Structure of a Composite Volcano
Main Vent - The Main vent is the path taken by the liquid rock from the Magma Chamber to the surface. It is like a pipe up which the lava can flow. Sometimes the main vent has branches which, if they reach the surface, may for secondary cones or fumeroles. When a volcano erupts, lava, gasses and fragments of rock travel right up the main vent and are thrown out through the crater. When the eruption finishes the lava may drop back down the pipe or form a lava lake in the crater.
Lava Flow - When magma reaches the surface it is called lava (pronounced 'larver'). If the level of lava in the crater overflows the rim, it will begin to flow down the sides of the volcano. Different types of lava are more runny than others
and the most runny types can flow down the volcano at over 30 miles per hour.
Although the rock is liquid, is is very dense. If you stood on it, you wouldn't sink, but you would burn. The rock surface may seem to have cooled and set, but below the very thin crust the lava can remain liquid for a long time, at temperatures of 500°C or more.
Ash and Lava strata - The volcano in the diagram is made of layers, coloured light and dark grey. These are the layers of ash and lava which have been thrown out of the volcano during its life. Ash consists of small fragments of rock, some as fine as tiny dust particles, other chunks being bigger than your fist. The layers of lava are old lava flows which cooled and set as they flowed down the volcano. The volcanic ash is usually thrown out of the volcano before the lava. It settles to the ground and forms a steep sided pile. You can see the same effect by pouring salt onto a flat surface. It will form a pyramid, the steepness of which is determined by the size and shape of the individual salt grains. The ash is soft and is easily worn away by wind and rain. The layers of lava which flow over the ash protect it from erosion and thus allow the volcano to keep growing upwards.
Secondary Cone - When the main vent develops branches, the volcano may begin to grow secondary cones. The rock and ash layers which make up the volcano are often cracked and weakened by the explosions that occur during eruptions. If these cracks form a path from the main vent to the surface, magma is able to move up the new pipe and reach the surface. As it erupts the ash and lava is sprayed into the air like a fountain. Splashes of lava mix with the ash to form a new cone. Given time, a secondary cone may divert so much of the magma that it becomes the main vent and the original cone becomes less important.
Magma Chamber - the soucre of the molten rock.
Fumerole - A fumerole is a crack in the surface through which steam and gas can escape. The magma below the surface heats water to the point where it turns to steam and is able to dissolve minerals from the surrounding rock. As the gas reaches the surface it is both hot and under pressure. It cools and expands, depositing the dissolved minerals around the vent. In some parts of the world local industries have built up collecting the sulphur deposited around fumerole vents.
Crater - A volcanic crater is a funnel shaped hollow at the top of the vent. It is formed as lava, gas and ash are blasted upwards from the main vent. Material falls back down to earth around the vent and slowly piles up forming a rim around it. The inside of the crater is kept clear by the force of upward moving material constantly removing any debris which falls there.
Ash and Gas Clouds - airborne materials from the volcano.
A composite volcano is also commonly called a stratovolcano. These volcanoes usually form in subduction zones. Stratovolcanoes form on the continental side of a subduction zone where the melting oceanic crust forms pools of molten rock in a magma chamber. The magma, molten rock in the magma chamber is lighter than the volcano rocks that surround it. The magma works its way to the summit where it erupts as tephra or in thick lava flows.
Andesite, dacite and rhyolite are the three major types of lava that create these large volcanic mountains on the continents. These three types of lava have a high percentage of quartz and are associated with large violent eruptions. Often these volcanoes will alternate between explosive eruptions and thick lava flows. This combination of eruptions produces the classical look of a composite volcano.
The summit of a stratovolcano has steep sides. Andesite, dacite and rhyolite create slowing thick lava flows that usually cool on the upper slopes of a volcano. The base of these volcanoes is wide and has gentle slopes due to erosion. As the volcano grows in size large amounts of the pyroclastic material erodes away. The eroded pyroclastic material is carried down the slopes by erosion where it is deposited at the base of the volcano.
When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface--a telltale remnant of the vanished volcano.
Andesite, dacite and rhyolite are the three major types of lava that create these large volcanic mountains on the continents. These three types of lava have a high percentage of quartz and are associated with large violent eruptions. Often these volcanoes will alternate between explosive eruptions and thick lava flows. This combination of eruptions produces the classical look of a composite volcano.
The summit of a stratovolcano has steep sides. Andesite, dacite and rhyolite create slowing thick lava flows that usually cool on the upper slopes of a volcano. The base of these volcanoes is wide and has gentle slopes due to erosion. As the volcano grows in size large amounts of the pyroclastic material erodes away. The eroded pyroclastic material is carried down the slopes by erosion where it is deposited at the base of the volcano.
When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface--a telltale remnant of the vanished volcano.