It was Horrible, but now it's dead

EXTINCT: South Canterbury's Mt Horrible
EXTINCT: South Canterbury's Mt Horrible

Mt Tongariro reminded everyone on Monday night that, as volcanoes go, it is very much alive - but experts say Timaru's own volcano, Mt Horrible, is dead.

James White, a vulcanologist and associate professor at the University of Otago, said Mt Horrible can be "confidently considered extinct."

But when it was active, about 2 million years ago, it would have been a spectacular thing to witness.

Evidence points to only one eruption, or a series of eruptions in a short period of time, in Mt Horrible's relatively brief career, Mr White said.

That eruption produced a sheet of lava, about 5 metres thick, that flowed over about 130 square kilometres.

"Everything in that area would have been burned," he said, likening it to what now occurs at Kilauea in Hawaii.

Based on observations at Kilauea, the Mt Horrible lavas could have been produced quickly - in a few years or less.

It was likely that the lava flows' gradual spreading would have been very noticeable from day to day, Mr White said.

That was partly because it was the pahoehoe kind of lava, a "thin, runny lava" that flowed easily, he said.

(To see just how easily, search for "pahoehoe lava" on YouTube.)

Most North Island volcanoes are likely the result of melting related to subduction - one tectonic plate moving under another.

But the South Island volcanoes, of which Mt Horrible is the youngest, probably formed in the same way as the Auckland volcanic field, Mr White said - through melting within the plate.

Just what caused the formation of that kind of melting and volcano remains a bit of a mystery, Mr White said.

But Mt Horrible was unusual, for its short but very productive life.