1. Igneous rocks
Rocks that form from cooling and solidifying molten magma and/or lava
Magma—molten rock beneath the Earth’s surface
Lava—molten rock above the Earth’s surface

2. Coarse (large crystals) Cool slowly Form underground
Intrusive (Plutonic) Igneous Rocks
Coarse (large crystals)
Cool slowly
Form underground
Have interlocking, random crystals

3. gas pockets (gas holes)
Extrusive (Volcanic) Igneous Rocks
Fine (small crystals) OR Glassy (no crystals)
Cool quickly
Form above ground
May be vesicular or non-vesicular—meaning the rock has
gas pockets (gas holes)

4. gas pockets (gas holes)
Intrusive (Plutonic) Igneous Rocks
Coarse (large crystals)
Cool slowly
Form underground
Extrusive (Volcanic) Igneous Rocks
Fine (small crystals) OR Glassy (no crystals)
Cool quickly
Form above ground
May be vesicular or non-vesicular—meaning the rock has
gas pockets (gas holes)

5. ESRT page 6

6. ESRT page 6 Earth surface
This chart is broken into 3 pieces. Two are shown here: Environment of formation and the characteristics shown along the arrows.
Think of the line separating the intrusive from the extrusive rocks as the Earth surface. The intrusive rocks form below the Earth’s surface and take a long time to cool. The extrusive rocks form above the Earth’s surface and take a short amount of time to cool.

7. The top arrow refers to the overall color of the rock.
Earth surface
The top arrow refers to the overall color of the rock.
The lighter the rock, the further to the left it will be on the chart. The darker the rock, the further to the right it will be on the chart.

8. The middle arrow refers to the density of the rock.
Earth surface
The middle arrow refers to the density of the rock.
The lower the density of the rock, the further to the left it will be on the chart. The higher the density of the rock, the further to the right it will be on the chart.

9. Earth surface
The bottom arrow refers to the composition of the rock. Here, geologists refer to the elements that make up the rock.
Felsic refers to a rock that has Silicon and Aluminum in it.
Mafic refers to a rock that has Fe and Mg in it. What do these stand for?
Fe = iron
Mg = magnesium

10. Earth surface
Rocks on the left side of the chart (obsidian, pumice, granite, etc.) are light colored, low density, and felsic (have a lot of Aluminum in them)

11. Earth surface
Rocks on the right side ( gabbro, basalt, dunite, etc.) are dark colored, high density, and mafic (have a lot of iron and magnesium in them)

12. The third part of the chart deals with the MINERALS that make up the rocks. Each of the minerals has a symbol on this crazy graph.

13. All the rocks in the first column have different amounts of the minerals colored in blue.

14. Not all rock samples have the exact same composition!!!
If a sample of Andesite contains quartz, it will NOT also contain pyroxene. This follows our straight vertical line rule for determining composition.
Not all rock samples have the exact same composition!!!