• Rural NZ
• Assistance / Emergencies
• People issues
• Profitability
• Research
• Rural Proofing
• SLM Hill Country Erosion Programme
• Community Irrigation Fund
• Stats & forecasts
• Sustainability
• Legislation
• Media Centre
• Publications
• What's New

• 3.2.1 Near-vent Destructive Hazards
• 3.2.2 Distal Hazards

---

3.2 Impacts of Volcanic Hazards

Typically a number of types of hazards will result from a volcanic eruption. The most threatening hazards include pyroclastic falls, pyroclastic density currents, lava (flows and domes), lahars and flooding, debris avalanches and volcanic gases. These hazards can be divided into two categories; near-vent destructive hazards and distal damaging and/or disruptive hazards.

3.2.1 Near-vent Destructive Hazards

Pyroclastic Falls

Large fragments (blocks and bombs > 64 mm) follow ballistic trajectories and are here termed projectiles. Large ballistic projectiles rarely land more than 1-2 kilometres from a vent in a brittle or molten state and are capable of starting fires. The impact of ballistic clasts will cause damage to buildings (including ignition), with the degree of damage dependent on projectile mass, temperature and velocity. Projectiles present a high risk of death or injury to people and animals.

Finer material (ash < 2 mm and lapilli 2-64 mm) is convected upwards in an eruption column (Self & Walker 1994) and dispersed by the wind (see distal hazards section). Finer material can form thick pyroclastic fall deposits capable of overloading roof strength causing collapse and possible death or injury to people or stock inside. Since building collapse usually requires ash thicknesses in excess of 100 to 300 mm, an area affected will usually be limited to within a few kilometres or tens of kilometres of the volcano (except in the case of very large rhyolite eruptions). Deaths and injuries are also likely to result from falling branches or other accidents.

Pyroclastic Density Currents

Pyroclastic density currents (flows, blasts, and surges) often travel at speeds up to 200 km/h, and cause total destruction in the areas they cover. Surges and blasts are more dilute, turbulent, and widespread in their effects than flows (White 1996). Flows are more concentrated and are topographically controlled (Sparks 1976). Pyroclastic density currents from magmatic eruptions are usually very hot (several hundred ^oC) and can start fires. Pyroclastic surges from hydrothermal and phreatomagmatic eruptions are cooler (usually less than 300^oC) and often deposit sticky wet mud. Pyroclastic flows and surges have been produced by many eruptions from New Zealand volcanoes and represent one of the most destructive manifestations of volcanic activity.

People or animals caught in the direct path of pyroclastic density currents are most unlikely to survive and any survivors will probably receive severe injuries. Buildings offer some protection on the periphery of the flow but will not guarantee survival as the building may be destroyed or severely damaged. The best protection is to evacuate the area prior to the event. Pyroclastic density currents will cause destruction of vegetation caught in its path, often removing the forest cover by uprooting and stripping foliage, branches and bark. Heat damage to plants may also occur Heat damages the hydrated tissues of plants. Damage to buildings and other structures depends on the temperature, duration of flows and amount of solid material it carries.

Lahars

Lahars are mudflows formed by mixing of volcanic particles and water. They can be generated by the collapse or overtopping of a volcanic barrier impounding a lake or river, or simply heavy rain washing unconsolidated volcanic material from slopes, or directly from pyroclastic flows or debris avalanches (Neall 1996). Lahars have densities greater than normal river flows and travel at greater velocities; therefore they are more energetic and are highly erosive to river banks.

People or animals caught in the path of a lahar have a high risk of death from severe crush injuries, drowning or asphyxiation. Lahar events will cause destruction of buildings, installations and vegetation caught in their path. Depending on their densities and flow velocities, lahars may destroy structures, or bury them in situ. People have survived lahars by climbing onto the roofs of houses which have remained intact despite inundation by the lahar.

Lava

Lava flows are streams of molten rock that flow downslope from a volcano. The distance lava travels depends on the viscosity of the lava, output rates, volume erupted, steepness of the slope, topography and obstructions in the flow path (Cas and Wright 1987). Basalt flows have low viscosity (flow easily) and have been recorded to travel more than 50 km from a volcano but usually only flow 5-10 km. Andesite flows are more viscous and rarely travel more than 5 km. Dacite and rhyolite lavas have high viscosity and typically form short, thick flows or domes.

Lava flows will also cause complete destruction of vegetation, often igniting trees and shrubs. Lava flows will seldom threaten human life because of their slow rate of movement. The steep fronts of flows may became unstable and can collapse, causing small pyroclastic flows. Lava flows will also cause total destruction of buildings and other infrastructure in their path.

Sector Collapses and Debris Avalanches

A debris avalanche is a sudden and rapid movement of rock and associated materials due to gravity (Siebert 1984). Debris avalanches usually occur at larger over-steepened volcanoes and are one of the most hazardous of volcanic events. Debris avalanches can travel considerable distance from the summit area and destroy everything in their path. As debris avalanches can occur with little or no warning and can travel at high speeds, prior evacuation is the only safe option for areas that might be affected if an avalanche is anticipated.

Volcanic Gas Hazards

Magma generated at the base of the earth's crust attains buoyancy through density differences between the molten material and adjacent solid, or semi-solid crustal rock. Density differences are attributable to two factors. First, the liquid phase is of intrinsically lower density than the adjacent solid rock, and secondly, as the magma ascends through the crust, confining pressures decrease leading to exsolution of a discreet volatile phase from the magma. It is widely argued in the literature as to where in the crust exsolution occurs, but its contribution to magma buoyancy is tremendous, and results in accelerated ascent of the magma. As the magma approaches the surface, it is common for the volatile phase to decouple from the magma, and continue on to the surface independently of the molten material. Although the processes have been vastly simplified here, this illustrates how significant quantities of magmatic volatiles can be released from a volcano between, or even in the absence of, eruption events.

The magmatic gas stream typically consists of, in order of decreasing abundance, H₂O, CO₂, SO₂, H₂S, HCl, HF and a number of other minor species, including metal-bearing chloro-, fluoro- and sulfido- gas species. All of these gases enter into chemical equilibrium relationships with each other, and with the enclosing hydrothermal system. The various equilibrium relationships are discussed by Giggenbach (1996), Symonds and Reed (1993), and Christenson & Wood (1993).

Plume gases released from volcanoes such as Ruapehu and White Island contain acidic constituents from essentially two end-member sources, including those coming directly off the magma, and those derived from the hydrothermal system which typically envelops (and/or covers) the magma column. These end-member fluids become thoroughly mixed in the vigorous, high temperature environment within the eruption vent, and within the plume itself.

Chemical data for the major fumarolic gas species from Ruapehu and White Island are listed in Table 3.2. By virtue of the fact that these samples were collected during relatively quiescent periods from the respective volcanoes, they clearly show evidence of interaction with shallow surface waters, and therefore are not perfect analogues of the main vent gases. Never-the-less, the component ratios for the sample for Ruapehu sample site "B" are close to theoretical values derived for magmatic gas streams from other volcanoes (eg White Island Giggenbach and Sheppard, 1989; Table 3.2), suggesting that this particular discharge is representative of the carbon, chlorine and sulphur contents of the magmatic gas from this volcano.

There are three agricultural hazards associated with volcanic gases. The first, and usually most benign in an agricultural context, results from direct exposure to gases emanating from active volcanoes. Symptoms from such exposure are restricted to respiratory and eye irritation in humans and animals, and significant exposure is likely to be restricted to zones within about 5-7 km of the active vents. Although there have been instances where heavy gases released from active vents have flowed considerable distances down confined channel-ways (valleys) or slopes from volcanic vents, leading to suffocation and/or poisoning of humans and animals in the pathway, this is not expected to be a serious threat in the New Zealand context.

The second threat to livestock and crops stems from the interaction between volcanic gases and atmospheric environment adjacent to the volcano leading to the development of vog (volcanic fog) and/or acid rain. Under the appropriate atmospheric conditions, the soluble acidic components of the gases (SO₂, H₂S, HCl, HF) can be readily absorbed into aerosol droplets (condensed steam, cloud or rain), and subsequently dispersed downwind to rain out on livestock and crops. Whereas the effect on livestock is not considered to be a serious threat, acid rain and fog can cause considerable damage to crops and farm plant/machinery (ie. Corrosion).

The third threat posed by volcanic degassing results from the uptake of the water soluble plume gas species described above onto aerosol particles and ash, which subsequently falls out onto pastures and forests. As shown in the 1995 eruptions of Ruapehu, ash with even moderate soluble chemical burden proved lethal to sheep and deer. The characteristics of these leachable components are described below (Tables 3.2, 3.3).

Table 3.2 Fumarolic Discharge Chemistry. All values as mmol/mol total discharge. ^1. Data from Christenson (1998). 2. Data from Giggenbach and Sheppard (1989).

Volcano	Date	Site	T(°C)	CO2	St	Sn	H2S	SO2	NH3	HF	HCl	He	H2	O2	N2	CH4	Ar	CO	H2O
Ruapehu1	6.12.95	A	281	16134	12274	3.00	2040	10234	2.5	32.9	611	0.14	184.5	0.82	108	-	0.12	-	970652
Ruapehu1	18.1.96	A	92	10950	3311	2.74	693	2618	0.2	0.8	30	0.11	123.8	0.49	80	-	0.10	-	985504
Ruapehu1	26.2.96	A	94	30816	1089	4.00	0	1095	2.8	0.5	61	0.35	38.08	0.00	667	0.174*	5.77	0.13*	967319
Ruapehu1	26.3.96	B	190	33135	15772	0.59	8964	6808	16.5	4.5	7456	0.25	18.28	599.27	3517	0.015*	39.46	0.10*	939443
Ruapehu1	26.3.96	C	215	18694	6465	1.92	2245	4220	58.7	73.8	1518	0.15	79.82	1.37	151	0.015*	0.15	0.42*	972958
Ruapehu1	10.5.96	A	92	9416	4300	-1.95	4266	34	0.6	0.4	29	0.06	109	0.00	74	0.390	0.20	-	986071
Ruapehu1	28.2.97	D	154	15469	5249	2.59	1236	4013	17.5	na	346	0.07	509.0	<.02	151	-	0.18	6.18	978252
White Island2	13.12.76	3	100	146300	14430	4.10	0	14430	10.3	16	2460	-	46	<0.8	763	1.0	<.5	1.3	983000
White Island2	21.5.81	3	396	268000	44560	2.70	9655	34905	10.0	37	4150	2.78	1040	6.7	1600	20.1	97.15	-	665000
White Island2	22.7.83	3	685	115200	23250	2.90	4263	18988	18.1	424	10460	.332	888	36.5	711	1.7	2.42	67.2	849000
White Island2	Magmatic Gas Phase			89000	18000						9000								885000

Table 3.3 Crater Lake Water Chemistry, 1995-1996. All values are mg/l.

Date	Tm(°C)	Ta(°C)	pH	Li	Na	K	Ca	Mg	Fe	Al	Cl	SO4	B	SiO2	H2S	F
15.8.95	29	16	0.67	0.51	432	202	906	584	666	1960	8154	26000	18.1	290	<0.05	420
20.9.95	48	21	0.63	0.62	467	196	1037	713	888	2000	8619	30300	20.3	430	<0.05	450
6.12.95	57.7	23	0.7	1.07	766	143	1944	903	556	1160	12536	11400	17.7	415	<0.05	380
20.12.95	60	23	1.04	0.86	582	124	1448	615	528	1030	8127	7070	13.9	471		260
18.1.96	49.6	23	1.06	0.54	332	61	1512	367	362	580	5664	5780	14.2	370	<0.05	240
26.2.96	55	17	1.2	0.26	173	26	1020	183	236	220	2684	5251	5	222		110
26.3.96	49.6	17	1.07	0.53	326	26	1120	378	318	377	4343	5483	17.7	261		90
10.5.96	65.6	20	0.99	0.86	655	52	1529	705	500	615	8103	12019	14.8	442		130

3.2.2 Distal Hazards

Pyroclastic Fall Deposits

Fine ash can be deposited hundreds to thousands of kilometres from its source, making volcanic ash the product most likely to affect the largest area and the most people during an eruption. The physical and chemical properties of volcanic ash are discussed in more detail in the following section of this report.

Sedimentary Response

The impact of ash fall on hydrologic systems depends on a number of factors, including: thickness of the deposits; grain-size distribution; nature of the substrate i.e. slope angle and degree of vegetation cover; and climate, in particular the intensity of precipitation. There are two main classes of impact: (1) hydrologic effects such as run-off, flashier stream discharges and higher flood peaks, due to enhanced surface run-off and reduced infiltration rates in catchments, and (2) erosion and resedimentation processes, which may be partly a function of the hydrologic effects and which act to remobilise and redistribute the ash.

Contact for Enquiries

Manager
North Island Regions
Sector Performance Policy
MAF Policy
Hamilton
NEW ZEALAND

Phone: +64 7 957 8313
Contact this person