This depression is usually less than 40 kilometers in diameter. These volcanoes form when "wet" granitic magma quickly rises to the surface of the Earth. When it gets to within a few kilometers of the surface the top of the magma cools to form a dome. Beneath this dome the gaseous water in the magma creates extreme pressures because of expansion.
When the pressure becomes too great the dome and magma are sent into the Earth's atmosphere in a tremendous explosion. On the island of Krakatau , a caldera type volcano exploded in ejecting 75 cubic kilometers of material in the air and left a depression in the ground some 7 kilometers in diameter.
A potentially very destructive caldera covering an area of about square kilometers exists under Yellowstone National Park in the United States Figure 10n Investigations have discovered that over the last 2 million years this volcano has exploded on a regular interval of about , years.
The last eruption occurred , years ago and the next could take place anytime. When the Yellowstone caldera last erupted, it blasted 1, cubic kilometers of volcanic ash and rock into the atmosphere. The ash ejected into the atmosphere created climatic havoc on a global scale. The ash would have blocked sunlight from being received at the ground surface for a few years. A reduction in the reception of solar radiation would have caused the global climate to cool significantly.
Over time this ash settled back to the Earth's surface covering more than half of North America. Study Guide. Additional Readings. Created by Dr. Volcanism - Volcanoes A volcano is generally a conical shaped hill or mountain built by accumulations of lava flows , tephra , and volcanic ash.
Figure 10n-1 : Location of the Earth's major volcanoes. Most occur along tectonic plate boundaries where plate subduction creates rising plumes of magma. The volcanoes that do not occur along plate boundaries are the result of localized asthenosphere hot spots that melt through the Earth's crust.
The Hawaiian Island chain of volcanoes was create by a hot spot. Figure 10n-2 : Location of major volcanic deposits laid down during the last 2 million years.
Hawaiian eruptions are similar to Icelandic eruptions because both eruption types have many fissures bringing the lava to the surface. Both types of eruptions are known for their beautiful fire fountains like the one shown above. The lava that flows from both types of eruptions is very hot, thin, and runny which allows for fast flowing lava flows.
The main difference lies in the fact that most Hawaiian eruptions have the greatest quantity of lava pouring out of the main vent at the volcano's summit, not along side fissures. These summit eruptions build the cone steeper and higher.
The volcano above was formed from Hawaiian eruptions. Shield cones were named by Icelandic people because the cone's shape reminded them of a warriors shield layed down. Shield cones form from hot, runny lava that is erupted from the the volcano through its summit and the many side vents and fissures throughout the volcano's flanks Sides.
Shield cones are low, very broad, and gently sloping volcanoes. The volcano pictured above is Mauna Kea, which is located on the big island of Hawaii. 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 it was formed. Mauna Loa is 13, feet above sea level but over 17, feet of mountain lies under the water.
This volcanic mountain is over 30, feet tall from sea floor to the summit. Maua Loa started to form above the Hawaiian hot spot about one million years ago and broke the surface of the ocean about , years ago. Strombolian and Vulcanian eruptions are more explosive than Icelandic and Hawaiian eruptions. Strombolian eruptions are named for the volcanic island off of the coast of Italy. Stromboli has erupted over many centuries almost constantly. Stromboli has been named the "Lighthouse of the Mediterranean" because it erupts every 20 minutes or so.
Strombolian eruptions are short lived explosive eruptions that shoot very thick and pasty lava into the air along with bursts of steam and gas. Strombolian eruptions usually produce little or no lava. Because of this the cones that are produced by this type of eruption is a very steep sided cone called a cinder cone.
The photo shows a strombolian eruption taking place from a 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. These cinders and ash settle around the main vent and build a steep sided cone. Very little lava is erupted from a cinder cone. Cinder cones very rarely rise to more than 1, feet above the surrounding landscape.
Cinder cones are known for their very violent, explosive, exciting eruptions. Paricutin in Mexico and Mt. Vesuvius in Italy are famous cinder cones. Vulcanian eruptions are more violent and explosive than strombolian eruptions. Vulcanian eruptions are named after the island of Vulcano off the coast of Italy.
This is the same island that gave us the name "Volcano". Vulcanian eruptions contain high dark clouds of steam, ash, and gas. The ash plume builds a cauliflower shaped head and a thinner more treetrunk-like base. When the volcano quits erupting ash and gases it then ejects thick pasty lava. Cinder cones rarely reach meters in height but they have steep sides.
Cinder cones grow rapidly, usually from a single eruption cycle. Cinder cones are composed of small fragments of rock, such as pumice, piled on top of one another. The exact composition of a cinder cone depends on the composition of the lava ejected from the volcano. Cinder cones usually have a crater at the summit. Cinder cones are often found near larger volcanoes. Supervolcano eruptions are extremely rare in Earth history.
A supervolcano must erupt more than 1, cubic km cubic miles of material, compared with 1. Helens or 25 km3 for Mount Pinatubo, a large eruption in the Philippines in Not surprisingly, supervolcanoes are the most dangerous type of volcano. Supervolcanoes are a fairly new idea in volcanology. The exact cause of supervolcano eruptions is still debated, however, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion.
This creates a huge hole or caldera into which the surface collapses. Yellowstone sits above a hotspot that has erupted catastrophically three times: 2. Yellowstone has produced many smaller but still enormous eruptions more recently.
Long Valley had an extremely hot and explosive rhyolite explosion about , years ago. An earthquake swarm in alerted geologists to the possibility of a future eruption, but the quakes have since calmed down. A supervolcano could change life on Earth as we know it.
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