Adapted by Lyndsay R. Hauber & Joyce M. McBeth (2018) University of Saskatchewan from Deline B, Harris R & Tefend K. (2015) “Laboratory Manual for Introductory Geology”. First Edition. Chapter 8 “Igneous Rocks” by Karen Tefend, CC BY-SA 4.0. View source. Last edited: 8 Jan 2020
All rocks found on the Earth are classified into one of three groups: igneous, sedimentary, or metamorphic. This classification is based on the origin of each of the rock types. The focus of this chapter will be on igneous rocks, which are the only rocks that form from what was once a molten or liquid state. Therefore, igneous rocks are defined as those rock types that form by the cooling of magma or lava. Each igneous rock has a name that distinguishes it from other igneous rocks; so, these igneous rocks differ from each other primarily due to:
- the original composition of the molten material from which the rock is derived, and
- the cooling process of the molten material that ended up forming the rock.
These two parameters define the classification of igneous rocks, which are simplified into composition and texture. Igneous rock composition refers to what is in the rock (the chemical composition or the minerals that are present), and texture refers to the features that we see in the rock such as the mineral sizes or the presence of glass, fragmented material, or vesicles (holes) in the igneous rock.
3.1.1 Learning Outcomes
After completing this chapter, you should be able to:
- Classify igneous rock types based on colour, texture, and mafic colour index
- Identify, when possible, the minerals present in an igneous rock
- Determine the cooling history of the igneous rock
3.1.2 Key Terms
- Felsic (Silicic)