Movement and emplacement mechanisms of the Rio Pita volcanic debris avalanche, and its role in the evolution of Cotopaxi volcano
This research investigates a previously unidentified geological deposit at Cotopaxi Volcano, Ecuador: the Rio Pita Formation. The thesis is in two parts. The first defines and identifies the Formation, and shows that it is a volcanic debris avalanche deposit. It also investigates the nature of other deposits in the Rio Pita area, and thereby builds up a revised evolutionary history of Cotopaxi volcano. The second part questions more closely two controversial aspects of volcanic debris avalanches: their mode of travel, including the reason for their great mobility; and the development of their characteristic topography, the mound field. Despite earlier studies which suggested otherwise, Cotopaxi is shown to have produced highly silicic magmas (72&'37 SiO2) as well as more basic material. The Rio Pita volcanic debris avalanche occurred about 4000 years ago, probably as a result of a high level silicic magma intrusion. It was associated with a blast and a rhyo-dacite eruption, and travelled at more than 28ms-1 (100kmh-1). Its H/L ratio was 0.1, and it expended a total energy of more than 9.5 x 1016J. After the avalanche, the volcano was dormant or non-explosive for about 1000 years, before producing a series of increasingly more basaltic tephras (59&'37 - 56&'37 SiO2). Cotopaxi's last major eruption was in 1904. This study tests various hypotheses proposed for volcanic debris avalanche movement by comparison with the field evidence at the Rio Pita Formation. The movement hypotheses best able to explain the sedimentary features of the Formation are sliding, for the block facies, and a combination of fluidised flow with grain flow at the base for the matrix facies. It is proposed that the Rio Pita volcanic debris avalanche was fluidised by a mixture of gas and dust. This fluidising medium was created partly by the avalanche itself, due to friction and clast comminution during movement, and partly by the volcanic blast which accompanied the avalanche. The avalanche was only rapidly mobile for as long as the depressurising fluidising agent stayed within the avalanche: a period of between 3 and 12 minutes. During this time, the avalanche was inflated by a factor of five, and had developed a head. The escaping gas and dust tended to blow through the avalanche debris along preferential routes, a process termed 'perflation'. Perflation is also able to account for formation of the mound-field. In the past, there had been various differing ideas regarding mound-field development. However, the fact that the mounds differ in character to those of non-volcanic debris avalanches and landslides suggests that they are formed by a process peculiar to volcanic events. Perflation would enhance mound development because the escaping gas and dust follows preferential routes through the avalanche, throwing up debris as it does so. Areas from which the debris has been thrown up are left as hollows, and the other areas from mounds. Simple experiments show that this effect does occur, but more experimental and theoretical work is required before the hypothesis can be generally accepted.