lesson 11: volcanoes Flashcards
general volcano knowledge
There are about 1500 potentially active volcanoes around the world with some erupting each day. Eruptions are often not noticed, however as many of these active volcanoes live underwater, their eruptions producing about 3/4 of the total lava erupting from volcanoes each year. There are no active volcanoes in Australia. Mount gambier in South Australia last erupted about 4600 years ago and can probably be said to be extinct. A long period of inactivity does not always mean that a volcano is extinct, however. All volcanoes can be classified as either active or dormant. Active volcanoes erupt regularly. Dormant volcanoes are sleeping. Volcanic activity is still present, but they have lasted 20 to 5000 years without interruption. Dead volcanoes are those that cannot erupt. They are often classified as extinct and have no eruptions for the last 25,000 years. Most volcanoes occur at the edges of the tectonic plates. Pressure from gases in the mantle squeezes the molten rock upwards. The surface of the earth can swell until it cannot take any more pressure. It then explodes with lava, ash, and steam bursting through the surface. Eruptions may come from a single vent, or a group events. Others take place from long cracks called fissures. Most volcanoes just released clouds of steam, gas and smoke which is actually made up of fine rock dust or ash and rock. Magma is molten rock that forms in the magma chamber deep under the earth.
hotspots
Volcanoes are usually located at the weak edges of tectonic plates. Some are nowhere near an edge, however as these volcanoes are directly over hotspots or plumes. Although there are no obvious weaknesses in the plate above it comment the magma has so much pressure that can force its way through. The island of Hawaii lies 3200 kilometres from the nearest plate boundary. Underwater volcanoes form over a hot spot, eventually rising above the sea level to form islands, all of different ages. Although the hotspot never changes its position, the plate above it does, carrying all of the islands either east or West.
shield volcanoes
- Shield cones are very low and very broad shaped volcanoes
- these volcanoes erupt many times over the same area forming huge and thick lava plateaus
- Mauna Loa, which is also on the Big Island, is the largest volcano on earth and the tallest mountain in the world if measured from the floor of the ocean where is formed
cinder cone
- cinder cones get their name from the material that forms them, cinders
- cinder cones are the simplest volcanic formation. They form from explosions of red, hot magma, cinders, and ash
- Please send as an ash settle around the main vent and build a steep sided cone
- Very little lava is erupting from a cinder cone
stratovolcano
- Stratovolcanoes or composite cones are formed from a combination of eruptions
- 1st the volcano will have an explosive eruption that ejects huge amounts of steam, gas, and ash
- This will be followed by the ejection of lava
- A large strata volcano will be built with many layers of ash and lava
subduction zone volcanoes
- Formed at the boundaries of two plates, one overriding the other
- subduction zone volcanoes are the most violent under struct if of the volcano types
- for example mount Saint Helens, Krakatoa, and Mount Vesuvius
mid-ocean rift volcanoes
- form where two oceanic plates are spreading apart. There are more rift zone volcanoes than any other type. These mid ocean or rift zone volcanoes are the world’s longest continuous mountain chain. This mountain chain encircles the entire earth. It is more than 40,000 miles long
hotspot volcanoes
- Usually found under oceanic crust but can be located under continental crust
why are there no active volcanoes in australia
Australia does not lie near the edge of a plate (the Indo-Australian Plate). We have a small number of earthquakes. However, since the continent is a long way from a subduction zone or slip strike zone, volcanoes do not form.
