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.
(inside the earth)
outside the earth)
(folds or layers))
(no folds or layers)
Learn more about each type of rock
Sedimentary Igneous Metamorphic