ROCKS

How are rocks identified and classified? Geologists look at different things to help them. 

Formation:   What was the PROCESS that made the rock?
There are three processes that can make a rock.

  • Compaction and/or Cementation of sediments
  • Melting and Solidification of magma or lava (melted material)
  • Pressure and/or Heating of any rock (intense heat or pressure)


Composition:
Geologists look at and determine what the rock is made of.
Where did the minerals come from? How were they formed?

Texture: The grain size and grain shape make up the texture.
Texture or Grain size is divided into terms describing the texture-

                 Based on SIZE
                     Fine –  smooth feeling, unable to see the individual grains
                     Medium – sandy feeling, you can see the individual grains
                     Course (large) – Rough, you can easily see the individual grains
                Based on LOOK
                    Glassy – smooth shiny finish (how glass looks)
                    Vesicular – full of holes left from gas bubbles in the rock (like a sponge)
                    Foliated – having folds, layers, stripes, or bands (like a sheet that is folded)       

Color: The color of the rock helps to determine the mineral composition


Rocks are grouped by HOW they are made and then Classified by characteristics.

Rock type Process Classifications
Sedimentary Compaction
and/or
Cementation
Clastic
(rock sediments)
Non-Clastic
(other sediments)
Igneous Melting
and
Solidification
Intrusive
(inside the earth)
Extrusive
outside the earth)
Metamorphic Heat
and/or
Pressure
Foliated
(folds or layers))
Non-Foliated
(no folds or layers)

 

Learn more about each type of rock

Sedimentary        Igneous          Metamorphic