Types of Volcano
Types of volcano. The shape of a volcano is caused by the type of rocks it erupts and how explosive an eruption is.Shield volcanoes have long gentle slopes causd by highly fluid lava flows of basalt. In comparison stratovolcanoes / Composite cones are steep sided due to more viscous lava and are explosive.Supervolcanoes are often very large and form no cone at all as they are the remains of enormous magma chambers which have exploded violently leaving a caldera.Volcanic Rifts,fissures,domes and cinder cones are smaller scale examples of volcanic eruptions.
- Shield Volcano
- Strato/Composite Cone
- Cinder Cone
- Lava Dome
- Supervolcano/Caldera
- Rift/Fissure
Shield Volcanoes
Shield volcanoes have gentle slopes and large diameters. They are formed by the accumulation of low viscosity basalt lava flows. Because the flows have low viscousity they flow a long way. Lava tubes form which enable molten lava to flow long distances from the vent. They form where large volumes of basalt lava are erupted over along period of time from one location such as over a hot spot/mantle plume.
They are called shield volcanoes ecause they look like a warriors shield.
The most famous and largest shield volcanoes in the world are those in Hawaii. Mauna Loa is the largest volcano on earth , Mauna Kea is 16000m tall from base to summit thats greate than Everest.Kilauea is still active and pours lava into the sea everyday.Alll located on Big Island/Hawaii.
Strato/Composite Cone
Composite and Stratovolcanoes are one and the same. They are the volcano diagram that we all drew at school with layers of ash and lava. They are normaly located above
subduction zones and erupt explosively. Viscous lava, ash,tephra and pyroclastic flows build up a steep sided cone with a crater(s) at the summit. The voclano can erupt a variety of lava types from black basalt, through andsite and sticky dacite to the eruptive equivalent of granite- rhyolite. This variety of lavas is due to cooling and crystallisation of the magma within the volcano changing the composition of the melt when it is erupted.
The volcano may have smaller parasitic cones. Domes of lava may form in the summit craters.
Mt.St.Helens and Mt.Rainier in the Cascades are both Stratovolcanoes.
Cinder Cones
Cinder cones are compsed of rock fragments which have been ejected from a vent possibly as molten lava , cooled on their descent and dropped downwind of a volcanic vent. Often made of scoria and tephra ( small volcanic rock fragments) they from quickly perhaps over a few days. They can 100 of metres tall. The rock fragments are cooled quickly and glassy in texture. They require low viscosity lava to form such as basalt and can be found in Iceland and on Mt.Etna.
Lava Dome
Lava domes are composed of highly viscous lavas with high silica contents such as dacite or rhyolite. They are typically found in the craters of stratovolcanoes such as Mt.St. Helens or Soufriere Hills. The lava is exuded rather than erupted and is squeezed out like thick toothpaste. If they become unstable and collapse pyroclastic flows can be produced or tall eruption clouds.
Supervolcano Caldera
A supervolcano eruption is rare and only recently recognised. A supervolcano usually forms over a hot spot and under thick crust. They often appear on the surface as a large depression called a caldera which can be 10 s of km across. The depression is sometimes filled with a lake as at Taupo. The caldera is formed as the magma inside is violently blown out of the magma chamber below. Initially this is from the edges then the whole caldera floor collapses, leaving a flat bottomed depressio surrounded by steep sides.
Eruptions are incredibly violent and dramatic and although volcanoligists and TV crews would love to see one the rest of the world can wait!! Yellowstone has been much talked about in the media as due for an eruption but there are no signs of this just yet. Hooray!
A supervolcano has not erupted within the modern technological age. The most recent eruption that gets close is the 1815 eruption of Tambora in Indonesia wich ejcted 100 km3 of material. Previous to this the last true supervolcano eruption was Taupo New Zealand 26 000 years ago whcih ejected 800km3 of material. Both these eruptions dramativally effected the worlds climate and food availability.
Fissures and Rifts
Fissures and rifts are elongate cracks in the earths surface.Fissures are smaller than rifts. They typically form where the crust is under tension and is pulled apart allowing molten rock to pour out. Basalt is the predominate rock type found and fountains or curtains of "fire" can spectacularly erupt. Eruptions can pour out large volumes of basalt and eventually focus on one or two main vents.
Iceland has had spectacular fissure eruptions because the crust is under extensional tension due to the rifting or spreading as the North Atlantic and European plates pull apart.
Rifts are on a larger scale and involve great extensional tensions in the crust. Successful rifting can create new oceans and split apart contients. The NW coast of Scotland has plenty of volcanics which erupted when the North Atlantic ocean was formed.
