What Kind of Rock Is This?
ES0610  Rock Identification: Introduction

Rocks are classified according to how they formed. In most cases, we don't see a rock during its formation, so we rely on rocks’ observable clues to infer their formations. Two clues that indicate a rock’s formation are its composition and texture.

  • Composition refers to what a rock is made of. The color of a rock can provide a clue to the composition. Fragments of other rocks, fossils, and identifiable mineral grains are also aspects of composition.
  • Texture is a description of the rock material. It includes characteristics such as crystal size and shape, number of different grain sizes, and alignment of grains.

  !   Examine these images of rocks. Consider how you might describe the composition and texture of each one. Scroll down the page to begin identifying your own rock samples.

Igneous rock: granite
Igneous rock: andesite

Sedimentary rock: conglomerate
Metamorphic rock: gneiss
Matthew Nyman, TERC

This investigation presents an online key for identifying real rocks (not rock images). On the Web pages, you'll answer a series of questions about a rock to come up with its type (igneous, sedimentary, or metamorphic) and its name. Tools such as a hand lens or magnifying glass, dilute hydrochloric acid, and a nail for testing hardness will be necessary for some of the tests.

  !   Before you start, you'll need a rock sample in your hands. To begin using the rock identification key:

1. Record the rock's sample number on your table.

2. Describe the texture and color or composition of your sample.

 Step:   1   2