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Your message to the editors. Your email only if you want to be contacted back. Send Feedback. Thank you for taking time to provide your feedback to the editors. E-mail the story What is the difference between active and dormant volcanoes? Your friend's email. Your email. I would like to subscribe to Science X Newsletter. Learn more.
Your name. Note Your email address is used only to let the recipient know who sent the email. Your message. Your Privacy This site uses cookies to assist with navigation, analyse your use of our services, collect data for ads personalisation and provide content from third parties. Ok Cookie options. E-mail newsletter. It appears that you are currently using Ad Blocking software. The length of dormancy is tricky as well. A volcano could be dormant for tens to maybe even hundreds of thousands of years, provided there are some bouts of unrest that suggest it has the potential to erupt again.
Remember, lots of volcanoes also have very active hydrothermal systems that circulate water from the surface to depth where they are heated up by cooling magma. Even if the volcano has hot springs, mudpots, even geysers, those are hydrothermal features, not volcanic.
A body of magma of decent size that feeds an eruption could take half a million to a million years to cool and crystallize to reach equilibrium with the temperature of the rocks around it.
All that heat can drive a lot of hydrothermal action. Extinct: It takes a lot to be an "extinct" volcano. The rule of thumb I use is about 1 million years since the last eruption Sutter Buttes , just outside of Sacramento in California, is an oddity of a volcano in the Central Valley. This means that Sacramento likely doesn't need to worry about the Buttes because they are extinct.
This doesn't mean that a new volcano can't form in the same place. If you look at many volcanoes in the Cascades, such as Baker or Lassen Peak , they are built on the remnants of ancestral volcanoes that might be millions of years old.
There seems to be evidence that once a volcano is established in one location, it is a preferred path for magma to use for millions of years — but that might be in bursts of , years, then quiet for a million, then a new volcano.
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. 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. 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. 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. 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. 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.
There are two currently debated explanations of how this volcanism is generated:. 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. 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 kilometres south west of Perth in the Australian Antarctic Territory: Heard Island and the nearby McDonald Islands.
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.
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