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We report the first detailed study of recent tephra deposits at Irazú volcano, Cos-ta Rica. These ash-fall deposits consist of unconsolidated, moderately to well-sorted, mostly juvenile ash of porphyritic basalt to basaltic andesite. Ash accumulations are thickest SW of the crater, an area that includes the headwaters of the Reventado River, which flows through the city of Cartago. With increasing eruption intensities, deposition shifts more westerly—toward the capital city, San José. Of seventeen historic eruptions, only two have left distinct ash deposits. At least eight other ash-fall deposits from the past 2600 yr are preserved on the SW flank of Irazú. Carbon-14 based correlations of deposits indicate that the ash accumulation rate has been relatively consistent during this period (e.g., ≈18 cm/century, 5 km SW of the crater). This consistency combined with the historic preservation ratio and correlated prehistoric deposits implies that Irazú may have erupted >85 times during the past 2600 yr. Most of these would have been small, volcanic explosivity index (VEI) ≤2 eruptions, with only ten or so VEI = 3 eruptions likely occurring every 200–400 yr. The largest historic eruption occurred in 1963–1965, and we estimate a minimum tephra volume of 3 × 107 m3 for that eruption. The 1963–1965 eruption was not quite as energetic as some eruptions of the past 2600 yr, but it is of the same order of magnitude, and, based on its thickness, it approximates the size and duration of the larger eruptions of the past 2600 yr.

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