An alternative model of the Lassen hydrothermal system, Lassen Volcanic National Park, California
Xu W and Lowell RP. 1998. An alternative model of the Lassen hydrothermal system, Lassen Volcanic National Park, California. Journal of Geophysical Research. 103(B9):20,869-20,881
We describe an alternative two-dimensional numerical model for the Lassen hydrothermal system. As in earlier models, the new model consists of a vertical upflow zone with a low permeability caprock near its top and a permeable horizontal channel that connects to the upflow zone at depth. The new model, however, also includes a re-circulation region beneath the horizontal channel and a recharge zone. Magmatic heating is explicitly represented by specifying heat flows near the bottom of the upflow zone. Simulations show that a periodically appearing vapor-dominated zone, which sometimes consists of pure-steam, develops beneath the caprock and feeds steam-type discharges at high elevations. The model is able to reproduce the inferred ~1:1 discharge ratio between water and steam and to match estimates on current heat output from the system. The development of the vapor-dominated zone does not require dramatic changes in geohydrologic conditions. Our model also shows that both the vapor-dominated zone and the whole system evolve quasi-periodically with a period of ~10^3 years. These oscillations are related to the unstable two-phase hydrothermal fluid flow within the vapor-dominated zone. The observed high-frequency oscillations near the vapor-dominated zone superimposed on low frequency ones probably result from the oscillatory instability introduced by rapid changes in permeability of the low-permeability caprock. The results indicate that the system is probably still in a transient state.
- Type
- Journal Article
- Authors
- Xu, Wenyue; Lowell, Robert
- Units
- KLMN , LAVO
- Keywords
- Bumpass Hell, Hydrothermal, magmatic heating, upflow zone, vapor-dominated zone