Assignments

Do not proceed beyond this point before having read all the previous text. Otherwise certain information will have no meaning to you and the purpose of the exercise is defeated.

Exercise 1

Go to: Taranaki/Egmont Volcano Geology and read about the potential volcanic hazards at Egmont volcano (Mt Taranaki/Egmont).”

Exercise 2

Go to: Whakapapa to view the poster about lahar hazards at the Whakapapa skifield.

Exercise 3

The following pages detail the scenario prepared by Dr I Nairn, of a fictitious eruption of Okaitaina Volcanic Centre for the Nga Puia exercise held by Civil Defence in January and February 1992. Draw up a calendar on the left side of some blank sheets of paper covering each day of this exercise.

- Using the alert levels from the National Contingency Plan for Volcanic Eruption (see table below), read the scenario and note down on the right hand side of your calendar the times when you would have changed the alert levels.
- On a separate sheet of paper give your reasons for each change of alert level.

In this exercise it is imperative that you do not scan all the notes first. Rather you should begin at the start and prepare your decisions, giving reasons as you proceed.

Having completed this exercise do not go back and alter the dates and reasons for your decision making. This is a learning exercise! It puts you firmly in the position of the volcanologist interpreting all the information available at any given point in time, and making it relevant to Civil Defence authorities.

Current New Zealand Scientific Alert Signals to be used in Exercise 1.

Nga Puia volcanic hazards exercise 1992

Prepared by Dr. I. A. Nairn
Institute of Geological and Nuclear Sciences
With minor adaptations by V. E. Neall for 30.701 students

Haroharo Caldera has been experiencing an increased level of seismicity during the last 6 months of 1991. A year-long interval of relative seismic quiescence in the Okataina area was ended by a swarm of earthquakes on 10 July 1991. Twenty three events (7 felt), occurred with magnitudes between 3 and 3.9. These earthquakes could not be accurately located because the seismic station on Mt. Tarawera had been struck by lightning during an electrical storm on July 1. Lack of money and pressure of other works meant that the system had not yet been repaired. However, felt reports suggested that shaking had been most severe at Lakes Rotoehu, Rotoiti, Okataina and Okareka, and the earthquakes appeared to have been centred somewhere in the northern part of Haroharo Caldera. Only minor damage (some crockery breakage) was caused by these earthquakes.

A similar swarm occurred on 21-22 August, with 19 events located between Lake Okataina and Lake Rotoehu. These events were felt in the same areas as the July swarm. All were high frequency tectonic earthquakes with the largest of magnitude 4.1. The occurrence of two such swarms in consecutive months briefly aroused public and media interest, but lack of permanent effects meant that this interest quickly faded. No unusual deformation was recorded by the Lake Tarawera level recorders.

Following the August swarm, intermittent seismicity continued at a lower but still elevated level. Magnitude 3 and 4 events occurred at 2 to 3 week intervals. Another swarm of 49 events occurred on November 15, and was located in the same general area as the earlier swarms. There were indications that some of the November events were located at shallower depths than most of the earlier earthquakes. Again, no surface effects could be detected, and local residents were becoming used to the mild shakings. After another brief flurry of local media interest the events were largely forgotten by the public, particularly as no further seismicity ensued into and during December 1991. IGNS staff at Rotorua however, had recognised that Haroharo Caldera was undergoing a period of unrest, and had accelerated plans for the installation of three extra seismic stations around the northern part of Haroharo caldera.

1. Monday (December 30)

A magnitude 4.9 earthquake occurs at 10 km depth under Makatiti dome (in the centre of the Okataina Volcanic Complex) at 1145 hrs. Minor damage occurs to buildings and contents at Lakes Rotoiti, Okataina, Okareka and Spencer Road - Lake Tarawera; all places where the event is strongly felt. Rotorua City is also shaken. A few small landslides occur on the steep slopes above Lakes Okataina and Tarawera. Fishermen observe small seiches in both of these lakes. Aftershocks commence within 20 minutes, with 8 events of magnitudes between 3 and 4 up to midnight. All these earthquakes have high frequency (tectonic) signatures.

2. Tuesday (December 31)

Four aftershocks occur, with magnitudes between 3 and 4. Two portable seismographs are installed (at Matawhaura and Makatiti). A check of the lake level gauges on Lake Tarawera reveals that the Haroharo gauge appears to have subsided by 6 cm relative to gauges at Te Wairoa and Mt. Tarawera.

3. Wednesday (January 1)

Initial analysis of the seismicity indicates that the earthquakes are clustered along a 15 km-long zone extending between Te Horoa dome and Rotokohu dome. Three further aftershocks occur; magnitudes 2 to 3. Field checks of the forested epicentral area find no breakages on the few known faults, and no other displacements across roads on the Haroharo Volcanic Complex. Two tilt-levelling sites are installed on the complex, on Hainini dome (V16/14834) and the Waiti lava flow (V15/177405). No changes are noted to the solfataric activity at Pararoa (maximum fumarole temperature 90 oC) by investigating geochemists, who also measure temperatures and outflows in the warm spring area at Mangakotukutuku. No obvious changes are noted at this locality.

4. Thursday (January 2)

One aftershock, magnitude 3. Seismicity is tailing off. Completion of quick ground inspections, have found no evidence of surface effects of the earthquakes apart from the small landslides. Detailed analysis of seismic records shows that no long period (low frequency) earthquakes have occurred, and no volcanic tremor has been detected.

5. Sunday (January 5)

Portable seismographs removed. The Oruaroa hydrothermal area on the east shore of Lake Okataina is visited - no obvious changes to gas emission or lake edge water temperatures are seen.

6. Monday/Tuesday (January 6, 7)

Global Positioning Satellite (GPS) survey of marks in the Okataina area is repeated, about 12 months after the previous (first) survey.

7. Tuesday (January 7)

Seismicity resumes. A swarm of 11 magnitude 2 and 3 events, most in the Makatiti-Rotokohu area, but 3 events in the Pararoa-Rotoma area shake residents at Rotoehu and Rotoma. Initial results of the GPS re-survey are received. No significant (>2 cm) changes are detected.

8. Wednesday (January 8)

Portable seismographs are reinstalled at the previous sites. Seismicity increases with 65 events recorded (12 felt); epicentres are scattered between Te Horoa and Rotokohu. Magnitudes range between 1 and 3. The first long period earthquakes are recorded, with earliest arrivals and largest amplitudes on the portable instrument at Makatiti dome.

9. Thursday (January 9)

48 earthquakes are recorded (8 felt) with magnitudes 1 to 3.

10. Friday (January 10)

23 earthquakes are recorded (6 felt) with magnitudes 1 to 3.2. All are tectonic.

11. Saturday (January 11)

Seismicity is decreasing, with 10 events (magnitude 1 to 2.6) recorded, none felt. However, several short bursts (lasting 30 to 70 seconds) of low frequency (2 Hz) low amplitude (1 to 2 mm) vibrations are recorded on the Makatiti instrument during the (calm) night - possible volcanic tremor? Pararoa gas samples (collected on Jan 9) are reported to contain mostly CO2, minor H2S. No significant changes to the gas compositions found in 1987 are noted.

12. Sunday (January 12)

Seismicity continues to decrease, 5 discrete earthquakes are recorded, magnitudes 2 to 2.8. Noise from strong winds obscures the seismic records so that smaller earthquakes and any possible volcanic tremor cannot be recognised. Note: An instrument for detecting SO² in gas emissions, COSPEC, has been ordered from the Canadian manufacturers by the Ministry of External Relations and Trade. This was done to ensure the urgent order is filled as soon as possible.

13. Wednesday (January 15)

No significant seismicity for last 3 days. Field checks have found no surface effects anywhere within Haroharo Caldera (apart from early landslides). No change to fumarole gases at Pararoa. A 1 oC apparent rise measured in the warm springs at Mangakotukutuku (V16/193357) may be due to warmer air temperatures at the time.

14. Thursday (January 16)

A few small (magnitude 1 to 2) earthquakes have been recorded since January 15. No apparent tremor has been seen. Possible slight (c. 4 mm) uplift of the Haroharo gauge on Lake Tarawera, but at limits of accuracy of the technique.

15. Wednesday (January 22)

No further activity. Is this a sign that the period of unrest is over? The COSPEC has arrived from Canada on hire only.

16. Thursday/Friday (January 23, 24)

Earthquake swarm! 112 events recorded with magnitude >2.3, largest is 4.1. Most events cluster beneath the Makatiti-Tikorangi area, but a few occur at greater depth beneath the Okareka area and in the Tumunui Road area of the Ngahuru Graben to the southwest where a magnitude 4.0 event causes slight damage to farm buildings. Minor surface cracking is found on some of the fault traces in this area. A seiche in Lake Rotokakahi sends a surge of water across the road to Te Wairoa and down the Wairoa Stream to Lake Tarawera. Long period earthquakes (8) are detected on January 27 as the high frequency events decrease.

17. Friday (January 24)

Seismicity continues with 48 high frequency (tectonic) earthquakes recorded (13 felt), magnitudes 2.1 to 3.2. Five long period earthquakes are recorded.

18. Sunday (January 26)

Measurements are made at the portable lake level gauges installed in 1990 at Lake Rotokakahi. Minor tilt, down towards the caldera is found.

19. Monday (January 27)

Bursts of low frequency, low amplitude volcanic tremor appear on the Makatiti dome seismic record, each lasting for 30 to 60 seconds, and occurring at an average rate of about 1 per 3 hours. 35 high frequency earthquakes occur, with magnitudes between 2 and 4. Loud booming noises accompanying some of these earthquakes were heard by hunters in the field. Four long period earthquakes are recorded.

20. Tuesday (January 28)

A tilt-levelling site is installed at Tuahu and the initial measurements made. 42 high frequency and 6 low frequency volcanic earthquakes are recorded.

21. Wednesday (January 29)

Uplift of the Haroharo lake level gauge confirmed as exceeding 5 cm (c. 7 microradians) since January 22, relative to the Te Wairoa and Mt. Tarawera stations. 38 high frequency earthquakes, 4 low frequency events are recorded this day. Volcanic tremor reappears on the Makatiti seismograph.

22. Thursday (January 30)

The new lake level sites across Lake Okataina (at U16/063341, V16/101349 and V/16108377) suggest a few mm uplift of the eastern side has occurred in four days. The Tuahu tilt-levelling site indicates 10 microradian uplift to the southeast, towards a centre between Makatiti and Haroharo domes. Seismicity continues at rates similar to those prevailing since January 27, with 42 high frequency and 8 low frequency events. Note: Sites for portable lake level gauges are installed on the east and west shores of Lake Okataina and the initial measurements have been made. Grid references will be advised later.

23. Saturday (February 1)

Seismicity continues; 56 high frequency (hf) events, 6 low frequency (lf).

24. Sunday (February 2)

Increased outflow and temperature of the warm springs at Mangakotukutuku is found. Outflow of the warm springs appear to have doubled, with erosion of the spring outlets and the adjacent stream bed occurring. Temperature of the hottest spring has risen 5 oC to 30 oC. Gas discharge, and outflow of hot water at Oruaroa Beach (Lake Okataina) also appears to have increased, but no changes are found to the iron-depositing springs in Humphreys Bay, Lake Tarawera. The GPS survey is repeated. Significant uplift (>5 cm) of the Haroharo GPS station is reported from the initial analysis of results, with minor apparent uplift (2 cm?) of the Lake Tarawera outlet station. 42 hf earthquakes are recorded, with 8 lf events. Tremor continues on the Makatiti seismograph.

25. Monday (February 3)

Local people report areas of dead and dying vegetation at Pararoa, where increased temperatures (to a maximum of 120 oC) and increased gas discharge in fumaroles are confirmed by geochemists. Presence of SO2 gas is detected by nose. Seismicity continues with 32 high frequency earthquakes (M 2 - 4.1). 12 lf earthquakes are recorded. Tremor continues, with bursts of high frequency microearthquakes.

26. Tuesday (February 4)

Tilting measured at the Tuahu (V16/139394) site continues, up to the SE. 45 hf and 18 lf earthquakes are recorded, with M 4.0 the largest. Sharp bangs are heard accompanying some of the hf earthquakes during the Tuahu tilt-levelling. A 20 cm-high seiche is reported from Lake Okataina.

27. Wednesday (February 5)

The presence of SO² gas a Pararoa (V15/215435) is confirmed by the analysis of gas samples collected on Feb. 3. 50 hf and 4 lf earthquakes are recorded, with the largest of M 3.8. Tremor continues with microearthquakes. A tilt-levelling site at Pararoa has been installed and initial measurements made.

28. Thursday (February 6)

Minor uplift towards the south is found by re-levelling at Pararoa, ie. up-tilt towards the solfatara area. Tilting continues at more rapid rates at the Tuahu levelling site. 43 hf and 6 lf earthquakes are spread over the area between Lakes Rotokakahi and Rotoma. A seiche (to 30 cm) on Lake Rotoma is generated by the largest (M 3.9) hf event, which is located beneath the lake.

29. Friday (February 7)

The temperature of the hottest spring at Magakotukutuku has increased to 35 oC, while outflow has increased by an estimated 50% since the last measurement on Feb. 4. 52 hf and 9 lf earthquakes are recorded; the largest of M 3.8 is located beneath Haroharo dome. Lake Okataina level gauging shows further uplift of the eastern shoreline, with a cumulative rise of 18 mm (7 microradians) since installation on Jan 28.

30. Saturday (February 8)

70 hf earthquakes are recorded, the largest is M 3.8. There are 8 lf events. Volcanic tremor continues to be recorded on the Makatiti instrument. Ground cracking is discovered crossing Ruawahia Road at V16/149382; the cracks strike at 060oN (true). Small slips are occurring on the steep slopes in the vicinity.

31. Sunday (February 9)

185 hf and 20 lf earthquakes are recorded. Volcanic tremor continues. Cracks on Ruawahia Road widen to 2-3 cm across. Millimetre-wide cracks are also found on Rotoiti Road at V16/161388. The Haroharo level station on Lake Tarawera shows uplift of 15 cm (20 microradians) since January 22. Largest magnitude of hf earthquakes was 3.7.

32. Monday (February 10)

Small explosion occurs during hours of darkness opening a vent sited between Pukerimu and Haroharo domes. Seismic data suggests the explosion probably occurred about 0447 hrs when a 3 minute duration E-type earthquake is recorded. Small craters formed along a NE trending rift, c. 80 m long and 3-8 m deep, are emitting steam plumes to 300 m elevation when first observed (from the air) at 0820 hrs. Blast has knocked down trees to 100 m radius. Beyond that, trees are ash coated and have damaged limbs. Blocks of 1 m dimensions have been thrown to 150 m from the vents. Visible ashfall extends 3 km downwind. A thin dusting of fine ash is reported on cars and buildings at Lake Rotoiti. Volume of ejecta roughly 106 m3. Ejecta samples collected from the vent area by helicopter contain much fresh-appearing pumice and glassy rhyolite lava, but this appears to be derived from the older (prehistoric) pyroclastic and lava deposits known to underlie this area. No undoubted new magmatic ejecta can be identified. SO2 detected in plume by COSPEC is being emitted at about 5-10 tonnes per day. Seismicity increases considerably following this eruption, with 434 earthquakes recorded to midnight Feb 11. All are hf (tectonic) shocks, with magnitudes 1.0 to 3.6, and many appear very shallow. Other earthquakes occur near Paparoa and Lake Rotoma. No long period earthquakes are recognised during this period, and no low frequency volcanic tremor could be identified on the very noisy records of nearby instruments.

33. Tuesday (February 11)

Seismicity decreases to 105 hf events per day, largest magnitude 3.5. Volcanic tremor becomes visible on seismic records, with amplitudes ranging between 2 and 4 mm. Eight lf earthquakes occur. Steam emission from the new vents continues. Occasional small eruptions throw blocks to 60 m and small ash plumes are generated.

34. Wednesday (February 12)

Seismicity increases to 207 hf events per day, largest magnitude 3.8. 78 lf events are recorded. Semi-continuous volcanic tremor continues. Fissuring is found to extend across roads on both sides of the vent area, in an en echelon arrangement of discontinuous fractures, forming a 2.1 km-long NE-trending zone centred on the new vent area (V16/155383). Steam is being emitted from several places on these fractures which roughly define a 50 m wide graben, although its ends are largely obscured by trees. No significant eruptive activity occurs this day. Measurements are made at the tilt-levelling sites on the Haroharo Complex, and at Lake Okataina. Level gauges show a regional upward tilt, increasing towards the new vent area. SO2 emissions from the new vent area are measured at 20-50 tonnes per day.

35. Thursday (February 13)

Sesimicity decreases to 157 events per day (largest M3.9), but lf events are again recorded, with 8 on the two closest seismographs. Semi-continuous but weak volcanic tremor (1 to 3 Hz) is recorded on the Makatiti seismograph, now being serviced by helicopter. Steam plumes rise from the new fracture zone, but no significant ash eruptions occur. SO2 emission continues at 20-50 tonnes per day from this area, and about 5 tonnes per day from the Pararoa area. Flow of the Mangakotukutuku warm springs has redoubled, with temperatures of the hottest spring increased to 42 oC.

36. Friday (February 14)

Seismicity increases to 521 events to 2130 hrs, largest magnitude 4.1, with epicentres located between Lakes Rotoma and Rotokakahi. Most events, and the shallowest are clustered between Lake Okataina and Rotokohu dome. 33 lf events are recorded while volcanic tremor continues with increased amplitude (to 250 mm peak to peak). Microearthquakes are recorded at 10 mm peak to peak. A seismic crisis occurs between 2130 and 2330 hrs when local instruments (at Makatiti, Matawharua, and Whakapoungakau) are saturated by a continuous stream of hf earthquakes, M2-4 (determined by more distant instruments). Observers camped on Whakapoungakau summit report no eruptive activity is visible, but they are feeling frequent earthquakes. Many of the earthquakes are felt in Rotorua with intensity MMV-MMVI. Changes to hydrothermal features are reported with increase in flow and temperature of some Rotorua hot springs.

37. Saturday (February 15)

An aerial inspection at 0645 hrs reveals lateral extension of fissuring and deepening of the graben formed around the new vents. New fractures extend up through the Feb 10 deposits, as 0.2-0.8 m wide gaping fractures on the margins of the graben. Further subsidence of the graben has occurred; in places it is now more than 20 m deep. Fracturing extends SW onto the slopes of Pukerimu cone, and NE under the lavas of Haroharo dome. Steam and gas plumes rise to 300-500 m. No changes can be seen at Pararoa, but increases in the outflow and temperature of the Waitangi hot spring near the SE shore of Lake Rotoehu are reported by telephone. Seismicity declines after the end of the crisis. High frequency events occur at 20-30 per hour; lf earthquakes at 2-3 per hour (M 2-3.5). Volcanic tremor remains at roughly constant intensity, when it can be seen on the records. SO2 discharge from the new vent area is measured at 50 tonnes per day.

38. Sunday (February 16)

Seismicity continues to increase above the levels of the last few days. Fine ash is present in the steam plumes rising to 300-500 m. No magmatic ejecta is recognised in helicopter-sampled ash near vent ejecta. SO2 discharge is measured at 50 tonnes per day at the new vents and 10 tonnes per day at Pararoa, where fumarole temperatures have increased to 250 oC and areas of dead vegetation are rapidly spreading. Earthquakes are still clustered around the new vent area, e.g. they extend between Lake Okataina and Rotokohu dome, but several events are located in the Lake Rotoma-Pararoa area and 6 between Te Horoa dome and Lake Okareka (largest magnitude 3.8). Outflow of the Mangakotukutuku springs has doubled again, and temperature has increased to 50oC. Gas discharge and water temperatures have also increased at Oruaroa Beach in Lake Okataina, and gas bubbles are visible rising in the lake off-shore where not previously noticed. Re-measurement of the lake level gauges at Okataina finds accelerated up-tilt to the east. Re-levelling of tilt sites near Pukerimu cone, and on the Waiti lava flow shows sharp up-tilt to the new vent area since the last measurement on Feb 12, and tilting appears to be going on while the measurements are being made. The Waitangi hot spring temperature is measured to have increased from its normal 50 oC to 58 oC, and outflow has increased from 50 L/s to 80 L/s.

39. Monday (February 17)

Earthquakes become larger and more frequent after 0500 hrs. Eight events >M3.8 occur between 0800 and 1100 hrs; the largest is M4.2. There are numerous smaller events. Seiches are generated in Lakes Okataina and Rotoma. Loud bangs and rumblings accompany some earthquakes, but do not correlate with any observed surface effects as observed from the Whakapoungakau observation post, and the air. At 1143 hrs a M4.6 earthquake is recorded, accompanied by some disturbance (seen from the air) and landsliding of the steep slopes cut by the eastern part of the new vent area, followed within a few seconds by the outbreak of steam, quickly followed by an ash-laden steam plume which rises to 1 km. Major explosions follow within 2 minutes, and rapidly generate a vertical eruption column which expands laterally as it rises to 10 km altitude by 1150 hrs. [By flying upwind at maximum speed the two aircraft from which close observation of the new vent area had been made during the initial stages of the eruption, managed to escape the expanding eruption cloud. The near vent area was hidden from view.]

The initial eruptions are phreatomagmatic, as the numerous fractures formed at shallow depth by the rising magma intrusion allowed abundant local ground water to explosively interact with the magma. A fine ash-dominated, water-rich eruption cloud spreads at low elevations (mostly below 19 km) depositing light grey fine ash to beyond 30 km downwind from the vent. Partial column collapse produces small pyroclastic flows and surges which flow north 4 km across the Haumingi and Waiti lava flow surfaces. Most come to rest on this surface, but a small part of the flow is channelled down the valley between the Haumingi and Waiti lava flows, where it accelerates down the steep valley above the Rotoiti settlement and becomes turbulent.

An ash-cloud surge spreads across the area between Gisborne Point and Hinehopu, knocking down most buildings in the area before spreading across the lake surface. Fires break out. Only a few cm of hot fine ash is deposited by this surge. Similar small pyroclastic flows and surges travel to the SE and S of the vent area, down the valleys occupied by Rotoiti Road and the Mangakotukutuku Stream, to reach the Tarawera River. Again only a few cm of ash are deposited, and flow of the river is unaffected. Other flows and surges travel NE into the northern part of the Lake Okataina basin where they destroy buildings. All trees within 3 km of the vent are knocked down; further out, any trees left standing are stripped bare. The wet ash cloud does not rise high into the atmosphere but is carried downwind at below 10 km. Total darkness prevails beneath the cloud, except for local illumination by nearly continuous lightning flashes generated within it, which cause total disruption of local radio communications (including the radio-telemetered seismograph signals). Hundreds of fires are started by lightning in the forest areas near the vent, but these fires do not spread due to the blanketing effect of the ash. The ash is initially lithic-dominated but has a steadily increasing component of pumice ash and lapilli.

Time is now 1320 hrs.

The eruption grows in intensity as the vent is widened by shallow explosions and a clear conduit is established down to the rhyolite magma body. Distant observers, and airport primary radars at Auckland and Ohakea, record a sudden increase in column height from about 15 km at 1325 hrs to reach 30 km at 1335 hrs, continuing to rise to reach 40 km at 1350 hrs. Coarse pumice ash and lapilli shower out of the eruption cloud as it is carried off downwind. Darkness prevails everywhere beneath the ash cloud. Lightning strikes are nearly continuous, starting many fires. All nearby seismographs have been put out of action by 1230 hrs, either by ash damage to antennas or lightning strikes. Distant seismic stations (not radio-telemetered) e.g. at Whakatane and Arapuni, continue to record continuous earthquake activity (the records are largely unreadable) until 1500 hrs, then decreasing to occasional breaks in seismicity. Signals from the Utuhina, Paeroa, and Ohaki telemetered stations continue to be received in Rotorua.

The whole area inside the Haroharo Caldera rim is obscured by rolling ash clouds and total darkness. Partial collapse of the eruption column generates pyroclastic lows which travel radially away from the vent in all directions except to SW, where the high country around Makatiti dome diverts the flows to NW and SE. Thick pyroclastic flows reach the south shore of Lake Rotoiti between Hauparu Bay and Hinehopu, and plunge into the lake, generating waves which travel west down the lake with diminishing height until they are funnelled by the narrows to north of Tikitere and regain amplitude. Waves between 1 and 3 m high flood the built up shorelines to the west of Ngarehu Point and a surge of water passes over the Okere outlet control structure and down the Kaituna River. Larger waves are generated by the passage of pyroclastic flows into Lake Okataina. Pyroclastic flows which travel to the SW of the vent area are funnelled between Haroharo, Parewhaiti and Hainini domes to flood into the Te Haehaenga basin where they infill the Tarawera River channel to depths of 20-30 m. "Rootless" steam explosions occur in these deposits, forming craters up to 100 m in diameter and sending steam and ash plumes up to 2 km in height. Co-ignimbrite eruption clouds of fine ash roll over the entire surrounding area.

Emission of the high plinian eruption column continues through midnight, and into early Tuesday morning.

40. Tuesday (February 18)

An elliptical area within 20 km downwind of the vent has been covered by more than 1 m-deep ash and pumice fall deposits by 1000 hrs on this day. Outside this zone, ashfall exceeds 0.3 m thickness out to 60 km downwind, and to 15 km upwind (due to wind shear at high altitudes). Ashfall to 0.1 m thickness extends to 120 km downwind. The plinian eruption continues, with a column height fluctuating between 20 and 40 km. Deposition rates and coarseness of the ash and pumice fall-out downwind fluctuate in delayed synchronisation with the column heights. Insufficient seismic data is being received in the Rotorua command centre to enable location of eruption-located earthquakes. Seismographs at Taupo and Whakatane record continuing earthquakes in the Haroharo area, but these events cannot be accurately located. Small floods of pumice-laden water pass down the Tarawera River, as local pyroclastic dams burst, carrying away road bridges in the Tarawera Forest. Further large waves on Lake Rotoiti signal the passage of new pyroclastic flows into that lake. Ponding is occurring behind the main pyroclastic dam in the Tarawera River valley, upstream of the Waterfall Road bridge. At 1000 hrs, a moderately large hydrothermal eruption occurs at Pararoa, ejecting hot hydrothermally altered rocks to 2 km downwind from the source. Base surges are funnelled 2 km down to the valleys north and south. All farm buildings in the area are destroyed.

41. Tuesday (February 18)

A dense black column of ash, steam and liquid water rises to 10 km above the Rotoma lake basin, while ash clouds roll over the surrounding countryside, depositing non-vesiculated, glassy fine ash as plastic mud. These eruptions are similar to those which deposited the Rotongaio Mud at Taupo 1800 years ago. Column collapse occurs frequently, and dense base surges fill the Rotoma basin, destroying all buildings and devastating forests out to 4 km from the lake shore. Large waves pass over the western low points to the lake basin, and flood into the swampy area between Rotoehu and Rotoma, lowering the level of Lake Rotoma. Base surges flow through the low saddles into the Rotoehu basin to the NW; cross the low divide to north of Lake Rotoma to flow into the headwaters of the Waitahanui Stream where they destroy forests. Other surges flow through the valley occupied by SH30 to the SE of the lake, and down the Waikanapiti and Ruruanga Stream valleys towards Kawerau. These surges are largely wet and cool, and although isolated fires are started, they do not spread. The eruptions are accompanied by intense rain-out of liquid water separating from the aerosol eruption clouds, and the just-deposited mud is eroded by floods of water which pass as lahars down all the valleys encompassed by the eruption cloud. As Lake Rotoma becomes shallower due to removal of water in the eruption column, and by slopping over the western basin rim, the eruption column becomes drier and rises higher in the atmosphere. A ring of ejecta dams the water from around the vent, and vesiculated ash and pumice begin to appear in the ejecta a the degree of interaction with lake water diminishes. By midnight, the Rotoma eruption column is rising to 25 km, taking over as the main plinian eruption as the Pukerimu column begins to wane. Small pumice pyroclastic flows are generated by intermittent partial collapse of this column, but are confined within the lake basin. The erupted material is estimated at 2 km3 DRE (Dry Rock Equivalent - ie. equivalent volume of non-vesiculated rock).

42. Wednesday (February 19)

The Rotoma eruption column is sustained at 20-25 km heights until 0930 hrs. Ash and pumice falls are dispersed downwind, but the eruption is smaller than that from the Pukerimu vent four days earlier. An elliptical area within 15 km downwind of the Rotoma vent has been covered by >1m of tephra. Outside this zone, tephra is > 0.3 m thick to 50 km downwind (and 10 km upwind), and > 0.1 m to 90 km downwind. Rainfall associated with the eruption (either meteoric, or generated by the eruption) causes damaging lahars to pass down all the valleys where the tephra deposits are thick. Earthquakes accompanying the eruption have caused fault movements on the many faults to the N and NE of the Rotoma area, and roads are cut by 0.2. to 0.5 fault displacements. Floods of pumice continue to pass down the Tarawera River in surges as local pyroclastic dams resulting from the Pukerimu eruption are overtopped. The plinian eruption column from Rotoma slowly decreases in height after 1200 hrs, and the plinian eruption is essentially over by 1700 hrs. Low ash and steam columns persist from both the Pukerimu and Rotoma vents for the next four days, and minor steam explosions are common at Rotoma as residual lake water seeps into the cone of ejecta surrounding the vent. These explosions throw ballistic blocks out to 3 km from the vent and present a hazard to investigating scientists and their helicopter transport.

43. Wednesday (February 19)

Eruptive activity continues to decline. Small hydrothermal eruptions continue at Pararoa. Lava dome extrusion commences at the Pukerimu vent.

44. Days 1 to 6 following the main eruptions (Thursday/Wednesday 20-26)

Ash columns rise to a few hundred metres above the vents. Earthquake activity diminishes, and events are located at greater depths (8-13 km). Only 3-5 events exceed M3 on most days. Volcanic tremor continues at low levels. Ash is analysed to determine its effects on people and agriculture. The main road bridge across the Tarawera River at Kawerau is removed by the largest lahar of the eruption, which also damages other bridges downstream. There is a risk of larger floods on the Tarawera flood plain. Nobody is allowed in the danger area. Rootless explosions continue in the Tarawera valley ponding area. Fires started by the eruptions continue to burn in the surrounding forests; smoke haze adds to the ash loading of the atmosphere. After 5 days, seismicity has fallen to below the immediately pre-eruption levels. No events exceed M3. Steam emission continues, as do rootless phreatic explosions at Rotoma and in the Tarawera River valley. The lava dome continues to grow in the Pukerimu vent.

45. Day 7 Thursday (February 27)

New ash eruption occurs from the Pukerimu vent, as an explosion destroys much of the small growing lava dome. The eruption column rises 10 km in 10 minutes. The eruption was preceded by 10 minutes of increased volcanic tremor amplitudes but no increase in discrete volcanic earthquakes was recorded.

Exercise 4

Choose any overseas volcanic eruption that has occurred in history and about which you can obtain ample information. Find out which New Zealand volcano is most similar to that which you selected. Now write an account (no more than 10 pages) of the impact of the eruption scenario, you chose, if it were located at the analogous New Zealand volcano. After introducing the eruption scenario, take each volcanic hazard in turn and discuss its likely impact on the community surrounding the New Zealand volcano.