Previous page     Contents     Next page

During rapid decompression of lava domes and cryptodomes caused by collapse events, fragmentation of highly viscous magma can occur. To study this phenomena experiments were recently performed on fragmentation of gray dacite of the May 18, 1980 Mount St. Helens eruption, at temperatures up to 915°C and a pressure differential of 12 MPa using a recently developed high-temperature shock-tube type apparatus. Brittle disruption of the dacite was observed over the entire experimental temperature range (15 � 915°C) implying that during rapid decompression, the behaviour of highly viscous vesicular dacite is similar to the behaviour of solid vesicular dacite. Experiments on dacite fragmentation at moderate temperatures offer the obvious advantage of direct measurements and observations during fragmentation, which are very difficult to conduct at high temperature. To understand the mechanism of highly viscous vesicular magma fragmentation by rapid decompression at high temperature conditions it is important to know the mechanism of fragmentation of such vesicular rocks under room temperature conditions.

The main objectives of the present experiments on dacite fragmentation at room temperature were therefore: (1) to determine the value of the pressure drop and the moment of the onset of fragmentation, (2) to measure the duration of the fragmentation event and the fragmentation velocity, (3) to constrain further a mechanistic model for fragmentation.

The fragmentation dynamics of vesicular gray dacite have been studied at room temperature. The experimental apparatus consists of a high pressure section and a low pressure section vented into a large tank (Annual Report 1995). Cylindrical dacite samples (diameter = 17 mm, length = 50 mm) are placed in the high pressure section of the apparatus, saturated by argon gas and rapidly depressurized from initial pressures of 1.0 - 18.5 MPa down to 0.1 MPa via the disruption of a diaphragm located between the high and low pressure sections. Samples of the 1980 Mt. St. Helens cryptodome dacite, whose physical properties of have been well documented (Annual Report 96) were used in the experiments. The high-pressure section was equipped with two piezoelectric pressure transducers installed near the free surface of the sample and beneath the sample, respectively. These transducers provide a record of the pressure history during fragmentation.

The main results may be summarized as follows: (1) Fragmentation occurs when the value of the pressure drop on the sample surface P exceeds the critical fragmentation pressure P_fr 8.5 - 12.0 MPa (Fig. 3.7-2). The value of P_fr increases with an increase in the initial pressure differential between the high and low pressure sections P₀; (2) The time interval between the beginning of sample decompression and the beginning of fragmentation, t₁ 0.5 - 2.0 ms, decreases with increasing P₀. The time interval between the beginning of sample decompression and the end of sample fragmentation t₂ 0.8 - 11.4 ms also decreases with increasing P₀. The effective decompression rate corresponding the fragmentation onset | dP/dt |_fr 5.0 - 23.0 MPa/ms increases with the increase of P₀; (3) The fragmentation velocity, N, is in the range of ~5 - 170 m/s and increases with an increase in P₀ (Fig. 3.7-3); (4) Dacite fracturing is accomplished by the propagation of a fragmentation wave front that generates a layer-by-layer
 

Fig. 3.7-2: The pressure history of a fragmenting sample near the upper end of the sample, recorded by transducer T1, and near the lower end of the sample recorded by transducer T2; experiment at T0=20°C and P0=13.7 MPa. General view, showing the time of fragmentation onset (t1) and the time of fragmentation cessation (t2), and peak on the pressure curve (Pfr) recorded by transducer T1, corresponding to the fragmentation onset.

Fig. 3.7-3: Fragmentation velocity (N) vs. the value of initial pressure differential between high- and low-pressure sections (P0).

fracturing of dacite and ejection of fragmentation products by a high pressure gas expanding towards the low pressure zone. At P₀ only slightly exceeding the threshold for fragmentation the process can exhibit a multiple or sequential character and is characterized by very low effective fragmentation velocities ( ~5 m/s).