4.2 Near Vent Destructive Hazards
Pyroclastic Falls
As outlined in Section 3, pyroclastic falls 1-2 kilometres from a vent are very destructive and present a high risk to people and animals. Farm land and stock close to a vent would be destroyed by a significant eruption. Rehabilitation of this land would be very difficult and costly due to the coarse nature of the material and amount deposited. Farms at risk include land surrounding Taupo, Okataina and Taranaki vents.
Pyroclastic Density Currents
Pyroclastic flows or density currents can result in devastation up to 20 to 30 kms from the vent, destroying stock, buildings, pastures and crops. Restoration of land affected would be dependent on the nature of the material. Where the flow deposited wet sticky mud the land could take 10-20 years for vegetation to be re-established. Pyroclastic flows may occur from a major eruption of Mt Egmont, Okataina or Taupo.
Lava
Although lava flows are very destructive to land the damage to farm land is likely to be small as they generally only travel 5-10 kms from a volcano with only a small area of farm land likely to be affected.
Lahars
Erosion and reworking of volcanic material can impact on land many kilometres from the original volcanic deposit. Alluvial deposits and mud flows may occur for many years after the eruption.
Ash covered watersheds (normally undisturbed tussock and pasture lands, bush and forests) will affect the supply or continuity of water to rivers and streams. Restricted infiltration in areas with significant ash deposition will disrupt the natural hydrology of the region. Water from rain and snowmelt will be shed more rapidly and will be high in sediment. This will increase the hazards of flooding and erosion. At this time , it appears that little can be done through management to modify the potential effects of ash on land at high ash depths or on steeper land with less ash deposits. Increased runoff and sediment laden water could persist for many years (RJ Cook et al 1981).
For example, a major eruption of Taupo or Okataina could result in the Rangitaiki Plains being unsuited for intensive agriculture for at least 10 years from on going flooding and alluvial deposits of volcanic origin (D Johnston pers comm). It was from alluvial deposits of volcanic material that the Rangitaiki Plains in the Bay of Plenty were originally formed.
| © MAF 1999 | ||
| MAFnet Help | Last updated: 28-Nov-2002 | Important Disclaimer |
Contact for Enquiries
Manager
North Island Regions
Sector Performance Policy
MAF Policy
Hamilton
NEW ZEALAND
Phone: +64 7 957 8313
Contact this person
