Most known magma chambers are located close to the Earth's surface, usually between 1 km and 10 km deep. Lava: Lava is the silicate rock that is hot enough to be in liquid form, and which is expelled from a volcano during an eruption.
The source of the heat that melts the rock is known as geothermal energy — i. As it makes contact with air and flows downhill, it eventually cools and hardens.
Main Vent: A volcano's main vent is the weak point in the Earth's crust where hot magma has been able to rise from the magma chamber and reach the surface.
The familiar cone-shape of many volcanoes are an indication of this, the point at which ash, rock and lava ejected during an eruption fall back to Earth around the vent to form a protrusion. Throat: The uppermost section of the main vent is known as the volcano's throat. As the entrance to the volcano, it is from here that lava and volcanic ash are ejected. Crater: In addition to cone structures, volcanic activity can also lead to circular depressions aka.
A volcanic crater is typically a basin, circular in form, which can be large in radius and sometimes great in depth. In these cases, the lava vent is located at the bottom of the crater. They are formed during certain types of climactic eruptions, where the volcano's magma chamber empties enough for the area above it to collapse, forming what is known as a caldera.
Pyroclastic Flow: Otherwise known as a pyroclastic density current, a pyroclastic flow refers to a fast-moving current of hot gas and rock that is moving away from a volcano. Pyroclastic flows normally hug the ground and travel downhill from their eruption site. Their speeds depend upon the density of the current, the volcanic output rate, and the gradient of the slope. Given their speed, temperature, and the way they flow downhill, they are one of the greatest dangers associated with volcanic eruptions and are one of the primary causes of damage to structures and the local environment around an eruption site.
Ash Cloud: Volcanic ash consists of small pieces of pulverized rock, minerals and volcanic glass created during a volcanic eruption. These fragments are generally very small, measuring less than 2 mm 0. This sort of ash forms as a result of volcanic explosions, where dissolved gases in magma expand to the point where the magma shatters and is propelled into the atmosphere. The bits of magma then cool, solidifying into fragments of volcanic rock and glass.
Because of their size and the explosive force with which they are generated, volcanic ash is picked up by winds and dispersed up to several kilometers away from the eruption site.
Due to this dispersal, ash an also have a damaging effect on the local environment, which includes negatively affecting human and animal health, disrupting aviation, disrupting infrastructure, and damaging agriculture and water systems. Ash is also produced when magma comes into contact with water, which causes the water to explosively evaporate into steam and for the magma to shatter.
Volcanic Bombs: In addition to ash, volcanic eruptions have also been known to send larger projectiles flying through the air.
Known as volcanic bombs, these ejecta are defined as those that measure more than 64mm 2. These cool before they hit the ground, are thrown many kilometers from the eruption site, and often acquire aerodynamic shapes i.
While the term applies to any ejecta larger than a few centimeters, volcanic bombs can sometimes be very large. There have been recorded instances where objects measuring several meters were retrieved hundreds of meters from an eruptions. Small or large, volcanic bombs are a significant volcanic hazard and can often cause serious damage and multiple fatalities, depending on where they land.
Luckily, such explosions are rare. Secondary Vent: On large volcanoes, magma can reach the surface through several different vents. Where they reach the surface of the volcano, they form what is referred to as a secondary vent. Where they are interrupted by accumulated ash and solidified lava, they become what is known as a Dike.
And where these intrude between cracks, pool and then crystallize, they form what is called a Sill. Secondary Cone: Also known as a Parasitic Cone, secondary cones build up around secondary vents that reach the surface on larger volcanoes.
As they deposit lava and ash on the exterior, they form a smaller cone, one that resembles a horn on the main cone. Yes indeed, volcanoes are as powerful as they are dangerous. And yet, without these geological phenomena occasionally breaking through the surface and reigning down fire, smoke, and clouds of ash, the world as we know it would be a very different place.
More than likely, it would be a geologically dead one, with no change or evolution in its crust. I think we can all agree that while such a world would be much safer, it would also be painfully boring! Explore further. More from Earth Sciences. Use this form if you have come across a typo, inaccuracy or would like to send an edit request for the content on this page.
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You cannot download interactives. According to the United States Geologic Survey, there are approximately 1, potentially active volcanoes worldwide. Most are located around the Pacific Ocean in what is commonly called the Ring of Fire. A volcano is defined as an opening in the Earth's crust through which lava, ash, and gases erupt.
The term also includes the cone-shaped landform built by repeated eruptions over time. Teach your students about volcanoes with this collection of engaging material. A caldera is a large depression formed when a volcano erupts and collapses.
A volcano is a feature in Earth's crust where molten rock is squeezed out onto the Earth's surface. Along with molten rock, volcanoes also release gases, ash, and solid rock. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Article Vocabulary. Tuesday, December 16, A caldera is a depression created after a volcano releases the majority of the contents of its magma chamber in an explosive eruption.
Without any structural support below, the land around the erupting volcanic vent or vents collapses inwardly, creating the bowl-shaped caldera. Depending on their intensity and duration , volcanic eruption s can create calderas as much as kilometers 62 miles wide.
A caldera-causing eruption is the most devastating type of volcanic eruption. It permanent ly alter s the environment of the surrounding area. A caldera is not the same thing as a crater. Craters are formed by the outward explosion of rock s and other materials from a volcano.
Calderas are formed by the inward collapse of a volcano. Craters are usually more circular than calderas. Calderas may have parts of their sides missing because land collapses unevenly. Craters are also usually much smaller than calderas, only extending to a maximum of one kilometer less than a mile in diameter. Crater-lake calderas result from the collapse of a stratovolcano after a Plinian eruption , the most explosive type of volcanic eruption. Plinian eruptions release massive amounts of lava , volcanic ash , and rocks.
Fractures can also act as conduits for escaping volcanic gases , which are released at the surface through vent openings called fumaroles. Summit Crater Parasitic Cones Fumarole. Although every volcano has a unique eruptive history, most can be grouped into three main types based largely on their eruptive patterns and their general forms.
The form and composition of the three main volcano types are summarized here:. A fissure eruption , for example, can generate huge volumes of basalt lava; however, this type of eruption is not associated with the construction of a volcanic edifice around a single central vent system.
Vent types related to hydrovolcanic processes generate unique volcanic structures, discussed separately under hydrovolcanic eruptions. For a description of each of the main volcano types , see:. Classifying a volcano as active, dormant, or extinct is a subjective and inexact exercise. A volcano is generally considered active if it has erupted in historic time.
This definition, however, is rather ambiguous, because recorded history varies from thousands of years in Europe and the Middle East, to only a few hundred years in other regions of the world, like the Pacific Northwest of the United States.
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