Ruapehu unrest and the school ski-field trip
At 6:00 am on a Tuesday, Ruapehu's monitoring network shows Crater Lake temperature at 42°C, up from 15°C three weeks earlier. Volcanic tremor has risen steadily for four days, and 38 small volcanic earthquakes (M1.2–2.8) were recorded in the past 24 hours. SO2 flux is 320 tonnes per day, above the usual 50–150. The crater lake is at a high level after heavy rain, and the lahar alarm is armed. A Year 12 ESS class is due at Whakapapa at 10:00 am for a ski-field geology trip; the forecast is clear but cold. The school's policy says trips may proceed if the official volcanic alert level is 2 or lower. The alert level is currently 2. The teacher has one hour to decide. Local iwi and DOC have not issued an evacuation order. The bus is already booked.
1. Should the trip go ahead? Use the monitoring data to justify your decision, and state the main uncertainty.
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Decision: postpone. The data show unrest, not proof of eruption. Lake heating, tremor, shallow quakes and elevated SO2 can all be produced by magma moving or by hydrothermal fluid. Alert level 2 means unrest; it does not certify safety. The most diagnostic combination is sustained tremor plus high SO2 plus a quake swarm, because together they suggest magmatic gas and heat are reaching the lake. The high lake level and armed lahar alarm matter because a lahar could occur without a major eruption, and Whakapapa is downslope. The school policy is a floor, not a full risk assessment: a bus of students is harder to evacuate than a few staff. The main uncertainty is whether this is hydrothermal or magmatic. Additional data (SO2/CO2 ratio, lake chemistry, GPS deformation, seismic character) would help. Because the cost of being wrong is high, postpone.
2. What additional measurements would best reduce uncertainty, and how would each change the interpretation?
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SO2/CO2 ratio: magmatic gas has a higher SO2/CO2 ratio; if it rises with tremor, a magmatic source is more likely. Lake chemistry (Mg/Cl, temperature): hydrothermal heating changes lake chemistry differently from fresh magma injection. GPS/InSAR deformation: uplift or extension indicates pressurisation at depth; no deformation suggests shallow hydrothermal activity. Seismic character: low-frequency tremor versus volcano-tectonic quakes; hybrid events can indicate fluid movement. Lake level and outlet: a high level raises lahar risk, especially with heavy rain. Rainfall and snowmelt can cool or dilute the lake, so temperature must be corrected for weather. Each measurement tests a specific hypothesis, rather than simply adding more of the same data.