Chapter 8, Problem 1RQ

To determine

The way in which metamorphic rocks are different from igneous and sedimentary rocks.

Answer to Problem 1RQ

Metamorphic rock are formed when a pre-existing rock undergoes higher pressure-temperature conditions. Igneous rocks are formed by the crystallization of magma, and sedimentary rocks are formed by the accumulation of sediments. Metamorphic rocks are characterized by a texture called foliation and both igneous and sedimentary rocks lack this texture.

Explanation of Solution

When a rock is exposed to a condition that is entirely different from an environment in which they have formed, then the mineralogy, texture, and rarely chemical composition will be changed. This process of converting from one rock into another with the action of temperature, pressure, and chemically active fluids is termed as metamorphism and the resultant rock type is known as metamorphic rocks. When all the three agents of metamorphism act on a pre-existing rock, it recrystallize to form metamorphic rocks. The pre-existing rock can be igneous, sedimentary or metamorphic itself.

An important texture that can be used to identify metamorphic rocks from other rock types is foliation. It is a texture in which the mineral grains are arranged in a parallel pattern. This texture is mainly associated with intensely deformed metamorphic rocks and it is developed due to compressional stress. Metamorphic rocks are mainly identified based on the type of foliation type and mineral assemblages.

Igneous rocks are the rocks that are formed by the crystallization and solidification of hot molten material and sedimentary rocks are formed by the deposition of sediments. When a pre-existing rock undergoes erosional processes or weathering, then the sediments are eroded away. Those sediments are carried and transported by various geological agents such as air, water, and glacier and they are deposited in some basin. Later, they undergo various processes such as lithification, compaction, and cementation to form sedimentary rocks. Both igneous and sedimentary rocks lack foliation.