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
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