Peperite

In the Northern Owyhee Mountains (SW Idaho), a >200-m-thick flow of the Miocene Jump Creek Rhyolite was erupted on to a sequence of tuffs, lapilli tuffs, breccias and lacustrine siltstones of the Sucker Creek Formation. The rhy-olite lava flowed over steep palaeotopography, resulting in the forceful emplacement of lava into poorly consolidated sediments. The lava invaded this sequence, liquefying and mobilising the sediment, propagating sediment subvertically in large metre-scale fluidal diapirs and sediment injectites. The heat and the overlying pressure of the thick Jump Creek Rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the Sucker Creek Formation units, and the formation of metre-scale loading structures (simple and pendulous load casts, detached pseudonodules). Density contrasts between the semi-molten rhyolite and liquefied sediment produced highly fluidal Rayleigh-Taylor structures. Local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment. The result is a 30–40-m zone beneath the rhyolite lava of extremely deformed stratigraphy. Brittle failure and folding is recorded in more consolidated sediments , indicating a differential response to loading due to the consolidation state of the sediments. The lava-sediment interaction is interpreted as being a function of (1) the poorly consolidated nature of the sediments, (2) the thickness and heat retention of the rhyolite lava, (3) the density contrast between the lava and the sediment and (4) the forceful emplacement of the lava. This study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture.

Quench fragmentation is a non-explosive process that occurs when molten magma is super-cooled to glass upon contact with ambient water. This occurs when coherent lavas are erupted subaqueously, when they flow into water, when magma intrudes into water-saturated sediments, and when magma and water interact explosively during phreatomagmatism.  Quench fragmentation also occurs alongside explosive phreatomagmatic activity. Although products of quench fragmentation (hyaloclastite sensu stricto) have been discussed qualitatively in the volcanological literature, compared to explosive fragmentation processes very little is known about the exact dynamics of quench fragmentation of magma and how this relates to the rheology and physical properties of volcanic glass. Based on literature from materials engineering, we present a detailed review of the processes by which glass forms, the properties of glass, and the fracture mechanics that cause it to fragment non-explosively. We also consider how this can be applied to understanding the dynamics behind the volcanological processes of in-situ glass fragmentation during quenching in wet environments and phreatomagmatism. Important parameters for the occurrence of quench fragmentation are the temperature difference between the magma and the ambient water and how much the ambient water is superheated above its Leidenfrost temperature. The geometry of the lava or magma intrusion, the thermal conductivity and the thermal expansion are also of great importance. The resistance of the magma against fragmentation can be increased with the presence of crystals provided the thermal expansion of the crystals does not greatly exceed that of the glass; vesicles have the opposite effect, unless the magma is highly vesicular. This overview then provides a solid basis for further quantitative study of quench fragmentation and hyaloclastite formation.

Peperites are recognised as a mixture of magma and sediment, which is generated by an interaction of partly emergent domes and lava flows with unconsolidated sediments. They commonly occur in back-arc and rift settings where progressive subsidence of
ba~ins occurs contemporaneously with magmatism . This presented research deals with peperites formed along the ephemeral lacustrine sediml!nts and the overlying smallvolume ultrapotassic lava flows in western Turkey. Ultrapotassic rocks occur as domes,
dykes and lava flows and include peperitic basal contact zones. The peperitic contact zone with lava flows is defined by up-to-a metre thick layer of magma and sediment mixture . Magma clasts display fluidal and blocky morphologies with variable sizes
ranging from few mm to a few decimetres. Fluidal clasts predominate with respect to blocky varieties . They were locally in-situ fragmented into blocky clasts that form a typical jig-saw texture. Blocky clasts are variably shaped and include digitate margins.
Tapered clasts also occur along foliation planes of lava flows. Peperitic textures related to ultrapotassic lava flows closely resemble those generated by olivine basalt flows. Clast morphologies suggest a progressive fragmentation due to decreasing temperature
along with increasing viscosity during peperite formation. Sediment fluidisation and low viscosity are presumed to be major controlling factors for fluidal clast generation and non-explosive emplacement. Common occurrences of peperite-bearing volcanic edifices in western Turkey suggest that magmatism developed within a progressively subsiding basin, probably corresponding to rift or back-arc extensional / transtensional environment.

Quench fragmentation is a non-explosive process that occurs when molten magma is super-cooled to glass upon contact with ambient water. This occurs when coherent lavas are erupted subaqueously, when they flow into water, when magma intrudes into water-saturated sediments, and when magma and water interact explosively during phreatomagmatism. Quench fragmentation also occurs alongside explosive phreatomagmatic activity. Although products of quench fragmentation (hyaloclastite sensu stricto) have been discussed qualitatively in the volcanological literature, compared to explosive fragmentation processes very little is known about the exact dynamics of quench fragmentation of magma and how this relates to the rheology and physical properties of volcanic glass. Based on literature from materials engineering, we present a detailed review of the processes by which glass forms, the properties of glass, and the fracture mechanics that cause it to fragment non-explosively. We also consider h...

Paleoproterozoic peperite-like rocks of Lyudikovian level in Onega trough (Baltic shield) are formed at 1984±8-1975±24 Ma (Filippov et.al., 2007, Kulikov, 1999) as a result of interaction of shungit rocks with high content of carbon, and major dolerite sill in shallow sedimentary sequence of trough-basin in dissipate penetrating of continental crust terms and conditions. In the initial phase evolution magma was extruded on the surface with origin of intraplate volcanoes and basaltic lava flows and last phase sills was formed with partical truncation of lava flows (western part of trough). At central part of the trough sills was intruded presumptive in ductile mudstones with generate of peperite-like rocks, that formed at two stages. Firstly, at a ductile condition, isometric fragments with glassy zones are generated. The shapes of some fragments are arched and curved. Orientation of these deformed fragments let reconstruct the joint movement with host rocks. At a next step, that reproduced a brittle condition without formed of glassy zones in angulated clasts. In the sill-contact zone the long strain fractures that filled by carbon-bearing rocks are formed at this stage. Two types of fragments and clasts are scattered inside upper exocontact zone dolerite sill body in carbon-bearing rocks. Distribution of these fragments and clasts allow for is reconstructing of conditions of sill intrusion in a ductile host rock and dispersion with convective agitation of fragments and clasts. In this interpretation, the peperite-like rocks are indicates of shallow conditions of sill intrusion for Lyudicovian age and indicates of fluid barrier location – the level of sills intrusion origin in dissipate penetrating of continental crust terms and conditions.