Lab 4: Igneous Rocks

Lab Structure

Recommended additional work None
Required materials Mineral ID kit, Rock Kits 1 and 2, pencil

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, 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 mineral crystallization according to the Bowen reaction series.
  • Describe the origins of phaneritic, porphyritic, and vesicular rock textures.
  • Apply the criteria for igneous rock classification based on mineral proportions.
  • Use observations of mineralogy and texture to correctly identify and name an igneous rock.
Key Terms
  • Magma
  • Lava
  • Partial melting
  • Crystallization
  • Intrusive
  • Extrusive
  • Felsic
  • Intermediate
  • Mafic
  • Ultramafic
  • Phaneritic
  • Aphanitic
  • Porphyritic
  • Vesicular
  • Glassy
  • Phenocrysts
  • Groundmass

A rock is a consolidated mixture of 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. 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:

  1. Igneous: formed from the cooling and crystallization of magma (molten rock)
  2. Sedimentary: formed when weathered fragments of other rocks are buried, compressed, and cemented together, or when minerals precipitate directly from solution
  3. Metamorphic: formed by alteration (due to heat, pressure, and/or chemical action) of a pre-existing igneous or sedimentary rock

For the next few weeks you will learn about each of these categories of rock in the lab, beginning with igneous rocks in Lab 4. You will practice identifying minerals and textures, and you will use your observations to classify samples of rocks. Finally, by Lab 6, you will use the processes of the rock cycle to link all three categories of rocks together.



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A Practical Guide to Introductory Geology Copyright © 2020 by Siobhan McGoldrick is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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