Classification Of Volcanoes
There are two main types of volcano, shield volcanoes and strato-volcanoes, both of which have differing impacts on the environment due to the different materials which they eject.
Shield Volcanoes
Shield volcanoes are characterised by gentle slopes of less than about 10 degrees, formed by the effusive eruption of mafic basalt lavas which are very fluid, meaning that they flow and spread out in all directions before solidifying. The gradual build-up of solidified layers of basaltic lava leads to the production of a broad-shaped volcano.
Shield volcanoes occur at diverging plate boundaries, where the plates move apart, such as at the boundary between the Eurasian and North American plates, known as the Mid-Atlantic Ridge. Shield volcanoes frequently occur at mid-ocean ridges due to convection cells within the Asthenosphere. Here, hotter, less dense molten material rises to the surface where it cools and then begins to sink again, diverging away from the ridge and effectively 'dragging' the lithospheric plates away from the ridge with it. Some of this rising molten material, however, reaches the surface of the sea floor at the ridge where it erupts as pillow lavas. The diagram below shows these convection cells within the Asthenosphere and lower mantle.
Shield volcanoes occur at diverging plate boundaries, where the plates move apart, such as at the boundary between the Eurasian and North American plates, known as the Mid-Atlantic Ridge. Shield volcanoes frequently occur at mid-ocean ridges due to convection cells within the Asthenosphere. Here, hotter, less dense molten material rises to the surface where it cools and then begins to sink again, diverging away from the ridge and effectively 'dragging' the lithospheric plates away from the ridge with it. Some of this rising molten material, however, reaches the surface of the sea floor at the ridge where it erupts as pillow lavas. The diagram below shows these convection cells within the Asthenosphere and lower mantle.
Below are a selection of photos of shield volcanoes, their shallow slopes are clearly visible.
Strato-Volcanoes
Strato-volcanoes are tall, conical-shaped volcanoes with steep sides which develop along converging plate boundaries. At these boundaries, an oceanic plate (2.9 g/cm³) is subducted beneath a less dense continental plate (2.7 g/cm³)[1]. Friction during subduction causes the top of the oceanic plate to partially melt, producing magma which then rises upwards. Intermediate or Silicic lavas are usually ejected at strato-volcanoes which don’t flow very far before solidifying due to their high viscosity, hence producing the steep-sided slopes characteristic of strato-volcanoes. However, lava is less common at strato-volancanoes, strato-volcanoes predominantly eject large amounts of pyroclastic material, making them the most hazardous type of volcano.
Below are a selection of photos of strato-volcanoes, their steep slopes are clearly visible.
[1] OCR Geology AS/A2 Debbie Armstrong, Frank Mugglestone, Ruth Richards and Frances Stratton Page 35