From Jicha (2009)
[1]: Koniuji is a nascent stratovolcano, with 40Ar/39Ar ages that indicate subaerial lavas and domes erupted over the last 15,000 years, including at least three eruptions within the last 6000 years. It has an estimated volume of 1 cubic km, with approximately 25% of that volume subaerial. Dated products include older basaltic andesite lavas and younger andesite dome rocks.
Wood and Kienle (1990)
[2] note that in 1981, "geologists from the USGS research vessel S.P. Lee landed in a small boat to collect samples. The collected rocks are from andesite flows, and pyroclastic deposits. A northeast-trending fault cuts the island on the southeast. A possible vent occurs just north of the highest point on the island."