Question

The rock mass and material characteristics of an igneous rock are greatly influenced by the chemistry of magma and the tectonic settings in which they are formed. Based on the statement given, answer the following questions. a) Briefly explain the main difference between the texture of volcanic and plutonic igneous rock? (3b) Discuss how silica content affects the chemistry of magma? c) Explain the difference between the typical rock mass characteristics of plutonic and volcanic igneous rock? (d) Discuss the suitability of the following rock types in terms of their potential for Alkali Silica Reaction (ASR) when used as aggregates in concrete mix. i) Granite ii)

Answer 1

a. Volcanic rocks have very fine crystals due to rapid cooling of magma. This results in aphanitic texture. Aphantic texture means glassy texture with no visible crystals. Eg- Basalt

Plutonic rocks have large or coarse grained crystals due to slow cooling. This results in phaneritic texture. phaneritic texture means there are visible crystals. Eg- Granite

b. Magma's flow resistance depends on its internal friction. This internal friction is caused by the chemical bonds in magma. Oxygen is the only anion and silicon is the most abundant cation in magma. So, the Si-O bond is the most important factor in determining the degree of a magma's viscosity.

Also,in the liquid state, the bonding of SiO₄ results in the polymerization of the liquid, which will increase the internal friction of the magma, thus increasing the viscosity.

So, magma that has a high silica content will exhibit greater degrees of polymerization, and it will have higher viscosity, than those with low-silica contents.

c. Volcanic rocks are hard. So, they are used in pavement construction. Plutonic rocks are used as building stones.

Volcanic rocks have high amounts of iron. So they are dark coloured rocks and plutonic rocks are lighter coloured rocks compared to volacanic rocks.

d. Granite can be used as aggregate in concrete mix for high strength concrete production. But this graite should not have any alkali reactive materials in it. It depend on the area where the stone is formed and the type of volcano from which it is formed.

Some granites have amorphous silica or micro crystalline quartz which can cause Alkali Silica Reaction.

There are some places that use polished granite to replace some of the aggregates in concrete and it shows wonderful results.