• IGNEOUS: Igneous rocks form as molten rock cools and solidifies. Two environments are distinguished:
  • underground - in which case the melt is called 'magma' and the rock that results from its solidification is described as 'intrusive'.
  • on the surface - in which case the melt is called 'lava' and the rock that results is described as 'extrusive' or 'volcanic'.
• SEDIMENTARY: Sedimentary rocks form as grains of sediment are attached to each other by a cement or by the interlocking of grains. The environment in which this occurs is at or close to the earth's surface and is characterized by relatively low temperatures and pressures.
• METAMORPHIC: Metamorphic rocks form as pre-existing rocks respond to a radical change in environment, most commonly an increase in temperature, pressure, and/or an infusion of hot, mineralized fluids.

It is important to note that in general, these rock-forming processes and the environments in which they take place cannot be observed. Solidification of magma, most changes in temperature and pressure sufficient to cause metamorphism, and most processes that attach sediment grains to one another occur out of sight beneath the earth's surface*. They are, therefore, not observed 'facts', but hypotheses that geologists in the 18th and 19th centuries constructed to explain features of rocks that they examined. Gradually, these hypotheses gained acceptance and became elevated to the status of theories. Today they are accepted by all geologists. Our task is to examine the basis for this belief.

*Exceptions include the eruption and cooling of lava to form rock, the transformation of snow to ice, and the formation of salt rock in evaporating saline lakes, such as the Dead Sea or the Great Salt Lake, all of which can be observed.

Examples of rock-forming processes will next be considered in more detail.