When contrasting lava from composite volcanoes to lava from shield volcanoes.? 1

Question

When contrasting lava from composite volcanoes to lava from shield volcanoes.?

1)composite volcanoes most often produce basaltic lavas with high viscosities, whereas shield volcanoes produce silica-rich lavas with low viscosities.

2)both types of volcano produce silica-rich lavas, but the lavas of composite cones have low viscosities, whereas the lavas of shield volcanoes have high viscosities.

3)composite volcanoes most often produce silica-rich lavas with low viscosities, whereas shield volcanoes produce basaltic lavas with high viscosities.

4)composite volcanoes most often produce silica-rich lavas with high viscosities, whereas shield volcanoes produce basaltic lavas with low viscosities.

5)composite volcanoes most often produce basaltic lavas with low viscosities, whereas shield volcanoes produce silica-rich lavas with high viscosities.

Explanation / Answer

Composite volcanoes get their name from the composite stratified sequential structure of these volcanoes. These are characterized by the sequential layers of volcanic materials formed from effusive and explosive volcanism. These are conical structures that may consist of a collapsed summit(known as Caldera).

Composite volcanoes(also known as Strato volcano) are generally found at the convergent plate boundaries, where an oceanic plate subducts under a continental or another oceanic plate. At these positions, dewatering from the subducting oceanic plate releases water at depths into the asthenosphere. This water lower the melting point of the surrounding materials, and produces melt. This melt, due to its lower density than the surrounding, rises upward and pools down the continental crust. Assimilation from the continental crust causes the composition of the rising melt to shift towards more silicic nature. As a result, the final composition of the modified magma is generally intermediate to silicic in nature.

Shield volcanoes, on the other hand are generally characterized by the low profile of these volcanoes, that give them the appearance of a shield lying on the ground. From this structure of these volcanoes they have derived the name. Shield volcanoes are formed by the gentle and effusive type of eruption of very low viscosity basaltic lava. After the eruption of these types of low viscosity lava, it flows for a considerable distance before solidification and formation of the characteristic structure of the volcanoes.

So,4)composite volcanoes most often produce silica-rich lavas with high viscosities, whereas shield volcanoes produce basaltic lavas with low viscosities.

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