Unit 30

MINERALS AND IGNEOUS ROCKS

Unit Overview

This unit examines minerals (the building blocks of rocks) and the primary rock type, igneous rocks. The main sections are as follows:

  • Minerals and rocks
  • Classification of rock types
  • Igneous rocks

A mineral is a naturally occurring inorganic element or compound having a definite chemical composition, physical properties, and usually, a crystalline structure. Approximately 100 different types of minerals can be identified based on their chemical composition, hardness, cleavage/fracture, colour/streak, and luster.

Rocks are comprised of mineral assemblages, and the three types of rocks are igneous, sedimentary, and metamorphic. Igneous rocks form first (i.e. they are primary rocks), and they consist of intrusive and extrusive forms. Jointing and exfoliation of igneous rocks facilitates their weathering and subsequent erosion. This wearing away of igneous rocks produces material that can be incorporated into sedimentary rocks.

Unit Objectives

  • To understand the relationship between rocks and their constituent minerals
  • To briefly investigate the important properties of minerals and to provide an elementary scheme for their classification
  • To discuss some important aspects of igneous rocks and their influence on landscape form


Glossary of Key Terms

Batholith A massive, discordant body of intrusive igneous rock (pluton) that has destroyed and melted most of the existing geologic structures it has invaded.
Concordant (intrusion) Intrusive magma that did not disrupt or destroy surrounding, existing geologic structures but conformed to them.
Crystalline Atoms arranged in a regular, repeating pattern.
Dike A discordant intrusive igneous form in which magma has cut vertically across preexisting strata, forming a kind of barrier wall.
Discordant (intrusion) Intrusive magma that did not conform to but cut across or otherwise disrupted surrounding, existing geologic structures.
Exfoliation A special kind of jointing that produces a joint pattern resembling a series of concentric shells, much like the layers of an onion; caused by the release of confining pressure, the outer layers progressively peel away and expose the lower layers.
Extrusive igneous rock Rocks formed from magma that cooled and solidified, as lava or ash, on the Earth's surface.
Igneous rock The (primary) rocks that formed directly from the cooling of molten magma; igneous is Latin for "formed from fire."
Jointing The tendency of rocks to develop parallel sets of fractures without any obvious movement such as faulting.
Laccolith A concordant intrusive igneous from in which a magma pipe led to a subterranean chamber that grew, dome-like, pushing up the overlying strata into a gentle bulge without destroying them.
Lava Magma that reaches the Earth's surface.
Magma The liquid molten mass from which igneous rocks are formed.
Metamorphic rock The (secondary) rocks that were created from the transformation, by heat and/or pressure, of existing rocks.
Mineral Naturally occurring inorganic element or compound having a definite chemical composition, physical properties, and usually, a crystalline structure.
Rock Any naturally formed, firm, and consolidated aggregate mass of mineral matter, of organic or inorganic origin, that constitutes part of the planetary crust.
Sedimentary rock The (secondary) rocks that formed from the deposition and compression of rock and mineral fragments.
Sill A concordant intrusive igneous form in which magma has inserted itself as a thin layer between strata of preexisting rocks without disturbing those layers to any great extent.
Stock A discordant pluton that is smaller than a batholith.


Unit Outline

  • Minerals and rocks
    • Elements are the most basic substances-they cannot be broken down further
    • A mineral is a crystalline, naturally occurring inorganic element or compound with a definite chemical composition, physical properties, and structure
    • Rocks are composed of assemblages of minerals
    • Mineral properties
      • chemical composition identified by a one or two-lettered symbol
      • hardness
      • cleavage/fracture - tendency to break
      • colour/streak - streak is mineral's colour when it is rubbed on porcelain in powdered form
      • luster (sheen)
    • Mineral types
      • silicates - contain silicon and oxygen
      • nonsilicates - carbonates, sulfates, sulfides, halides
  • Classification of rock types
    • Igneous rocks formed by the cooling and solidifying of magma
    • Sedimentary rocks are produced by the deposition and compression of rock fragments
    • Metamorphic rocks are formed when existing rocks are modified by heat or pressure
    • Sedimentary and metamorphic rocks are secondary rocks
  • Igneous rocks
    • Formed by cooling of lava (magma)
    • Igneous rocks are a complex mix of many minerals and gases
      • intrusive igneous rocks form from magma that never reached Earth's surface
      • extrusive igneous rocks form from magma that spilled out onto Earth's surface
    • Intrusive forms
      • intrusions are discordant if they disrupt existing structures
        • batholith
        • stock
        • dike
      • intrusions are concordant if they do not cut across existing rock
        • sill
        • laccolith
    • Jointing and exfoliation
      • jointing is the tendency of rock to form parallel fractures without any obvious movement
      • joint planes are planes of weakness and separation
      • exfoliation is a special kind of jointing that forms concentric circles, caused by release of overlying pressure on rock, and subsequent expansion
    • Igneous rocks in the landscape
      • resist weathering and erosion
      • mesa
      • dike
      • volcanoes


Review Questions

  1. List five of the properties of minerals that aid in their classification.
  2. What is the difference between igneous and sedimentary rocks?
  3. List and describe a few of the intrusive forms in the landscape, using Fig. 30.4.