A volcano is a vent or chimney which transfers molten rock (known as magma) from depth to the Earth's surface through eruptions. Magma erupting from a volcano is called lava. Lava builds up around the vent and forms a cone.
A volcano is currently active if it is erupting lava, releasing gas or generating seismic activity. An active volcano is labelled dormant if it has not erupted for a long time but could erupt again in the future. When a volcano has been dormant for more than 10 000 years, it is considered extinct. Volcanoes can remain inactive, or dormant, for hundreds or thousands of years before erupting again. During this time, they can become covered by vegetation, making them difficult to identify.
How explosive a volcanic eruption is depends on how easily magma can flow or trap gas. If magma is able to trap a large amount of gas, it can produce explosive eruptions.
Volcanoes can have many different appearances. Some volcanoes are perfect cone shapes while others are deep depressions filled with water. The shape of a volcano provides clues to the type and size of eruption that occurred. Eruption types and sizes depend on what the magma is made up of. Three common volcano forms are:
Shield volcanoes have a broad, flattened dome-like shape created by layers of hot and runny lava flowing over its surface and cooling. When magma is very hot and runny, gases can escape easily. Eruptions of this type of magma are gentle, with large amounts of magma reaching the surface to form vast lava flows.
Because the lava flows easily, it can move down gradual slopes over great distances from the volcanic vents. The lava flows are slow enough for humans to outrun or outwalk them. This type of magma ranges in temperature between 800 °C and 1200 °C and is called basaltic magma.
Examples of shield volcanoes in the Asia-Pacific region include Taveuni Volcano in Fiji, Dunedin Volcano in New Zealand and Tweed Volcano in Australia.
Composite volcano (Strato)
Also known as strato-volcanoes, composite volcanoes are formed from explosive eruptions. These eruptions create steep sided cones.
When magma is slightly cooler it is viscous (thick and sticky), which makes it harder for gas bubbles to expand and escape. The magma in these eruptions has higher silica content than the magma that forms shield volcanoes. When there is high concentration of silica in lava, the silica molecules link together by sharing oxygen atoms. These bonds are very strong and make the liquid magma act more like a solid.
Examples of composite volcanoes in the Asia-Pacific region include Mayon and Pinatubo volcanoes in the Philippines, Tambora, Merapi and Sinabung volcanoes in Indonesia and Mount Ruapehu and Mount Taranaki in New Zealand.
These volcanos erupt so explosively that little material builds up near the vent. Eruptions partly or entirely empty the underlying magma chamber which leaves the region around the vent unsupported, causing it to sink or collapse under its own weight. The resulting basin-shaped depression is roughly circular and is usually several kilometres or more in diameter. The lava erupted from caldera volcanoes is very viscous and generally the coolest with temperatures ranging from 650 °C to 800 °C and is called rhyolitic magma. Although caldera volcanoes are rare, they are the most dangerous. Volcanic hazards from this type of eruption include widespread ash fall, large pyroclastic surges and tsunami from caldera collapse.
Examples of caldera volcanoes in the Asia-Pacific region include Rabaul Volcano in Papua New Guinea and Krakatoa Volcano in Indonesia.
Volcanic hazards include explosions, lava flows, bombs or ballistics, ash or tephra, pyroclastic flows, pyroclastic surges, mudflows or lahars, landslides, earthquakes, ground deformation, tsunami, air shocks, lightning, poisonous gas and glacial outburst flooding known as jökulhlaups. Each hazard has a different consequence, although not all occur in all eruptions or in association with all volcanoes.
Volcanic eruptions are measured using a simple descriptive index known as the Volcano Explosivity Index (VEI) which ranges from zero (non-explosive) to eight (catastrophically explosive). The index combines the amount of material ejected (by volume) with the height of the eruption column and the duration of the eruption.
Volcanic activity frequently occurs at the boundaries of the Earth's tectonic plates. The movement of these plates plays a significant role in the type of volcano formed, which influences its shape.
Spreading plate margins
Areas of the Earth where plates move away from each other are called spreading or divergent plate margins. In these areas, volcanic eruptions are usually gentle extrusions of basaltic lava. Most of these eruptions occur underwater where magma rises from great depth below to fill the space created by seafloor spreading. This occurs at a rate of about 10 centimetres a year.
Subducting plate margins
At subducting plate margins, one plate is pushed under a neighbouring plate as they squeeze together. In these margins, wet sediment and seawater is forced down in addition to the old, weathered plate. The addition of this sediment and seawater creates andesitic lava and more violent eruptions containing ash. These volcanoes form classic cone shapes.
Locations of anomalous volcanism (i.e. forming at great distances from plate boundaries) are commonly referred to as 'hot spot' volcanoes. There are two currently debated explanations of how this volcanism is generated:
- from hot mantle upwellings or plumes which rise from great depth (hence the term 'hot spot')
- from passive rising of melt from shallow depths that is not actually anomalously hot.
Examples include Hawaii, Iceland and Yellowstone. If the plate overlying the plume moves away from the hot spot, a new volcano can be formed. The previous volcano cools to become dormant and eventually extinct. This sequence forms a volcanic chain such as with the Hawaiian Islands
Sometimes volcanoes can be two types. Iceland is an example of a volcano that falls into two categories. It is a spreading plate margin volcano as well as a hotspot volcano.
Volcanoes in Australia
Active volcanoes generally occur close to the major tectonic plate boundaries. They are rare in Australia because there are no plate boundaries on this continent. However, there are two active volcanoes located 4000 kilometres south west of Perth in the Australian Antarctic Territory: Heard Island and the nearby McDonald Islands.
The other active volcanoes nearest Australia are in Papua New Guinea, New Zealand, Indonesia and the Philippines. Gas-rich sticky magmas dominate the Asia Pacific, making composite volcanoes and calderas the most common varieties in the region. These types of volcanoes severely threaten lives, property, agricultural lands and lifelines throughout south east Asia and the Australian region.
Evidence for volcanism throughout geological time can be found in every Australia state and territory, with extensive volcanism down the east coast during the past 60 million years. This volcanic activity can be divided into areas where large amounts of lava flowed from dykes and pipes over a wide area and others where volcanism was produced from either a single central vent or a cluster of vents.
It is thought that the central volcanoes were produced as the Australian continent moved over a hot spot in the underlying mantle which melted through the plate to form the volcano. As the continent moved northward, the stationary hot spot formed volcanoes further to the south on the continent. As a result, the rocks of central volcanoes down the east coast become younger as you move southward.
Understanding how volcanoes work and how their eruptions can be predicted is essential for the well-being and preservation of people who inhabit volcanically vulnerable areas. Eruptions can occur without any preceding signals, making them extremely difficult to predict. However, sometimes there are useful clues for judging when a volcano is likely to erupt.
A volcano's eruptive history can provide some clues. However, because only a small number of the world's volcanoes have a known history it is extremely difficult to predict future eruptions, particularly for certain types of volcanoes. Scientists use the repose period, or the time between eruptions, to indicate the expected size and strength of an eruption. Consistently long repose periods may indicate that a volcano's eruptions are usually large and explosive. However, sometimes there is no clear relationship in the length of time between eruptions and the nature of the eruptions.
Earthquake activity around a volcano can provide valuable information. An eruption can be preceded by hundreds of small earthquakes know as earthquake swarms. Earthquakes also can indicate that magma is moving beneath a volcano. However, eruptions can occur with no perceivable change in seismic activity.
Small changes in the shape of a volcano such as bulging may indicate that magma is rising. Accurately measuring the summit and slopes of a volcano is one of the most important tools used for forecasting an eruption. Temperature changes in surface lakes or the groundwater near a volcano also can be a valuable early detection tool, although not all large changes in temperature are related to volcanic eruptions.
Gases emitted at, or near a volcano may show that a magma chamber is refilling or that a new type of magma is rising from depth. Changes in the volume or type of volcanic gases produced also may be an indicator of magma activity.
- Volcanic ash clouds can damage aircraft engines but ash is not visible by radar, the main navigation aid for aircraft. There are nine Volcanic Ash Advisory Centres around the world which use satellites to help track volcanic ash clouds and provide warnings for aircraft. The Bureau of Meteorology operates Australia's Ash Advisory Centre.
- On average, one or two eruptions each year worldwide originate from volcanoes with no previous historical activity or after being dormant for thousands of years. The most violent eruptions occur after long periods of inactivity. Eruptions from these types of volcanoes represent some of the worst natural disasters.
- The interior of many volcanoes stay hot for a long time. This heat can warm underground water in the vicinity of the volcano. The heated water then reaches the surface to form geysers, fumaroles, bubbling mud pools and hot springs. They also create occurrences of sulphur and other mineral deposits.
- There is a volcanic province in southeast Australia called the Newer Volcanics Province, which is a collection of volcanoes (almost 400) covering a 15 000 square kilometre area. The most recent eruptions were around 5000 years ago at Mount Schank and Mount Gambier. The area is considered dormant, which means it is not currently active, but could become active at any time.
- There are also active volcanoes located 4000 kilometres south west of Perth on the Australian territories of Heard Island and McDonald Islands. There have been several eruptions in recent years, however, these eruptions have not affected mainland Australia.
- Volcanic eruptions are measured using a simple descriptive index known as the Volcano Explosivity Index (VEI) which ranges from zero (non-explosive) to eight (catastrophically explosive). The index combines the amount of material ejected (by volume) with the height of the eruption column and the duration of the eruption.