Abstract The mid to late Miocene Misaki Formation sediments of the Miura and Boso peninsulas, south central Japan, were deposited in the Pacific-facing forearc region of the proto Izu-Bonin arc at bathyal depths. The hemipelagic background facies, composed mainly of calcareous biogenic and pumiceous voicaniclastic material, are interbedded with thin to thick scoriaceous beds of turbiditic and pyroclastic fall origin. Careful study in the field and in the laboratory of these fine-grained background sediments has revealed the marked influence of bottom currents at certain horizons in producing intervals with distinct muddy contourite characteristics. These include: a general absence of primary structures due to intense bioturbation, some diffuse layering, irregular concentrations of coarser-grained material, sharp and erosive top and bottom lamina contacts, rare micro-cross-lamination disrupted by bioturbation that was continuous with deposition, and a mixed pelagic biogenic (commonly fragmented), volcaniclastic and terrigenous composition. Small-scale cyclicity of variations in grain-size and structural features can be related in part to episodic volcaniclastic input and in part to fluctuation in bottom current strength. Evidence for bottom current reworking of the tops of thin-bedded sandy turbidites is equivocal, and further work is required to resolve this debate. The recognition of Miocene-age contourites from the NW Pacific provides further evidence for the existence of active deep-ocean circulation in the Pacific at this time. However, it is not possible to determine which current system, Antarctic Bottom Water or deep Kuroshio Current, was responsible for these outcrop examples of fossil contourites.

Approximately 21,700 yr B.P., after a period of quiescence of ∼4800 yr, Nevado de Toluca volcano erupted, producing the Lower Toluca Pumice deposit. The activity generated a 24-km-high Plinian column that lasted ∼11–13 h and dispersed 2.3 km 3 (0.8 km 3 dense rock equivalent) of tephra toward the NE, blanketing the Lerma basin, an area occupied today by the city of Toluca, with up to 5 cm of ash. Subsequent eruptive pulses were sub-Plinian in style, accompanied by phreatomagmatic explosions that emplaced surge deposits. Finally, the column collapsed toward the NE with the emplacement of a pumice flow deposit. The high vesicularity of the pumice from the basal Plinian layer, up to 83% by volume, indicates that exsolution was dominantly magmatic, and that pressurization of the magma chamber was probably due to a magma mixing process. Evidence for this includes the compositional range of juvenile products (61–65 wt% SiO 2 ), as well as the presence of two types of plagioclase, one in equilibrium and the other one with disequilibrium textures and reverse zoning. This suggests input of an andesitic liquid into the dacitic magma chamber. Based on the eruptive record, the most likely future eruptive activity at Nevado de Toluca volcano will be Plinian. Although quiet for more than 3250 yr, Plinian activity could occur after a long period of quiescence, and it could represent a hazard for the entire Toluca basin, where more than one million people live today.