Rock Types

A rock is made up of one or more minerals.  A mineral is naturally occurring, inorganic, and with a specific crystalline structure.  As a result it has physical characteristics like color, hardness, and specific gravity that can be used for identification.

Rock type Key characteristics
Sedimentary Weathering and erosion, transport, deposition, and lithification
Igneous Melts to create a magma, and then solidifies by crystal growth
Metamorphic Recrystalization without melting to from new minerals from the application of heat and pressure

These are tied together in the rock cycle.  Any kind of rock can be the source of any other.  This idea of recycling goes back to at least the work of Hutton, and is also central to plate tectonics.

There are transitional cases, such as a volcanic tuff that falls into a lake (sedimentary or igneous), or a gneiss (metamorphic) that is just starting to melt into a granite (igneous), and you can see characteristics of both.  These just show that most human classifications are arbitrary, trying to put order into a complex world, and if we look at these hybrid rocks we clearly see the processes that lead to the two different classifications.

Last revision 8/24/2016