volcano.gif The Formation of a Volcanoe


Volcanoes form when hot material from below rises and leaks into the crust. This hot material is called magma, comes from either from a melt of subducted crustal material, which is light and buoyant after melting, or it may come from deeper in the interior of a planet and is light and buoyant because of its extreme temperature. Over time the layers of magma continue to pile up and they eventually form a volcanoe!
What makes a volcanoe
Within a volcanoe, regardless of the type, there are a few common features that play a large role wiht the devolopment of an eruption.

1) A MAGMA CHAMBER- acts as a resevoir for the molten material.
2) A PIPE- the peassageway for magma to rise to the surface.
3) A VENT- is a opening at the Earth's surface
4) A CRATER- the bowl-like shape depresssion at the top of a volcanoe where the volcanic material are released!


How a volcanoe erupts. A volcano erupts when the pressure of magma within the volcano gets so high that it breaks its way to the surface. When molten rock is below the surface it's called "magma" and once it has reached the surface it's called "lava" if it is flowing or "pyroclastic material" if it is being blown apart and thrown into the air. Many volcanoes have magma chambers that always have a certain amount of magma inside them. Some inflation events result in the ground surface moving upwards by as much as 2 meters! When the pressure becomes too great, the side of the magma chamber breaks and magma can escape into the flanks of the volcano or up to the summit. Sometimes it breaks to the surface to cause an eruption and sometimes the magma just intrudes into the flank of the volcano and stops there. At other volcanoes that are not as active. This allows the magma in the chamber to "differentiate", meaning that many crystals form and sink to the bottom of the chamber and at the same time gas bubbles form and float to the top of the chamber. Sometimes the pressure from all these gas bubbles is sufficient to cause an eruption, and other times it isn't, but is just sitting there waiting. Then a new batch of magma comes after so long. Even if this new batch is only a small volume, the added heat is enough to trigger an eruption of the magma that has been sitting around for a while.

volcano parts

Parts of a volcano
There are just a few parts of a volcano. The first part is the magma resevoir. It is located towards the bottom of the volcano. There is also a sill and a dike they are like pipes. The "throat" is the pipe that leads the magma up to crater of a volcano. The crater is at the summit of the volcano. THere is also a flank on both sides of thr volcano. The flank is like the volcano's clothes. It is the outer layering of the volcano. Then you have the base. Its name says what it is. It's the bottom of the volcanoe that supports the volcano! Over all the parts of the volcano are basic. To look at the parts of a volcano look at the picture above.

The three main types of volcanoes

There are three main types of volcanoes:shield, composite, and cinder-cone!
The shield volcanoe

Shield volcano
Shield volcanoes are large volcanoes that are built almost entirely of fluid lava flows. It has broad sloping sides and is usually surrounded by gently sloping hills in a circular or fan shaped pattern, that looks like a warrior's shield.The volcano is produced by the action of the gas with heat from the earth's core. This action melts rock turning it into magma. The pressure from the heat of the gas pushes the magma upwards till it explodes. Molten magma shoots upward from deep below the ocean floor and breaks through the plates to form shield volcanoes. Lava flows gently and continuously out of the central volcanic vent or group of vents. This lava is very runny, and can't be piled up into steep mounds. It gradually comes together and cools around the volcano. The volcanoes are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava. The lava spread widely over great distances, then cools as thin gently dipping sheets. Lavas also erupt from vents along fractures (rift zones) that form on the flanks of the cone. Some of the largest volcanoes in the world are Shield volcanoes.Shield volcanoes may be produced by hot spots which lay far away from the edges of tectonic plates. Shields also occur along the mid-oceanic ridge, where sea floor spreading is in progress and along subduction related volcanic arcs.In northern California and Oregon, many shield volcanoes have diameters of three or four miles and heights of 1,500 to 2,000 feet. A good example of a shield volcano is the Island of Hawaii. The Big Island is formed of five coalesced volcanoes of successively younger ages. The Hawaiian Islands are composed of linear chains of volcanoes including Kilauea and Mauna Loa on the island of Hawaii. That is the shield volcano!
The cinder-cone volcano
cinder cone volcano

Cinder cone volcanoes are the most common kind of volcanoes.They are steep sided cones of basaltic fragments and are smaller and simpler than composite volcanoes. Streaming gases carry liquid lava blobs into the atmosphere that fall back to earth around a single vent to form the cone. The volcano forms when ash, cinders and bombs pile up around the vent to form a circular or oval cone.Cinders are melted volcanic rock that cooled and formed pebble-sized pieces when it was thrown out into the air. They are ejected from a single vent and accumulate around the vent when they fall back to earth.Bombs are melted volcanic rock that cooled and formed large pieces of rock when it was thrown out into the air before landing on the ground.Cinder cones are chiefly formed by Strombolian eruptions. They grow rapidly and soon reach their maximum size. Cinder cones can occur alone or in small to large groups or fields. Most have a bowl-shaped crater at the summit. The longer the eruption, the higher the cone. They rarely exceed 250 meters in height and 500 meters in diameter, although some may rise to as high as 650 meters or more. If gas pressure drops, the final stage cinder cone construction may be a lava flow that breaks through the base of the cone. If a lot of water in the environment has access to the molten magma, their interaction may result in a maar volcano rather than a cinder cone. The shape of a cinder cone can be modified during its life. When the position of the vent alters, aligned twin cones develop. Nested, buried or breached cones are formed when the power of the eruption varies. A example of a cinder cone is Paricutin in Mexico. In Iceland, Surter I and Surter II cinder cone volcanoes
. The composite volcano
composite volcano
Sometimes called Stratovolcanoes, Composite volcanoes form when runny lava escapes through a fissure and flows a long way. Composite volcanoes are tall cone-shaped mountains that are typically steeply-sided, symmetrical cones of large dimensions. The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash etc. are added to its slopes.

Composite volcanoes erupt in different ways at different times. These volcanoes are built in layers by multiple eruptions, sometimes recurring over hundreds of thousands of years, sometimes over a few hundred. Andesite magma (the most common but not the only magma type), tends to form composite cones. During some eruptions, cinders, bombs and blocks form a mountain or add height to one that earlier volcanic eruptions had built. During other eruptions, lava flows cement these rocks together. Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lava either flow through breaks in the crater wall or from fissures on the flanks of the cone. Lava, solidified within the fissures, form dikes that act as ribs which help to strengthen the cone. They may rise as much as 8,000 feet above their bases. Depending on the type of volcanic material it is composed of, some can grow to such heights that their slopes become unstable and are susceptible to collapse from the pull of gravity.

When volcanic activity ceases, erosion begins to destroy the cone. After thousands of years, the cone is stripped away and the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) become exposed, and it too is slowly reduced by erosion. Finally, all that is left is the plug or "volcanic neck" and dike complex projecting above the land surface.

Some composite volcanoes occur in chains and are separated by several tens of kilometers. There are many composite volcano chains on earth, notably around the Pacific rim, known as the "Rim of Fire".

Other examples of composite volcanoes and their locations are:

Mount St. Helens - Washington State

Mount Rainier - Washington State

Mount Vesuvius - Italy