Tassie quake where hotspot snoozes

08 April 2002

Was the recent earthquake that occurred in February off the coast of north-west Tasmania a signal that one of Australia's hotspots is about to reawaken?

According to Geoscience Australia, the earthquake happened at 11:48 p.m. (Eastern Summer Time) on Thursday 7 February and measured 4.5 on the Richter scale. It occurred roughly 50 kilometres south of King Island. There was one foreshock and several aftershocks.

This particular quake occurred in a region between Victoria and the west coast of Tasmania - an area that scientists calculate is one of the most likely spots for the birth of a new Australian volcano.

Hotspot volcanoes: a way of cooling off

Humans have sweat glands, while the Earth has hotspots. They are one way our planet gets rid of internal heat.

A hotspot is a long-lived plume that is hotter than its surroundings. It originates from deep in the mantle, which surrounds the Earth's core. The mantle is rock that behaves plastically - somewhat like warm plasticene - because of the heat and pressure involved.

The hotspot plume is buoyant and rises until it meets the brittle outer part of the Earth, called the crust. The crustal rocks heat up and melt, creating large amounts of magma. Then it's the magma's turn to rise. And a volcano forms when it reaches the surface.

But the Earth's crust is constantly moving via the process of plate tectonics. It means this active volcano moves away from the fixed hotspot and becomes extinct after a few million years, and a new volcano forms above the hotspot.

Over time, this process of rising, melting and moving creates a sweeping chain of volcanoes - the pattern is similar to the scorch marks made on a piece of paper that is slowly and steadily moved across the top of a candle flame. The volcanic chain can tell us how fast a piece of crust has been moving and in which direction.

Hotspot system in our backyard

We have a hotspot system of our very own. Australia's hotspot currently lies under Victoria, Bass Strait, Tasmania, and the floor of the Tasman Sea at a latitude of about 40°S. It's one of more than a hundred systems identified around the world.

As far as hotspots go, the one in our backyard is slumbering. Present hotspot activity is possibly confined to the triggering of earthquakes in predicted areas, such as the recent event off the coast of north-west Tasmania, and deep gas discharges under Victoria and Tasmania. But earthquakes alone don't mean an eruption is imminent.

Don't be alarmed - there are no active volcanic edifices forming just yet. They may sprout up in the next few tens of thousands of years (if at all). It's quite soon in geological terms, but for most of us this is the distant future.

Our continent has moved compared to its hotspot system. With Australia moving northwards away from Antarctica at about seven centimetres per year, the hotspot system has spawned volcanic chains along the east coast.

One chain of about thirteen volcanoes begins in north Queensland. The largest in this chain is the Tweed Volcano, where Mount Warning represents the main vent. The chain extends south from the Cape Hillsborough Volcano in north Queensland through to the Macedon Volcano in Victoria. Other volcanoes in the chain include the Glasshouse Mountains, the Warrumbungles and Canobolas near Orange. The volcanoes are quite young. The oldest ones are found in north Queensland, while the youngest are in Victoria - the most northern volcano formed around 33 million years ago.

Another chain of hotspot volcanoes extends along the floor of the Tasman Sea and includes the Gascoyne seamount.

Topic contact: media@ga.gov.au Last updated: October 4, 2013