Tephra deposits for the past 2600 years from Irazu Volcano, Costa Rica; Volcanic hazards in Central America

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Peer Reviewed


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Journal/Book/Conference Title

Special Paper - Geological Society of America

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We report the first detailed study of recent tephra deposits at Irazu volcano, Costa 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 Jose. 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 Irazu. Carbon-14 based correlations of deposits indicate that the ash accumulation rate has been relatively consistent during this period (e.g., nearly equal 18 cm/century, 5 km SW of the crater). This consistency combined with the historic preservation ratio and correlated prehistoric deposits implies that Irazu may have erupted >85 times during the past 2600 yr. Most of these would have been small, volcanic explosivity index (VEI) < or =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 3X10 (super 7) m (super 3) 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.

Published Article/Book Citation

Special Paper - Geological Society of America, 412 (2006) pp.225-234.

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