Shield and Composite Volcanos

How, where and why shield and composite volcanos are formed.

Shield Volcanos1

Formed at Constuctive (tensional) margins.

For example, imbetween UK and USA, the North American plate moves away from the Eurasian plate, so magma rises above to fill the gap. This forms the Mid-Atlantic Ridge.

Volcanos are made from basic lava (with a low silica content), which is quite runny so the volcanos don't usually make it above ground, and form quite gentle slopes. One of the many exeptions to this is Hawaii, created at a tension margin. Eruptions usually are:

  • Regular and frequent
  • Rarely violent
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Shield Volcanos 2

Key Words (consult a text book to see where they are on a diagram):

Gentle slopes


Magma chamber


Wide Base

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Composite Volcanos 1

Formed at Destructive (Compressional) margins.

For example, the denser oceanic Nazca plate, subducts under and crushes theless dense continental South American plate; which cripples and folds upwards, creating fold mountains (Andes) and volcanos.

Another example is the formation of Mt. Etna, the northward moving African plate subducts under the southward moving Eurasian Plate.

What happens to create the volcanoes?

Well, the denser oceanic plate is pushed downwards, under the opposing plate into a "subduction zone". Great HEAT and PRESSURE melt the rock which forms a pool of magma (remember! magma is what we call lava when it is underground), and forces the magma along a crack upwards untill it erupts at the surface.

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Composite Volcanos 2

Features of a Composite cone volcano:

  • Tall cone with narrow base and steep sides

-> Because at a destructive margin, acid lava (high silica content) is created which is more viscous then its basic counterpart and travels a shorter distance before cooling.

  • Formed in layers of ash and dust (ash is released from the volcano during an eruption)
  • Irregular, long dormant periods
  • Violent explosions possible

Other Key Words (consult a text book to see where they are on a diagram):

Subsidery cone

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