How are composite volcanoes and shield volcanoes different?

Answer 1

Composite volcanoes or Stratovolcanoes are your typical cartoon image of volcanoes, very tall, narrow craters at the top, steeper sides often snow capped peaks.

Shield volcanoes tend to not be as tall and have much more shallow sloping sides.

The reason for this is due to their products and location.

Composite volcanoes produce lava which is much more viscous (thicker) than shield volcanoes, which tend to produce a thinner runnier lava.

This is due to the Silica (SiO2) content of the lavas. Composite volcanoes tend to be produced by more Acidic lavas (with a higher Silica content) where as shield volcanoes are produced by lavas with a more Basic composition, (a lower silica content).

Composite volcanoes (Mt St Helens, Vesuvius, Mt Fugi) tend to be formed near destructive plate boundries where one tectonic plate is being subducted beneath another. When the subducted plate reaches roughly 700 km depth it begins to enter the Aesthenosphere, a layer of more ductile rock deep in the mantle.

The water content of the subducted plate causes the aesthenosphere to partially melt. The molten magma begins to rise in plumes towards the surface. As the magma plume rises it has a lot of contact with the surrounding rocks it passes through, and it absorbs silica from them, arriving at the surface as a viscous silica rich melt.

Shield volcanoes (Mauna Loa, Kiluea, Hekla) are more common over constructive plate margins where two plates are pulling apart. When the plates pull apart it de-pressurises the mantle beneath the plates and causes it to partially melt.

The Magma rises up the cracks between the two retreating plates, with little contact to the surrounding rocks, and so picks up very little silica content. This causes it to reach the surface as a runny basic lava.

The runny nature of shield volcanoes means that lava flows travel a long way from the volcano crater, causing little build up on the surrounding flanks of the volcano.

The viscous nature of Composite volcanoes means that the lava doesn't travel very far and builds up on the sides of the volcanoes making them much steeper.

Shield volcanoes are more often active than composite volcanoes, some erupt near constantly for many decades.

Composite volcanoes erupt far less often, and can be mistakenly thought of as being extint for hundreds of years between eruptions.

Composite volcanoes tend to have much more explosive eruptions than shield volcanoes, which tend to be more effusive in nature.

Composite volcanoes are capable of pyroclastic flows (Mt St Helens) and huge eruptions which can entirely destroy the volcano (Pinatubo, Krakatoa).

Shield volcanoes tend to only erupt in lava flows, which can create enormous shallow volcanoes (Mauna Loa, Olympus Mons)