Earth Science

Materials of Earth’s Crust

Rocks

Study Tip
Remember that although nearly all rocks are made of minerals, a few are made of materials which do not fit the definition of minerals.

What are Rocks?

A rock is a naturally formed, non-living Earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass.

  • Rocks are made of minerals
  • Identified primarily by the minerals they contain and by their texture
  • Texture is a description of the size, shape, and arrangement of mineral grains
Study Tip
Remember that although nearly all rocks are made of minerals, a few are made of materials which do not fit the definition of minerals.
Rocks
As seen in Table below, these two rocks have the same chemical composition and contain mostly the same minerals, but they do not have the same texture. Sample 1 has visible mineral grains, but Sample 2 has some visible grains in a fine matrix. The two different textures indicate different histories. Sample 1 is a diorite, a rock that cooled slowly from magma (molten rock) underground. Sample 2 is an andesite, a rock that cooled rapidly from a very similar magma that erupted onto Earth’s surface.
Sample
Minerals
Textures
Formation
Rock Type
Sample 1
plagioclase, hornblende, pyroxene
Crystals, visible to naked eye
Magma cooled slowly
Diorite
Sample 2
plagioclase, hornblende, pyroxene
One type of crystal visible, rest microscopic
Magma erupted and cooled quickly
Andesite
Concept Check
  • Name a rock type that is not made of minerals and state how a rock could not be made of minerals
  • Can a rock be made of only one type of mineral, or do rocks need to be made of at least two minerals?
  • Why is texture so important in classifying rock types?