Products of Ignimbrite Eruptions

Many ignimbrite flow units show a reverse grading of large pumice clasts and a normal grading of large lithic clasts. Each ignimbrite flow unit has a basal layer finer grained than the body of the ignimbrite, with a ground surge deposit commonly underlying the ignimbrite, and a fine ash-fall deposit commonly overlying it. These two types of deposit, although not an integral part of the ignimbrite, are produced by the same eruptive act, and they and the ignimbrite constitute the several and varied products of a Peléan-phase eruption. Volcanic eruptions in which ignimbrite is generated show the following sequence of events so often that it may be regarded as the normal one: (1) a highly explosive, often Plinian, phase, producing a pumice-fall deposit; (2) a Peléan phase; and (3) an effusive phase, producing a lava flow. This sequence is believed to represent the tapping of progressively deeper levels in the magma chamber and the escape during the eruption of magma of progressively lower gas content.