KEY POINTS:
William Pike, the man seriously injured when Mt Ruapehu erupted on Tuesday, has suffered renal failure and was operated on for the fourth consecutive day yesterday.
His father, Barry Pike, said through a Waikato Hospital spokeswoman that the breakdown in his son's kidney functions was being treated and he remained in a stable condition.
The 22-year-old climber's right leg was amputated below the knee after it was crushed by debris from a lahar, which included a rock at least the size of a bowling ball.
The family, including William's mother Tracy and brother Andrew, 18, have been at his bedside all week, and have been joined by a group of friends.
One of those people, Jamie Anderson, said through Mairangi Bay Surf Life Saving Club spokesman Grant Morrison he could not talk about his friend at the express wish of the Pike family.
But Mr Morrison said the events of the week had been tragic and clubmates were devastated at what had happened.
Mr Pike was an active member of the club and took part in surf boat rowing.
Mr Morrison said the news of renal failure was a worry. "We were all set to leap into support mode and go and wish him well and visit him in hospital. We can't do that at the moment, he's not in a position to receive guests."
Renal failure takes two forms, chronic and acute, and it is likely that Mr Pike has suffered the latter.
Acute renal failure is serious and is usually treated as a medical emergency. If treated quickly it can be reversible. It comes under three distinct categories, pre-renal, renal and post-renal.
Pre-renal failure is caused by problems with the blood supply, and can be triggered by shock or dehydration and fluid loss.
Renal failure is caused by damage to the kidney itself, either by way of infection, involvement of toxins or medication, or breakdown of muscle tissue and red blood cells.
Breakdown of muscle tissue, called rhabomyolysis, is caused by injury, commonly crush injuries and extensive blunt trauma. It can also be brought on by raised levels of statins, stimulants and some other drugs.
Post-renal failure is caused by obstructions to the urinary tract, which can be the result of medication, prostate cancer, kidney stones or an obstructed urinary catheter.
Chronic renal failure is a progressive loss renal function over a period of months or years. It is not reversible, and leads to severe illness. It requires therapy such as dialysis.
Ruapehu - there's good and bad news
Volcanologists think there is only a 10 per cent chance of Mt Ruapehu erupting again this month - but they cannot rule out more activity in the near future.
According to Geonet, the alert level of the country's most active volcano remains at 2 (with 5 being the most hazardous level), meaning an "onset of eruptive activity, accompanied by changes to monitored indicators" and "minor eruptive activity".
GNS volcanologist Brad Scott said that Tuesday's eruption was different from those of 1995 and 1996.
"They were magmatic in nature. This one was more the result of activity in the hydrothermal system."
That meant a build-up of pressure beneath the crater lake surface where rocks, fluids and gases mixed at depths within 2km.
"You could think of the hydrothermal system as being much like a stove. The magma below the earth would be the element on the stove, and the crater lake is like the water in a pot, where a lot of chemicals are reacting in the hydrothermal system."
The crater lake had over time "clogged up" the system, much like a pot lid that had been put on too tightly for too long. Because of pressure, the lid finally blew off this week.
The eruption was different from that of 11 years ago because there had been no evidence of magmatic activity deeper down.
Testing showed that "the element had not been turned up" this time round, Mr Scott said.
GNS Science volcanologist Craig Miller, based at Wairakei, said Ruapehu presented complications for people trying to predict its behaviour.
"The lake can absorb many of the gases, making it [magmatic activity] difficult to measure. Gas on the surface is hard to detect because the lake is in the way."
This was unlike Ruapehu's cousin, White Island, where carbon dioxide and sulphur dioxide were easy to measure. Scientists there knew that magma was present at a depth of about 2km because of the gases at the surface.
Mt Ruapehu and White Island are two of four active volcanoes in New Zealand, the others being Mt Ngauruhoe and Raoul Island, 1000km north-east of the Bay of Plenty coastline.
How often?
Average frequency of eruptions:
* Mt Ruapehu: Once every five to 10 years.
* Mt Ngauruhoe: Once every five years.
* White Island: Once every two years.
* Raoul Island: Once every 50 years.
