Volcanoes

Introduction

The Australian continent is generally not regarded as a land of volcanic activity, as there has not been an eruption in over 4000 years. Even though there have been no recent eruptions, Australia has had a rich history of volcanic activity. The majority of volcanoes are located on the east coast, along a 500 kilometre band. This volcanic area stretches from Mount Hypipamee in north Queensland, running south to Cradle Mountain in Tasmania, and spreading west to Mount Gambier in South Australia. Over the past 40 million years, close to one-third of this area has experienced scattered to intense volcanic activity. See image 1

The formation of volcanoes

A volcano is a geological landform created by magma, or molten rock, that comes up from beneath the Earth's crust. The magma either explodes out of the volcano in a violent, destructive eruption or flows through the mouth of the volcano. Once the magma pours out of the volcano it is called lava. There is no chemical difference between lava and magma. Volcanic activity generally occurs at the boundaries of tectonic plates. See image 2

Rifting and subduction

Volcanoes are formed at plate boundaries through two conditions, rifting and subduction. Rifting occurs at divergent boundaries where two tectonic plates separate, which allows magma to surge to the surface. Subduction takes place at convergent boundaries when one tectonic plate slides underneath another plate. The eruptions that occur as a result of subduction are often explosive, spewing remelted crust, ash, lava and rock together with significant quantities of pressurised steam. Since the border of the Indo-Australian plate is far away from the Australian mainland, there are not very many volcanoes in Australia that formed under these conditions. See image 3

Plumes and hot spots

Not all volcanoes are located on tectonic plate boundaries. Some exist in the centre of plates and are sourced by plumes or hot spots. Plumes are upwardly mobile bodies of magma rising from the lower mantle. They pass through the upper mantle and slowly push towards the surface by melting the overlying rock. Hot spots form when plumes remain connected to the lower mantle, allowing for a continuous source of magma on which the volcanoes feed. As the tectonic plates drift over the hot spots, volcanoes activate on the surface. Most of the Australian volcanoes active in the last 35 million years have developed from plumes or hot spots.

Active, dormant or extinct

Volcanoes can be categorised according to three states: active, dormant or extinct. Scientists consider a volcano active if it is erupting or showing signs of unrest, such as abnormal earthquake activity or significant gas emissions. Dormant volcanoes are those that are not currently active, but could potentially erupt again. Volcanoes deemed extinct are those that are unlikely to re-erupt. Australian volcanoes are either dormant or extinct. The most recent volcanic activity on the Australian mainland was the eruption of Mount Gambier volcano in South Australia around 2900 BC.

Although there are no longer volcanoes erupting on the Australian mainland, volcanoes are still a part of the Australian landscape. The remnants of volcanic mountains can be seen along Australia's east coast and in Tasmania. Some of the oldest volcanoes can be seen as volcanic plugs, which are volcanoes that have been so eroded that the only part of them left standing is the strong volcanic rock found in the volcano's core. Some examples of volcanic plugs include the Warrumbungle Mountains in New South Wales and the Glass House Mountains in Queensland. Volcanoes have also shaped Australia by providing volcanic ash that later formed into rich, fertile soil.