Introduction

Learning Objectives

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

  • Describe the rock cycle and the types of processes that lead to the formation of igneous, sedimentary, and metamorphic rocks, and explain why there is an active rock cycle on Earth
  • Explain partial melting and the geological processes that lead to melting
  • Describe, in general terms, the range of chemical compositions of magmas
  • Discuss the processes that take place during the cooling and crystallization of magma, and the typical order of crystallization according to the Bowen reaction series
  • Explain how magma composition can be changed by fractional crystallization and partial melting of the surrounding rocks
  • Apply the criteria for igneous rock classification based on mineral proportions
  • Describe the origins of phaneritic, porphyritic, and pegmatitic textures
  • Identify plutons on the basis of their morphology and their relationships to the surrounding rocks
  • Explain the origin of a chilled margin
Figure 3.1 A fine-grained mafic dyke (dark green) intruded into a felsic dyke (pink) and into coarse diorite (grey), Quadra Island, B.C. All of these rocks are composed of more than one type of mineral. The mineral components are clearly visible in the diorite, but not in the other two rock types. [SE photo]

Figure 3.1 A fine-grained mafic dyke (dark green) intruded into a felsic dyke (pink) and into coarse diorite (grey), Quadra Island, B.C. All of these rocks are composed of more than one type of mineral. The mineral components are clearly visible in the diorite, but not in the other two rock types. [SE photo]

A rock is a consolidated mixture of the same or different minerals. By consolidated, we mean hard and strong; real rocks don’t fall apart in your hands! A mixture of minerals implies the presence of more than one mineral grain, but not necessarily more than one type of mineral (Figure 3.1). A rock can be composed of only one type of mineral (e.g., limestone is commonly made up of only calcite), but most rocks are composed of several different minerals. A rock can also include non-minerals, such as fossils or the organic matter within a coal bed or in some types of mudstone.

Rocks are grouped into three main categories based on how they form:

Igneous: formed from the cooling and crystallization of magma (molten rock)

Sedimentary: formed when weathered fragments of other rocks are buried, compressed, and cemented together, or when minerals precipitate directly from solution

Metamorphic: formed by alteration (due to heat, pressure, and/or chemical action) of a pre-existing igneous or sedimentary rock