THE ORIGIN OF IGNIMBRITE

Ignimbrite is a kind of pyroclastic rock formed by volcanic explosive-effusive eruption. This term was formally proposed by P. Marshall in 1932 after he had studied the rhyolite plateau in the North Island, New Zealand. Since then its origin has always been the subject discussed by geologists throughout the world. In this paper, on the basis of reviewing the hypotheses proposed by predecessors, the author enumerates 15 factors according to the concrete conditions of acid and intermediate acid ignimbrites in Zhejiang province. He specially emphasizes that the clasts must be formed in such a manner as tuffs do, i. e. there must be severe explosive fragmentation, and that the rocks must be formed in such a manner as lavas do, i. e. the melts must become flows after bursting through the crater. Besides, ignimbrites should possess such features as transition towards lavas, intrusive contact and emptiness of the magme chamber, crater collapses and finally choking of the centre of the vent by contemporaneous molten magmas and termination of the activity. Hence the"hypothesis of explosive-effusive origin"is proposed. Its concept is as follows: When high-temperature, high-pressure, high-viscosity and volatile com- ponents-highly saturated acid and intermediate acid melts at the deep part of the crust are intruded upwards to the neighbourhood of a crater or the shallow part of a vent, as temperature and pressure drop abruptly, large amounts of high-pressure gases and volatile components stored in the melts suddenly expand and froth up and then escape; as a result, the melts that have cooled abruptly and are to be solidified into glass are violently exploded to clasts like broken bubble walls, and meanwhile phenocrysts and wall rocks that have been brought up by the melts are also exploded to clasts together. The volcanic suspensions composed of rock, crystal and glass fragmentsand a large volume of gases and water occur as gigantic pyroclastic flows and flow all around. They are partly squeezed into wall rocks to exhibit an intrusive contact, partly ejected into the air, and mostly become flows to flatten out and spread all around at high speed to the lowlying land along the slope of the ground in the course of continuous gas escape and self-explosion. Then pyroclasts （mainly glass fragments） that are in a candescent state and have not yet been completely solidified undergo plastic deformation under the flowing and overburden pressures. After localized, they, depending on the remaining heat stored in themselves and the static pressure of the rocks, are packed tightly and weld themselves together to form ignimbrites. Afterwards owing to voluminous pyroclastic eruptions, pressures and gases contained decrease greatly and the magma chamber is emptied, thus resulting in collapse of the crater and opening of the vent. At that time, the melts can neither continue to blow nor be exploded to bits, but be only expressed by relatively quiet effusive eruptions. Finally, the centre of the vent is commonly choked by stagnant contemporary melts or hypabyssal intrusions and then the activity is terminated.