 Complete the following phrase:  Scientific knowledge of the geologic history of Vesuvius comes from core samples taken from a plus bore hole on the flanks of the volcano, extending into Mesozoic rock. Cores were dated by potassiumargon and argonargon dating. The area has been subject to volcanic activity for at least 400,000 years; the lowest layer of eruption material from the Somma caldera lies on top of the 40,000-yearold 