Deposits of eroded volcanic ash surrounding Mt. Pinatubo were observed by the USGS team and the remains of buried and burned trees in the ash were collected and dated using the radiocarbon method. What did these pieces of evidence indicate about the past eruptions of the volcano? a. they were big! b. they have occurred about once every 50 years c. the volcano typically had small eruptions in the past d. lava flows alternated with pyroclastic flows.