Abstract The Quaternary as a System is traditionally considered to be the Ice Age - an interval of oscillating climate extremes (glacials and interglacials) encompassing the Pleistocene and the Holo-cene as Series. The term was formally introduced by Desnoyers ( 1829 ). The basic principles used in subdividing the Quaternary into chronostratigraphic units are the same as for other Phaner-ozoic units which require boundary definitions and the designation of boundary stratotypes ( Salvador 1994 ). However, in contrast to the rest of the Phanerozoic, the division of Quaternary sequences on the basis of climatic changes documented in the sedimentary record is fundamental and has a long tradition. Classifications based on climatostratigraphic units such as ‘glacials’ or ‘interglacials’ are reasonably well-established in different countries or areas of Central Europe, and are accepted as regional chronostratigraphic standards ( Gibbard & West 2000 ; Gibbard & Kolfschoten 2004 ; Litt et al. 2005 ). The climatostratigraphic terms ‘interglacial’ and ‘interstadial’ were first defined by Jessen & Milthers (1928) for periods with characteristic records of non-glacial climate, as indicated by palaeobotanical evidence for major vegetation changes. Following these suggestions, interglacials in Central Europe are classified as temperate periods with a climate optimum at least as strong as the present interglacial (Holocene) in the same region. Interstadials are assumed to have been either too short or too cold to reach the climate level of interglacial type in the same region. In North America, the fundamental units of geological-climate classification have been defined as follows ( American Commission on Stratigraphic Nomenclature 1961 ): ‘A glaciation is a climatic episode during which extensive glaciers developed, attained a maximum extent, and receded. A stadial is a climatic episode, representing a subdivision of a glaciation, during which a secondary advance of glaciers took place. An interstadial is a climatic episode within a glaciation during which a secondary recession or standstill of glaciers took place. An interglacial is an episode during which the climate was incompatible with the wide extent of glaciers that characterize a glaciation’ (see also Gibbard & Kolfschoten 2004 ). The glacially based terms, however, are very difficult to apply in regions which were not directly affected by ice activity. Furthermore, cold rather than glacial climates have often tended to characterize the periods between interglacial events. Therefore, the term ‘cold stage’ has been adopted instead of ‘glacial’ or ‘glaciation’ ( Gibbard & West 2000 ). The status of the Quaternary, long regarded as a geological period, has recently been questioned as a formal stratigraphic unit, with proposals for its abandonment or modification ( Grad-stein et al. 2004 ; Pillans 2004 ; Steininger 2002 ). Gibbard et al. (2005) , however, argue that the Quaternary should be retained as a formal period of geological time following the Neogene which would be formally subdivided into the Pleistocene and Holocene epochs (see also Bowen & Gibbard 2006 ). A formal decision on its chronostratigraphic status is pending.

We review the most important types of volcanic hazards that have occurred in Nicaragua during the past ∼40,000 yr and that are expected to occur in the future. Population density within the potential hazard area is clearly essential in defining and understanding volcanic hazard and risk. There are three main groups of volcanic events that pose major hazards: Group 1 comprises several types of explosive volcanic eruptions that impact society (people and infrastructure) directly. The most hazardous types are pyroclastic surges, particularly those generated by water-magma interaction, pyroclastic fallout, and pyroclastic flows, as well as tsunamis generated by volcanic eruptions within and close to Nicaragua's large lakes. Group 2 includes nonexplosive volcanic activity such as lava flows and the permanent or episodic emission of volcanic gases from open vents. Group 3 comprises chiefly lahars generated by mixing of volcanic debris with water and volcano flank collapses (landslides) sometimes unrelated to synchronous volcanic eruptions but being conditioned chiefly by the stability of a volcanic edifice. We discuss the present database on the age and type of the most recent eruptions emphasizing those that potentially pose major hazards to the populated areas. These include volcanogenic tsunamis in Lake Managua and Lake Nicaragua, scoria cone and maar formation chiefly in the western part of Managua, and major explosive eruptions of Chiltepe and Masaya volcanoes, a large eruption from Masaya volcano having devastated the entire area of present Managua only ∼2000 yr ago. We discuss the most important techniques for monitoring volcanoes to detect unrest and predict the time and magnitude of upcoming eruptions, emphasizing techniques presently employed in Nicaragua. Finally, we address the subjects of risk assessment, including hazard and risk maps, and the importance of long-term development plans to reduce vulnerability.

40 Ar/ 39 Ar dates, field observations, and geochemical data are reported for Irazú volcano, Costa Rica. Volcanism dates back to at least 854 ka, but has been episodic with lava shield construction peaks at ca. 570 ka and 136–0 ka. The recent volcanic record on Irazú volcano comprises lava flows and a variety of Strombolian and phreatomagmatic deposits, with a long-term trend toward more hydrovolcanic deposits. Banded scorias and hybridized rocks reflect ubiquitous magma mixing and commingling. Two distinct magma batches have been identified. One magma type or batch, Haya, includes basalt with higher high field strength (HFS) and rare-earth element contents, suggesting a lower degree melt of a subduction modified mantle source. The second batch, Sapper, has greater enrichment of large ion lithophile elements (LILE) relative to HFS elements and rare-earth elements, suggesting a higher subduction signature. The recent volcanic history at Irazú records two and one half sequences of the following pattern: eruptions of the Haya batch; eruptions of the Sapper batch; and finally, an unusually clear unconformity, indicating a pause in eruptions. In the last two sequences, strongly hybridized magma erupted after the eruption of the Haya batch. The continuing presence of two distinct magma batches requires two active magma chambers. The common occurrence of hybrids is evidence for a small, nearer to the surface chamber for mixing the two batches. Estimated pre-eruptive temperatures based on two-pyroxene geothermometry range from ∼1000–1176 °C in basalts to 922 °C in hornblende andesites. Crystallization occurred mainly between 4.6 and 3 kb as measured by different geobarometers. Hybridized rocks show intermediate pressures and temperatures. High silica magma occurs in very small volumes as banded scorias but not as lava flows. Although eruptions at Irazú are not often very explosive, the pervasiveness of magma mixing presents the danger of larger, more explosive hybrid eruptions.