Igneous Rocks

We’ve already learned about the different types of volcanoes and the hazards associated with them. This week, we will explore the type of rocks that are produced when molten rock cools and crystallizes—igneous rocks! In this activity, we will investigate the different identifiers that geologists use to characterize igneous rocks in the field.

The Giant's Causeway in Northern Ireland, where hexagonal-shaped basalt columns lock together from an ancient eruption.
Figure 4.1 The Giant’s Causeway in Northern Ireland, where hexagonal-shaped basalt columns lock together from an ancient eruption. Image by Chmee2, CC BY-SA 3.0, Wikimedia Commons

While igneous rocks are made when molten rock cools, the source of that magma or lava was once preexisting rock. Igneous rocks are essentially sourced from old sedimentary rocks, metamorphic rocks, and even other igneous rocks that became melted under high temperatures and eventually cooled and crystallized to form a new material. This interconnectedness of the three different rock types – sedimentary, metamorphic, and igneous rocks – is called the rock cycle. Through geologic processes, any rock type can eventually be transformed into another!

LEARNING OBJECTIVES

After completing the exercises and reading the text in this chapter, you should be able to…

  • Explain how molten rock cools and crystallizes to form igneous rocks with different appearances
  • Explain how magmas and lavas with different chemical compositions form igneous rocks with different colors
  • Compare and contrast the textures of common igneous rocks and use them to identify hand specimens
  • Characterize the common minerals found in igneous rocks

 

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