Abstract Round Butte (Hopi Buttes volcanic field, Arizona) exposes a diatreme 170–190 m across, 190 m below the pre-eruptive surface. The central part of the massif is 130–150 m in diameter, displaying 20–30 m-high subvertical cliffs. The well-known layer-cake stratigraphy of the sedimentary rocks of the Colorado Plateau permits identification of the largest lithic fragments preserved in the Round Butte diatreme. We define three main groups of pyroclastic facies: undisturbed beds, disturbed beds and non-bedded rocks. Two other minor facies groups were mapped: megablocks (blocks over 2 m in long axis), and small-volume debris avalanche deposits. Pyroclastic megablocks are finer grained and richer in lithic clasts than most diatreme rocks surrounding them. These pyroclastic megablocks are interpreted as subsided portions of the maar ejecta ring. Sedimentary megablocks originate either from above, or from the same level, relative to their current location, i.e. no megablock has a net upward displacement. Small-volume debris avalanche deposits are poorly sorted deposits resulting from gravitational destabilization of the surrounding country rocks into the syn-eruptive crater. Small-volume debris avalanches and individual megablock collapse are the main ways in which the crater grew in size laterally during the eruption. We combine the componentry of the disturbed bedded pyroclastic facies, the non-bedded pyroclastic facies and the pyroclastic megablocks with a series of conceptual models for country rock fragmentation. This exercise further allows us to estimate diatreme wall slopes of 70° below the Bidahochi Formation to approximately the depth of the root zone around 440 m below the pre-eruptive surface. Lithic fragments at the current level of exposure come from elevations up to 190 m above (i.e., up to the pre-eruptive surface) and up to 250 m below (i.e., down to the root zone) their current locations. Pyroclastic units displaying the richest content of lithic clasts with a deep origin are typically the non-bedded facies interpreted to have formed from debris jets during the eruption.

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什么岩屑和富含岩屑的相可以告诉我们关于火山岩过程的信息：以亚利桑那州纳瓦霍族霍皮巴特斯火山场 Round Butte 为例

摘要 Round Butte（Hopi Buttes 火山场，亚利桑那州）在喷发前表面下方 190 m 处暴露了一个直径 170-190 m 的火山口。地块中央部分直径130-150 m，显示20-30 m高的近垂直悬崖。科罗拉多高原沉积岩著名的层饼地层学允许鉴定保存在圆形小山岩层中的最大的岩屑碎片。我们定义了三个主要的火山碎屑相组：未扰动层、扰动层和非层状岩。绘制了另外两个小相组：巨型块体（长轴超过 2 m 的块体）和小体积碎屑雪崩沉积物。火山碎屑巨块比周围的大多数火山岩具有更细的颗粒和更丰富的岩屑。这些火山碎屑巨块被解释为玛尔喷射环的下沉部分。沉积的巨型区块要么来自上方，要么来自相对于它们当前位置的同一层，即没有巨型区块具有向上的净位移。小体积碎屑雪崩矿床是由于周围围岩重力不稳定而进入同喷火山口的分类不良的矿床。小体积碎片雪崩和单个巨型块体坍塌是火山口在喷发期间横向扩大的主要方式。我们将扰动层状火山碎屑相、非层状火山碎屑相和火山碎屑巨块的成分与一系列围岩破碎的概念模型相结合。这项工作进一步使我们能够估计 Bidahochi 组下方 70° 的火山岩壁坡度，大约相当于喷发前表面下方 440 m 处的根区深度。当前暴露水平的岩屑碎片来自海拔高达 190 m（即，高达喷发前的表面）和高达 250 m（即，向下到根区）的当前位置。火山碎屑单元显示出最丰富的岩屑碎屑含量，通常是非层状相，被解释为在喷发期间由碎片喷射形成。