆罗等人。(2008) 描述了在厄瓜多尔 800 年 BP Quilotoa 演替中记录这种过渡状态的混合岩相。U1单元的A2亚段由交替的碎屑支撑的浮石层和层状灰、浮石和岩屑层组成。相近地，相是交叉分层的。在远端，出现退行和渐进床形式，并且相横向分级为浮石青金石床。 1912 CE Novarupta 近端演替（美国阿拉斯加）中的交替相由 Houghton 等人解释。（2004）反映同时代的制度，而不是羽流振荡。Unit Fall 2/PDC 2 由多达 7 个 PDC 床层组成，中间有薄薄的 lapilli 瀑布，用于记录在离散 PDC 单元之间的间隔期间从同时保持浮力和非浮力状态的羽流中沉积的沉降物。具有沉降物和沉降物特征的沉积物PDC 流程可以通过返工来创建。在喷发中断期间，可以通过环境风或水对辐射单元进行改造（例如，美国黄石公园；Myers 等人，2016 年）。强风流的协同作用可能导致普林尼沉降物沉积物与完全稀释的 PDC 沉积物之间缺乏明确的区别（Wilson 和 Houghton，2000 年）。Wilson 和 Hildreth (1998) 描述了加利福尼亚州 Bishop Tuff 的混合秋季矿床，以可变的交叉层理和亚圆形浮石的存在为特征，这被解释为记录了由气流驱动的风涡流将普林尼沉降物重新沉积到同时代的 PDCs 中。近端火山碎屑地层学的研究很少，很大程度上是因为没有保存由于火山喷发打蜡期间火山口塌陷或侵蚀。然而，在保存完好的地方，近端暴露可以为复杂的沉积过程提供重要的见解（例如，Druitt 和 Sparks，1982；Houghton 等，2004）。我们报告了在拉斯加纳达斯火山口（特内里费岛）和潘泰莱里亚发现的近端混合岩相（通篇称为 xspB）。 273 ka Poris 喷发的近端沉积物暴露在距离可能的喷口位置不到 4 公里的拉斯加纳达斯，在1.9 公里宽的 Diego Hernandez 墙（Smith 和 Kokelaar，2013 年；图 1A)。远端 Poris 暴露发生在 15-20 公里外的沿海 Bandas del Sur（例如，Brown 和 Branney，2004）。近端 Poris 组包括平行分层到交叉分层的浮石块相（xspB；图 1B）。xspB 通常厚度小于 2 米，由 50-800 毫米厚的富含浮石的床层和小于 100 毫米厚的富含灰的床层组成（Smith 和 Kokelaar，2013 年）。浮石床分选不良（σΦ = 1.7；粒度分布见图 1C），通常含有 70%–80% 的浮石和块状（5–300 毫米），带有稀有的岩屑。在一个位置，浮石床完全由碎屑支撑（图 2A）。直径约 20 毫米的浮石是亚圆形的，而大的青金石和块（20-300 毫米）是亚角到角的。浮石块没有显示弹道冲击的证据（例如下垂结构或拼图断裂）。浮石层显示平面和低角度交叉分层（图1B）和浮石碎屑偶尔的内部交叉分层（图2B）。三维切割显示 xspB 床在横向和纵向上变薄和变厚。xspB 相与下方的层状富岩性青金石凝灰岩呈渐变至侵蚀接触，其上覆有层状至块状青金石凝灰岩，具有局部侵蚀性接触（图 1B）。它的灰分和岩屑含量较差（在图 1 位置分别为 15% 和 14%），并且比块状青金石凝灰岩的分选效果更好。xspB 相与浮石块演替底部的层状浮石 lapilli 不同（图 2A），分选较差（σΦ = 1.7 vs. σΦ = 1.3；图 1C），交叉分层和可变的灰分含量（图1B）。在远端孔隙形成暴露中，两个离散的碎屑支撑的浮石青金石相记录了普林尼沉降物（Brown and Branney [2013] 的成员 1 和 5）。近端 xspB 相在地层上与上部远端沉降物相关（Smith，2012 年）。从 Cinque Denti 破火山口壁（距离喷口 < 3 公里）到沿海部分（< 7 公里从通风口；Williams，2010；Williams 等人，2014）。在 Bagno dell'Acqua 的 Cinque Denti 墙中（图 3A），近端绿色凝灰岩组包含碎屑支撑、分选不良（σΦ = 1.6；图 3B 和 3C）、交叉分层、浮石的不连续层位。块相（xspB）。相由角状浮石和块体（<275 mm）和次要的多石块体和块体（<77 mm；图1）组成。2C) 不小于浮石碎屑。单元内出现局部富含岩石和浮石的透镜体（图 2D）。xspB 中的交叉分层是相对较大的角度（~20° 到 30°），不是单向的并且与推断的电流方向横向。局部，富岩屑冲刷厚度<300 mm，宽<500 mm，岩屑>40%，发生在xspB底部，底部接触切入下方单元（图3B）。xspB相从a垂直分级与局部侵蚀接触的大量浮石青金石相；xspB 与下伏单元的不同之处在于它包含较大的浮石和岩屑块并且表现出较差的分选（图3C），具有更广泛的岩屑碎屑成分，并且是交叉分层的。它在成分（Zr ppm）和地层上与沿海部分的浮石（或灰烬）坠落层相关（Williams 等人，2014 年）。上覆相为熔结火成岩。xspB 相不同于近端富岩质角砾岩（例如，Druitt 和 Sparks，1982）。它以浮石为主（除了在 Pantelleria 的少量富含岩石的透镜体），不包含分级或淘析管，并且不横向分级成火成岩。xspB 相与贫细火成岩有相似之处（例如，Walker 等，1980）；它比块状青金石凝灰岩分选得更好，也更粗糙，而且分选不如相关的普林尼沉积物。然而，xspB 不是大规模的，不只是局部发生（在特内里费岛，它连续穿过火山口壁），并且与普林尼沉降物横向相关。交叉分层使 xspB 与报告的近端沉降物不同，例如 1912 年 Novarupta 喷发的粗糙的、分类不良的“Sed S”，它记录了来自低喷泉喷射物“项圈”的复杂沉降物（Fierstein 等，1997 ）。然而，与床 S 一样，xspB 包含明显粗糙且分选差的浮石块。xspB 相表现出沉降物和 PDC 沉积物的特征。亚角浮石块、区域连续性、（可变的）镂空纹理以及与普林尼单位的相关性都暗示了沉降（例如，Walker，1971）。交叉分层、相对较差的分选、侵蚀基础和相邻碎屑之间缺乏空气动力学等效性表明 PDC 沉积（例如，Branney 和 Kokelaar，2002）。xspB 相不同于先前报道的混合相。与相关的 Plinian 沉降物相比，它具有不同的粒度（图 1B 和 3B），并且在 Pantelleria，它显示出比由 Valental 和 Gianneti（1995 年）描述的由沉降到稀释的 PDC 中产生的混合相产生的更高角度的交叉分层。它并不总是直接覆盖普林尼沉降相（特内里费岛），也不包含互层地层（参见 Di Muro 等，2008）。然而，浮石块的优势类似于在 Novarupta 交替的 Fall 2/PDC 2 序列中的粗近沉降层（Houghton 等，2004）。xspB 相被解释为记录初级火山沉积；由于没有集水区或上坡水源，近端位置不太可能进行大范围的水体改造。xspB 的成分不同于底层和同时代的浮石沉积物，使这些相的晴空返工不太可能（参见 Wilson 和 Hildreth，1998 年）。在特内里费岛和潘泰莱里亚，xspB 的晶粒尺寸相对于下伏相的增加记录了喷口处较粗物质的流入。这可能是由于喷口扩大和破碎变浅（潘泰莱里亚岩相内的粗岩屑和特内里费岛下伏富含岩屑的层状凝灰岩就是证明；Smith 和 Kokelaar，2013 年）。当粗物质进入柱子时，大块会从放射性尘埃的低喷泉领中沉积（如 Fierstein 等人 [1997] 所引用的），较小的物质会在同期喷泉形成的 PDC 中运输。 Poris 喷发，PDC 活动在 xspB 沉积之前就开始了（记录在下面的凝灰岩沉积物中；Brown 和 Branney，2004；Smith 和 Kokelaar，2013 年），但它不稳定且以起伏为特征，导致跳动距离发生变化。在 xspB 相沉积期间（距离可能的喷口约 4 公里），远端 PDC 活动的中断允许在海岸记录同时期的普林尼沉降物（Dowey 等人，2020）。在 Pantelleria 上，xspB 标志着 PDC 活动的开始，表明通风口扩大事件可能引发了柱塌陷。近端电流没有传播很远（<1 km）；xspB 在纵向上并不广泛，并且在远端位置不存在。这里报道的 xspB 相主要包含具有可变细度的粗材料，并且表现出交叉分层。交叉分层表明流边界带的床型以牵引力为主的沉积和迁移（sensu Branney 和 Kokelaar，2002 年）。这通常与完全稀释的 PDC（也称为浪涌）有关，但在密集的颗粒电流中也可能发生（例如，Smith 等人，2020）。xspB 中明显的粒度范围和较小浮石磨损的证据表明，所涉及的水流没有完全稀释或富含灰分（参见 Valentine 和 Gianneti，1995 年）。少量富含细粒的区域可能是由流动边界区域的供应变化或沉降物质的可变影响产生的。我们认为，这里报道的混合 xspB 相形成于一个短暂的阶段，其中非常近的沉降物与湍流相互作用密度电流，在类似于瓦伦丁（2020）设想的“撞击区”的环境中。这项研究提供了一个在复杂的近端区域同时发生初级火山沉积的新例子，代表了混合沉积光谱中以前未报告的部分。探索近端点火形成过程的数值模型表明，粗材料流入塌陷柱可以转化为近端“冲击区”中密集流动的形成，这些流动被覆盖被驱逐罚款的稀释电流（Valentine，2020 年，以及其中的参考资料）。该模型可以解释此处报道的 xspB 相的细粒性。更好地识别岩石记录中的混合过程可以为未来的建模提供信息，使我们能够更自信地了解沉降物和流动如何相互作用并影响火山周围的危险评估。识别野外火山学中的“灰色区域”非常重要。人们普遍认为，诸如旁路和侵蚀之类的复杂性是 PDC 活动的固有方面，在岩石记录中可能是隐秘的（例如，Brown 和 Branney，2004 年）。混合过程可能同样神秘。只有在不被后续 PDC 侵蚀的地方才能保留在中断期间通过返工产生的混合相。那些将 Plinian 沉降物记录到 PDC 中的人可能只记录在电流减弱到足以使沉降物支配沉积物或 Plinian 材料粗糙/占主导地位足以被识别的情况（本研究）。我们建议应将混合过程视为固有的普林尼火山爆发并给予更多考虑。在火山周围的不同位置和喷发的不同阶段都可能发生不同的混合过程。这种复杂性的快照如图 4 所示。因此，混合火山碎屑单元可能比报道的更常见。在记录的情况下，它们可能难以与沉降物或 PDC 沉积物区分开来。对 ignimbrite 的解释可能导致放射性尘埃的参与被低估，而作为放射性尘埃的识别可能导致研究地点存在 PDC 被忽视。无论火山上的哪个位置，正确的危险识别都是理想的，但承认此类研究强调的潜在复杂性和不确定性对于危险建模和评估可能同样重要。我们承认自然环境研究委员会的学生身份 NE/G523855 /1 和 NER/S/A/2006/14156。我们感谢 P. Rowley、D. Brown、G. Valentine 和 C. Wilson 的评论。因此，混合火山碎屑单元可能比报道的更常见。在记录的情况下，它们可能难以与沉降物或 PDC 沉积物区分开来。对 ignimbrite 的解释可能导致放射性尘埃的参与被低估，而作为放射性尘埃的识别可能导致研究地点存在 PDC 被忽视。无论火山上的哪个位置，正确的危险识别都是理想的，但承认此类研究强调的潜在复杂性和不确定性对于危险建模和评估可能同样重要。我们承认自然环境研究委员会的学生身份 NE/G523855 /1 和 NER/S/A/2006/14156。我们感谢 P. Rowley、D. Brown、G. Valentine 和 C. Wilson 的评论。因此，混合火山碎屑单元可能比报道的更常见。在记录的情况下，它们可能难以与沉降物或 PDC 沉积物区分开来。对 ignimbrite 的解释可能导致放射性尘埃的参与被低估，而作为放射性尘埃的识别可能导致研究地点存在 PDC 被忽视。无论火山上的哪个位置，正确的危险识别都是理想的，但承认此类研究强调的潜在复杂性和不确定性对于危险建模和评估可能同样重要。我们承认自然环境研究委员会的学生身份 NE/G523855 /1 和 NER/S/A/2006/14156。我们感谢 P. Rowley、D. Brown、G. Valentine 和 C. Wilson 的评论。在记录的情况下，它们可能难以与沉降物或 PDC 沉积物区分开来。对 ignimbrite 的解释可能导致放射性尘埃的参与被低估，而作为放射性尘埃的识别可能导致研究地点存在 PDC 被忽视。无论火山上的哪个位置，正确的危险识别都是理想的，但承认此类研究强调的潜在复杂性和不确定性对于危险建模和评估可能同样重要。我们承认自然环境研究委员会的学生身份 NE/G523855 /1 和 NER/S/A/2006/14156。我们感谢 P. Rowley、D. Brown、G. Valentine 和 C. Wilson 的评论。在记录的情况下，它们可能难以与沉降物或 PDC 沉积物区分开来。对 ignimbrite 的解释可能导致放射性尘埃的参与被低估，而作为放射性尘埃的识别可能导致研究地点存在 PDC 被忽视。无论火山上的哪个位置，正确的危险识别都是理想的，但承认此类研究强调的潜在复杂性和不确定性对于危险建模和评估可能同样重要。我们承认自然环境研究委员会的学生身份 NE/G523855 /1 和 NER/S/A/2006/14156。我们感谢 P. Rowley、D. Brown、G. Valentine 和 C. Wilson 的评论。对 ignimbrite 的解释可能导致放射性尘埃的参与被低估，而作为放射性尘埃的识别可能导致研究地点存在 PDC 被忽视。无论火山上的哪个位置，正确的危险识别都是理想的，但承认此类研究强调的潜在复杂性和不确定性对于危险建模和评估可能同样重要。我们承认自然环境研究委员会的学生身份 NE/G523855 /1 和 NER/S/A/2006/14156。我们感谢 P. Rowley、D. Brown、G. Valentine 和 C. Wilson 的评论。对 ignimbrite 的解释可能导致放射性尘埃的参与被低估，而作为放射性尘埃的识别可能导致研究地点存在 PDC 被忽视。无论火山上的哪个位置，正确的危险识别都是理想的，但承认此类研究强调的潜在复杂性和不确定性对于危险建模和评估可能同样重要。我们承认自然环境研究委员会的学生身份 NE/G523855 /1 和 NER/S/A/2006/14156。我们感谢 P. Rowley、D. Brown、G. Valentine 和 C. Wilson 的评论。但承认此类研究所强调的潜在复杂性和不确定性可能对灾害建模和评估同样重要。我们承认自然环境研究委员会的学生身份 NE/G523855/1 和 NER/S/A/2006/14156。我们感谢 P. Rowley、D. Brown、G. Valentine 和 C. Wilson 的评论。但承认此类研究所强调的潜在复杂性和不确定性可能对灾害建模和评估同样重要。我们承认自然环境研究委员会的学生身份 NE/G523855/1 和 NER/S/A/2006/14156。我们感谢 P. Rowley、D. Brown、G. Valentine 和 C. Wilson 的评论。